Pipeline Design Essential in Making Pigging Plans

Bi-directional and instrumented scraper traps

Pipeline design essential in making pigging plans

Pipeline design essential in making pigging plans

Pigs Traps have gotten an unfortunate reputation for getting stuck in pipelines. As a result, for many years few pigged their pipelines and consequently, many companies are paying the price to repair or replace their corroded pipelines. It is currently considered a necessary evil to run pigs to improve pipeline efficiency and prevent corrosion.

Some pipelines were not designed to run pigs and occasionally the wrong type of pig is selected to run in a particular pipeline, increasing the chances of sticking a pig.  A pipeline properly designed for pigging along with proper pig selection greatly reduces chances of sticking a pig.

Design Factors & Considerations

There are three elements to be determined when designing a new pipeline for pigging: First, what is the goal of the pigging program (removal of liquids from a wet gas system, liquids that have accumulated in product pipelines, paraffin that has built up on the walls of crude oil pipelines, batch products, etc.). Second, select the type of pig that will best perform the desired service. Third, design a pipeline to accommodate the pigs to be selected.
If an existing pipeline is to be pigged, review each design feature to see what type of pig will negotiate it for the best results. The pig must maintain a seal as it passes through the entire pipeline. If the pig loses its seal anywhere in the pipeline, it may get stuck. Minimize the internal pipeline diameter variations through design whenever possible. It may be tempting to use in-stock or readily available pipe even if it has a drastically different wall thickness, but this may create future problems. Never use different nominal sizes of pipe in a pipeline that may require pigging.
Many factors affect the type of pig to be used. Changes in pipe diameter will require a dual-diameter pig and out-of-round pipe may require a pig with conical shaped cups. If there are mitered bends, only a soft foam pig has a good chance to traverse the bend and maintain a seal. Short radius bends (less than 3D), will require special pigs (short or flexible).
Improper branch connections will increase the chances of a pig getting stuck. Paraffin or other foreign material that build up on tile pipe walls need special pigging equipment and techniques. Partially closed valves, control valves, orifice or slip plates, inline strainers, thermowells protruding into the line, etc. may make it impossible to pig a pipeline.

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