Pigging in the maintenance of pipelines refers to the practice of using pipeline inspection gauges or 'pigs' to perform various operations on a pipeline without stopping the flow of the product in the pipeline. Pigs get their name from the squealing sound they make while traveling through a pipeline. These operations include but are not limited to cleaning and inspection of the pipeline. This is accomplished by inserting the pig into a 'pig launcher' - a funnel shaped Y section in the pipeline. The launcher is then closed and the pressure of the product in the pipeline is used to push it along down the pipe until it reaches the receiving trap - the 'pig catcher'.
Pigging has been used for many years 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.
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, and foodstuffs.
Pigs are used in lube oil or painting blending: they are used to clean the pipes to avoid cross-contamination, and to empty the pipes into the product tanks (or sometimes to send a component back to its tank). Usually pigging is done at the beginning and at the end of each batch, but sometimes it is done in the midst of a batch, e.g. when producing a premix that will be used as an intermediate component.
Pigs are also 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 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.
Without the need for line flushing, pigging offers the additional advantage of a much more rapid and reliable product changeover. Product sampling at the receipt point becomes faster because the interface between products is very clear, and the old method of checking at intervals, until the product is on-specification, is considerably shortened.
Pigging Systems can also be operated totally by a Programmable Logic Controller (PLC).
A few pigging systems utilize a "captive pig", and the pipeline is only opened up very occasionally to check the condition of the Pig At all other times, the pig is shuttled up and down the pipeline at the end of each transfer, and the pipeline itself is never opened up during process operation. These systems are not common.
Modern pigging systems are highly sophisticated sets of equipment that consist of a standard 5-6 finned pig with an intelligent transmitter that has a global positioning system fixed on it to tell the exact location of the pig inside the pipeline while it is on the move. Along with the GPS positioner there are a host of other instruments like the internal camera that takes live video of the pipe condition inside while the pig is moving, the thickness gauge that constantly measures the thickness of the wall of the pipe as the pig moves.
As the pig moves inside the pipe data like the speed of the pig, flow, rate of fluid inside etc is measured at regular intervals. So by the time the pigging process is over the complete set of data for all the measurable parameters is ready outside.
This data is compiled and used for post pigging analysis about the condition of the pipeline from inside.
For GPS positioning, the pig has a transmitter that sends a signal outside the pipe where a recorder (above ground marker) records the time the pig passes by that precise location. The pig actually has a gyro on it that records angles and combines that with distances and when combining all 3 data sets the GPS positioning can determined. It is difficult for a GPS receiver to be located on a pig since it may be several feet below ground and/or surrounded by steel.