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In Piping Plan 52 there is an external reservoir providing buffer liquid for the outer seal of an Arrangement 2 seal. 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 to and from the reservoir by means of an internal circulating device. To maximize buffer liquid circulation, the piping losses should be minimized through the proper selection of pipe size, elimination of fittings, use of large radius bends, and reduction in the length of piping runs.
Piping Plan 52 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. The buffer liquid is contained in a seal reservoir that is vented to a collection system, thus maintaining the buffer system pressure close to atmospheric. Piping Plan 52 works best with clean, nonpolymerizing, pure products that have a vapor pressure higher than the buffer system pressure. Leakage of higher vapor pressure process liquids into the buffer system will flash in the seal reservoir and the vapor can escape to the collection system.
Inner seal process liquid leakage will normally mix with the buffer fluid and contaminate the buffer liquid over time. Maintenance associated with seal repairs, filling, draining, and flushing a contaminated buffer system can be considerable.