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A88438-22 Critical Procedure 07-01 - Nitrogen purging - A5X9W8

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Nitrogen Purging
Critical
Doc. No.:
Revision No.:
Release Date:
4894
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Feb-20/2014
TRANS-NORTHERN PIPELINES INC.
PURPOSE and SCOPE
This subject outlines the procedure for the displacement of line fill using nitrogen
as the displacement medium. The aim is to evacuate the line section with an
inert gas to allow for the safe and efficient removal or replacement of line pipe or
other maintenance/construction activities.
The procedure includes steps for executing nitrogen-purging activities, isolating
the line to complete the work safely and finally, re-commissioning the line to
return it to service.
A job specific execution plan will be required to cover specifics not detailed in this
general procedure. A written plan for the specific displacement operation should
be prepared by the activity co-ordinator.
Completion of a What If risk assessment may be required to help identify
potential hazards and problems specific to the scope of work. This will also allow
for the development of mitigating controls that can be included in the job specific
execution plan.
REFERENCES
Procedure - Safe Work Permits
Procedure - Lockout and Energy Isolation
Procedure- Pigging, Launch and Receive Pigs
ASSOCIATED DOCUMENTS
Form - Safe Work-Confined Space Entry Permit
Form - Job Specific Execution Plan
Form - Equipment Lockout Record
Form - Lockout and Drawing List
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DEFINITIONS
Calculations
Calculations that should be consider (but are not limited to) the following:
1. The volume of product to be displaced.
2. The pipeline profile to determine the maximum change in
elevation over the displacement line section.
3. The specific gravity of the product to be displaced.
4. The volume of nitrogen to be used during the displacement
(include a minimum 10% contingency factor).
5. Nitrogen injection pressures to be maintained.
6. Displacement rate and anticipated duration.
7. Minimum back pressures to be held at terminus, consideration
being given to elevations along the pipeline.
8. Blowdown requirements and duration.
Competent or Qualified
An appointed, authorized or designated person who has knowledge and
understanding, through experience and training on specific equipment that
is sufficient to meet all job requirements as determined by the employer.
Displacement Pigs
Foam or high density wrapped polypigs are suitable for short line sections.
Longer line sections require cupped polyethylene scrapers. Spheres are
not suitable for nitrogen displacement.
The downstream pigs, which form the seal between the nitrogen and the
product being displaced should be over-sized in accordance with the
manufacturer's specifications. This will minimize nitrogen/product
blending ahead of the pigs.
Some nitrogen bypass around displacement pigs is unavoidable. The
amount of bypass can be minimized by choosing the most appropriate pig
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for the specific application. A minimum of two abutting pigs should be
used. Excessive by-pass may create "air lock" ahead of the displacement
pigs and significantly slow down the displacement rate.
Special consideration should be given to the untrimmed internal flash on
the longitudinal pipe weld when specifying displacement pigs. The flash
will accelerate external pig surface wear.
All pigs should be equipped with Electronic Tracking devices to enable
tracking by technicians or other suitable devices such as Rattling devices.
Order spare pigs.
Written Plan or Job Specific Execution Plan
A written plan for the specific displacement operation should be prepared
by the activity co-ordinator. Refer to Form: Job Specific Execution Plan.
A preliminary plan should be reviewed by all departments involved,
including the nitrogen vendor. A revised plan should be issued and signed
off by all parties prior to mobilizing the displacement operation.
Strict adherence to the written plan is required in order to evaluate the
prescribed procedures. Any deviation from the written plan should be
approved by all parties before implementation. Such deviations, if
approved, should be documented as a revision to the existing plan and
given consideration when preparing future displacement plans.
PROCEDURE DETAILS
Hazards and Precautions:
1. Address Contingencies: i.e.
a. Should the displacement pigs become lodged during the
displacement operation, a new injection point may be
required in order to continue the displacement.
b. Similarly, alternative blow-down sites may be required to
expedite venting and to relieve pressure locked in by line
elevation changes.
2. Public Awareness:
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a. Obtain necessary access from adjacent landowners and
acquire adequate work space
b. Notify landowners and public authorities having jurisdiction in
the affected areas of potential noise, blow down fumes and
additional traffic. (Variance may be required for Municipal
Noise By-Laws).
3. Acquisition of "specialty" materials and outside services may
include, but are not be limited to, venting (blow-down) connections,
separation tank, batching pigs and tracking devices.
4. Ensure that all piping and pressure relief devices are rated for the
specific application.
5. Nitrogen interfaces should be delivered to the nearest downstream
terminal. Pumping stations between the injection point and the
terminus should be by-passed.
