systems are designed to pig the pipeline in either direction. The pig is pushed either with a gas or a liquid; if pushed by gas, some systems can be adapted in the gas inlet in order to ensure pig's constant speed, whatever the flow pressure is. The pigs must be removed, as many pigs are rented, pigs wear and must be replaced, and cleaning (and other) pigs push contaminants from the pipeline such as wax, foreign objects, hydrates, etc., which must be removed from the pipeline. There are inherent risks in opening the barrel to atmospheric pressure so care must be taken to ensure that the barrel is depressurized prior to opening. If the barrel is not completely depressurized, the pig can be ejected from the barrel and operators have been severely injured when standing in front of an open pig door. A pig was once accidentally shot out of the end of a pipeline without a proper pig receiver and went through the side of a house away. When the product is sour, the barrel should be evacuated to a flare system where the sour gas is burnt. Operators should wear a self-contained breathing apparatus when working on sour systems. A few pigging systems utilize a "captive pig", and the pipeline is only opened 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 is never opened up during process operation. These systems are not common. Pigging operations See diagram on the right representing a pig launcher. In normal operation valves V2 and V4 will be open, all other valves and the door are closed and locked. To launch a pig Ensure pig launcher is drained and vented. Unlock and open, then close and relock the vent and drain valves. Unlock and open door, load pig into launcher, close and lock door. Unlock and open valves V1 and V3, to create a flow path through the launcher. Partly close ‘kicker’ valve V2 to launch the pig. The launch alarm XA will indicate when the pig has been launched Open valve V2 fully. Close and lock valves V1 and V3. Unlock and open vent and drain valves. Allow launcher to depressurise and drain. Close and lock vent and drain valves. Pig launcher is then ready for the next pig to