Page 27 - Pipeline Pigging Technology

Pipeline Pigging Technology

brushing would be flushed out in front of the pigs and kept in suspension by
the turbulent flow. The pipeline would then be flushed and swept out by
batching pigs until the particulate matter in the flow had reduced to accept-
able levels. Fig.l shows typical brush and batching pigs.
Following brushing, the longer the pipeline the longer it will take to flush
and sweep out the particles to an acceptable level. Gel slugs are used to pick
up the debris into suspension, clearing the pipeline more efficiently. Gels are
specially-formulated viscous liquids which will wet the pipe surface, pick up
and hold particles in suspension. A slug of gel would be contained between
two batching pigs and would be followed by a slug of solvent to remove any
traces of gel left behind.

Flooding for hydrotest

In order to demonstrate the strength and integrity of the pipeline, it is filled
with water and pressure tested. The air must be removed so that the line can
be pressurized efficiently as, if pockets of air remain, these will be com-
pressed and will absorb energy. It will also take longer to bring the line up to
pressure and will be more hazardous in the event of a rupture during the test.
It is therefore necessary to ensure that the line is properly flooded and all the
air is displaced.
A batching pig driven ahead of the water forms an efficient interface.
Without a pig, in downhill portions of the line, the water would run down
underneath the air trapping pockets at the high points. Even with a pig, in
mountainous terrain with steep downhill slopes, the weight of water behind
the pig can cause it to accelerate away leaving a low pressure zone at the hill
crest. This would cause dissolved air to come out of solution and form an air
lock. A pig with a high pressure drop across it would be required to prevent
this.
Alternatives to using a pig include flushing out the air or installing vents at
high points. For a long or large-diameter pipeline achieving sufficient flushing
velocity becomes impractical. Installing vents reduces the pipeline integrity
and should be avoided. So for flooding a pipeline, pigging is normally the best
solution.

Dewatering and drying

After hydrotest the water is generally displaced by air, although sometimes
nitrogen or the product are used. The same arguments apply to dewatering
as applied to flooding. A pig is used to provide an interface between the air

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