Page 80 - Pipeline Pigging Technology

Pre-inspection-survey activities

PIPE-CLEANING PIGGING

The pipe-wall surface condition can only finally be assessed by the use of
pigging, although pigs only produce consequential evidence. However, as
stated in the introduction, a single pig run does not produce conclusive
information.
The reason for this is that the results of pigging are assessed by the amount
and quality of debris that is accumulated in the receiving pig trap, and by the
physical condition of the pig after the run. These results provide a certain
amount of information, but leave three unknowns:

pig performance on this run;
debris quantity;
debris quality.

These unknowns are further qualified by the following factors:

pigs wear down in the pipeline and, as such, their performance
capability reduces during the run (cup/disc wear is very much
affected by the vast amounts of dust in gas pipelines);
greasy pipe walls lubricate the cups/discs, reducing the pig perform-
ance;
temperature differences influence the stiffness of the cups/discs;
the amount of debris may exceed the pig capacity (in long lines);
the adhesion of debris to the pipe wall may be greater than the pig can
scrape off.

It is for these reasons, among others, that more than one pig run is required
to assess the pipe-wall condition.

Pig performance

Pig performance can only be assessed by comparing one type with
another. However, they will never have identical running conditions; the
added complication of the dual function of the pig (scraping off and pushing
out debris over long distances), makes a true comparison impossible, and
assessment very difficult.

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