The gas properties, seal arrangement and the criticality of the service may dictate a need for a protective shutdown. The purpose of the protective shutdown would be to either prevent a hazardous release of gas and/or prevent damage to the seal. In the case of a double seal arrangement, loss of seal (barrier) gas could result in reverse pressurization of the primary seal, leading to possible face damage and/or release of gas. A protective trip on low seal gas supply pressure is strongly recommended for this application. An exception can be made if there is specific testing to assure reverse pressure capability, and the process gas is non-toxic.
On tandem seals in toxic services, a protective shutdown is also strongly recommended for personnel protection. The shutdown should activate when the primary seal vent pressure exceeds the capabilities of the nitrogen buffer to the intermediate labyrinth. For other tandem seals or a single seal, a protective shutdown on high primary seal vent pressure can be used to minimize seal damage or limit gas leakage to atmosphere. Since gas seals have demonstrated rapid failure in some instances, the impact of a compressor shutdown with little or no warning must be considered when deciding on protective shutdowns and their set points.