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In Piping Plan 21 there is recirculation from a high-pressure region of the pump (typically the pump discharge or the pump discharge piping) through a flow control orifice and cooler, then into the seal chamber.
With purchaser approval, the flow control orifice may be omitted if it is not needed to achieve the required flush flow rate.
In Piping Plan 21, product in the seal chamber is isolated from that in the impeller area of the pump by a throat bushing. Piping Plan 21 provides a cool flush to the seal. This may be needed to improve the margin to vapor formation, to meet secondary sealing element temperature limits, to reduce coking or polymerizing, or to improve lubricity (as in hot water). The benefit of Piping Plan 21 is that it not only provides a cool flush but also has sufficient pressure differential to allow good flow rates. The drawback is that the cooler duty is high, leading to fouling and plugging on the water side and potential plugging on the process side if the fluid becomes too viscous. Piping Plan 21 works best in dry climates where a vertically oriented, finned air cooler is used instead of a water cooler. Piping Plan 21 also uses more energy than Piping Plan 23, because the pumped fluid that is used for the flush is repumped from suction back to the discharge. A close clearance throat bushing is recommended to isolate the cooled process fluid from the pump fluid. In pumps with low differential head or pumping high-viscosity fluids, the required flow rate for the piping plan may be achieved without the flow control orifice. Care shall be taken when using this plan with polymerizing fluid that may plug the orifice and connecting piping.
Malavazi NOTE: For liquids with crystallization characteristics, Quench is recommended due to micro leakage on the faces, otherwise plug the injection connection leaving the drain free to prevent environmental disasters, since the floating ring will act as a safety point.