FMDS0788 FLAMMABLE LIQUIDS STORAGE TANKS

FM Global
Property Loss Prevention Data Sheets
7-88
October 2011
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FLAMMABLE LIQUID STORAGE TANKS
Table of Contents
Page
1.0 SCOPE .................................................................................................................................................... 3
1.1 Changes ........................................................................................................................................... 3
2.0 LOSS PREVENTION RECOMMENDATIONS ........................................................................................ 3
2.1 Construction and Location ............................................................................................................... 3
2.1.1 General .................................................................................................................................... 3
2.1.2. Aboveground tanks ............................................................................................................... 6
2.1.3 Buried Tanks ........................................................................................................................... 9
2.1.4 Indoor Tanks ......................................................................................................................... 10
2.1.5 Intermediate Bulk Containers (IBC) ..................................................................................... 11
2.1.6 Protection against Flooding ................................................................................................. 12
2.1.7 Earthquake ........................................................................................................................... 13
2.2 Occupancy ...................................................................................................................................... 13
2.2.1 General ................................................................................................................................. 13
2.2.2 Normal and Emergency Venting .......................................................................................... 14
2.2.3 Asphalt Tanks ....................................................................................................................... 23
2.3 Protection ....................................................................................................................................... 24
2.3.1 Indoor Tanks ......................................................................................................................... 24
2.3.2 Outdoor Tanks ...................................................................................................................... 24
2.3.3 Water Supply ........................................................................................................................ 25
2.4 Operation and Maintenance ........................................................................................................... 26
2.4.1 Repair, Reconditioning, and Abandonment ...................................................................... 27
2.5 Ignition Source Control .................................................................................................................... 27
3.0 SUPPORT FOR RECOMMENDATIONS .............................................................................................. 28
3.1 Background information ................................................................................................................. 28
3.1.1 Hazards ................................................................................................................................ 28
3.1.2 Types of Tanks .................................................................................................................... 29
3.1.3 Indoor Tanks ......................................................................................................................... 31
3.1.4 Tank Spacing and Containment ............................................................................................ 32
3.1.5 Manifolded Vents ................................................................................................................. 32
3.1.6 Asphalt Tanks ....................................................................................................................... 34
3.1.7 Fire Protection ..................................................................................................................... 35
3.2 Loss History .................................................................................................................................... 35
3.2.1 Storage Tanks ...................................................................................................................... 35
3.2.2 Manifolded Vents .................................................................................................................. 36
4.0 REFERENCES ...................................................................................................................................... 36
4.1 FM Global ........................................................................................................................................ 36
4.2 NFPA ................................................................................................................................................ 37
4.3 Others ............................................................................................................................................. 37
APPENDIX A GLOSSARY OF TERMS ....................................................................................................... 38
APPENDIX B DOCUMENT REVISION HISTORY ....................................................................................... 40
APPENDIX C HYDROCARBON FIRE DURATION .................................................................................... 41
APPENDIX D HAZARDS ............................................................................................................................ 43
List of Figures
Fig. 1. Horizontal aboveground tank .............................................................................................................. 5
Fig. 2. Buried tank with flame arrester ........................................................................................................ 10
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Flammable Liquid Storage Tanks
FM Global Property Loss Prevention Data Sheets
3. Enclosed indoor tank ........................................................................................................................ 12
4. Cone roof vertical tank ..................................................................................................................... 13
5. Safe gauging methods ..................................................................................................................... 15
6. Manifolded tanks .............................................................................................................................. 22
7. Required pipe sizing if detonation arrester is smaller than nearby piping ...................................... 22
8. Improper piping around detonation arrester ..................................................................................... 22
9. Open top double deck ...................................................................................................................... 30
10. Open top pontoon .......................................................................................................................... 30
11. Pan-type covered tanks .................................................................................................................. 31
12. Detonation arrester ......................................................................................................................... 32
13. Storage tank with flame arrester .................................................................................................... 33
14. End-of-line flame arrester ............................................................................................................... 33
15. End-of-line flame arrester with pipe-away flange ........................................................................... 34
16. Backflash interrupter ....................................................................................................................... 34
17. Typical conservation vent ................................................................................................................ 38
List of Tables
Table 1. Support (Saddle) Width for Horizontal Steel Flammable Liquid Tanks ............................................ 4
Table 2. Spacing for Flammable Liquid Storage Tanks and Loading/Unloading Stations ............................ 6
Table 3. Spacing for Flammable Liquid Tank Containment Dikes ................................................................ 7
Table 4. Indoor Tank Quantity Limits ........................................................................................................... 10
Table 5. Size of Opening for Normal Venting ............................................................................................... 16
Table 6. Required Thermal (Normal) Venting Capacity 1 ............................................................................. 17
Table 7. Typical Vent Line Size for Buried Tanks ......................................................................................... 18
Table 8. Capacities for Emergency Relief of Excessive Internal Pressure for
Aboveground Tanks Operating at 1 psig (7 kPa) or less1 ............................................................................ 19
Table 9. Values for L (M)1/2 ........................................................................................................................... 20
Table 10. Environmental Factors for Emergency Venting Capacity (use only one factor) .......................... 21
Table 11. Sprinkler Density for Storage Tank Rooms, gpm/ft2(mm/min) ...................................................... 24
Table 12. Hose Stream Demand for TANKS Storing Flammable Liquids 1 ................................................. 26
Table 13. Estimated Water Demand for Fixed Foam Protection for a full Surface Fire. ............................. 26
Table 14. Safety Distances for Hot Work, Open Flames, Maintenance, Repair or Modification ................. 28
Table 15. Losses over US$100,000 by Occupancy Class ........................................................................... 35
Table 16. Losses over US$100,000 by Engineering Peril ............................................................................ 36
Table 17a. Relationship Between Fuel Volume, Pool Size, and Fire Duration (English) ............................. 41
Table 17b. Relationship Between Fuel Volume, Pool Size, and Fire Duration (metric) ............................... 42
Table 18a. Flow Rate, Pool Diameter, Heat Release Rate, and Flame Height for a
Flowing Kerosene Fire (English) ................................................................................................. 42
Table 18b. Flow Rate, Pool Diameter, Heat Release Rate, and Flame Height for a
Flowing Kerosene Fire (Metric) ................................................................................................... 42
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1.0 SCOPE
In this data sheet, the term ‘‘flammable liquid’’ is used synonymously for all three classes of liquids. Where recommendations vary because of flash point, that is indicated.
The recommendations in this data sheet apply to chemically stable or unstable flammable liquids when stored
in atmospheric pressure (operating at less than 1 psig [0.07 barg]) or low pressure (operating over 1 psig
[0.07 barg] and less than 15 psig [1 barg]) tanks.
This data sheet applies to storage in horizontal and vertical tanks usually constructed of metal and located
aboveground, underground, or inside buildings. Storage in floating roof tanks is not addressed. For guidance on floating roof tanks, refer to NFPA 30, Flammable Liquid Storage in Portable Containers, or equivalent national or international standard.
This data sheet provides requirements for intermediate bulk containers (IBCs), when used to supply liquids
to a process. Storage of IBC is covered by DS 7-29, Flammable Liquid Storage in Portable Containers.
This data sheet does not cover all aspects of pumping operations as represented by load and unload racks,
pump pads at tank farms, or fuel pumping and transfer systems in buildings. Spacing criteria for some of
these peripheral operations are provided in Table 2. For other aspects, refer to DS 7-32, Flammable Liquid
Operations.
The recommendations for drainage, fire protection, separation, or diking do not apply to day tanks or other process tanks. Locate and protect those tanks in accordance with the appropriate FM Global data sheet, such
as Data Sheet 7-14, Fire and Explosion Protection for Flammable Liquid, Flammable Gas, and Liquefied
Flammable Gas Processing Equipment and Supporting Structures; Data Sheet 7-30, Solvent Extraction
Plants; Data Sheet 7-32, Flammable Liquid Operations, or Data Sheet 7-43/17-2, Loss Prevention in Chemical Plants.
This data sheet does not apply to flammable liquids or gases stored in pressure vessels above 15 psig (103
kPa). See Data Sheet 7-55, Liquefied Petroleum Gases, for such storages.
This data sheet addresses methods to prevent flame propagation throughout low-pressure flammable liquid storage tanks that are manifolded together to reduce atmospheric emissions where the presence of an
ignitable vapor-air mixture in normal operation is likely.
This data sheet does not address preventing flame propagation in fuel gas piping systems (see Data Sheet
6-10, Process Furnaces) in systems handling acetylene (see Data Sheet 7-51, Acetylene) or in fume collection systems for process equipment (see Data Sheet 7-78, Industrial Exhaust Systems).
1.1 Changes
October 2011. The reference in Table 10, note 1 was corrected from 2.2.2-5 to 2.1.2-5.
2.0 LOSS PREVENTION RECOMMENDATIONS
2.1 Construction and Location
2.1.1 General
1. Construct atmospheric tanks (operating at less than 1 psig [0.07 barg]) in accordance with the following
recognized engineering standards or suitable national or international equivalents:
a) API (American Petroleum Institute) Standard 650, Welded Steel Tanks for Oil Storage
b) UL (Underwriters Laboratories) 142, Standard for Steel Aboveground Tanks for Flammable and Combustible Liquids
c) UL 2080, Fire Resistant Tanks for Flammable and Combustible Liquids
d) UL 2085, Protected Aboveground Tanks for Flammable and Combustible Liquids
e) UL 2244, Standard for Aboveground Flammable Liquid Tank Systems
f) UL 58, Standard for Steel Underground Tanks for Flammable and Combustible Liquids
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2. Construct low-pressure tanks (operating at more than 1 psig [0.07 barg] but less than 15 psig [1 barg])
in accordance with the following recognized engineering standards or suitable national or international equivalents:
a) API Standard 620, Design and Construction of Large, Welded, Low-Pressure Storage Tanks
b) Code for Unfired Pressure Vessels, Section VIII, Division 1 of the ASME Boiler and Pressure Vessel
Code
c) EN BS 14015, Specification for Design and Manufacture of Site Built, Vertical, Cylindrical, FlatBottomed, Aboveground, Welded, Steel Tanks for the Storage OF Liquids at Ambient Temperatures and
Above
d) EN BS 12285, Part 1, Workshop Fabricated Steel Tanks — Horizontal Cylindrical Single and Double
Skin Tanks for Underground Storage of Flammable and Non-Flammable Water Polluting Liquids
e) EN BS 12285, Part 2, Workshop Fabricated Steel Tanks — Horizontal Cylindrical Single and Double
Skin Tanks for Aboveground Storage of Flammable and Non-Flammable Water Polluting Liquids
3. Design supports for horizontal cylindrical tanks to minimize settlement or lateral movement that could result
in overstress or rupture of the tank or associated pipe and fittings.
a) Provide supports of fire-resistive construction (e.g., saddles of reinforced concrete as shown in Fig. 1),
with at least one-third of the circumference of the tank bearing on the supports. Protect reinforcing steel
in concrete saddles with at least 2 in. (50 mm) of concrete.
b) Design saddles in accordance with the following table.
Table 1. Support (Saddle) Width for Horizontal Steel Flammable Liquid Tanks
Capacity, gal (m3)
Tank diameter,
in (mm)
Saddle width,
in (mm)
≤ 550 (2.1)
48 (1220)
> 550 ≤ 1100
(2.1 – 4.2)
64 (1625)
> 1100 ≤ 9,000
(4.2 – 34)
76 (1930)
> 1100 ≤ 35,000
(4.2 – 133)
144 (3660)
> 35,000
≤ 50,000
(133 – 189)
144 (3660)
4.5 (115)
6 (150)
6 (150)
9 (230)
10 (255)
c) Provide fireproofing of tank supports that are structural steel with a material having a fire resistance
of 2 hrs (concrete in accordance with DS 1-21 or an FM Approved coating rated for process structure or
tank protection) or protected with automatic water spray nozzles in accordance with DS 4-1N.
d) Provide bracing to prevent movement in locations subject to earthquakes.
e) In an area subject to flooding, anchor tanks to prevent either full or empty tanks from floating during
a rise in water level up to the maximum flood stage. Details are given in Section 2.2.6, Protection against
Flooding.
4. Pressure vessels and low-pressure tanks may be used as atmospheric storage tanks. Where unstable liquids are stored, see 2.2.2.2 – 7.
5. Fixed tanks of combustible construction (usually glass fiber-reinforced plastic) may be used:
a) for underground installation.
b) where required by the properties of the liquid stored.
c) for liquids with flash point greater than 200°F (93°C) stored outdoors where not exposed to the leakage of liquids with lower flash point.
d) for liquids with flash point greater than 200°F (93°C) (or any flash point if required as in b) stored indoors
with suitable automatic sprinkler protection and containment and installed in accordance with 2.1.4, below.
6. When glass fiber-reinforced plastic (FRP) tanks are used:
a) Construct the tank in accordance with the following recognized engineering standards or suitable
national or international equivalents:
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Fig. 1. Horizontal aboveground tank
1. UL 1316, Standard for Glass Fiber Reinforced Plastic Underground Storage Tanks for Petroleum
Products, Alcohols, and Alcohol-Gasoline Mixtures
2. API Specification 12P, Fiberglass Reinforced Plastic Tanks
3. ASTM D3299-Standard Specification for Filament Wound Glass Fiber Reinforced Thermoset Resin
Chemical Resistant Tanks.
4. ASTM D4097-Standard Specification for Contact Molded Glass Fiber Reinforced Thermoset Resin
Chemical Resistant Tanks.
5. EN BS 13121 GRP Tanks and Vessels for Use Aboveground.
Part 1: Raw materials – specification and acceptance conditions
Part 2: Composite materials – chemical resistance
Part 3: Design and Workmanship
Part 4: Delivery, installation and maintenance
b) Install aboveground tanks on a concrete pad in the vertical position only.
c) Store only chemically stable liquids, compatible with the reinforced plastic.
d) Provide separate dikes for all reinforced plastic tanks over 2,500 gal (9.5 m3).
e) Provide spacing for all reinforced plastic tanks in accordance with Table 2.
f) On tanks containing liquids with flash point less than 100°F (38°C), install conductive metal fill and discharge lines, supported internally and extending to within 3 in. (76 mm) of the tank bottom, and provide
a static ground to dissipate charges that can accumulate during filling operations.
g) Where tanks are located indoors, provide automatic sprinkler protection designed in accordance with
section 2.3.1, below. Automatic sprinkler protection may be omitted in low-value buildings that have
adequate space separation from important buildings and structures.
h) Install buried tanks in strict conformance to the manufacturer’s recommendations.
