Pig launchers are essentially vessels used for launching of a pipe pig or a pig tool into the pipeline for cleaning or inspection purpose. After being launched into the pipeline, the pig tool is pushed through the pipeline by a driving fluid. The pipeline is cleaned along the way by brushing action from the pig. On the other end the pig tool is received by a pig receiver. Pig receiver is also a vessel used for receiving the pipe pig tool. Pig launchers and receivers are designed to meet the specific requirements of a pipeline.
Pig traps can be of horizontal, vertical or inclined type. For ease of operation, horizontal pig traps are preferred. When space constraints become critical, vertical or inclined pig traps are installed instead of horizontal.
The schematic shown here is for a horizontal pig launcher. A horizontal pig receiver would be almost identical to this schematic, except for the position and size of certain nozzles such as drain, kicker line and PSV nozzle.
For any pig trap, be it horizontal, vertical or inclined pig launcher or receiver, basic structure is similar and consists of -
1. Major barrel which is larger than the pipeline size
2. Minor barrel which matches with the pipeline size
3. A reducer barrel connecting the major and minor barrels
4. A quick opening door on the major barrel end of the pig trap
5. Full bore ball valve on the minor barrel end of the pig trap
6. Following nozzles - kicker line, relieving line (PSV), pressure gauges on minor and major barrels, pig signaler on the minor barrel, purge connection, vent and drains.
7. As a safety measure, quick opening door should be mechanically interlocked with isolation valves on kicker line and main line, so that the door cannot be opened when pig trap is still connected to these pressurized lines. This safety precaution can also be accomplished by interlocking the door with vent or depressurizing line, so that the door can be opened only when the pig trap is depressurized.
For vertical pig traps, the reducer barrel is of concentric type, so that the center lines of major and minor barrels match with each other. Vertical pig traps are always installed to have the major barrel above the minor barrel. The tentative position of the nozzle shown in the schematic, may change for vertical pig traps.
Following certain dimensions are important for pig trap design and have to be specified in the pig trap datasheet, before sending it to the supplier.
1. Major barrel size and length
2. Minor barrel length and size (same as pipeline size)
3. Reducer barrel length.
4. Inclination type of pig trap (horizontal or vertical)
5. Slope in case of horizontal pig trap
6. Location of certain nozzles such as Kicker line, Relieving line, drain, vent, pig signaler.