6. Daylight is preferred for displacement/ tracking, night shift is
preferred for blow down activities, so that work may start early the
next morning. This reduces public exposure to the activity and
leaves daylight hours for tie-in activities.
7. Arrange to have a suitable tank available, into which the displaced
nitrogen/liquid mixture may be safely diverted, taking into account
the rapid expansion rate of nitrogen. Avoid tanks with floating
roofs.
8. Ensure that thermometers, gauges and sampling apparatus are
installed at appropriate locations. Meters may be required at the
injection point for nitrogen and at the terminus for displaced
product. Line Control monitors flow during displacement runs.
9. Existing scraper traps may require modifications to launch or
receive the displacement pigs.
10. Use geophones to aid in tracking the pigs/scrapers.
11. Ensure that injection and blow-down sites can accommodate
tankers and pumpers.
12. Ensure that all valves that must be operated during the procedure
are manually controlled and that all valves through which a pig
must pass unattended are locked "fully" open. Similarly, ensure
that all valves that are to be closed during line displacement are
locked closed.
13. Ensure all station by-pass valves are operational.
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14. For blowdown sites where noise generation must be minimized,
use Pulsco Silencer Vent Model BVS 18-4, c/w 2", 600#, RFSO
flange for line connection. CAUTION: Weight of silencer vent is
approximately 500 lbs. Silencer vent must be supported by skid
deck when mounted on line. Silencer must also be free draining
during use.
Safety:
1. Nitrogen can cause rapid asphyxiation and death if released in
confined, poorly vented areas. Readings should be taken
periodically with a hazardous atmosphere detector capable of O2
readings.
2. Nitrogen as a compressed gas or liquid may cause severe frost bite
to exposed flesh. Piping connecting the pumper to the injection
manifold should be cordoned off with warning tape.
3. Nitrogen as a compressed gas expands rapidly when heated and
may violently dislodge loose fittings. Ensure that all fittings are
secured, whipchecks installed, before commencement of nitrogen
injection.
4. The use of Nitrogen involves the hazard of released energy stored
in compressed gas in the event of a breach of containment. Parts of
mechanically assembled systems should not be adjusted while the
system is under pressure.
5. The vendor's safety precautions and MSDS should be read and
understood by all personnel involved in the injection and venting
activities. These documents should be kept on site throughout the
injection and venting activities.
6. Continuous oxygen deficiency monitoring is required at all
blowdown locations.
7. The venting of heated nitrogen is accompanied by an extremely
loud noise that may damage hearing. Hearing protection as
specified by Canada Noise Control Regulations and CSA Z94.2-94
should be worn by all personnel on site.
8. Noise level readings should be taken periodically in order to ensure
that personnel do not exceed exposure limits as specified in the
Canada Noise Control Regulations.
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PROCEDURE:
Nitrogen Purging can vary greatly depending on the scope of work and equipment being
utilized. The following procedure therefore contains phrases such as: if required and if
applicable in many of the steps. This procedure is intended to allow the user to follow the
steps appropriate to the work being done. Each step should be considered and if applicable
be followed as part of the procedure. The written Job Specific Execution Plan should outline
any steps required for the purge that are not covered in this general procedure.
Nitrogen Purging activities must be performed by competent and qualified persons, whether
TNPI employees or contractors.
Applicable
Yes/No
1)_____ DETERMINE scope of work for purging operation.
a. _____ ESTABLISH time frame for work.
b. _____ INTERFACE with appropriate groups (i.e. Line Control,
Contractors) to schedule timing.
2)_____
COMPLETE a Job Specific Execution Plan or Written Plan for
scope of work. (Should be done well in advance of scheduled
start date to allow for changes and acceptance)
a. _____ ENSURE all required forms are completed (i.e. Request
for Line Down Time, Lockout and Drawing List)
b. _____ OBTAIN all required permits, agreements and approvals.
c. _____ CONDUCT a site visit with appropriate people to review
scope of work and understand specific job requirements.
d. _____ REVIEW plan with appropriate people.
e. _____ ENSURE plan is signed off by all appropriate people.
3)_____
STAKE and/or DAYLIGHT the line upstream of the injection
point as required to allow for adequate warning that the batching
pigs are approaching the injection point.
4)_____
STAKE and/or DAYLIGHT, If the terminus of the purge is located
at a mainline station, upstream of the station if required to coordinate the shutdown/bypass of the station.
5)_____
INSTALL required vent fittings, PSV’s, piping for blowdown tanks
and other required hardware as per written plan.
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Responsible
Person
Nitrogen Purging
Critical
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a. _____ POSITION blowdown tanks/trucks at the specified
location(s). (See typical valve and piping schematics prepared
with the written plan.)
