Page 30 - Pipeline Pigging Technology

Why pig a pipeline?

however, have the advantage that it proves the line is clear and there is no wax
build up which might cause problems for a line which is only pigged
occasionally.

Line cleaning

Similar arguments about improving pumping efficiency apply to any
products prone to depositing solids on the pipe wall. Gas line efficiencies can
be improved by removing dust or using a smooth epoxy-painted internal
surface.

Condensate clearance

In gas lines, conditions can occur where liquids condense and collect on
the bottom of the pipeline. They can be swept up by the gas to arrive at the
terminal in the occasional large slug, causing problems with the process
facilities. Slug catchers which are basically large separators are used to absorb
these fluctuations. However, it is normal to limit the potential size of the
condensate slugs by regular sphering, and thus reduce the size of the slug
catcher required.

Corrosion inhibition

Inhibitors are used to prevent the product attacking and corroding the
pipeline steel. In some cases, particularly in liquid lines, small quantities of
inhibitor are added to the flow. However, in other cases it is necessary for the
inhibitor to coat the whole inside surface of the pipe at regular intervals. This
is accomplished by retaining a slug of inhibitor between two batching pigs.
This method also ensures that the top of the pipe is coated.

Meter proving

In order to calibrate flowmeters during operation, a pig is used to displace
a precisely-known volume of fluid from a prover loop past the flowmeter.
Normally a tightly-fitting sphere is used for this purpose, and the run is
repeated until consistent results are obtained.

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