Access into pressure vessels is a potentially hazardous exercise. Residual pressure, volumes of residual liquids or gases all can be harmful – typical examples of these potential hazards include pig traps, slug catchers and filter housings. Locking the vessel closure in the closed position ensures access into the vessel can be achieved only under controlled safe conditions. The operating key is held in a secure place – e.g. Control Room or Supervisor’s office or is retained (trapped) in some other related interlocked item of equipment.

As a minimum, vessel closures are usually interlocked with vessel venting and draining functions – the interlocking arrangement can be extended to incorporate all process items of equipment relating to the vessel’s function