6)_____
INSTALL portable lighting plants as required at the injection and
venting sites prior to connecting the pumper.
7)_____
CONFIRM the status of the communication link with Line Control
before commencing nitrogen injection.
8)_____
CONFIRM operation of tracking transmitters and receivers
before launching pigs.
9)_____ ENSURE proper pig separation as per calculations.
a. _____ LINE SHUTDOWN may be required to provide time to
load pigs.
b. _____ ADDITIONAL pig separation may be required to allow for
pig slippage during travel.
10)_____
LAUNCH pigs into a product batch from existing traps upstream
of the injection point. (REFER TO PROCEDURE: Pigging,
Launch and Receive Pigs)
a. _____ VERIFY separation, for pigs launched upstream, at a
predetermined location upstream of the nitrogen injection point.
This location is based on mainline rate and corresponding pig
travel speed.
OR
11)_____
ALTERNATIVELY LAUNCH from a temporary trap installed at
the injection point. (REFER TO PROCEDURE: Pigging, Launch
and Receive Pigs)
a. _____ VERIFY pig separation at predetermined location
downstream of the launch site.
12)_____ MONITOR closely thereafter.
a. _____ CONFIRM mainline rate with Line Control.
At the same time:
13)_____ VERIFY:
a. _____ Nitrogen pumpers are commencing cool-down period.
b. _____ Commencement of a reduced rate if required.
c. _____ Adequate separation between displacement and
recommissioning pigs to permit closing of upstream isolation
valve between the two.
d. _____ Initiation of an open line of communication with Line
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Control.
14)_____
ADJUST line rate if required to allow for acceptable positioning
of pigs.
a. _____ GIVE special consideration, If a trap is terminus of
nitrogen purge section: i.e.:
i. reduction in line rate,
ii. venting upstream
iii. ensuring a back pressure cushion in the trap when
receiving pigs.
15)_____
GIVE predetermined notification to the on-site personnel for final
alert.
NOTE: The location of the pigs/scrapers should be known at all times during the displacement
operation. Should displacement pigs stop moving through the pipeline, it may be extremely difficult
to locate and remove them.
16)_____ ENSURE All quick-action valves on the nitrogen injection piping
are secured in the required (open/closed) position.
17)_____
ADVISE Line Control to commence shutdown of upstream
booster when displacement pigs have reached the five-minute
position upstream of the injection point.
18)_____
ADVISE Line Control, to close the upstream isolation valve and
begin nitrogen injection, once the displacement pigs are
confirmed to be past the injection point.
a. _____ ADVISE downstream operations, upon confirmation, that
nitrogen injection is on-line.
19)_____ MONITOR closely:
a. _____ nitrogen rate,
b. _____ nitrogen pressure,
c. _____ downstream suction pressure
d. _____ mainline displacement rate.
20)_____ CONTINUOUSLY MONITOR pressure at the nitrogen injection
location to ensure that over-pressuring of the line does not
occur.
a. _____ MAKE every effort to maintain specified line rate and
downstream suction pressure.
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21)_____
CONTINUOUSLY TRACK the displacement pigs at critical
locations:
a. _____ MLV's,
b. _____ Stations,
c. _____ Tracking points.
22)_____
CLOSE VALVE and notify Line Control to shutdown downstream
units immediately once displacement pigs pass downstream
valve.
a. _____ IF the downstream valve is located at a station, when the
interface arrives at the sixty-minute location upstream of the
station, the station is to operate on bypass to ensure nitrogen will
not enter mainline units.
b. _____ NOTIFY injection crew.
c. _____ MAKE Alteration of station sequencing (software) as
required.
d. _____ CLOSE station bypass valve and shutdown line once
displacement pig passes station
e. _____ CLOSE required upstream and downstream valves to
provide proper isolation.
NOTES:
The rate of injection must be slow enough to maintain control of the purging operation. Back
pressure, held at the terminus, is the primary means of controlling purging rate. This rate should not
exceed the meter capacity.
The temperature of the nitrogen to be injected should be as close as practicable to existing ground
temperature.
Do not exceed MAOP pressure rating of the pipeline.
23)_____ BYPASS OR REMOVE, At an appropriate time before the
nitrogen/liquid interface arrives, the meter from the stream and
replaced with spools if required.
24)_____
DIVERT the stream to a suitable receiving tank upon arrival of
the nitrogen/liquid interface.
a. _____ The time required to receive the nitrogen/liquid interface
will depend upon the viscosity and temperature of the mixture
25)_____
ENSURE the displacement pigs are constantly moving, if at all
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possible.
a. _____ A steady decrease in the displacement rate may be
indicative of pig bypass and "air lock".
