Loading and Launching
First, a pig is loaded into a device known as a "Launcher" or "Pig Trap" on one end of the pipeline. In order to facilitate pigging safely, and using smart pigs, a temporary launcher and receiver may be utilized on the pipeline if permanent facilities are not in place or are inadequate. The pig is loaded while the launcher is isolated from the pipeline and any pressure sources. Then, the pig trap is pressurized until it is equalized with the pipeline pressure. At this time, the launcher is opened to the pipeline, and flow is routed using valves to drive behind the pig and launch the pig into the pipeline. The flow continues to propel the pig through the pipeline.
Tracking and Flow Control
For many applications including smart pig inspection, line cleaning, and post-hydrotest drying, tracking the speed and location of the pig is important. Many modern pigs employ transmitters that trackers can use to locate the pig while it transits the pipeline from launcher to receiver, sometimes hundreds of miles. Audio equipment can also be utilized to locate pigs. If the pig is moving too fast, flow can be reduced to slow the pig. Conversely, if the pig is moving too slowly, the speed can be increased by increasing the pipeline throughput during the pigging operation. A common flow speed for pigging is 5 feet per second, but speeds vary.
In anticipation of receiving a pig, the receiver is open to the pipeline while the pig is in transit. When the pig enters the receiver "trap", the equipment is designed such that the pipeline flow can bypass the pig, leaving the pig stationary inside the trap. After the pig is received, using valves, the pipeline flow is routed around the receiver so that the trap can be isolated from the pipeline flow. Then, the receiver is depressurized (using venting, draining, flaring, or ZEVAC) so that the pig and any cleaning debris may be removed from the receiver.