Page 470 - Pipeline Pigging Technology

Ftyging research

the effects of by-pass and optimum by-pass configuration;
driving cup/disc (i.e. seal) performance, materials and configuration;
the effects of the differential pressures across the seals;
the optimum type, arrangement, loading and materials for cleaning
elements.

The expertise to carry out such research exists - as do most of the facilities;
Fig. 1 shows some of the pigging test loops currently available. What is lacking
are the financial resources and, often, an appreciation of the advantages to be
gained from such an R&D programme.
Further discussion of just the first two aspects listed above may provide
some indication of the current situation and the advantages to be gained from
a formal research and subsequent development programme.

VELOCITY EFFECT AND OPTIMUM PIG SPEED

Enough is already known about the effects of pig speed to be able to state
unequivocally that it is very important.
One of the more obvious problems is that of "speed excursions". This is an
area where British Gas On Line Inspection Centre has done a lot of research.
When pigging low-pressure lines, the pig will hold up at a weld bead or
other obstruction until the gas pressure builds up behind it sufficiently to
overcome the obstacle. It then accelerates away - often attaining speeds of
well over 60mph before coming to rest once more and repeating this cycle.
This not only results in negligible pigging efficiency, but is also highly
dangerous. Pigs have been known to rip open and exit a pipe on a bend when
travelling under these conditions.
It is known that pipeline pressure and velocity determine whether a speed
excursion will occur, but an even better understanding could help in the
development of methods for speed control for use where it is impractical to
create the optimum running conditions.
Perhaps the most important factor concerning speed is its effect on the
sealing efficiency of a pig. The importance of creating and maintaining a good
seal is obvious for the separation of dissimilar fluids (batching) in products'
pipelines, for condensate removal in gas lines, for commissioning and, more
recently, for providing secondary barriers for pipeline isolation.
Less obvious, but equally important, is the film thickness left behind the
pig when applying in situ coatings or when performing batch inhibition.

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