Page 28 - Pipeline Pigging Technology

Why pig a pipeline?

and the water so that the water is swept out of the low points. Sometimes a
bi-directional batching pig is used to flood the line, is left during the hydrotest,
and is then reversed to dewater the line.
In some cases it is necessary to dry the pipeline. This is particularly so for
gas pipelines, where traces of water may combine with the gas to form
hydrates, waxy solids which could block the line. Following dewatering the
pipe walls will be damp, and some water may remain trapped in valves and
dead legs. The latter are normally eliminated by designing dead legs to be self-
draining, and by fitting drains to valves where necessary.
One way to dry the pipeline is to flush the water with methanol or glycol.
The latter chemical also acts as an inhibitor, so that traces of water left behind
do not form hydrates. To fill the pipeline with methanol would be prohibi-
tively expensive; instead a slug or slugs of methanol are sent through the
pipeline between batching pigs.
Vacuum drying is increasingly being used as an alternative to methanol
swabbing for offshore gas lines. Here vacuum pumps reduce the internal
pressure in the pipeline so that the water boils and the vapour is sucked out
of the line.

PIGGING DURING OPERATION

If pigging is required during operation, then the pipeline must be designed
with permanent pig traps, especially when the product is hazardous. As was
mentioned above, it is far better to avoid pigging if possible, but for some
operations it is the safest and most economical solution. Typical applications
for pigging in operational lines are illustrated in Fig.3, and include separation
of products, flow improvement, corrosion inhibition, meter proving and
inspection.

Separation of products

Some applications demand that a pipeline carries a number of different
products at various times. It is basically a matter of economics and operational
flexibility as to whether a single line with batches of products in series is to
be preferred to numerous exclusive lines where the products can flow in
parallel.
As with flooding and dewatering, a batching pig provides an efficient
interface between products, minimizing cross contamination. To ensure that