It is widely acknowledged that the single greatest hazard associated with accessing pressurized vessels, such as pig traps, is the risk of opening the closure while the vessel is still pressurized or contains residual hazardous product. For this reason, the only reliable way to prevent the possibility of unsafe opening is to interlock the pig trap opening mechanism with the vessel vent and/or drain valves.
The principle of the key-exchange interlocking system is very simple: when work needs to be carried out on an interlocked pig trap, a specially-coded permit key is issued from the control room. The insertion of the permit key into the first valve of a sequence unlocks the valve assembly, allowing operation of the valve. When the valve has been operated to its new position, it may then be locked by the removal of a previously-trapped second key. This key is coded in common with the next designated valve lock in the sequence and the procedure is repeated until the vessel has been totally isolated, vented, drained, and purged. This ultimately releases the closure lock key, usually after locking-open the vent valve
,THE DL/3 key-exchange interlocking system from Smith Flow Control has been designed to control the safe operation of pig-trap closures by eliminating human error or deliberate abuse, and forms part of a comprehensive range of safety systems specially designed for both the on- and offshore hydrocarbon and chemical processing industries.
