4.5.8 Pipeline Pigging
4.5.8.1 General
Pipeline pigs and spheres are used for a variety of purposes in both liquids and natural gas pipelines.
Pigs and spheres are forced through the pipeline by the pressure of the flowing fluid. A pig usually consists of a steel body with rubber or plastic cups attached to seal against the inside of the pipeline and to allow pressure to move the pig along the pipeline. Different types of brushes and scrapers can be attached to the body of the pig for cleaning or to perform other functions.
Figure 4.28 illustrates a variety of pipeline pigs
Pipeline pigging is done for the following reasons:[link Point to another website Only the registered members can access]
• To clean up pipelines before use (foam pigs);
• To fill lines for hydrostatic testing, dewatering following hydrostatic testing, and drying and purging operations (spheres and foam pigs);
• To periodically remove wax, dirt and water from the pipeline (scraper pigs and brush pigs);
• To sweep liquids from gas pipelines (spheres)
• To separate products to reduce the amount of mixing between different types of crude oil or refined products (squeegee pigs and “Go-Devil” pigs);
• To control liquids in a pipeline, including two-phase pipelines (spheres and foam pigs);
• To inspect pipelines for defects such as dents, buckles or corrosion (“intelligent-pigs or caliper pigs).
(Figure 4.29 illustrates a kaliper pig.)
[link Point to another website Only the registered members can access]
Differential pressure is required to move a pig or sphere through the pipeline. The force required depends on elevation changes in the pipeline, friction between the pig and the pipe wall and the amount of lubrication available in the line. (A dry gas pipeline provides less lubrication tan a crude oil pipeline, for example).
Cups are designed to seal against the wall by making them larger than the inside diameter of the pipe. As the cups become worn, the amount of blow-by fluid by-passing the pigs increases because the seal is not as effective.
In the case of spheres, a certain amount of over-inflation is required to provide a seal. (In two-phase pipelines, spheres are sometimes under-inflated to allow some blow-by to lower the density of the fluid ahead of the sphere).
Pigs and spheres travel at about the same velocity as the fluid in the pipeline and travel speed is relatively cons
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