NOTE:
Should the nitrogen supply be exhausted before the completion of the displacement, the movement
of the pigs can continue at a slower rate while additional nitrogen is being obtained. The
continuance of the displacement at the reduced rate may be possible due to the expansion of the
nitrogen in the pipeline.
In such cases, do not close the line section, as the increasing pressure may exceed the allowable
maximum operating pressure of the line section.
This increase may be severe if the temperature of the nitrogen originally injected was much lower
than the product temperature.
26)_____ SECURE the purged line section by isolating the appropriate
valves. ( REFER TO PROCEDURE: Lockout and Energy
Isolation)
a. _____ COMMENCE blowdown. (A nitrogen locked-in pressure
range of 125-550 psig may be encountered and therefore
blowdown may be lengthy and noisy.)
b. _____ REGULATE the flow to control foaming and potential
over-pressure.
27)_____
SET UP continuous oxygen monitoring as required at blowdown
locations.
28)_____
MONITOR the rate of pressure decrease on blowdown.
29)_____ VENT nitrogen, but NOT directly to atmosphere if at all possible.
a. _____ Where practical: Blowdown tanks should contain sorbent
pads.
b. _____ OPEN blowdown valves slightly to confirm product will not
be expelled to atmosphere.
c. _____ USE muffling devices as required.
d. _____ CONSIDER using additional vents to expedite blowdown.
30)_____ CHECK the pipeline for LEL. LEL must be below 10 %.
a. _____ CHECK LEL at both pig traps and other vents or drain
valves that are available while venting to ensure a successful
purge.
31)_____
INSTALL isolation blinds as required. ( Refer to Form: Lockout
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and Drawing List)
32)_____
IMPLEMENT the required work.
Re-commissioning:
33)_____
INSPECT pipeline for completeness of scope of work.
34)_____
REMOVE all installed blinds and locks ( Refer to Form: Lockout
and Drawing List)
35)_____
INFORM Line Control that the pipeline is being re-commissioned
and will be returned to normal service.
36)_____
ENSURE that upstream valves are open prior to linefill start up.
37)_____
ENSURE downstream isolation valve remains closed in order to
compress air between the displacement pigs. (This will provide
backpressure for the introduction of product into the line.)
38)_____
LAUNCH re-commissioning pigs as required by written job plan
with appropriate interfaces and spacing. (Nitrogen, Liquid
product.)
a. _____ REFER to procedure: Pigging; Launching and Receiving
Pigs.
b. _____ CONSIDER the line may require de-watering depending
on scope of work completed.
39)_____
VENT air at the first practical downstream opportunity (MLV with
traps, manifolds) to the displaced line section.
a. _____ VENT into vacuum truck at specified location if
appropriate.
b. _____ AVOID venting into marketing terminal tanks.
40)_____
TRACK the displacement pigs at predetermined locations.
41)_____
SAMPLE for product/nitrogen interface, as linefill nears
completion.
42)_____
CONTINUE VENTING into downstream blowdown tank until
liquid appears.
a. _____ This step may have to be repeated several times if
excessive bypass has occurred.
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43)_____ TRACK the displacement pigs to the downstream receiving trap.
a. _____ Timing for appropriate station bypass must be previously
arranged with Oil Movements and Field Services.
b. _____ In the planning stages, consider the total length of the
pigs vs. the length of the trap to allow for receiving all pigs at one
time if possible.
c. _____ It may be necessary to remove each pig from the trap as
it comes in.
44)_____
MONITOR all locations including drains, vents and flanges for
any signs of leakage.
45)_____
ADVISE downstream Terminals of potential nitrogen
entrainment.
46)_____ PRESSURIZE worked on line section:
a. _____ CHECK all work sites for leaks.
b. _____ ARRANGE for appropriate testing. (HOLS test)
47)_____
NOTIFY Line Control when the pipeline has been re-filled and
normal operations can resume.
48)_____
FORWARD all log sheets and other vital information to the
assigned supervisor and project co-ordinator.
a. _____ Logging of pressures and temperatures should be
documented at predetermined points along the pipeline route as
identified in the work plan.
b. _____ Volume and flow rates should be recorded at the nitrogen
injection point and the displacement terminus.
c. _____ Individuals assigned to track and log the progress of
pigs/scraper through the pipeline should do so in accordance
with a reporting procedure suitable to the needs of the specific
displacement operation.
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APPENDIX:
Form: Job Specific Execution Plan
Form - Equipment Lockout Record
Form - Lockout and Drawing List
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