launch. See diagram on the right representing a pig receiver. To receive a pig Unlock and open valves V1 and V3, to create a flow path through the pig receiver. Partly close ‘kicker’ valve V2 to induce flow through the receiver. The receive alarm XA will indicate when the pig has been received. Open valve V2 fully. Close and lock valves V1 and V3. Unlock and open vent and drain valves to empty the receiver. Close and lock vent and drain valves. Unlock and open door, remove pig from receiver, close and lock door. Pig receiver is then ready to receive the next pig. The pigging installation shown (right) is known as an Above Ground Installation (AGI). It is part of the UK's National Transmission System for natural gas. It shows two pig launcher/ receivers. There are at the end of two diameter pipelines that carry gas under the River Thames between East Tilbury Essex and Shorne in Kent. Should one pipeline be damaged, by a ship's anchor for example, that line can be isolated and the second pipeline allows gas to flow across the river. Mechanical interlocks There are many reports of incidents in which operators have been injured or even killed while performing a pigging operation. Common causes of such events are: opening of the closure door while the vessel is still pressurized; opening of the main process valve while the closure door is not fully closed; opening of the closure door while a high concentration of H₂S or other toxins remains inside the vessel; the vent valve remaining open while the vessel is being pressurized with its medium. All these causes are directly related to improper operation of the process valves and the closure door. A common method of avoiding these kinds of incidents is to add valve interlocks, which has been adopted by all global oil and gas operating companies. Safety during pigging relies on strict adherence to safety procedures, which give detailed description of a safe valve operating sequence. By physically blocking incorrect operations, valve interlocks enforce such sequences. Valve interlocks are permanently mounted to both manual and motor operated valves and the closure door. The interlocks block operation of a valve or door, unless the appropriate keys are inserted. The principle of valve interlocking is the transfer of keys. Each lock is equipped with two keys: a key for the locked open position and