Page 257 - Pipeline Pigging Technology

Pipeline Pigging Technology

construction from that of standard manufacturers, in that we utilize a steel
body fitted with over-size polyurethane butterfly discs together with overlap-
ping thin spring steel plates. These plates protect the butterfly discs from
abrasion, assist with the cleaning operation, and give added support to the pig
whilst it is in the pipeline.

Pressure by-pass pig

One of the most notable new pig designs to emerge in recent years is the
pressure by-pass pig produced by ourselves. It was specifically developed for
pre on-line inspection pigging, and is now used for both proving and cleaning
operations. The front of the pig is fitted with what is effectively a pressure-
relief valve, having a diameter of around 40% of the internal bore of the
pipeline and set to open at a pre-chosen differential pressure. If, during a
proving or cleaning run, the pig builds up a large accumulation or slug of
debris ahead of it, the differential pressure across the pig will obviously rise
as the pig begins to work harder. If a conventional cleaning pig was being
used, the accumulation of debris ahead of it might well increase until the pig
became stuck or substantially damaged. This cannot happen with a pressure
by-pass pig, since once the pre-set differential pressure is reached, the by-pass
valve opens, thereby allowing a substantial volume of fluid or gas to flow
through the pig body. This results in the debris being jetted or blown away
from the front of the pig, after which time the differential required to run the
pig will drop, the by-pass valve will close, and the pig will move on. This
sequence may take place many hundreds of times during a run in a particu-
larly-dirty pipeline before the pig reaches the receiver. Also, it is most unlikely
that the by-pass pig can ever block the pipeline in the event that it becomes
totally stuck, since the by-pass facility allows continuous by-pass of the
propelling medium. To date we have designed and supplied many by-pass
pigs, ranging in size from 6in to 42in diameter.

Magnetic cleaning pig

Whilst the presence of ferrous debris, such as welding rods and the like,
does not generally present a major problem in an operational pipeline, it is
essential that such debris is removed if on-line inspection is to take place. Most
major pig manufacturers offer magnetic cleaning pigs, which are generally
standard swabbing pigs with permanent magnets attached. Under normal
circumstances such pigs might be adequate and will generally remove the
debris during several runs through the pipeline. However, if the presence of

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