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In Piping Plan 72 there is externally supplied buffer gas for Arrangement 2 seals. Buffer gas is maintained at a pressure less than seal chamber pressure. In normal operation, the buffer gas pressure should not exceed 0.07 MPa (0.7 bar) (10 psi).
Piping Plan 72 can be used on Arrangement 2 unpressurized dual seals that use a dry-running containment seal. This plan may be used alone or in conjunction either a Piping Plan 75 or Piping Plan 76. The buffer gas can be used to sweep inner-seal leakage away from the outer seal to a collection system and/or dilute the leakage so the emissions from the containment seal are reduced.
The Piping Plan 72 system is intended to function as follows: from the source supply provided by the plant operator, the buffer gas enters the system, mounted on a plate or panel, provided by the seal vendor. A coalescing filter to remove any particles and liquid that might be present follows an inlet block valve on the panel. The gas then flows through a forward-pressure regulator that is set at the Piping Plan 75 or Piping Plan 76 alarm point, or at least 0.04 MPa (0.4 bar) (5 psi) above the normal flare pressure. Next comes a pressure transmitter with a local indicator ahead of an orifice; these are used to set the regulator. They : also ensure the source pressure always maintains a flush flow over the operating range of the combined 3 systems (when used with Piping Plan 75 or Piping Plan 76) and does not pressurize the seal chamber or i affect the alarm settings of the exit flush plan. The orifice provides flow regulation and is followed by a flow : transmitter to measure flow. The orifice also serves to control buffer gas loss in the event of an outer seal failure. (The use of a needle or globe valve for the orifice to allow flow regulation is optional.) The flow , transmitter with a local indicator will display buffer gas flow rate and alarm on high flow. The last element on the panel is a check valve. Buffer gas is then routed through tubing to the seal.