 The separate metal lid for the central opening provides a clamp mechanism that closes and seals the lid against the keg with a large rubber O-ring.The lid also provides a pressure relief valve that is typically identified by a circular pull-ring (like a key ring).The relief valve can be manually opened to release the pressure in the keg; in the event of an unsafe overpressure in the keg, the relief valve automatically opens to avoid the hazard of bursting the cylinder.