Pigging refers to maintenance practice for pipelines using 'pipeline pigs', for cleaning or inspection of pipeline without stopping operation of the pipeline. Pipeline pigs are capsule shaped objects which travel through the pipeline, cleaning the inner walls of the pipeline by brushing action. Pigs get their name from the squealing sound they make while traveling through a pipeline.
Pigging usually means inspection and cleaning of the pipeline. The pig is inserted into a pig launcher, which is essentially a vessel used to for launching the pig into a pipeline using by creating a pressure differential. Refer to EnggCyclopedia's article on typical pig launcher P&ID arrangement. The pressure differential pressure between the two ends of pig launcher is created by partially closing the bypass line on the pig launcher. After launching the pig into the pipeline, pig launcher is closed and the fluid pressure is then used to push this pig through entire length of the pipeline, cleaning the inner walls all the way to the other end. On the other end of the pipeline, this pipe pig is received by a pig receiver, which has a similar structure and arrangement as the pig receiver. After receiving the pipe pig, pig receiver is closed, depressurized and then pig is removed along with the dirt and sludge.
If the pipeline contains butterfly valves, the pipeline cannot be pigged. Full bore ball valves cause no problems because the inside diameter of the ball can be specified to be the same as that of the pipe.
Pigging has been practiced for a long time to clean larger diameter pipelines in the oil industry. Today, however, the use of smaller diameter pigging systems is now increasing in many continuous and batch process plants as plant operators search for increased efficiencies and reduced costs.
Pipe pigging can be used for almost any section of the transfer process between, for example, blending, storage or filling systems. Pigging systems are already installed in industries handling products as diverse as lubricating oils, paints, chemicals, toiletries, cosmetics and foodstuffs.
Pipe pigs are very frequently used in oil and gas pipelines: they are used to clean the pipes but also there are "smart pigs" used to measure things like pipe thickness and corrosion along the pipeline. They usually do not interrupt production, though some product can be lost when the pig is extracted. They can also be used to separate different products in a multiproduct pipeline.