7. Construct tanks that have special features, such as corrosion resistance, with strength equivalent to that
of steel tanks.
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8. Concrete tanks without liners may be used for the storage of liquids with flash points higher than 100°F
(38°C) and specific gravities greater than 0.8.
9. Concrete tanks with combustible or noncombustible liners may be used for the storage of liquids with flash
points lower than 100°F (38°C) when designed in accordance with recognized engineering standards.
Choose the type and thickness of the liner depending on the properties of the liquid to be stored.
10. Provide rectangular steel tanks with internal braces to withstand hydrostatic loads in accordance with recognized engineering standards.
11. Where combustible plastic insulation is used on storage tanks, install a proper fire protective coating
over the insulation or install FM Approved Class 1 insulation. See Data Sheet 1-57, Plastic in Construction
, for additional guidance.
12. Prior to being placed in service, test all tanks in accordance with the standard under which they were constructed; generally, by filling the tanks with water and observing them for leakage. (PRIOR 2.1.1.5)
2.1.2. Aboveground tanks
1. Locate aboveground tanks on ground sloping away from main facility buildings and plant utility installations. On hilly terrain, provide drainage or dikes to bypass buildings or installations at lower levels.
2. Locate tanks with respect to buildings and other tanks in accordance with Table 2.
Table 2. Spacing for Flammable Liquid Storage Tanks and Loading/Unloading Stations
Stable liquids, tank to bldgs of non combustible or better
construction (See Appendix A) or open process structures (3)
Stable liquids, tank to buildings of combustible construction
(See Appendix A)
Stable liquids in listed UL 2080, 2085 and 2244 containers
Unstable liquids, tank to bldgs of any construction
Stable liquids, tank to tank
Unstable liquids, tank to tank
Tank truck and railcar loading/unloading to tank, (4)
Tanks (single or multiple) to LPG storage
Liquid Flash Point (1)(2)
≤ 140°F (60°C)
> 140°F (60°C)
1 D (min 75 ft, 23 m)
0.5 D (min 50 ft, 15 m)
2 D (min 125 ft, 38 m)
1 D (min 75 ft, 23 m)
See 2.1.2 – 6
2 D (min 125 ft, 38 m)
1 D (min 75 ft, 23 m)
0.5 D (min 3 ft, 0.9 m)
0.5 D (min 3 ft, 0.9 m)
1 D (min 5 ft, 1.5 m)
1 D (min 5 ft, 1.5 m)
75 ft (23 m)
50 (15 m)
minimum 100 ft (30 m) or 1 D
Notes
1
Where tanks are equipped with internal heating systems and store liquids subject to boil over, froth over, or slop over, evaluate as if containing liquids with flash points = 140°F (60°C), regardless of their flashpoint.
2
D refers to the diameter of the largest flammable liquid tank.
3
Open process structure refers to areas of one or multiple levels used to manufacture chemicals. Intermediate tanks considered part of
the process are excluded from this spacing requirement.
4
For separation between loading/unloading facilities and buildings, see DS 7-32.
3. Provide containment for tanks containing flammable liquids with flash points below 200°F (93°C) by remote
impounding, dikes around the tanks, or secondary containment. (Environmental or other government regulations may require containment for smaller tanks.)
4. Construct dikes used to provide containment around the tanks according to the following criteria:
a) Size dikes to hold 100% of the contents of the largest tank within the diked area. The volume occupied by this tank below the top of the dike may be considered part of the dike capacity unless the liquid
stored is subject to boil over. The volumes of all other tanks below the top of the dike must be deducted
when calculating dike capacity.
b) Construct dike walls of earth, steel, concrete, or solid masonry, designed to be liquid-tight and to withstand a full hydrostatic head by release of tank contents.
c) Provide earthen walls 3 ft (1 m) or more in height with a flat section at the top not less than 2 ft (0.6
m) wide with the wall slope consistent with the angle of repose of the material of which the wall is constructed.
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d) Control vegetation, desirable protection against erosion, so as not to impede fire fighters or add to
the fire hazard.
e) Limit the height of dikes, regardless of construction, to 6 ft (2 m) to minimize the chances of pocketing flammable vapors and to facilitate fire fighting.
f) Provide drainage to remove water from within diked areas at a minimum uniform slope of 1% away
from tanks toward a sump, a drain box, or other means of disposal located at a safe distance from the
tank.
g) Design drains to prevent liquids from entering natural water courses, public sewers, or drains. Trap
drain lines and provide valves on the lines, outside the dike, so they are accessible under fire conditions. Protect the traps from freezing.
h) Limit dikes to contain an aggregate capacity of 5,000,000 gal (18,900 m3), except were individual tank
capacity exceeds 5,000,000 gal (18,900 m3) in which case, ensure the dike contains only one tank.
i) Subdivide any dike containing two or more tanks by intermediate dikes or channels as follows:
1. Stable liquids in weak seam roof tanks
a. Subdivision for each tank over 420,000 gal (1,590 m3)
b. Subdivision for each group of tanks with total capacity up to 630,000 gal (2,390 m3), none individually > 420,000 gal (1,590 m3)
2. Stable liquids in horizontal tanks or vertical cone roof tanks
a. Subdivision for each tank over 100,000 gal (380 m3)
b. Subdivision for each group of tanks with total capacity up to 150,000 gal (570 m3), none individually >100,000 gal (380 m3)
3. Unstable liquids in any type of tank need individual subdivision.
4. Unstable liquids in any type of tank protected by water spray in accordance with Data Sheet 4-1N
can follow the subdivision requirements in “2” above.
j) Build intermediate dikes at least 18 in high.
k) Provide separation between a tank and the dike wall of at least one-half the tank diameter.
l) Provide separation between buildings and dike wall in accordance with Table 3.
m) Where tanks are arranged in more than two adjacent rows or in an irregular pattern, provide greater
spacing between tanks, additional dikes, or roadways so all tanks are accessible for firefighting.
Table 3. Spacing for Flammable Liquid Tank Containment Dikes
Stable liquids, dike wall to buildings of noncombustible or
better construction (See Appendix A) or open process
structures (3)
Liquid Flash Point (1)(2)
≤ 140°F (60°C)
> 140°F (60°C)
1 D (min 75 ft; 23 m;
0.5 D (min 50 ft; 15 m;
max. 200 ft, 61 m)
max. 200 ft, 61 m)
Stable liquids, dike wall to buildings of combustible construction
(See Appendix A)
2 D (min 125 ft, 38 m;
300 ft, 91 m)
1 D (min 75 ft, 23 m; 300
ft, 91 m)
Unstable liquids, dike wall to buildings any construction
2 D (min 125 ft, 38 m;
300 ft, 91 m)
1 D (min 75 ft, 23 m; 300
ft, 91 m)
Notes
Where dikes contain tanks equipped with internal heating systems and store liquids subject to boil over, froth over, or slop over, protect
as liquids with flash points = 140°F (60°C) regardless of their flashpoint.
2
D usually refers to the longest dike dimension, length, width, or diameter (if circular).However, where a non-circular dike is present, base
the spacing to the exposure on the exposing dimension, i.e., the side that most directly faces the exposed structure, vessel or other dike,
not necessarily the longest dimension.
3
Open process structure refers to areas of one or multiple levels used to manufacture chemicals. Intermediate tanks considered part of
the process are excluded from this spacing requirement.
1
5. Design remote impounding used for containment in accordance with the following criteria:
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a) Provide drainage within diked areas at a minimum uniform slope of 1% away from tanks toward the
impounding basin.
b) Route drainage between the tanks and the impounding basin so that if the liquid in the system is ignited,
it will not seriously expose tanks or important buildings (DS 7-83 can provide valuable guidance on the
design of the drainage system).
c) Provide the impounding basin with a minimum capacity equal to twice the largest tank that could drain
to it.
d) Equip the impounding basin with means to drain off accumulations of water from precipitation.
e) Separate the impounding basin from important buildings and facilities according to the size of the basin
and the exposure potential to the building, as follows:
1. From buildings of ordinary or combustible construction (or from buildings containing hazardous materials) having extensive window areas or associated combustible yard storage, spacing distance = 1.8
× basin diameter or diagonal.
2. From buildings of fire resistive or noncombustible construction not having extensive window areas,
hazardous materials, or associated combustible yard storage, spacing distance = 0.6 × basin diameter or diagonal.
3. From flammable liquid storage tanks, spacing distance = 0.3 × basin diameter or diagonal.
f) Provide each diked and/or subdivided area with drainage systems leading to the impounding basin.
Hydraulically design the drainage system from each diked or subdivided area as follows:
1. Provide drainage capacity from each subdivision in a dike capable of carrying off liquid at a rate
not less than that which could be released assuming a break in a bottom connection from the largest
full tank within the subdivision, or the maximum tank fill rate, whichever is greater.
2. Use drainage system piping that is a minimum of 24 in. (60 cm) diameter.
3. Route piping under subdivisions and dikes to culverts or open channels leading to the impounding
basin.
4. Design culverts or open channels with the capacity to carry off liquid from all the drainage connections within the diked area having the largest single tank, with the connections flowing at their maximum capacity.
5. Design the culverts or channels with additional capacity to carry off the maximum expected discharge of water from fire fighting operations.
6. Locate open channels a minimum of 50 ft (15 m) from important buildings and facilities.
7. Provide roads with culverts over the channels at intervals to permit access to the tanks for maintenance or emergencies.
8. Pave channels with asphalt or concrete, or line them with smooth stone, metal, or compacted clay
to prevent growth of vegetation that could restrict liquid flow.
9. Provide a minimum of 1% slope for channels and culverts. Steeper slopes are advisable to reduce
culvert or channel size.
6. Secondary containment tanks (double skinned) listed as meeting the requirements of UL 2080, 2085, 2244,
and EN BS 12285, Part 2:
a) are limited to a capacity of 12,000 gal (45 m3) (Locate tanks exceeding 12,000 gal (45 m3) in accordance with Table 2 and meet all of the remaining criteria in b through i).
b) are limited to the storage of stable flammable liquids.
c) are spaced a minimum of 5 ft (1.5 m) from building walls or openings.
d) are spaced a minimum of 3 ft (1 m) from adjacent tanks of the same type.
e) are protected against vehicle impact by suitable barriers except where the tank is specifically listed
and marked as having passed vehicle impact testing.
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f) are provided with a means to prevent siphon flow from the tank.
g) are provided with a means, accessible to the delivery operator, for determining the level of liquid in
the tank.
h) are provided with a means to prevent overfilling by sounding an alarm when the liquid level in the tank
reaches 90 percent of capacity and by automatically stopping delivery of liquid to the tank when the liquid level in the tank reaches 95 percent of capacity, without restricting or interfering with the proper functioning of the normal vent or the emergency vent.
i) do not need additional spill containment by way of impounding basins or drainage.
2.1.3 Buried Tanks
1. Locate buried tanks at least 5 ft (1.5 m) from building foundations and 2 ft (0.6 m) from other tanks and
pipelines. Where a choice of location is offered, choose the one farthest removed from below-grade open
areas such as pits and basements under important buildings.
2. Anchor the tanks where groundwater conditions are bad or where flooding is possible (Fig. 2). Details
are given in Section 2.1.6, Protection Against Flooding, below.
3. Cover buried tanks with 2 ft (0.6 m) of earth, except under concrete paving at least 4 in. (100 mm) thick,
where 1 ft (0.3 m) of earth is sufficient. In either case, provide an additional 1 ft (0.3 m) of cover at tank locations over which heavy vehicles pass. Reinforce paving over the tank and extend at least 1 ft (0.3 m) beyond
the tank perimeter in all directions to transmit the superimposed load to foundations beside the tank.
4 The equivalent of a location below ground may be obtained with a substantial portion of a tank above grade.
Earth is placed over the tank to form a 1 to 2 ft (0.3 to 0.6 m) cover at the angle of repose of the fill used.
A concrete retaining wall or lock-sheet steel piling may be placed around the tank and filled with earth to
reduce space requirements.
5. Protect tanks against corrosion as follows:
a) Provide at least 6 in. (150 mm) of well compacted clean gravel or sand around the tank.
b) Locate the tanks above the groundwater level.
c) Provide a protective coating on steel tanks. The base coat, usually applied by the manufacturer, acts
as a primer. The outer coating, applied in the field, needs to be compatible with the base coat.
d) Patch-paint portions of the protective coating damaged when the tank is installed.
e) Cathodic protection is an acceptable alternate to protective coatings.
6. Provide openings for normal venting in accordance with Section 2.2.2.1.2. Venting for fire exposure is
unnecessary.
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Fig. 2. Buried tank with flame arrester
2.1.4 Indoor Tanks
1. Limit the quantity of flammable liquids in indoor tanks in accordance with Table 4 below.
Table 4. Indoor Tank Quantity Limits
Liquid Flash Point
≤ 200°F (93°C)
>200°F (93°C)3
Location of Tank(s)
Upper floor
Grade level
Basement
Upper floor
Grade level 2
Basement
Maximum Indoor Storage, One or More Tanks
gal
m3
2,000
7.5
2,000
7.5
Not permitted
Not permitted
5,000
20
50,000 1
190 1
5,000
20
Notes
Not over 25,000 gal (95 m3) in one tank
Limit FRP tanks to 5,000 gal (20 m3) and to liquids with flash points greater than 200°F (93°C) in accordance with section 2.1.1 – 5.
3
Tanks containing liquids heated to within 25°F (14°C) of their flash point are evaluated in the ≤ 200°F (93°C) category.
