Stuck pigs can be costly- due to the risk of a shut-down and subsequent removal of the stuck pig. Here are the common reasons that lead to pigs plugging a pipeline.
Pigging is key to pipeline maintenance and integrity management strategy. Inspections, maintenance, removal of wax, etc. are all made possible thanks to pigs. However, stuck pigs can be costly- due to the risk of a shut-down and subsequent removal of the stuck pig.
Here we look at the main reasons why pigs get stuck:
1. Pigs Plugging a Line
Wrongly designed pigs or choosing an unsuitable pig for a line can lead to the pig plugging the line. Often the more pressure that is applied to move the plug, more jammed the pig gets eventual requiring a pig cut-out and repair of the line.
2. Buildup of wax
Build up of wax to a critical level and hardening of the wax as the liquid gets squeezed out by the pig can lead to plugging of the line. Wax plugs often withstand extremely high pressures before yielding. One of the ways to avoid pigs getting stuck in the wax is to include bypass in the pig.
3. Unintentional bypass
Unintentional bypass happens when the drive product flows past the pig due to a damage to the seals or due to the position of the pig in relation to the line components.
Jack-knifing can occur with dual-module pigs when the drive is transferred to the rear module instead of the front module. This results in seals moving off the centerline and causing the pig to stall.
5. Excessive wear
Excessive wear of the pig seals can happen due to abrasive system, high differential pressure, rough internal pipeline surface, low viscous fluid in the pipeline, low diameter of the pipeline or low pig velocity. Use of lubricants, reduction in differential pressure, use of high wear kits are some of the ways to prevent excessive wear of the pig.
6. Mechanical Damage
Unexpected situation while pigging or design problems can lead to loss of integrity to the pig’s components. Velocity excursions can lead to damage at bends, snatch loads or tensile loadings can cause damage to the couplings between pig modules, high velocities can lead to burning of the PU seals, excessive line pressure can cause the body of the pig to collapse.