Page 252 - Pipeline Pigging Technology

Pigging and chemical treatment

somewhat more effective on very soft deposits, but are not very effective on
sticky or hard waxes, as they have a tendency to "flex" and run over debris.
Spring-loaded brushes will also flex, but they will cut into hard deposits much
better than blades. It should also be noted that spheres are not cleaning tools,
and can press deposits further against the pipe wall. Polly Pigs have some
effect on paraffins and scale if they are made from high-density foam and have
wire brushes or other scraping surfaces.
When moving large volumes of deposits through a long pipeline, care must
be taken in not pushing so much debris that the pig becomes stuck. It is
recommended that there be some amount of by-pass around the pig, to assist
in suspending debris out in front of the pig and to help keep blades and
brushes clean. All pigs have some degree of by-pass; however, it is possible to
increase this amount by controlling the size of the pig's sealing area or by
providing by-pass ports through the pig.
Use of the progressive pigging technique allows large amounts of debris to
be removed safely by removing a little at a time in a progressive manner. The
technique utilizes foam pigs of different sizes, coatings, and densities to
gradually remove deposits, rather than attempting to remove them all in one
pass. Starting with soft, low-density, pigs, the condition of the line is assessed
by examining the condition of the pig after passing through the line. By
gradually increasing the density and diameter of the subsequent pigs, removal
of deposits is controlled.
For removal of settled liquids or for batching chemicals, a good sealing pig
should be used. There are many such designs available, such as Polly Pigs,
spheres, cup or disc pigs. Conical cups are deemed to be very good for sealing,
although any pig with four cups should be adequate. If a disc pig is used, it is
recommended that the configuration is equipped with guide discs to help
support the mandrel weight. This will reduce the potential of by-pass around
the softer sealing discs. Spheres can be inflated so that a tight seal is realized;
however, spheres offer the least amount of surface bearing area and minimal
wiping ability of any pig. A criss-cross coated Polly Pig offers a good seal, but
may not have as much usable life as offered by the other designs. When
batching chemicals, it is advisable to use two pigs, one in front and one behind
the slug of chemicals, to help contain the liquid in a full column form. This is
very important when batching in a downhill slope. A brush pig can be used
as the front pig to help prepare the pipe surface for the treatment.
In order for any pig to perform its task sufficiently, it must be in good
operating condition. Parts such as cups, disc, springs, brushes, and blades
should be routinely inspected for wear and fatigue. Replacement of these
parts should be made when it is determined that they are no longer useful in
sealing and cleaning, or in supporting the weight of the pig. Using a worn or

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