An introduction to pipeline pigging

[Freeman]

AN INTRODUCTION TO PIPELINE PIGGING 2

PIGGING FUNCTIONS


Fluid Separation Pigs form a solid barrier between dissimilar fluids, e.g. liquid and gas. As such, pigs can be utilised in a train to "batch" chemicals or other fluids to perform a function. A good example of this would be the following pig train.

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The first two slugs of fresh water provide desalination for a line previously flooded with
seawater, while the glycol slugs aid in dehydration and hydrate inhibition upon the
introduction of product. The whole train is driven by nitrogen gas.
Displacement

As there is a solid interface formed between the pipe wall and the pig sealing element, any fluid in the line (liquid or gas) is displaced from the line as in the pig train above. Inevitably there is some bypass due to surface roughness, weld penetration, and seal bypass.
Cleaning

Like the process for displacement, the positive interface between the pig and the pipe wall
imparts a cleaning action on the pipewall. This can be further enhanced by the addition of
brushes, scrapers, or even more aggressive tools to the pig. For lines where ferrous debris is expected, magnets attached to the pigs can add a pick-up action for removal of magnetic debris.

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The turbulence within the fluid flow will hold any small, solid debris in suspension,
effectively sweeping it out of the line. The use of bypass ports through the pig can aid this
sweeping effect.
Waxes and sludges tend to adhere to the pig brushes and scrapers and are generally
"ploughed" through the line.
Gauging

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In order to identify any major restriction in flow area through a line a simple metal plate is
attached to the pig to provide an internal line gauge. Generally sized to 95% of the pipeline internal diameter (95% nominal or 95% minimum, dependant on specification being used), the gauge plate tends to be made of a soft metal such as aluminium and is chamfered on the leading edge. It is common to "petal" the gauge plate with short, radial cuts to minimize the risk of the pig becoming stuck. These radial cuts through the gauge plate allow the plate to bend more easily if an obstruction is encountered.