Pipe pigs are capsule like objects used for cleaning of pipeline by pushing them through the pipeline using fluid pressure. The pipe is cleaned by brushing action from the pig tool. This cleaning process is called pigging. Pigging is also done for inspection of pipelines and not only for cleaning pipelines.
The pipe pig is launched using a pig launcher at the beginning of the pipeline. The pig tool is pushed through the length of the pipeline by the upstream fluid pressure, cleaning the pipeline along the way. At the end of the pipeline, the pig is received in a pig receiver.
Pig tools are also used to separate different products in a multiproduct pipeline. Use of pipe pigs for separation between two products results in saving of product and time required for switching between two different products and line flushing. Use of pig tool instead of line flushing for cleaning of pipelines in between batch operations also translates to elimination of environmentally harmful cleaning agent liquids.
Pipe pigs are used for performing following types of tasks
1. Cleaning of pipeline and displacement of sludge
2. Separation between batches of different products in the same pipeline
3. Inspection tool for the pipeline
The type of pig to be used and its optimum configuration for a particular task depends on the purpose of pigging, contents in the pipeline and characteristics of the pipeline such as size, minimum bend radius, types of valves etc. Based on these factors a type of pig tool is chosen. Pipe pig types can be classified into following categories.
1. Utility Pipe Pig - These pig tools are used for cleaning and product separation. Based on the utility or the purpose of the pig tool, these are classified into 'cleaning pig tools' used for cleaning and 'sealing or separation pig tool' used for separation between two products. Based on the pig structure, utility pig tools are further classified as, 'foam pigs', 'mandrel pigs' and 'spherical pigs'. Foam pigs are capsule shaped and made out of polymer foam. Mandrel pigs are dumbbell shaped with a central tube called 'mandrel'. Different kinds of components can be attached to this mandrel to customize the pig tool to suit different operations. Spherical pig tools are either solid spheres made from polyurethane foam or inflatable spheres inflated using glycol and water.
2. Inspection Pig Tool - They are also known as 'intelligent pig tool' or 'smart pig tool'. They are used for inspection of the condition of the pipe and/or its contents. These pig tools are equipped with sensors for gathering various kinds of data, as the pig tool travels through the pipeline. The electronics and sensors on an intelligent pig tool are powered by batteries on the pig. Surface pitting, corrosion of inner walls, cracks and weld defects in pipelines are often detected using magnetic flux leakage (MFL) pigs. Other intelligent pig tools use electromagnetic acoustic transducers to detect pipe defects. Caliper pigs can measure the roundness of the pipeline to determine areas of crushing or other deformations.
3. Gel Pig Tool - Gel pigs are a series of gelled liquid systems which have been developed for use in pipeline operations, either during initial commissioning, or as a part of a continuing maintenance program. Most pipeline gels are water-based, but a range of chemicals, solvents, and even acids can be gelled. Some chemicals can be gelled as the bulk liquid and others only diluted in a carrier. Gelled diesel is commonly used as a carrier of corrosion inhibitor in gas lines. As a liquid, although highly viscous, the gel can be pumped through any line which will accept liquids. Gel pigs can be used alone (in liquid lines), in place of batching pigs, or in conjunction with various types of conventional pigs. When used with conventional pigs, gelled pigs can improve overall performance while almost eliminating the risk of sticking a pig. Gel pigs do not wear out in service like conventional pigs. They can, however, be susceptible to dilution and gas cutting. Care must be taken, therefore, when designing a pig train that incorporates gel pigs to minimize fluid bypass of the pigs, and to place a conventional pig at the back of the train when displacing with gas.