1
2
2. Arrange tanks meeting the quantity limits of Table 4 as follows:
a) When located at grade level, provide a cut-off room for the purpose of containing the liquid storage
tank(s). Provide concrete or masonry construction with a minimum one-hour fire-rating for the cut-off room,
including similarly rated doors for any openings into the main building. Important structural steel needs
to be similarly protected for one-hour fire resistance. Design the covering specifically for a hydrocarbon fire
exposure. Locate the room along an outside wall with openings accessible to firefighters.
b) When located above or below grade level, provide a room separated from other occupancies by a wall
of at least 3-hour fire-rated concrete or masonry construction and a 3-hour fire-rated resistive covering
on any exposed steel. Design the covering specifically for a hydrocarbon fire exposure.
c) Repair spalled areas of fire-resistive coatings on structural framing if the spalled area exceeds more
than 4 in2 (25 cm2).
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d) Provide hard-piped fill lines terminating outside the building.
e) Provide overflow protection using a high-liquid-level device that automatically shuts down filling operations on detection of a high liquid level in the tank using an automatic safety shut-off valve. Locate the
safety shut-off valve as close to the tanker truck connection as possible.
f) Arrange the discharge line to exit the top of the tank. For tanks with bottom discharge lines, provide a fusible link-operated safety shut-off valve (SSOV) at the tank outlet.
g) Provide fire detection that automatically interrupts fill or discharge operations using automatic safety
shut-off valves. Approved water flow alarms, heat, smoke, or flame detection are acceptable means of
detecting a fire.
h) Provide detection for a liquid spill that automatically interrupts fill or discharge operations using automatic safety shut-off valves where the tank room is in a non-industrial occupancy, e.g., retail, office, education, or residential.
i) Provide properly sized normal and emergency relief vents hard-piped to a safe location outside the building. See Section 2.2.2 below.
j) Provide drainage designed to dispose of the discharge from all sprinklers in the room, as well as spilled
liquids, in accordance with Data Sheet 7-83, Drainage Systems for Flammable Liquids.
k) Where drainage to a safe location is impractical, provide containment sufficient to hold the contents
of all spilled liquid plus a minimum of a 2 in. (5 cm) freeboard and an FM Approved special protection system. Containment can be provided with curbs, dikes, and existing walls. Ensure floors and walls are liquidtight for the height of the required containment. Spilled liquid can include release from the storage tanks
or from an uncontrolled release during filling operations.
l) Support tanks either directly on the floor or in accordance with Section 2.1.1 – 3.
3. An alternative arrangement to “2” above is to locate the tank in a liquid tight, concrete or brick-walled enclosure with the space between the tank and the wall filled with sand to a depth of 1 ft (0.3 m) above the tank
as in Figure 3. In this case, there are no quantity limitations.
a) Provide hard-piped fill lines terminating outside the building.
b) Provide overflow protection using a high-liquid-level device that automatically shuts down filling operations on detection of a high liquid level in the tank using an automatic safety shut-off valve. Locate the
safety shut-off valve as close to the tanker truck connection as possible.
c) Provide properly sized normal and emergency relief vents hard piped to a safe location outside the building. See Section 2.2.2.1.2 below.
4. Arrange pumps located inside buildings as follows:
a) Install positive displacement pumps.
b) Arrange the pump to take suction from the top of the tank. Elevate the pumps to the same elevation
as the top of the tank or provide an anti-siphon device. Locate the anti-siphon device as close to the tank
outlet as possible. (Some volatile liquids may require special pumping arrangements.)
c) Where a pump takes suction from the bottom of a tank, or when the pump is a centrifugal type, provide a safety shut-off valve at the tank, interlocked to shut down the pump in the event of a leak or fire.
d) Provide a pressure-relief valve down stream of any positive displacement pump and pipe it back to the
tank.
e) Implement all other requirements for flammable and combustible liquid transfer systems as required
in DS 7-32 under “transfer by pumping”.
2.1.5 Intermediate Bulk Containers (IBC)
1. Store metal, composite, or plastic IBCs inside or outside in accordance with the requirements of DS 7-29,
Flammable Liquid Storage in Portable Containers.
2. Treat IBCs used to supply flammable liquid to any type of process as indoor tanks and implement the recommendations in Section 2.1.4.
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Fig. 3. Enclosed indoor tank
2.1.6 Protection against Flooding
1. Locate tanks (aboveground or buried) so that at least 30 percent of their allowable storage capacity extends
above the 100-year flood level, or secure the tank by one of the following methods:
a) Anchor the tank to resist movement.
b) Attach the tank to a foundation of steel and/or concrete having sufficient weight to provide adequate
load for the tank when submerged by flood water to the 100-year flood level (Fig. 2). If the tank can be
water-loaded, the anchoring load can be calculated assuming a full tank; otherwise assume an empty tank.
c) Adequately secure the tank from floating by other means.
d) Fill the tank (buried), or to at least 70% capacity (above-ground), with water from a dependable supply (if that is not impractical or hazardous due to the contents of the tank).
2. Construct any tank that is assumed to be submerged empty to safely resist external pressures.
3. Extend tank vents or other openings that are not liquid-tight above the 100-year flood level.
4. Provide tight closures at tank openings to prevent displacement of the tank contents by flood waters.
5. Where water filling is required to prevent tank floating, develop an emergency plan that includes a water
supply for water loading that is independent of public water or power supplies to allow for filling the tanks
to increase their weight.
6. Prior to water loading, close all valves not used in connection with the filling operation.
7. Facility fire protection water may be used for filling if:
a) other water supplies are lost.
b) normal plant operations are terminated and a fire watch is started.
c) the loading operation is done through hoses and is constantly attended.
d) no fire pump, divisional, or sprinkler control valves have to be shut
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e) the water supplies are not drawn down to seriously affect the required duration of sprinkler or hose
stream discharge.
f) the loading operation is immediately shut down in the event of a fire emergency.
8. When filling is complete, close and lock all valves on connecting pipe.
2.1.7 Earthquake
1. In FM Seismic Zones 150 and under, provide restraint and appropriate flexibility in piping connections
and associated tanks, pipe headers and piping systems per the requirements in DS 1-11, Fire Following Earthquakes.
2. Where tanks are located indoors, arrange all liquid transfer operations to be shut down during a seismic
event using seismic shutoff valves.
2.2 Occupancy
2.2.1 General
1. Make pipe connections to tanks above the normal liquid level.
2. Extend fill, return, and similar pipes below the lowest level of liquid in the tank or within about 6 in. (150
mm) of the tank bottom,(Fig. 1).
3. Where bottom connections are present:
a) provide steel shutoff valves bolted or welded to the first flange connection on the tank.
b) keep valves closed except when liquid is being transferred. (Fig. 4).
c) for tanks exceeding 10,000 gal (28 m3), provide valves that are manually controllable from a remote location.
d) provide a liquid-tight closure, such as a valve, plug, or blind, or a combination of these, on connections below the liquid level through which liquid does not normally flow.
Fig. 4. Cone roof vertical tank
4. Where pumps are provided, implement the requirements for flammable and combustible liquid transfer systems as required in DS 7-32 under “transfer by pumping‘‘.
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5. Do not permit piping in dikes to pass through a dike wall to an area containing other tanks which could
allow a spill or fire to spread to adjacent tanks.
6. Provide manway openings with a bolted, gasketed cover that is kept closed except when the tank is opened
for examination or maintenance (Fig. 4).
7. Provide level-gauging or measuring devices for all tanks.
8. Where manual gauging connections are present:
a) Where liquids with flash point below 100°F (38°C) are present, use a method that will not expose the
vapor space to outside atmosphere.
b) Avoid gauging equipment that will release large quantities of liquid if the equipment is damaged mechanically or by an exposure fire.
c) Where a rod and gauging well is provided, extend a pipe down into the tank below the level of the suction intake (Fig. 5[a]) to provide a liquid seal at the bottom of the well that prevents vapors above the
main body of liquid from escaping during gauging.
d) Install FM Approved devices for safe gauging (level measurement) of tanks .
9. Provide high-level alarms that sound at an attended location.
10. Arrange heating equipment for tanks as follows:
a) Provide heat only in the vicinity of the suction intake for tanks storing liquids with flash point below
200°F (93°C).
b) Provide only enough heat to ensure free flow of the liquid.
c) Arrange suction pipe or outlet pipe connections to ensure that heating coils will always be submerged.
d) For metal tanks, use steam, hot water or FM Approved electric heaters. For reinforced plastic tanks,
use only steam or hot water.
e) Steam heating coils are commonly used on tanks containing No. 5 and No. 6 fuel oil and similar liquids to reduce their viscosity for pumping. In one acceptable arrangement, a horizontal open-ended shell
or box contains the steam coils, and suction is taken from inside the shell. Another arrangement consists of a vertical spiral steam coil located around a top-connected suction pipe; this is acceptable if the
fill opening is trapped or the fill pipe is extended below the level of the suction intake (Fig.1).
f) Provide a steam pressure-relief valve close to the tank, set at about 5 psi (35 kPa) over normal working pressure, if steam is supplied through a reducing valve.
g) Provide FM Approved low-liquid- level and high-temperature interlocks to shut off the heating system.
11. Closely monitor all fill operations either by operator standing by or remote reading level gauges at an occupied location.
2.2.2 Normal and Emergency Venting
1. Provide normal venting to permit the intake and discharge of air during emptying and filling operations
and to permit expansion and contraction of vapor due to temperature changes.
2. Provide emergency venting to prevent the tank becoming overpressurized by fire exposure.
3. Where a mixture of several liquids is stored in the same tank, use the most volatile for the design basis.
4. Normal and emergency venting can be provided by one opening with a minimum capacity equivalent to
the emergency vent requirement.
5. Provide normally closed venting devices (conservation vents) on tanks storing liquids with flash points
less than 73°F (23°C) and boiling points less than 100°F (38°C).
6. Provide normally closed venting devices (conservation vents) or an FM Approved flash arrester on tanks
storing liquids with flash points above or equal to 73°F (23°C) and below 100°F (38°C), and with boiling points
above 100°F (38°C) or liquids that can be heated to their flash points under normal operating conditions.
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Fig. 5. Safe gauging methods
7. Prevent condensation in flame arresters on tanks containing liquids that solidify during cold weather by providing a heating arrangement such as a steam coil at the arrester.
8. Where polymerization of a material may occur at the arrester, provide a dual arrester equipped with a threeway valve so one arrester is always in service.
9. Where vent pipes are necessary to conduct vapors to a safe location, install them as follows:
a) Terminate vents close enough above the tank to avoid imposing a dangerous liquid head on the tank
if liquid overflows through the vent.
b) Extend vent pipe connections from indoor tanks to outside the building.
c) Terminate vents at a location free of potential ignition sources and away from openings through which
vapors can leak back into the building or locations where combustible construction would be exposed
by a fire burning at the end of the vent.
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d) Arrange horizontal runs of pipe to drain back to the tank.
e) Arrange the outlet and drains of vents on tanks operating at pressures in excess of 2.5 psig (17 kPa)
so they do not direct vapor discharge back onto the tank.
f) Terminate open vents either with a weather protective hood or a U-bend to keep out rain and provide
coarse screens to prevent foreign matter from obstructing the pipe.
g) Do not permit manifolding of tank vents for vapor recovery or air pollution control except in accordance with Manifolded Vents in Section 2.2.2.3.
2.2.2.1 Normal venting
2.2.2.1.1 Aboveground Tanks
1. For tanks with less than 50,000 gal (189 m3) capacity, the vent opening to meet normal venting requirements can be in accordance with Table 5 but at least as large as the largest of the fill or withdrawal connection.
Table 5. Size of Opening for Normal Venting
Tank Capacity, gals (m3)
Less than 2,500 (9.5)
2,500 – 3,000 (9.5 – 11)
3,001 – 10,000 (11 – 38)
10,001 – 20,000 (38 – 76)
20,001 – 35,000 (76 – 132)
35,001 – 50, 000 (132 – 189)
Minimum diameter, nominal pipe size, in. (mm)
1 ¼ (30)
1 1⁄2 (40)
2 (50)
2 1⁄2 (65)
3 (75)
4 (100)
2. For tanks with a capacity exceeding 50,000 gal (189 m3) provide venting as follows:
a) Provide inbreathing (vacuum) capacity of 1 ft3/hr free air for each 7.5 gal/hr of the maximum emptying rate (1 m3/hr inbreathing capacity for each 1 m3/hr emptying rate) plus the thermal venting capacity
given in Table 6.
b) For tanks storing liquid with a flash point ≤ 100°F (38°C), provide outbreathing (pressure) capacity of
1 ft3/hr free air for each 3.5 gal/hr of the maximum tank filling rate (1 m3/hr free air for each 0.47 m3/hr of
the maximum tank filling rate) plus the thermal venting capacity given in Table 6.
c) For tanks storing liquids with a flash point >100°F (38°C), provide outbreathing (pressure) capacity of
1 ft3/hr free air for each 7.0 gal/hr of the maximum tank filling rate (1 m3/hr free air for each 0.94 m3/hr
of the maximum tank filling rate) plus the thermal venting capacity given in Table 6.
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Table 6. Required Thermal (Normal) Venting Capacity
gal
Tank Capacity
42-gal
barrels
m3
42,000
84,000
126,000
168,000
210,000
420,000
630,000
840,000
1,050,000
1,260,000
1,470,000
1,680,000
1,890,000
2,100,000
2,520,000
2,940,000
3,360,000
3,780,000
4,200,000
5,049,000
5,880,000
6,720,000
7,560,000
1,000
2,000
3,000
4,000
5,000
10,000
15,000
20,000
25,000
30,000
35,000
40,000
45,000
50,000
60,000
70,000
80,000
90,000
100,000
120,000
140,000
160,000
180,000
160
320
480
640
800
1,600
2,400
3,200
4,000
4,800
5,600
6,400
7,200
8,000
9,600
11,200
12,800
14,400
16,000
19,200
22,400
25,600
28,800
Vacuum
All Stocks
m3/hr
ft3/hr
1,000
28
2,000
57
3,000
85
4,000
113
5,000
142
10,000
280
15,000
420
20,000
570
24,000
680
28,000
790
31,000
880
34,000
960
37,000
1,050
40,000
1,130
44,000
1,250
48,000
1,360
52,000
1,470
56,000
1,590
60,000
1,700
68,000
1,930
75,000
2,120
82,000
2,320
90,000
2,550
1
Pressure
Liquid Flash Point
≤ 100°F (38°C)
>100°F (38°C)
ft3/hr
m3/hr
ft3/hr
m3/hr
1,000
28
600
17
2,000
57
1,200
34
3,000
85
1,800
51
4,000
113
2,400
68
5,000
142
3,000
85
10,000
280
6,000
170
15,000
420
9,000
255
20,000
570
12,000
340
24,000
680
15,000
420
28,000
790
17,000
480
31,000
880
19,000
540
34,000
960
21,000
590
37,000
1,050
23,000
650
40,000
1,130
24,000
680
44,000
1,250
27,000
760
48,000
1,360
29,000
820
52,000
1,470
31,000
880
56,000
1,590
34,000
960
60,000
1,700
36,000
1,020
68,000
1,930
41,000
1,160
75,000
2,120
45,000
1,270
82,000
2,320
50,000
1,420
90,000
2,550
54,000
1,530
1.
