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In Piping Plan 66B, an orifice plug in the drain port minimizes the seal leakage leaving the seal gland and allows for detection of a seal failure.
Piping Plan 66B is intended for use in Arrangement 1 applications where it is required to limit leakage in case of a seal failure or it is required to monitor excessive leakage. In this plan, normal seal leakage can flow freely out the drain port. If the seal leakage is excessive however, the orifice plug in the drain limits the amount of leakage leaving the seal gland. As the leakage rate increases, the pressure will increase on the upstream side of the orifice plug. This pressure will be monitored by the pressure transmitter and can be used to monitor seal performance and trigger an alarm indicating a seal failure. Since the drain cavity will become pressurized, a floating throttle bushing should be used to limit leakage past the bushing. Leakage out of the drain port is collected and piped to a liquid recovery system or sump.
NOTE This plan is most commonly used on pipeline applications.