POWER September 2011 - 66

PLANT COMMISSIONING
the pig along down the pipe until it reaches
the receiving trap, the " pig catcher. " Pigging
has been used for many years to clean largerdiameter
pipelines in the oil industry.
If the pipeline contains butterfly valves or
other line obstructions, the pipeline cannot be
pigged. Full-port ball valves typically cause
no problems because the inside diameter of
the ball can be specified to be the same as
that of the pipe.
The pipe cleanliness resulting from pigging
is less than that achieved by the gas
blow method. Typically, the gas supply line
leading up to the power plant gas metering
station is cleaned via natural gas blows. It is
becoming more common that the line up to
the yard is pigged in lieu of gas blows. If a
gas blow is not conducted for the gas supply
line up to the gas yard, the cleanliness
of the overall system is expected to be equal
because particles left after pigging the main
line could be transferred to the pipe downstream
of the metering yard.
The cost of pigging is estimated to be
roughly of the same magnitude as gas blowing.
However, this cleaning process requires
additional gas line strainer cleaning during
initial commissioning compared with gas
blows. Overall, we estimate that relying on
pigging to achieve the same pipeline cleanliness
as gas blows will require about three
to five additional days. The cost of this longer
cleaning period should also consider the
additional combustion turbine operations at
reduced or restricted load because of the increased
strainer cleaning frequency.
Water Blasting. Water blasting (hydrolazing)
can also clean the piping system using
high-pressure water between 10,000 and
20,000 psig. This process uses a machine that
controls the cleaning hose feed rate through
the pipe system while the cleaning nozzle remains
forced against the pipe wall. Fluor has
historically utilized this process on process
piping systems such as main steam, turbine
extraction lines (heater bleed system), feedwater,
and condensate systems.
We estimate that the pipe cleanliness produced
by hydrolazing is equal to the cleanliness
achieved by the gas blow method.
Assuming that the pigging process is used to
clean the gas supply line leading up to the gas
yard, the internal cleanliness of the plant-side
piping is expected to exceed that of the supply
line. Hydrolazing requires additional pigging
to remove excess water from the piping
system prior to line purge and layup.
The cost to hydrolaze gas piping is roughly
two to three times that of natural gas blows
and is not expected to have any substantial
schedule impact, if planned well.
Air Blows. The air blow pipe cleaning process
uses oil-free high-pressure compressed
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air to clean the process piping systems. This
process uses temporary high-pressure air
compressors to pressurize the system and
high-speed relief valves to rapidly vent the
piping systems. The pressurization and depressurization
cycles are repeated until the
piping systems are determined to be clean.
Fluor has previously used the air blow
process in lieu of steam blows to clean main
steam, cold reheat, hot reheat, and auxiliary
steam systems. A similar technique is often
used to clean the instrument and service air
piping systems, but permanent plant air compressors
are used as the supply source.
For gas pipeline cleaning, the effectiveness
of air blows is not expected to be high due to
the lack of sufficient storage space (for the
compressed air) upstream of the piping system
to be cleaned. The air blows performed
in lieu of steam blows for steam line cleaning
used the boiler as a storage device prior
to the steam piping systems. Unfortunately,
when blowing the natural gas supply line, the
storage volume in the pipeline is much less.
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POWER September 2011

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