Based on API Standard 2000, Venting Atmospheric and Low Pressure Storage Tanks, 5th Edition, 1998.
(These requirements are also in NFPA 30)
2.2.2.1.2 Buried Tanks
1. Provide vent pipes sized in accordance with Table 7 for the maximum flow in or out of the tank, but not
less than 1.25 in. (30 mm) nominal inside diameter to prevent blowback of vapor or liquid at the fill opening while the tank is being filled.
2. Extend vents a minimum of 12 ft (3.7 m) aboveground level for liquids with flash points below or equal
to 100°F (38°C), and a minimum of 6 ft (1.8 m) aboveground level for liquids with flash points above 100°F
(38°C).
3. Arrange vent pipes without traps or pockets so liquid condensate can drain back to the tank.
4. Arrange vent pipes to discharge upward or horizontally away from adjacent walls.
5. Locate vent outlets so vapors will not be trapped by eaves or other obstructions and at least 5 ft (1.5 m)
from building openings and 15 ft (4.5 m) from powered air-intake devices.
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Table 7. Typical Vent Line Size for Buried Tanks
gpm
100
200
300
400
500
600
700
800
900
1000
Maximum In/Out Flow
m3/hr
50 ft
20
45
70
90
115
135
160
180
205
225
1-1⁄4 in
1-1⁄4 in
1-1⁄4 in
1-1⁄4 in
1-1⁄2 in
1-1⁄2 in
2 in
2 in
2 in
2 in
15 m
30
30
30
30
40
40
50
50
50
50
mm
mm
mm
mm
mm
mm
mm
mm
mm
mm
100 ft
1-1⁄4 in
1-1⁄4 in
1-1⁄4 in
1-1⁄2 in
1-1⁄2 in
2 in
2 in
2 in
2 in
2 in
Vent Pipe Length
30 m
200 ft
30
30
30
40
40
50
50
50
50
50
mm
mm
mm
mm
mm
mm
mm
mm
mm
mm
1-1⁄4 in
1-1⁄4 in
1-1⁄2 in
2 in
2 in
2 in
2 in
3 in
3 in
3 in
60 m
30
30
40
50
50
50
50
75
75
75
mm
mm
mm
mm
mm
mm
mm
mm
mm
mm
2.2.2.2 Emergency Venting
1. Provide aboveground storage tanks containing stable liquids with emergency relief venting in the form of
construction or a device to relieve excessive internal pressure that develops from fire exposure.
a) Relieving construction can be in the form of a floating roof or weak seam roof.
b) A relieving device can be in the form of a floating manhole arranged for relieving, an open pipe, or a pressure relief valve suitable for the service. (UL 142, July 2002, section 8.10 – 12, provides design criteria
for floating manways.)
c) Emergency relief venting can be provided by the same device used for normal venting, provided it has
adequate capacity and pressure rating.
d) Stamp each commercial venting device, regardless of type, with its start-to-open pressure, the pressure at which it reaches its full-open position, and the flow capacity of the device at that pressure. Express
all flow capacities in either cubic feet per hour of air at 60°F and 14.7 psia or cubic meters per hour of
air at 15°C and 100 kPa absolute.
e) Emergency venting is not required for FRP tanks as the tank will fail at around 200°F (93°C)
f) Emergency venting is not required for tanks over 12,000 gal (45 m3) capacity containing liquids with
flash points above 200°F (93°C) that are not exposed to spills from liquids with flash point less than or equal
to 200°F (93°C ). Note: Normal in-and out-breathing is still required.
2. Where stable liquids are stored in tanks operating at 1 psig (7 kPa) or less, provide relief capacity/size
of the relieving device or construction in accordance with Table 8 below. (Note: Tanks with weak seam roof
construction have adequate emergency venting but would need normal venting for in-breathing and outbreathing)
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Table 8. Capacities for Emergency Relief of Excessive Internal Pressure for
Aboveground Tanks Operating at 1 psig (7 kPa) or less1
Wetted area of tank 2
ft2
m2
20
30
40
50
60
70
80
90
100
120
140
160
180
200
250
300
350
400
500
600
700
800
900
1,000
1,200
1,400
1,600
1,800
2,000
2,400
2,800 and over5
Vent Capacity 3
m3 free air per
ft free air per
hour (ft3/hr)
hour (m3/hr)
21,100
597
31,600
894
42,100
1,191
52,700
1,491
63,200
1,789
73,700
2,086
84,200
2,383
94,800
2,683
105,000
2,970
126,000
3,570
147,000
4,160
168,000
4,750
190,000
5,380
211,000
5,970
239,000
6,760
265,000
7,500
288,000
8,150
312,000
8,830
354,000
10,020
392,000
11,090
428,000
12,110
462,000
13,070
493,000
13,950
524,000
14,830
557,000
15,760
587,000
16,610
614,000
17,380
639,000
18,080
662,000
18,730
704,000
19,920
742,000
21,000
3
1.9
2.8
3.7
4.6
5.6
6.5
7.4
8.4
9.3
11.2
13.0
14.9
16.7
18.6
23.2
27.9
32.5
37.2
46.4
55.7
65.0
74.3
83.6
92.9
112
130
149
167
186
223
260 and over5
Minimum opening, NPS 4
in
mm
2
2
3
3
3
4
4
4
4
5
5
5
5
6
6
6
8
8
8
8
8
8
8
10
10
10
10
10
10
10
10
50
50
75
75
75
100
100
100
100
125
125
125
125
150
150
150
200
200
200
200
200
200
200
250
250
250
250
250
250
250
250
1.
This table is based on hexane. For other materials, Table 9 can be used for vent capacity adjustments.
The wetted area of the tank is defined as 55% of the total exposed area of a sphere or spheroid, 75% of the total exposed area of a horizontal tank, and the first 30 ft (10 m) above grade of the exposed shell area of a vertical tank. Include the bottom surface area of vertical tanks mounted on supports, above grade.
3.
Based on atmospheric pressure of 14.7 psia and 60°F (100 kPa abs. and 15°C)
4.
Based on open vent pipes of the noted diameter not more than 12 in. (0.3 m) long with a tank venting pressure of not more than 2.5
psig (17 kPa).
5
For tanks operating at pressures less than 1 psig (7 kPa) and having wetted areas exceeding 2800 ft2 (260 m2), complete fire involvement is unlikely and overheating will probably cause loss of metal strength in the vapor space before the development of a maximum vaporevolution rate. For such tanks, the maximum listed relief capacity is adequate.
For tanks operating at more than 1 psig (7 kPa) and having wetted areas exceeding 2800 ft2 (260 m2), the venting requirements are provided in Section 2.2.2.2 — 3.
2.
3. For tanks operating at pressures greater than 1 psig (7 kPa) and having exposed wetted areas greater
than 2800 ft2 (260 m2), calculate the emergency venting capacity by one of the following formulae:
V = 1107 A0.82 (English)
V = 220 A0.82 (metric)
Where V = hexane vent requirement, ft3/hr or m3/hr (at standard conditions)
A = exposed wetted surface, ft2 or m2
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4. Where the stored liquid is other than hexane, adjust the emergency venting capacity as follows:
V’ = V 1337 / L (M)1/2
V’ = V 3110 / L (M)1/2
Where: V = hexane vent requirement from Table 8, ft3/hr or m3/hr
V’ = stored liquid vent requirement, ft3/hr or m3/hr
L = latent heat of vaporization of stored liquid, Btu/lb or kJ/kg
M = molecular weight of stored liquid, no units
Table 9 lists L (M)1/2 for a number of common liquids. Data on other liquids can be found in most handbooks.
Note: the vent capacity determined from Table 8 is conservative compared to the other liquids listed in Table
9; that is, if the capacity of the existing vents is adequate for hexane, it will be adequate for most other liquids.
Table 9. Values for L (M)1/2
Chemical
L (M)
English
(1)
1/2
Metric
English
Acetic Acid
Acetic Anhydride
Acetone
Acetonitrile
1350
1792
1708
2000
3140
4168
3973
4652
n-Amyl alcohol
iso-Amyl alcohol
Aniline
Benzene
n-Butyl acetate
n-Butyl alcohol
iso-Butyl alcohol
Carbon disulfide
2025
1990
1795
1493
1432
2185
2135
1310
4710
4629
4012
3473
3331
5082
4966
3047
Chlorobenzene
Cyclohexane
Cyclohexanol
Cyclohexanone
o-Dichlorobenzene
cisDichloroethylene
Diethylamine
Dimethyl
acetamide
Dimethyl amine
1422
1414
1953
1625
1455
1350
3308
3289
4543
3780
3384
3140
Ethyl acetate
Ethyl alcohol
Ethyl chloride
Ethylene
dichloride
Ethyl ether
Furan
Furfural
Gasoline
n-Heptane
n-Hexane
Methyl alcohol
Methyl ethyl
ketone
n-Octane
n-Pentane
n-Propyl acetate
n-Propyl alcohol
iso-Propyl alcohol
Tetrahydrofuran
1403
1997
3263
4645
Toluene
o-Xylene
1676
3898
(1)
L (M)1/2
Chemical
(1)
(1)
Metric (1)
1477
2500
1340
1363
3436
5815
3117
3170
1310
1362
1962
1370–1470
1383
1337
2680
1623
3047
3168
4564
3187–3419
3217
3110
6234
3775
1412
1300
1468
2295
2225
1428
3284
3024
3415
5338
5175
3322
1500
1538
3489
3577
L (heat of vaporization) in Btu/lb or kJ/kg
5. The venting capacity as determined by 2, 3 and 4 above, can be reduced for the effect of drainage, sprinklers, insulation and low heat of combustion liquids (alcohols) using the Environmental Factors presented
in Table 10.
6. The total emergency venting capacity can be provided with specific construction or devices alone or in combination with the opening(s) provided for normal venting.
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Table 10. Environmental Factors for Emergency Venting Capacity (use only one factor)
Environmental Factor (F) 1
Drainage 1
Water spray or sprinklers 2 & drainage
Water spray or sprinklers only
Insulated 3
Water spray & insulated
None of the above
Basic 4
0.5
0.3
0.3
0.3
0.15
1.0
For low heat of combustion liquids
0.25
0.15
0.15
0.15
0.15
0.5
4, 5
1
Adequate drainage to remote impoundment in accordance with 2.2.2 – 5 above
FM Approved water-spray installations in accordance with DS 4-1N or automatic sprinklers in accordance with Section 2.4.1 below.
3
FM Approved coating rated for process structure or tank protection
4
Use either basic credit or low heat of combustion credit, not both
5
Capacity reduction permitted for liquids whose heat of combustion and rate of burning are equal to or less than those of ethyl alcohol (ethanol)
2
7. Where unstable liquids are stored, provide tank-venting capacity that accounts for the effects of heat or
gas produced by polymerization, decomposition, or self reactivity and the possibility of a two-phase relief. Follow the design guidance for reactive systems in DS 7-49, Emergency Venting of Vessels.
2.2.2.3 Manifolded Vents
1. Do not manifold vent collection systems of tanks containing incompatible materials.
2. Do not manifold vent pipes from tanks containing liquids with flash points below or equal to 100°F (38°C)
with tanks containing liquids with flash points above 100°F (38°C).
3. Protect low-pressure storage tanks interconnected with fume recovery or collection systems against explosion propagation if they normally contain ignitable mixtures AND ignition sources could be (spontaneous heating) or are normally present (continuous flames as in flares, fume incinerators, etc.) by one of the following
methods:
a) Oxidant concentration reduction (e.g., inerting or purging). This method is limited to operations without open manway activities, such as sampling, liquid or solids addition, etc. (NOTE: Do not use inerting
in tanks with monomers containing inhibitors that require oxygen to maintain activity. Examples: hydroquinone and methyl ether of hydroquinone.) See Data Sheet 7-59, Inerting and Purging of Tanks, Process Vessels, and Equipment.
b) Combustible concentration reduction (e.g., ventilation). See Data Sheet 7-78, Industrial Exhaust Systems.
c) Explosion isolation (detonation arresters).
4. Where an explosion isolation system is needed, provide Approved detonation arresters as follows (Fig. 6):
a) At each tank, in the piping connecting it to the vapor recovery system.
b) At the end of the manifold immediately upstream of the feed nozzle for any vapor processing equipment; for example, incinerators and scrubbers.
Note: Detonation arresters may not be appropriate in systems where powders are handled or added on a
regular basis. The arrester could become plugged and fail to handle normal in-and-out breathing.
5. Provide detonation arresters with temperature sensors on each side, and as close as possible to the face
of the arresting element. Arrange the sensor to automatically close valves or initiate other actions that will
eliminate the possibility of a stabilized flame burning on the arrester element. Do not locate the sensor in a
thermowell that will delay its response. If the sensor is to be a metal-sheathed thermocouple, it must be of
small diameter, e.g., 1⁄4 in. (6 mm), and must be inserted bare through a suitable packing gland.
6. Within 120 pipe diameters of the detonation arrester, install piping of equal or smaller diameter than the
detonation arrester.
(Fig.s 7 and 8 are showing pipe sizing around dda to meet this criteria)
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Fig. 6. Manifolded tanks
Fig. 7. Required pipe sizing if detonation arrester is smaller than nearby piping
Fig. 8. Improper piping around detonation arrester
7. Where conditions of operation will significantly exceed approximately atmospheric pressure and temperature, specifically test detonation arresters under the actual operating conditions. Detonation arresters are
capable of successfully stopping detonation fronts only in systems initially at approximately atmospheric pressure and temperature.
