Troubleshooting of various pigging problems can be very frustrating. This is because the problems encountered are not necessarily due to the pig as the client suggests. Although related, normally the real problem lies elsewhere.
Sometimes it requires a well-founded knowledge of how pigs perform under varying pigging conditions, (including a pig’s limitations), an understanding of pipeline fittings, equipment, and physics to successfully identify the true problem. Other times it takes a lot of luck.
Although there are many reasons for pigging problems, we find that misapplication (the incorrect pig selection for a particular function) is the most common, with improper pressures and volumes running a close second. Lack of operator training and understanding is usually to blame.
Unfortunately, some operators have had barely enough experience in pigging within a single application or single industry to assume all pigging operations are the same. Others have heard experienced-pipeliner tales that helped them form a misunderstanding of general pigging procedures. They then attempt to over-compensate for the misconception that usually results in causing the problem they were trying to avoid, or a completely different problem all together.
General rules regarding pigs include:
- Standard utility pigs, including foam pigs, steel pigs, and solid-cast pigs, bypass a small portion the propelling medium during the pig run. This bypass averages anywhere from 2% to 15%, depending on the type of pig. It is important to note that when a pig displaces liquids during a run, there will normally be a wetted surface left on the pipe wall. However, if the propelling medium is air or nitrogen, an evaporation or drying process may take place rather quickly. Sometimes it is reported that the pigs are bypassing or leaving large quantities, as much as 25%, of the product in the pipe after having made a displacement run. Once an understanding of how pigs work in a pipeline is achieved, it is apparent that this is an indication that the problem may lie in the mechanical portion of the system, such as piping configurations.
- All pigs – regardless of style – are 1% to 5% oversize (i.e. larger than the internal pipe diameter). Often times, operators feel that if a pig fits in the pipe tightly, it is too large, but without the oversize it cannot do the job intended and a large amount of bypass will occur.
Following are many problems that have been experienced during usage of pigs, and recommended steps to take to solve the problem. Remember, pigs are not a difficult item to use once an understanding of how they work is gained. The key element is to ask questions (a lot of questions), and then apply reason, coupled with knowledge to arrive at a logical solution.
Please review the following examples to find the one that suits your current problem. If not found, click on the "Feedback & Suggestion" link at the bottom of the page, and tell us your specific problem. We will answer you as soon as possible.
Also, if you would like to offer any additional remedies to any of the following troubleshooting tips, we would greatly appreciate it. Simply send them to us through the "Feedback & Suggestion" link below.