This article provides guidance to do with the launching and retrieving of pipeline pigs (also known as scrapers or tools).
Below is a schematic of a launcher.
In normal operation the fluid (liquid or gas) moves along the line from left to right through the open Main Line valve. The kicker and isolation valves are both closed hence the barrel of the pig launcher is isolated from the Main Line.
To pig the line the pig is placed in the trap barrel, the Main Line valve is partially closed, and the kicker and isolation valves are opened. The fluid flows into the barrel and forces the pig into the Main Line, after which the Main Line valve can be opened fully and the trap isolated once more.
The following instructions provide more detail. Actual operation will, of course, vary from system to system.
- Close the isolation valve and the kicker valve.
- In liquid systems, open the drain valve and allow air to displace the liquid by opening the vent valve. In gas systems, open the vent and vent the gas in the launcher to reduce the pressure to atmospheric.
- When the pig launcher is completely drained (0 psig), with the vent and drain valves still open, open the closure door.
- Install the pig with the nose firmly in contact with the reducer between the barrel and the bore section of the launcher.
- Clean the closure seal and other sealing surfaces, lubricate if necessary.
- Close the trap door.
- Close the drain valve.
- Fill the trap by gradually opening the kicker valve and venting through the vent valve.
- When the barrel is full close the vent valve to allow pressure to equalize across the isolation valve.
- Open the isolation valve.
- The pig is ready for launching.
- Partially close the main line valve. This will increase the flow through the kicker valve and behind the pig. Continue to close the main line valve until the pig leaves the trap into the main line as indicated by the pig signaler.
- After the pig leaves the trap and enters the main line fully open the main line valve. Close the isolation valve and kicker valves.
Below is a schematic for a Pig Launcher.
The process for catching a pig at the end of its journey is described below using the above sketch as a reference.
- Ensure that the receiver is pressurized.
- Fully open the bypass valve.
- Fully open the isolation valve and partially close the main line valve.
- Monitor the pig signaler for pig arrival.
- Close the isolation valve and bypass valve.
- Open the drain valve and the vent valve.
- Check the pressure gauge on the receiver to assure the trap is depressurized.
- Open the trap closure and remove the pig from the receiver.
- Collect any liquids that come out of the barrel in a safe location.
- Clean the closure seal and other sealing surfaces, lubricate if necessary, and close and secure the trap door.
- Return the receiver to the original condition.
The material provided here is taken from the book Plant Design and Operations. One of our Safety Moments, Safety Moment #7: Dangers of High Pressure Gas, shows what can happen when the pigging process goes awry.
The YouTube video (Williamson 2010) illustrates the launching and trapping processes.