8. Install detonation arresters where easily accessible for maintenance and inspection.
9. Install vapor-collection system piping in accordance with ASME B31.3, Chemical Plant and Petroleum
Refinery Piping, or international equivalent, designed for a maximum allowable working pressure of 150 psig
(10 barg).
10. Provide the flow capacity in common portions of manifolded vapor collection piping for the maximum
flow of all vents connected to that portion of the system.
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11. Consider insulation and/or heat tracing of the system and arrester in cold climates where freezing or condensation of the vapor is possible. The heat tracing must be kept below the accepted operating range of
the arrester.
2.2.2.4 Indoor Tanks
1. Provide continuous low-level mechanical ventilation as specified in DS 7-32, Section 2.1.3.1, Ventilation.
2. Provide inert gas blanketing where tanks store liquid with a flash point less than 100°F (38°C).
2.2.3 Asphalt Tanks
Asphalt storage tanks have been a frequent source of fire or explosion events. In addition to the other criteria applying to outdoor tanks, the following represent good operating practice.
1. Ensure tank roofs are watertight.
2. Inspect tanks vents and the underside of the roof for accumulation of condensed material on a regular
basis and keep records of the inspection results.
3. Use tanks with weak seam roof (pressure relieving) construction per API 650 or similar.
4. Provide tanks with only one breather vent to minimize introduction of air into the vapor space.
5. Keep roof gauging and manway hatches closed to prevent unintended entry of air into the vapor space.
6. Use gauging hatches rather than manways when checking liquid level to minimize air entry into the tank
vapor space.
7. Do not use pressure-vacuum (conservation) vents as condensed materials could prevent operation of
the vent.
a) Where inerting of the vapor space is used, conservation vents will be needed.
b) Inject the inert gas below the vents to keep them free of accumulations.
c) Inspect the vents on a regular basis and keep records of the inspection results
8. Maintain tank liquid levels above any internal heating coils that could cause localized overheating, cracking of the liquids generating light ends and creating condensed deposits on the roof. Provide a reliable
method to monitor tank liquid level.
9. Route supply piping for heating systems below the lowest liquid surface level or insulate the pipe with a nonpermeable material.
10. Monitor the tank temperature with sensors located where it will be representative of bulk liquid temperature. Keep sensors away from tank walls, near submerged heating coils, or and below normal operating levels.
11. Maintain tank temperatures at safe levels with the following considerations:
a) Keep temperatures at least 25°F below the flash point (out of the flammable range).
b) Keep temperatures out of the range of 212°F–265°F (100°C- 130°C) to avoid water condensation.
c) Temperatures above 350°F (177°C) encourage asphalt condensation on the roof surface. Deposit can
oxidize, generate heat and possibly autoignite above 375°F (190°C).
d) Provide inert gas blanketing (oxygen concentration of 3% to 5%) for tanks operating at 350°F to 450°F
(177°C-232°C) to prevent oxidation of deposits.
e) Don’t store materials at temperatures above 450°F (232°C) which can promote cracking and production of light hydrocarbons and increase the likelihood of operation in the flammable range.
12. Don’t allow entry of piping or any fixtures to or through the tank roof which would hinder deployment of
the weak seam roof in an explosion.
13. Inspect internal tank heating coils for cracks, corrosion, and other damage whenever the tank is out of service and keep records of the inspection results.
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14. Take precautions to safely oxidize pyrophoric deposits before taking the tank out of service (see API
RP 2016 for details).
15. Follow a written procedure for returning long-idled tanks to service that addresses at least the following:
a) Water accumulations that could boil on heating.
b) Residual product that may heat irregularly with localized overheating until the entire contents have
reached a uniform temperature.
c) Lighter products that might have been previously in the tank and addition of hot material that could rapidly vaporize material and exceed vent capacity or cause the vapor space to enter the flammable range.
16. Develop an emergency response plan to address fire, explosion, and unexpected liquid release that identifies the hazards, site layout, protection equipment, shutoff valves, etc., as well as specific response to each
type of event. Ensure outside responders are familiar with the response plan.
2.3 Protection
2.3.1 Indoor Tanks
1. Provide automatic sprinkler protection in the tank room/vault designed as specified in Table 11 over the
entire tank room/vault area.
2. Provide sprinklers below tanks that are elevated and have greater than 3 ft (1 m) diameter or a plan area
of 10 ft2 (0.9 m2), or encase all tank supports in 3-hour fire-rated concrete.
3. Where pumps are present, extend a sprinkler down to within 2 ft of the pumps.
4. Provide an allowance of at least 500 gpm (1900 L/min) for hose stream use.
Table 11. Sprinkler Density for Storage Tank Rooms, gpm/ft2(mm/min)
Ceiling height
Flash point ≤ 200 °F (93 °C)
No drainage, no foam
With drainage, no foam
Foam with or without drainage
Flash point > 200 °F (93 °C)
No drainage, no foam
With drainage, no foam
Foam with or without drainage
≤ 15 ft (4.5 m)
> 15 ft (4.5 m) up to 30 ft (10 m)
Not permitted
0.3 (12)
As required by foam Approval*
Not permitted
0.3 (12)
As required by foam Approval*
0.3 (12)
0.2 (8)
As required by foam Approval*
0.4 (18)
0.2 (8)
As required by foam Approval*
*See Approval Guide listing for ″foam water sprinklers″.
2.3.2 Outdoor Tanks
The basic protection for tank farms is hose streams along with adequate spacing and containment as specified
in Section 2.1.2. This will generally limit fire involvement to all tanks within a common dike or three large
tanks that are individually diked. For large tanks or tanks farms, manual fixed foam protection may be appropriate (automatic foam is rarely justified).
Automatic water-spray protection is of value mainly for exposure protection of buildings where tanks are
located too close. An alternative protection method is fire-rated construction for the building.
1. Provide hydrants within 200 ft (60 m) of tanks so they can be reached by hose streams or monitor nozzles
from outside the dike.
2. Locate hydrants so every tank can be reached by hose or monitor streams from at least two directions.
3. Provide each hydrant with a minimum of two outlets controlled by individual valves.
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4. Provide FM Approved combination straight stream/water spray nozzles for each hose. A straight stream
discharge can cool exposed tanks or facilities, while a high-velocity spray discharge can control or extinguish
fires in liquids with flash points above 200°F (93°C).
5. Provide foam monitor nozzles or foam hose streams for exterior protection and spills in the dikes where
there are tanks that contain stable liquids with flash points below or equal to 200°F (93°C) or unstable liquids
of any flash point.
6. Provide fixed foam outlets and supply piping to a remote point outside the dike installed in accordance
with DS 4-7N on vertical cone-roof tanks storing stable or unstable liquids with flash points below or equal
to 200°F (93°C) when one or more of the following conditions exist:
a) The tank capacity exceeds 50,000 gal (190 m3) or there are multiple tanks in the same dike whose
aggregate capacity exceeds this value.
b) The tanks present a serious exposure to important buildings, process equipment, or utilities due to
inadequate spacing.
c) The tank-to-tank spacing and containment is deficient compared to the requirements of this standard.
d) The tank contents are of considerable value or are essential for continued operations and are not
readily replaceable. The contents can be readily salvageable after foam contamination.
e) Other unfavorable situations that cannot be corrected.
7. Where spacing between tanks and nearby buildings is inadequate (not in accordance with Section 2.1.2)
provide one of the following:
a) Provide building construction in accordance with DS 1-20 using guidelines for yard storage and consider
the tanks as high-hazard occupancy.
b) Provide deluge water spray (installed in accordance with DS 4-1N) on the exposed wall at a rate of
0.3 gpm/ft2 (12 mm/min) of exposed wall using the criteria in DS 1-20 to determine the extent of the
exposed wall. Include water supply duration for 2 hours and at least 500 gpm (1,900 L/min) for hose
streams.
8. Where spacing between adjacent tanks is inadequate (not in accordance with Section 2.1.2), provide
deluge water spray (installed in accordance with DS 4-1N) on all exposed tanks at a rate of 0.3 gpm/ft2 (12
mm/min) of tank surface. Include water supply duration for 2 hours and at least 500 gpm (1,900 L/min) for
hose streams.
9. Where spacing to rail or truck load/unload stations is inadequate (i.e., not in accordance with Section 2.1.2)
provide deluge water spray (installed in accordance with DS 4-1N) for the load/unload station (vehicle and
pumps) at a rate of 0.3 gpm/ft2 (12 mm/min) of tank surface. Include water supply duration for 2 hours and at
least 500 gpm (1,900 L/min) for hose streams.
2.3.3 Water Supply
1. Calculate the water demand as the sum of the following:
a) The hose stream demand for tanks storing all classes of liquids as determined from Table 12 and
supplied at a minimum pressure of 50 psi (345 kPa).
b) Water required for fixed foam equipment, when provided, supplied at minimum Approved pressure.
For purposes of estimation, see Table 13.
c) Provide water supply duration for a minimum of 4 hr for liquids with a flash points below 140°F (60°C)
and 2 hour for liquids with a flash points above or equal to 140°F (60°C).
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Table 12. Hose Stream Demand for TANKS Storing Flammable Liquids
Flash point of liquid
< 140°F (60°C)
≥ 140°F (60°C)
1
2
Largest Tank Involved in Fire
gpm
L/min
Largest Exposed Tank
gpm
L/min
2
10002
750
1
5002
250
3,800
2,800
1,900
950
2
Required flows may be reduced by half for horizontal tanks.
Add 250 gpm (950 L/min) for each 100 ft (30 m) increase in tank diameter above 100 ft (30 m).
Table 13. Estimated Water Demand for Fixed Foam Protection for a full Surface Fire.
Tank Diameter
ft
50
100
150
200
250
300
Water Demand
m
15
30
45
60
75
90
gpm
200
800
2,000
3,200
5,000
7,100
L/min
750
3,000
7,500
12,100
19,000
27,000
2.4 Operation and Maintenance
1. Implement a formal mechanical integrity program, as described in DS 7-43, Loss Prevention in Chemical Plants, for all flammable-liquid storage tanks.
2. Conduct monthly visual inspections of aboveground and indoor tanks for the following (where applicable):
a) Leaks, corrosion, settlement
b) Condition of attached piping, piping supports, gauging, level control systems, alarms, emergency and
breather vents, instrumentation, grounding, ladders, accessways
c) General housekeeping, water accumulation, and vegetation in dikes
d) The physical condition of berms/dikes/containment
e) Clear and operable drainage systems along with accessibility of any applicable valves
f) Operation of inerting systems
3. Conduct annual recorded inspections of tank vents, vent pipes, screens, and flame arresters and keep
them free from obstructions (e.g., stones, dirt, insect nests, polymerized material, etc.) that could prevent
proper operation and possibly overpressurization of the tank.
4. Conduct recorded inspections of detonation arresters in manifolded piping systems for damage and accumulations of debris caused by polymerization, condensation, corrosion, etc., which could impair operability. Replace damaged units (or repair if the damage does not affect their functionality) and remove
accumulations.
a) Conduct inspections at least quarterly during the first year and as experience dictates thereafter, but
at least annually (where practical).
b) Conduct inspections at least quarterly where powders are added to the system. (Arresters in this service are particularly susceptible to accumulations and may be inappropriate for use.)
5. Conduct annual recorded inspections of the performance of cathodic protection (CP) systems by qualified persons for the attainment of satisfactory CP criteria, proper functioning of equipment and that the level
of CP applied is properly controlling corrosion. Criteria for determining the effectiveness of CP include NACE
RP0169, 0285 and API 651.
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2.4.1
Repair, Reconditioning, and Abandonment
Prior to working on any tank that has contained flammable liquids, take the following precautions as appropriate:
1. Drain any residual liquids remaining in the tank.
2. Purge flammable-liquid tanks with steam or warm air before repairs are made or before the tanks are
reused. Route displaced flammable vapors to a safe location. Avoid excessive pressure or vacuum. (See
DS 7-59, Inerting and Purging OF Equipment)
3. Use an FM Approved flammable vapor indicator to determine whether vapors have been eliminated. Make
additional tests at frequent intervals.
4. Remove all remaining scale and sludge with nonferrous scrapers.
5. Fill the tank with an inert gas, such as carbon dioxide, or maintain positive continuous air movement through
the tank if cutting or welding torches are used on the outside of the tank.
6. Use a hot work permit system to control welding operations on a tank. (see Data Sheet 10-3, Hot Work Management)
7. Supervise workers and provide sufficient ventilation when welding is being done inside a tank. Station at
least one person outside near the manhole to watch the welder and assist in an emergency.
8. Remove, repair, or recondition underground flammable-liquid tanks that are no longer of any use. Prior
to removal, inert the tank. If removal of the tank is not possible, it may be left in place after doing the following:
a) Remove all of the liquid from the tank.
b) Purge the tank of flammable vapors.
c) Remove all suction, inlet, gauge, and vent lines.
d) Fill the tank with a solid inert material (e.g., sand, diatomaceous earth, perlite, etc.).
e) Cap all remaining underground piping.
f) Re-bury the tank and fittings.
2.5 Ignition Source Control
1. Install electric lights and other fixed or portable electrical equipment near storage tanks containing liquids with flash points below or equal to 100°F (38°C) or liquids with higher flash points heated to within 25°F
(14°C) of their flash point in accordance with the following:
a) Provide electrical equipment suitable for Class I, Division 1 or Zone 1 hazardous locations as defined
in Article 500 of the National Electrical Code when located within 3 ft (1 m) of vents.
b) Provide electrical equipment suitable for Class I, Division 2 or Zone 2 hazardous locations as defined
in Article 500 of the National Electrical Code when located between 3 and 5 ft (1 to 1.5 m) of vents.
c) Provide electrical equipment suitable for Class I, Division 2 or Zone 2 hazardous locations when located
within 10 ft (3 m) of any other tank openings or when located within a diked area.
d) Provide electrical equipment suitable for Class I, Division 1 or Zone 1 hazardous locations when tanks
are located in a room or vault.
e) Refer to Data Sheet 5-1, Electrical Equipment in Hazardous (Classified) Locations, and Data Sheet
5-7, National Electrical Code, for additional information regarding electrical installations.
