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In Piping Plan 55, there is an unpressurized external buffer fluid system supplying clean liquid to the buffer fluid seal chamber. Piping Plan 55 is used with Arrangement 2 liquid seals. The buffer liquid shall be maintained at a pressure less than seal chamber pressure and less than 0.28 MPa (2.8 bar) (40 psi).
Buffer liquid is circulated by an external pump or pressure system.
Piping Plan 55 is similar to Piping Plan 54 except the buffer liquid is unpressurized. Piping Plan 55 is used with Arrangement 2 seals, with a contacting wet containment seal (configuration 2CW-CW) utilizing a liquid buffer system. It is normally used in services where process fluid leakage to atmosphere should be minimized and contained. It is also used in applications where the process may solidify in contact with atmosphere or in applications where additional heat removal from the inner seal is required.
Piping Plan 55 differs from a Piping Plan 52 in that the buffer liquid is not self-contained and the buffer liquid circulation is created by an external pump or pressure system. If Piping Plan 55 is specified, carefully consider the reliability of the buffer liquid source and the possible contamination of the buffer flow with process liquid or vapor.