Purging a Pipeline
With the exception of purging service lines,
-Document the existence of a purge plan -
Factors to think about before purging:
-Equipment and/or piping to be purged, and how it should be isolated -The purge medium to be used -How the purge medium is introduced into the equipment and/or piped and vented -Method for testing the completeness of the purge and end point to be attained -Selection and assignment of a responsible project supervisor and operating personnel -
Examples of Source of Ignition
-flames from open lights, matches, lighters, laterns, etc -Sparks and arc from non-approved flashlights , torch igniters, enignes, static electricity, lightning, etc -heated mated materials like glowing metals, cinders, flaments, or electric lights
When purging out of service, the purge is considered complete when the reading is to
0-3% is obtained
Sources of Ignition
accidental ignition is most likely to occur at the outlet of the purge point.
Each small section, if it comes in contact with another dielectric, will
acquire its own local charge and potential.
The slug separates the gas from the
air in the pipeline
If you see a dramatic reduction in gas flow during purging operations,
an activated EFV is most likely the cause
When a vent stack is installed, it should be located in
an area that minimizes the hazards of vented gas
Always follow all relevant company procedures very carefully in order to
avoid hazards during the purging process
It is important that all possible sources of ignition
be eliminated
Whenever a main with attached service lines is to be purged into operation,
be sure that the service is turned off at the meter by making sure that all riser valves are closed
Purging should be performed on any pipeline that is
being taken out of service or placed into service
A facility to be purged is to
blocked or disconnected from the rest of the system
Before severing or disconnecting a pipe, a
bond wire should be attached to the metallic at two points to provide a connection across the proposed severance or disconnection
Before a service is purged, facility records should be reviewed to
check for the presence of Excess Flow Valves (EFV's)
An approved, calibrated combustible gas indicator is required to
check the percentage of gas-in-air during a purge operation and determine when the purge is complete.
If a purge is incomplete,
continue to purge, verify the CGI readings, and evaluate the situation
When purging a pipeline with gas or air, the flow rate should be
continuous over the course of the purge process.
Determining whether an inert gas is required in a purge process consider the
diameter and length of the pipe.
Readings should be done at a vent stack
during the purge and when the purge is complete
Static electricity on materials that are conductors of electricity may be
eliminated by grounding
Static Electricity
gas escaping from a pipeline can ignite in the presence of static electricity, which can lead to disastrous results
The purge velocity in the pipeline should be high enough to limit layering of
gases and create turbulent flow at the interface of the gas in the air
The vent stack should be a designated height above
grade, generally above the workers head
Ensure all machinery, pipes, vent stacks, tools, and other equipment are
grounded when charges may accumulate
Pyrophoric materials that can
ignite spontaneously in the presence of a gas-air mixture
Large Diameter pipes generally require
inert gas when purging into or out of service
Whenever any facility is purged, it is essential to
isolate it from the rest of the system
When purging from combustible gas to air, especially if old piping is being purged,
keep in mind that purging removes only gaseous and volatiale material
Direct communication between the introduction point and the discharge point must be
maintained at all times
The pipes being used should be
metallic, grounded, and properly supported all the time
Maintain the continuous flow rate when purging a pipeline
out of service
Prevent any leakage AFTER the purging operation when the facility is
out of service for inspection, repairs, or demolition.
Continue the operation until no air is
present in that being purged into service
Continue the operation until no combustible gas is
present in the equipment being purged out of service
The purpose for isolating the facility for it to be purged is to
prevent leakage DURING the purging operation
Refer to your company's policies and procedures for
proper handling of liquid or solid combustible material
It is important to make sure that the appropriate flow rate is maintained during
purging operations
Always follow your company's specific
purging procedures
If an EFV is activated during a service line purge,
reset the valve
Natural Gas is often used to purge a
small diameter pipe into service
Air is often used to purge a
small diameter pipe out of service
The vent stack outlet diameter should be
smaller than the pipe diameter
If water is discovered in the line during a purging operation,
stop the purge
If you get a reading of 100% but do not smell gas,
the gas is most likely under-odorized. If this is the case, be sure to contact the appropriate company personnel
It is essential that the combustible gas be diffused into the air without hazard to
the workers, public, or property
Important factors in choosing a vent stack location
wind direction, sources of ignition, building openings, and nearby traffic
During the blow down process, if you determine gas pressure is not dropping as expected,
you should stop the process and investigate
A person who is familiar with the properties and nature of the materials involved and the construction and function of the equipment and/or piping system to be purged
Responsibility and Authority
What can remain in the gas/pipeline after it is purge?
Liquid Combustibles, solid combustible material, and easily oxidized material
The charge cannot be drained by a ground connection
Nonconductor (Dielectric)
Safety Equipment
PPE, Fire Extinguisher
Location of active purge points should be planned to take into consideration
Potential ignition sources and building openings
If a sufficient amount of purging air or gas cannot be supplied to prevent the formation of a hazardous mixture of gas and air, a slug of inert gas must be released into the line first
Purging of Pipelines 192.629
States that the gas and air used to purge a line must be released into one end of the line in a moderately rapid and continuous flow.
192.629 Purging of Pipelines
When purging into service, the purge is considered complete when the reading is to
97-100% gas is obtained
Prior to purging gas mains, it may be necessary to contact company dispatch and/or local emergency officials.
Communication