2. Provide static grounding connections on tanks that are out of contact with the earth if piping is ungrounded
or nonconductive. (Ordinarily, special electrical grounding connections will not be needed. Adequate grounding for a tank is provided by its own contact or the contact of its connected piping with the earth.) (See DS
5-8, Static Electricity)
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3. Electrically bond all tank plates, internal structural members, fittings and isolated metal parts or pipe sections on tanks containing liquids with flash point less than or equal to 100°F (38°C) or liquids with higher
flash points heated to within 25°F (14°C) of their flash point to reduce the danger of internal sparks from lightning or charged liquid. (See Data Sheet 5-8, Static Electricity for further information on grounding and bonding.)
4. Prohibit the discharge of liquids with flash points below or equal to 100°F (38°C) or liquids with higher
flash points heated to within 25°F (14°C) of their flash point above the liquid level in the tank (usually called
’’splash filling‘‘) as it creates the possibility of static buildup and spark discharge to grounded components.
5. Prohibit hot work, maintenance, repair, or modification in or near (see Table 14) tanks, pumps, and other
handling equipment, tank truck or railcar loading and unloading, or fume-collection systems where flammable vapors could be present until the tank or system is isolated, drained, and purged or blanketed with
an inert gas. Use a hot work permit system to control the progress of such work. (See Data Sheet 10-3, Hot
Work Management.)
6. Prohibit smoking or open flames in or near (see Table 14) tanks, pumps, and other handling equipment,
tank truck or railcar loading and unloading, or fume-collection systems where flammable vapors could be
present. Provide designated safe areas for such activity.
Table 14. Safety Distances for Hot Work, Open Flames, Maintenance, Repair or Modification
Safety distances for hot work, open flames, maintenance, repair or modification*, ft (m)
Flash point
= 100°F (38°C) or heated to within
> 100°F (38°C)
25°F (14°C) of their flash point
Tanks outdoors
50 (15)
35 (10)
Within dikes or tank rooms
Not allowed
Truck or railcar loading/unloading
75 (22.5)
35 (10)
Pumps or other handling equipment
75 (22.5)
35 (10)
* allowed after hot work permit process is completed
3.0 SUPPORT FOR RECOMMENDATIONS
3.1 Background information
Tanks containing gasoline, alcohol, benzene, and other flammable and combustible liquids have been
involved in serious fires. The contents of a large tank can cause extensive damage if released during a fire.
The design and construction of such tanks needs to ensure a high degree of confinement and reliability.
3.1.1 Hazards
Flammable and combustible liquids are classified by various US and international regulatory bodies for the
purposes of packaging, transportation, and handling. The various definitions can make the application of
storage standards across a broad spectrum difficult. For the most part, this document limits differentiation
by using a breakpoint of 140°F (60°C) for spacing criteria and 200°F (93°C) for protection.
Crude oil (not addressed in this standard) and other liquids containing components with a wide range of boiling points, and some free water, present the additional hazards of boil-over, slop-over, or froth-over. Boilover is a phenomenon that may occur spontaneously during a fire in an open-top tank of crude oil that has
been burning for an extended period of time. In time, a sudden expansion of a steam-oil froth beneath the liquid surface can occur, resulting in a sudden explosion of hot residual oil from the tank. Generally, four conditions have to exist for a boil-over to occur:
1. The tank must contain free water or a water-oil emulsion near the tank bottom. This is a normal condition in crude-oil storage tanks as well as in some tanks storing heavier, residual oils.
2. The tank must be open-top. Experience indicates that fire in an open-top tank will result if an explosion
blows the roof off or if the pan or deck in a floating-roof tank sinks.
3. The oil must be capable of forming a heat wave of 300°F (145°C) or more. The heat wave is created
when lighter components in the liquid (e.g., pentane, hexane, etc.) distill off and burn at the liquid surface leaving a residue of higher density than the liquid just below it. This residue has a temperature in excess of 300°F
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(145°C) and, if it sinks at a rate substantially faster than the rate of regression of the liquid surface, the heat
wave is formed. The heat wave is created by convection (within the stored liquid) not conduction.
4. The oil must contain sufficient heavy ends to produce a persistent froth of oil and steam.
The boil-over tendencies of the oil can be evaluated by small-scale tests. While all crude oils are not susceptible to boil over, successive storages may exhibit boil-over potential. Thus, always design, install, and protect tanks storing crude oil recognizing the possibility of boil over.
Other liquids can exhibit slop-over or froth-over tendencies. Slop over occurs when a water stream is applied
to the surface of a burning viscous oil. The resultant frothing and ejection of liquid is generally much less
severe than a boil over because only the surface of the liquid is involved. It could present a hazard to fire fighters. Froth over occurs when a hot viscous liquid, such as asphalt or oil, floats on a water layer in a tank.
In time, the water is superheated and erupts, ejecting liquid from the tank. Unlike boil over or slop over, there
is no fire. Froth overs have occurred with sufficient violence to blow off tank roofs and spread the tank contents over a large area.
3.1.2 Types of Tanks
3.1.2.1 Atmospheric Tanks
Atmospheric tanks are used to store large quantities of liquids at pressures ranging from atmospheric to
1.0 psig (7 kPa). The following are the principal types of atmospheric tanks:
Cone roof tanks are the most widely used for flammable liquid storage. They are usually welded and may
have either weak roof or weak shell-to-roof seams designed to fail preferentially to the tank shell in the event
of a fire or internal explosion. Their major disadvantage is the vapor loss caused by breathing (the normal
expansion and contraction of the tank contents with atmospheric changes). The normal operating range of the
tank is ±11⁄2 in. of water (± 370 Pa).
Floating roof tanks are constructed with a roof floating on the liquid surface. The roof may be of doubledeck or pontoon-type construction (Figs. 9 and 10). By eliminating the vapor space, breathing losses become
negligible, and the fire and explosion hazard is greatly reduced. The seal provided between the roof edge
and the tank wall allows the roof to move freely within the shell. Drainage facilities are provided to prevent the
accumulation of water on the roof surface.
Covered floating roof tanks are similar in construction to cone roof tanks, except for a metal pan (or, occasionally, a double or pontoon internal roof) that floats on the liquid surface (Fig. 11). Since the floating cover
is protected from the weather, no provision for drainage or for rain or snow loading is required. Vents are provided around the periphery of the tank.
Lifter or expansion roof tanks resemble cone roof tanks, except the entire roof assembly has limited freedom to move up and down within the shell. A vapor-tight liquid seal, which maintains a slight pressure on
the contents of the tank, provides a seal between the roof assembly and the shell. The moving roof minimizes normal breathing losses. An expansion roof tank is occasionally used with a group of fixed roof tanks
to take up their composite vapor change.
Breather roof tanks are used where the liquid storage is not frequently disturbed. The horizontal flexible diaphragm, or roof, is attached to the top edge of the tank shell and maintains a variable vapor space by moving up and down. The roof, by confining the vapor, exerts a slight pressure upon the liquid, reducing
evaporation losses.
Vapordome tanks employ a dome containing a plastic diaphragm, which is free to move with the expansion of vapor in the tank. This is an effective method of reducing vapor loss from the top of the tank.
Cylindrical tanks are used for small quantities of liquids. Heads may be dished or flat. The long axis may
be either horizontal or vertical and the tank buried or aboveground.
3.1.2.2 Low-Pressure Tanks
Low-pressure tanks have a maximum working pressure of 15 psi (103 kPa). They are used to store volatile liquids, such as those with flash points below 73°F (23°C) and boiling points below 100°F (38°C) (Class
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Fig. 9. Open top double deck
Fig. 10. Open top pontoon
IA), under their own vapor pressure. Such tanks may be spheres, spheroids, or cylinders. In general, the
requirements applicable to atmospheric storage tanks apply to low-pressure storage tanks, with some modifications in construction, venting, and spacing.
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Fig. 11. Pan-type covered tanks
3.1.3 Indoor Tanks
Putting large quantities of a flammable liquid inside an important building is not recommended. Tank storage of flammable liquids creates the potential for many fire scenarios, including overflow during filling, overpressurization when exposed to fire, leak in a discharge line, or tank failure (a very low likelihood event,
but one that has the potential for significant consequences).
The main goals of the recommendations are to isolate the tank from non-flammable liquid occupancies, provide adequate protection for most fire scenarios and to ensure adequate access to the tank room for firefighters. Since all of the fire scenarios in a tank room involve a liquid release, adequate isolation must include
provisions for containment and emergency drainage.
In cases where tanks are not only inside a building, but are also located either below or above grade, additional safeguards are needed. Access to these tanks for manual firefighting will be very limited. The overall severity of a liquid release and fire involving the tank will be entirely dependent on what was provided for
active and passive protection around the tank. In buildings where the potential loss is significant, there is
a need to ensure any potential flammable liquid release/fire is contained to the tank room. The only reliable way to accomplish this is through the use of a 3-hour fire rated vault with only limited openings for fresh
air. This combination will limit the fire severity and help ensure survival of the room regardless of the size
of the liquid release.
The design goal for pumping and transfer systems is to ensure the liquid stays in the piping system and
can be shut down when necessary (e.g., leak or fire). The best way to accomplish this is to use welded steel
piping, positive displacement pumps and safety shut-off valves. There will always be several potential leakage sources in this type of system that can produce a liquid release and fire. The most likely source of leakage is the pump. Pump rooms must be isolated from other occupancies. Since the pumping system creates
similar hazards as the storage tank, it may be cost effective to locate the pumps in the tank room/vault. A
small fire at the pump can grow because the initial fire will produce additional failures. Sprinklers that are
extended from the ceiling to within 2 ft (0.6 m) of the fuel pumps can help to prevent those additional failures.
A second potential leakage source is flanged or threaded pipe joints/unions. Welded piping systems require
the use of flanged joints to permit equipment maintenance and repair. Leaks at flanged joints can be caused
by poor maintenance or fire exposure to a gasket that can melt. Threaded joints are inherently weaker
because the pipe wall thickness has been reduced. Locate flanged or threaded joints/unions in rooms that
are properly isolated and protected for a flammable liquid fire exposure.
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3.1.4 Tank Spacing and Containment
Tank spacing criteria were developed by comparison with existing standards and by analysis of pool fire
simulations for all three classes of liquid assuming a 20 mph wind during the event. The final simplified criteria were based primarily on heat flux predictions rather than existing standards.
3.1.5 Manifolded Vents
Environmental regulations have increased the use of emission control systems on tanks. The emission control
systems can include carbon bed adsorbers, scrubbers, condensers, incinerators, etc. (Fig. 6). In some cases,
the system could be handling vapors within the flammable range. An ignition at one point in the system could
cause a flame front to propagate throughout with damaging results. The ignition source could be static, lightning, an incinerator flame, etc. Proper design of the system can prevent such a situation.
Flame propagation is not possible in the manifold piping and connected vessels if the vapor-air mixture is
out of the flammable range. This is most often achieved by an inert gas system to decrease the oxygen to
an acceptable level. To accept such a system in lieu of arresters, it must be reliable. The criteria in Data Sheet
7-59, Inerting and Purging of Tanks, Process Vessels, and Equipment, will provide this reliability as long
as open manway operations do not occur.
The propagation velocity in a piping system containing a flammable mixture depends on the inherent turbulence in the system caused by flow, bends, valves, and fittings as well as the turbulence of the combustion process itself. It has been recognized that a deflagration flame front can transit to detonation velocities
with significant increase in the pressures within the piping and the potential for failure of the piping. Transition to detonation in pipe lengths of 50 to100 diameters are typically reported. Flame-arresting devices that
successfully stop the deflagration fail to stop the detonation or even a ’’fast‘‘ deflagration. Detonation arresters (Fig. 12) can stop detonation fronts, and test procedures are available to Approve/list these devices. Detonation arresters are rated for a specific gas or class of gases. Some classifications are based on National
Electrical Code groupings (A, B, C, D), while others are based on minimum experimental safe gap (MESG)
required to quench a flame.
Fig. 12. Detonation arrester
Detonation arresters are normally bidirectional; that is, they will stop a detonation front approaching from
either direction. Since it is not possible to ensure the direction of flame approach, use of unidirectional arresters usually is not appropriate.
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Testing has demonstrated that a detonation arrester is likely to fail if installed in piping whose diameter
increases within a distance of 120 pipe diameters from the detonation arrester in either direction.
Fig. 13. Storage tank with flame arrester
Fig. 14. End-of-line flame arrester
This data sheet, NFPA 30, Flammable and Combustible Liquids Code, and other standards specify the
requirements for installation of flame arresters on tanks. Although some FM Approved flame arresters are
equipped with flanges at both ends for short pipe-aways (Figs. 13 and 15) of the released vapors, they cannot be used in extended piping systems. The testing organization’s listing will detail limits between the open
pipe end (to atmosphere) and the arrester. They are based on the test conditions, and additional length could
permit a deflagration flame front to increase velocity, even up to a detonation front, and result in failure of the
device to stop the flame.
Conservation vents are installed on many low pressure tanks to minimize the release of vapors during tank
idle times while permitting release of pressure or vacuum created during filling or emptying. This data sheet,
NFPA 30, and other codes accept these devices in lieu of flame arresters where vented directly to atmosphere. The pressure setting (typically 3⁄4 in. water gauge [190 Pa]) and the device design create local velocities in excess of the propagation velocity of ordinary combustion flames, thus preventing flashback into the
tank. These velocities and the general construction are insufficient for stopping detonation propagation.
These are not acceptable alternatives to detonation arresters in manifolded piping systems.
Using rupture disks on elbows, or direction changes in the piping system, to provide explosion venting is
not considered effective in halting the progress of a flame front. Venting the piping in this manner will at least
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Fig. 15. End-of-line flame arrester with pipe-away flange
Fig. 16. Backflash interrupter
temporarily relieve the pressure but may not stop the flame front, which could continue on to interconnected vessels. The flame front will continue down the pipe and, if it is not vented at regular intervals (50
to 100 diameters), it could transit to detonation velocity. Other methods of explosion isolation are available.
