Page 81 - Pipeline Pigging Technology

Pipeline Pigging Technology

Hence, the only assessment that can be made is gathering field-perform-
ance feed-back and examining the design of the pigs. In regard to pig design,
the following points can be made:

bi-directional (bi-di) pigs with guiding and oversized sealing discs are
much more effective than conical-cup type pigs;
brushes with coil-type power springs are more effective than those
with leaf-type springs;
pig trains of three pigs are more effective than running three pigs
separately. (What is scraped off by one pig is pushed out by the next
in the train before the debris settles down again);
pigs with by-pass and spider noses push more debris out than those
without by-pass (provided sufficient flow is present; for a liquid 1 m/
sec minimum, and for a gas 3m/sec minimum);
increasing the number of guiding discs per pig has a more than
proportional effect on increasing the push-out performance;
mounting brushes on pigs in dry gas pipelines improves the stability and
reduces the disc wear; (the black dust in gas pipelines causes the
discs to wear down. This prevents the pig from rotating, causing
excessive and uneven wear);
the weight of the pig has little or no effect on the cleaning performance.

This means that for adequate pre-survey cleaning:

(a) in a pipeline that is relatively clean, a limited number of standard-type
pigs can satisfactorily prepare the line;
(b) in a pipeline where a good regular pigging programme is undertaken,
a simple increase in frequency can suffice (or maybe the use of a different type
of standard pig);
(c) in a pipeline with a recognized problem (wax, dust, over 100km in
length, etc.), a specially-designed pre-survey cleaning programme will be
required with specially-adapted pigs and the use of pig-train techniques.

Conditions in low-pressure/low-flow gas lines are not considered in the
review of cleaning problems outlined here. However, these operating condi-
tions result in uneven speed. Trial pigging should be carried out using
differential-pressure measurements and conscientious recording (low pres-
sure for pipelines below 14-in diameter is taken as 60bar; in pipelines from 16-
24in diameter, 30bar; and in pipelines above 24in diameter, 20 bar; low flow
is Im/sec or less).

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