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In Piping Plan 66A, the throttle bushings in the seal gland minimize the seal leakage leaving the seal gland and allow for detection of a seal failure.
Piping Plan 66A 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 to the drain port. If the seal leakage is excessive however, the inner bushing will restrict the flow to the drain and limit the amount of leakage leaving the seal gland. As the leakage rate increases, the pressure will increase on the upstream side of the inner bushing. 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. 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.