These include fast-acting valves, rapid discharge extinguishing (blocking) systems and flame-front diverters or backflash interrupters (Fig. 16). None of these devices presently are FM Approved and therefore are
not discussed in detail. There is limited information available on installation criteria and applicability limits.
All are designed to interrupt deflagrations, not detonations.
3.1.6 Asphalt Tanks
Loss history shows a disproportionate number of events involving tanks containing asphalt. Factors in these
events include:
• Tanks often operate at temperatures near the flash point.
• Material can condense on tank roof surfaces, overloading the roof.
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• Condensed material can be pyrophoric, iron sulfides, or carbonaceous.
• Water can accumulate on the bottom and froth over on addition of hot materials.
• Operating procedures are not always followed.
3.1.7 Fire Protection
The severity of flammable liquid tank fires requires that fire protection be provided. Normally, only hydrant
protection is required. Fixed or portable foam-making equipment or water spray systems may be required to
control fires where the quantities of liquids stored or tank sizes are unusually large. Special precautions are
required for the storage of crude oil and other liquids subject to boil over. Storage tank fires involving liquids with flash points of 100°F (38°C) or lower are difficult to control and extinguish and frequently burn for
days.
Do not consider provision of fixed foam or water spray systems as a substitute for adequate spacing of tanks
from important plant facilities.
Fixed foam systems have been effective in extinguishing fires in cone roof tanks, but have sometimes failed
for the following reasons:
a) Fires in tanks storing liquids with flash points of 100°F (38°C) or lower have originated with an explosion in the vapor space of the tank, damaging one or more foam distribution devices.
b) The roof support members have fallen into the liquid, preventing formation of a uniform foam blanket
over the liquid surface.
Even under these adverse circumstances, fixed foam systems may provide partial control until manual firefighting can be organized. Subsurface application could improve the operating experience of fixed foam extinguishing systems for fires in cone roof tanks. Some standards consider subsurface foam a requirement for
successfully extinguishing fires in tanks exceeding 200 ft (60 m) in diameter.
Floating roof tanks are less susceptible to serious fire loss than cone roof tanks. Seal fires in floating roof
tanks can be readily extinguished with either portable extinguishing equipment or fixed foam extinguishing
systems, depending upon the size of the tank.
For new installations, do not consider fixed foam systems as equivalent to adequate spacing and diking, selection of proper tank construction, or provision of exposure protection where needed.
3.2 Loss History
3.2.1 Storage Tanks
Loss history was developed for storage tanks handling flammable liquids for the period 1984 – 2004. A total
of 303 events at FM Global client locations resulted in a gross loss of US$280 million (all figures indexed
to 2005 dollars). Of these events, a total of 115 exceeded $100,000 gross loss, accounting for about 70% of
the total loss. The losses by industry group are shown in the Table 15. Metal working, plastic, wood and
paper and office, retail, and warehouses had the highest number of events. The largest single event in this
class was a tank explosion caused by cutting and welding at a paper mill.
Table 15. Losses over US$100,000 by Occupancy Class
Occupancy
None listed
Textiles
Metal Working
Plastic, Wood & Paper
Food & Beverage
Chemical & Pharmaceutical
Power Generation
Office, Retail, Warehouses
Residential
Misc Properties
Grand Total
Number
5
2
34
20
5
7
9
22
2
9
115
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Table 16 provides a breakdown of the engineering peril associated with the 115 losses.
Of the 115 losses reported over US$100,000 gross, losses caused by leakage and contamination were the
most common. There were a total of 57 incidents with a total gross loss of US$31 million.
There were a total of 29 fires and explosions with a total gross loss of US$133 million with fires accounting
for US$118 million. Of these, improper cutting and welding was often the cause.
“Oil” was the most common liquid involved in events, but asphalt was next most common with 12 events
and a total gross loss of about US$10 million.
Table 16. Losses over US$100,000 by Engineering Peril
Engineering Peril
Fire
Explosion
Escaped Liquid Damage
Riot & Civil Commotion
Collapse
Water-Liquid Damage
Implosion
Mechanical Breakdown
Impact
Miscellaneous
Total
Number
16
13
27
1
2
3
2
1
1
49
115
3.2.2 Manifolded Vents
There has been at least one FM Global loss involving manifolded vapor recovery systems on storage tanks.
In addition, a recent study in the province of Alberta, Canada, showed flame arrester failure in sour gas flaring operations (related to crude oil production) was responsible for 10 to 20 oil storage tank explosions per
year.
4.0 REFERENCES
4.1 FM Global
Data Sheet 1-57, Plastic in Construction
Data Sheet 4-7, Low Expansion Foam Systems
Data Sheet 5-1, Electrical Equipment in Hazardous (Classified) Locations
Data Sheet 5-7, National Electrical Code
Data Sheet 5-8, Static Electricity
Data Sheet 6-10, Process Furnaces
Data Sheet 7-14, Fire & Explosion Protection for Flammable Liquid, Flammable Gas, & Liquefied Flammable Gas Processing Equipment & Supporting Structures
Data Sheet 7-29, Flammable Liquid Storage in Portable Containers
Data Sheet 7-30, Solvent Extraction Plants
Data Sheet 7-32, Flammable Liquid Operations
Data Sheet 7-43/17-2, Loss Prevention in Chemical Plants
Data Sheet 7-51, Acetylene
Data Sheet 7-59, Inerting and Purging of Tanks, Process Vessels, and Equipment
Data Sheet 7-78, Industrial Exhaust Systems
Data Sheet 7-83, Drainage Systems for Flammable Liquids
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Data Sheet 10-3, Hot Work Management
4.2 NFPA
NFPA 11, Standard for Low, Medium and High Expansion Foam (2005)
NFPA 30, Flammable and Combustible Liquids Code (2003)
In section 4.4.2, reference is made to testing criteria for the integrity of secondary containment tanks. Below
is that reference:
4.4.2.3 Underground secondary containment tanks and horizontal aboveground secondary containment
tanks shall have the primary (inner) tank tested for tightness either hydrostatically or with air pressure at
not less than a gauge pressure of 20 kPa (3 psig) and not more than a gauge pressure of 35 kPa (5 psig).
The interstitial space (annulus) of such tanks shall be tested either hydrostatically or with air pressure at
a gauge pressure of 20 to 35 kPa (3 to 5 psig), by vacuum at 18 kPa (5.3 in. Hg), or in accordance with
the tank’s listing or manufacturer’s instructions. The pressure or vacuum shall be held for not less than 1
hour or for the duration specified in the listing procedures for the tank. Care shall be taken to ensure that
the interstitial space is not over pressured or subjected to excessive vacuum.
4.4.2.4 Vertical aboveground secondary containment–type tanks shall have their primary (inner) tank
tested for tightness either hydrostatically or with air pressure at not less than a gauge pressure of 10 kPa
(1.5 psig) and not more than a gauge pressure of 17 kPa (2.5 psig). The interstitial space (annulus) of
such tanks shall be tested either hydrostatically or with air pressure at a gauge pressure of 10 to 17 kPa
(1.5 to 2.5 psig), by vacuum at 18 kPa (5.3 in. Hg), or in accordance with the tank’s listing or manufacturer’s instructions. The pressure or vacuum shall be held for 1 hour without evidence of leaks. Care shall
be taken to ensure that the interstitial space is not over pressured or subjected to excessive vacuum.
NFPA 70, National Electric Code
4.3 Others
American Petroleum Institute, API 620, Design and Construction of Large, Welded, Low-Pressure Storage
Tanks, Tenth Edition, 2002
(American Petroleum Institute, API 650, Welded Steel Tanks for Oil Storage, Tenth Edition, 1998)
American Petroleum Institute, API 2000, Venting Atmospheric and Low Pressure Storage Tanks, Fifth edition, 1998
American Petroleum Institute, ANSI/API 2610, Design, Construction, Operation, Maintenance, and Installation of Terminal and Tank Facilities, Second edition, 2005
American Petroleum Institute, ANSI/API 651, Cathodic Protection of Aboveground Petroleum Storage Tanks,
Second edition, 1997
American Petroleum Institute, API Standard 2015, Requirements for Safe Entry and Cleaning Petroleum Storage Tanks
American Petroleum Institute, API Recommended Practice 2016, Guidelines and Procedures for Entering
and Cleaning Petroleum Storage Tanks
American Petroleum Institute, API Recommended Practice 2023, Guide for Safe Storage & Handling of
Heated Petroleum Derived Asphalt Products & Crude Oil Residua
American Society of Mechanical Engineers (ASME), Boiler and Pressure Code, Section VIII, Unfired Pressure Vessels, latest edition
American Society of Mechanical Engineers (ASME), B31.3, Chemical Plant and Petroleum Refinery Piping,
latest edition
ASTM International, ASTM D4206, Standard Test Method for Sustained Burning of Liquid Mixtures Using
the Small Scale Open-Cup Apparatus, 2001
Code of Federal Regulations, 33 CFR, Part 154, Appendix A, Guidelines for Detonation Flame Arresters
Code of Federal Regulations, 49 CFR, Chapter I, Subchapter C, Parts 171 – 180 Department of Transportation, Hazardous Materials Regulations
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International Standards Organization, ISO 2592, Determination of flash and fire points — Cleveland open
cup method, 2000
National Association of Corrosion Engineers, NACE RP-0169, Control of External Corrosion on Underground or Submerged Metallic Piping Systems
National Association of Corrosion Engineers, NACE RP-0285, Corrosion Control of Underground Storage
Tanks System by Cathodic Protection
APPENDIX A GLOSSARY OF TERMS
Conservation vents: These devices have both vacuum and pressure relief capacity. Vents usually open
when the positive or negative pressure in the tank reaches 3⁄4 to 1 in. water column (185 to 250 Pa). They
are normally closed and vent pipes equipped with conservation vents do not need flame arresters. The velocity through the openings is normally sufficient to prevent flashback. A typical conservation vent is shown in Figure 17.
Fig. 17. Typical conservation vent
Exposed Wall Categories:
Combustible Wall: A wall made of any combustible material, including overhanging wood eaves, any metal
faced plastic insulated sandwich panels that are not FM Approved, and any wall with single pane, annealed
(not tempered) glass windows. Increase separation by 25% for asphalt-coated metal walls.
Noncombustible Wall: Materials include FM Approved Class 1 insulated, steel, or aluminum faced sandwich panels with thermoset plastic insulation; EIFS assemblies having noncombustible insulation and gypsum board sheathing, and aluminum or steel panels that are uninsulated or insulated with noncombustible
insulation such as glass fiber, mineral wool, or expanded glass. It also includes cementitious panels or
shingles over steel or wood. There can be no overhanging wood eaves. Any windows should be multipane or tempered glass.
Fire Rated Wall: The wall should meet the required fire rating per FM Global Loss Prevention Data Sheet
1-21, Fire Resistance of Building Assemblies. Any openings should be protected with a comparably firerated door. Any windows should be fire rated to match the rating of the wall.
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Flammable Liquid: Any liquid rated as NFPA Class I, II or III or DOT/UN Class 3.
FM Approved: References to ‘‘FM Approved’’ in this data sheet mean the product or service has satisfied
the criteria for FM Approval. Refer to the Approval Guide, a publication of FM Approvals, for a complete listing of products and services that are FM Approved.
Intermediate Bulk Container (IBC): Any container that has a volumetric capacity of not more than 793 gallons (3,000 L) and not less than 119 gallons (450 L) as defined and regulated by the U.S. Department of
Transportation in CFR Title 49, Part 178, subpart N, and the United Nations Recommendations on the Transport of Dangerous Goods, chapter 6.5.
IBCs can be constructed of metal, plastic or a metal-plastic composite. In the UN and US DOT regulations,
metal IBCs are designated 31A, 31B, and 31N (for liquids, and the letter code is for steel, aluminum, and
other metals, respectively.), rigid plastic are designated by the codes 31H1, 31H2, and composite are 31HZ1,
31HZ2.
Listed: Equipment or materials included in a list published by an organization that maintains periodic inspection of production of listed equipment or materials and whose listing states that either the equipment or material meets appropriate designated standards or has been tested and found suitable for a specified purpose.
Stable liquid: Any liquid not defined as unstable.
Tank
Aboveground tank: A tank that is installed above grade, at grade, or below grade without backfill.
Atmospheric tank: A storage tank that has been designed to operate at pressures from atmospheric through
a gauge pressure of 1.0 psig (6.9 kPa) measured at the top of the tank.
Double-skinned tank: See Secondary Containment Tank, a term used in European Union (EN) standards.
Floating roof tank: An atmospheric tank intended for storage of high vapor pressure liquids such as crude
oil and gasoline with vapor pressure exceeding 15 psig (103 kPa or 1 bar gauge) with a roof floating on the
liquid surface. (Floating roof tanks are not covered by this standard.) Design according to the criteria in API
650, Appendix C or H, or other recognized equivalent standard.
External floating roof: A roof that sits directly on the liquid surface, usually on pontoons with a seal
attached to the roof perimeter to cover the annular space between the roof and the shell. Design criteria are in API 650, Appendix C. This type has inherent buoyancy and are difficult, though not impossible, to sink.
Internal floating roof: A roof similar to the external floater but with a fixed roof above, intended for weather
protection or quality assurance. The internal floater is often a simple pan or plastic membrane floating
directly on the liquid surface with little or no inherent buoyancy and is subject to sinking. Design criteria
are in API 650, Appendix H. Pontoon type roofs similar or identical to external floaters are possible but not
common. Unless the internal floater has the inherent buoyancy of a pontoon type, treat the tank as a
cone roof tank.
Low-pressure tank: A storage tank designed to withstand an internal pressure of more than 1 psig (6.9
kPa) but not more than 15 psig (103 kPa or 1 bar gauge) measured at the top of the tank.
Portable tank: Any closed vessel having a liquid capacity over 60 gal (230 L) and not intended for fixed installation. This includes intermediate bulk containers (IBCs) as defined and regulated by the U.S. Department
of Transportation in CFR Title 49, Part 178, subpart N, and the United Nations Recommendations on the
Transport of Dangerous Goods, chapter 6.5.
Protected aboveground tank: An aboveground storage tank that is listed in accordance with UL 2085, Standard for Protected Aboveground Tanks for Flammable and Combustible Liquids, or an equivalent test procedure that consists of a primary tank provided with protection from physical damage and fire-resistive
protection from exposure to a high-intensity liquid pool fire.
Secondary containment tank: A tank that has an inner and outer wall with an interstitial space (annulus)
between the walls and that has a means for monitoring the interstitial space for a leak.
Storage tank: Any vessel having a liquid capacity that exceeds 60 gal (230 L), is intended for fixed installation, and is not used for processing.
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United Nations Recommendations on the Transport of Dangerous Goods: Model Regulations directed
at providing safe packaging criteria but only related to the transport of all types of dangerous solids, liquids, and gases. Hazard class 3 addresses flammable liquids.
Unstable liquid: A liquid that, in the pure state or as commercially produced or transported, will vigorously
polymerize, decompose, undergo condensation reaction, or become self-reactive under conditions of shock,
pressure, or temperature. A liquid with an NFPA instability hazard rating of 2 or greater in accordance with
NFPA 704, Standard System for the Identification of the Hazards of Materials for Emergency Response.
Vent, normal: Pressure relief opening on a tank to permit the intake and discharge of air during emptying
and filling operations and to permit expansion and contraction of vapor due to temperature changes. Sometimes called breather vent.
Vent, emergency relief: Pressure relief opening on a tank to prevent overpressurizing the tank in the event
of fire exposure.
Weak seam roof (weak shell-to-roof joint construction): The attachment of the roof to the shell forms a
frangible joint that, in the case of excessive internal pressure, will rupture before rupture occurs in the tank
shell joints or the shell-to-bottom joint. Design criteria can be found in UL 142 or API 650.
APPENDIX B DOCUMENT REVISION HISTORY
October 2011. The reference in Table 10, note 1 was corrected from 2.2.2-5 to 2.1.2-5.
September 2010. Changes were made in Table 3, Spacing for Flammable Liquid Tank Containment Dikes.
March 2009. Minimum spacing requirements in Table 3, Spacing for Flammable Liquid Tank Containment
Dikes, were modified.
May 2008. Minor editorial changes were made for this revision.
January 2008. Minor editorial changes were made for this revision.
May 2007. Corrections were made to Table 2.
April 2007. Minor editorial changes were made to January 2007 version.
January 2007. The following changes were made:
• Removed the recommendation against bottom connections on FRP tanks to be consistent with referenced standards.
• Extended the recommendation for containment to all tanks with flash points below 200°F (93°C). Previously, no containment was required for tanks of less than 15,000 gal (57 m3) except to protect buildings.
• Simplified the spacing and diking recommendations.
• Upgraded the recommendation for indoor tanks, including automatic fire and leak-detection systems.
• Added recommendations for secondary containment tanks.
• Eliminated the exemption to the recommendations for cutoffs for small fuel oil tanks serving heating appliances.
• Added recommendations for IBCs when supplying flammable liquids to a process.
• Added earthquake recommendations, including seismic shutoffs for indoor tanks.
• Added section on asphalt storage tanks.
• Added recommendations for monitoring tanks during fill operations, level-gauging, and high-level alarms
to an attended location.
• Moved information on carbon disulfide to Data Sheet 7-23N, Hazardous Chemical Data.
• Added section on asphalt storage tanks.
• Added recommendations for monitoring tanks during fill operations, level-gauging, and high-level alarms
to an attended location.
• Moved information on carbon disulfide to Data Sheet 7-23N, Hazardous Chemical Data.
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September 2000. This revision of the document has been reorganized to provide a consistent format.
October 1994. Added information on manifold vents in systems, and detonation arrestors.
July 1976. Miscellaneous revisions and updating.
May 1971. Miscellaneous revisions and updating.
November 1967. Updated and consolidated material from handbook.
1959. original guideline in Factory Mutual handbook of Industrial Loss Prevention.
APPENDIX C HYDROCARBON FIRE DURATION
Spills in defined areas (e.g., curbed area in a room, tank contained by dike, etc.):
See Table B1 below.
1. Determine if the dike will contain the largest expected spill:
Volume = (Depth of Dike) x (Area of Dike)
2. Determine the depth of the spill in the confined area:
Depth of Fuel = (Volume of Spill) / (Area of Dike)
3. Determine the liquid fire duration:
Fire Duration = (Depth of Spill) x [(7 minutes) / (1 in.)] (English)
Or
Fire Duration = (Depth of Spill) x [(7 minutes) / (2.5 cm)] (metric)
Spills in undefined areas:
Assume an average spill depth of 1⁄16 in. (1.5 mm) for a relatively flat surface and use these equations to calculate the area of the spill. Thermal damage will occur to everything touched by the spill. The duration of
this type of liquid spill fire will be limited. See Table 17 below.
The spill area can be calculated as follows:
Area of Spill = (Volume of Spill) / 1⁄16 in. (English)
Or
Area of Spill = (Volume of Spill) / 1.5 mm (metric)
Table 17a. Relationship Between Fuel Volume, Pool Size, and Fire Duration (English)
Liquid Volume,
gal
Spill Area, ft2 for
1⁄16 in depth
100
200
300
400
500
600
700
800
900
1000
2600
5100
770
10300
12800
15400
18000
20500
23100
25700
Spill Depth (in.) for Liquid Pools of
Defined Area
1000 ft2
2000 ft2
0.2
0.1
0.3
0.2
0.5
0.2
0.6
0.3
0.8
0.4
1.0
0.5
1.1
0.6
1.3
0.6
1.4
0.7
1.6
0.8
Fire Duration (min) For Liquid Pools
of Defined Area
1000 ft2
2000 ft2
1.1
0.6
2.3
1.1
3.4
1.7
4.5
2.3
5.6
2.8
6.7
3.4
7.9
3.9
9.0
4.5
10.1
5.0
11.2
5.6
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Table 17b. Relationship Between Fuel Volume, Pool Size, and Fire Duration (metric)
Liquid Volume,
liters
Spill Area, m2 for
1.5 mm depth
380
760
1,100
1500
1900
2300
2600
3000
3400
3800
240
480
720
950
1200
1400
1700
1900
2100
2400
Spill Depth (mm) for Liquid Pools of
Defined Area
93 m2
190 m2
5
3
8
5
13
5
15
8
20
10
24
13
29
15
33
15
37
18
41
20
Fire Duration (min) For Liquid Pools
of Defined Area
93 m2
190 m2
1.1
0.6
2.3
1.1
3.4
1.7
4.5
2.3
5.6
2.8
6.7
3.4
7.9
3.9
9.0
4.5
10.1
5.0
11.2
5.6
Continuous Spills
Depending on the spill rate, a flammable liquid may be fully consumed before it reaches the floor or it will create a burning pool on the floor. The pool diameter is controlled by the rate at which the liquid is being consumed in the fire and the rate at which it is being released. The pool diameter will stop growing when these
two rates are equal. Table 18 below provides some expected pool sizes, heat release rates, and flame heights
for various flow rates of kerosene. Diesel fuel will produce similar results. Since even small spill rates will produce sizable fires, the key issue in deciding if building steel will be damaged is the fire duration. The duration of this type of fire is controlled by the volume of fuel available to be spilled and the rate at which it is spilled.
The spill fire duration can be calculated as follows:
Fire Duration = (Volume of Fuel) / (Spill Rate)
Table 18a. Flow Rate, Pool Diameter, Heat Release Rate, and Flame Height for a Flowing Kerosene Fire (English)
Flow Rate
(gpm)
1
2
3
4
5
10
15
20
25
Pool Diameter
(ft)
3
5
6
7
7
10
13
15
17
Pool Area
(ft2)
9
17
26
34
43
86
128
171
214
Heat Release Rate
(MW)
2
4
6
8
10
20
30
40
50
Flame Height
(ft)
13
16
19
21
23
30
34
38
41
Table 18b. Flow Rate, Pool Diameter, Heat Release Rate, and Flame Height for a Flowing Kerosene Fire (Metric)
Flow Rate
(l/min)
3.8
7.6
11.4
15.2
19.0
38.0
57.0
76.0
95.0
Pool Diameter
(m)
1.0
1.4
1.7
2.0
2.2
3.2
3.9
4.5
5.0
Pool Area
(m2)
1
2
2
3
4
8
12
16
20
Heat Release Rate
(MW)
2
4
6
8
10
20
30
40
50
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Flame Height
(m)
3.9
5.0
5.8
6.5
7.0
9.1
10.5
11.7
12.6
Flammable Liquid Storage Tanks
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FM Global Property Loss Prevention Data Sheets
Page 43
APPENDIX D HAZARDS
The National Fire Protection Association and various US federal and state regulations use the following liquid classifications:
1. Flammable liquids are defined as liquids having closed cup flash points below 100°F (38°C) and vapor
pressures not exceeding 40 psia (276 kPa) at 100°F (38°C). Flammable liquids are referred to as Class I liquids, and are subdivided as follows:
a) Class IA liquids are those having flash points below 73°F (23°C) and boiling points below 100°F (38°C).
b) Class IB liquids are those having flash points below 73°F (23°C) and boiling points at or above 100°F
(38°C).
c) Class IC liquids are those having flash points at or above 73°F (23°C) and below 100°F (38°C).
2. Combustible liquids are defined as liquids having closed cup flash points at or above 100°F (38°C). Combustible liquids are referred to as either Class II or Class III liquids and are subdivided as follows:
a) Class II liquids are those having flash points at or above 100°F (38°C) and below 140°F (60°C).
b) Class IIIA liquids are those having flash points at or above 140°F (60°C) and below 200°F (93°C).
c) Class IIIB liquids are those having flash points at or above 200°F (93°C).
The U.N. Recommendations on the Transport of Dangerous Goods has only defined flammable liquids (hazard class 3) as follows:
Flammable liquids are liquids, or mixtures of liquids, or liquids containing solids in solution or suspension
(for example, paints, varnishes, lacquers, etc., but not including substances otherwise classified on account
of their dangerous characteristics) which give off a flammable vapor at temperatures of not more than 60.5°C
(141°F), closed-cup test, or not more than 65.6°C (150°F), open-cup test, normally referred to as the flash
point. This class also includes:
(a) Liquids offered for transport at temperatures at or above their flash point; and
(b) Substances that are transported or offered for transport at elevated temperatures in a liquid state and
which give off a flammable vapor at a temperature at or below the maximum transport temperature.
Liquids meeting the above definition, with a flash point of more than 35°C (95°F) which do not sustain combustion need not be considered as flammable liquids for the purposes of these Regulations. Liquids are considered to be unable to sustain combustion for the purposes of these Regulations (i.e., they do not sustain
combustion under defined test conditions) if:
(a) They have passed a suitable combustibility test (see SUSTAINED COMBUSTIBILITY TEST prescribed in the UN Manual of Tests and Criteria);
(b) Their fire point according to ISO 2592 — 2000 is greater than 100°C (212°F) or
(c) They are water miscible solutions with a water content of more than 90% by mass.
The UN system applies to materials in transport.
The current U.S. Department of Transportation Code 49CFR 171 defines hazard class 3 slightly more broadly,
as follows:
(a) Flammable liquid. For the purpose of this subchapter, a flammable liquid (Class 3) means a liquid having a flash point of not more than 60.5°C (141°F), or any material in a liquid phase with a flash point at
or above 37.8°C (100°F) that is intentionally heated and offered for transportation or transported at or
above its flash point in a bulk packaging, with the following exceptions:
(1) Any liquid meeting one of the definitions specified in § 173.115 Class 2, Divisions 2.1 (flammable
gas), 2.2 (non-flammable, nonpoisonous compressed gas—including compressed gas, liquefied gas,
pressurized cryogenic gas, compressed gas in solution, asphyxiant gas and oxidizing gas), and 2.3
(gas poisonous by inhalation).
(2) Any mixture having one or more components with a flash point of 60.5°C (141°F) or higher, that
make up at least 99 percent of the total volume of the mixture, if the mixture is not offered for transportation or transported at or above its flash point.
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Flammable Liquid Storage Tanks
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FM Global Property Loss Prevention Data Sheets
(3) Any liquid with a flash point greater than 35°C (95°F) that does not sustain combustion according
to ASTM D 4206 or the procedure in appendix H of this part.
(4) Any liquid with a flash point greater than 35°C (95°F) and with a fire point greater than 100°C (212°F)
according to ISO 2592.
(5) Any liquid with a flash point greater than 35°C (95°F), which is in a water-miscible solution with a
water content of more than 90 percent by mass.
(b) Combustible liquid.
(1) For the purpose of this subchapter, a combustible liquid means any liquid that does not meet the definition of any other hazard class specified in this subchapter and has a flash point above 60.5°C (141°F)
and below 93°C (200°F).
(2) A flammable liquid with a flash point at or above 38°C (100°F) that does not meet the definition
of any other hazard class may be reclassed as a combustible liquid. This provision does not apply to
transportation by vessel or aircraft, except where other means of transportation is impracticable. An
elevated temperature material that meets the definition of a Class 3 material because it is intentionally heated and offered for transportation or transported at or above its flash point may not be reclassed
as a combustible liquid.
(3) A combustible liquid that does not sustain combustion is not subject to the requirements of this subchapter as a combustible liquid. Either the test method specified in ASTM D 4206 or the procedure
in appendix H of this part may be used to determine if a material sustains combustion when heated
under test conditions and exposed to an external source of flame.
The DOT system applies to materials in transport.
Finally, the European Union (EU) has the Classification, Packaging, Labeling and Notification of Dangerous Substances Regulations S.I. 116, 2003 with the following liquid flammability definitions:
Extremely flammable – liquid substances and preparations which have a flash point lower than 0°C (32°F)
and a boiling point (or in case of a boiling range the initial boiling point) lower than or equal to 35°C (95°F).
Highly flammable – liquid substances and preparations having a flash point below 21°C (70°F) but which
are not extremely flammable.
Flammable – liquid substances and preparations having a flash point equal to or greater than 21°C (70°F),
and less than or equal to 55°C (131°F).
This system covers only identification methods for these substances. Other regulations would apply to storage or transport.
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