Plan 21 - Single seals - By-pass from discharge through a heat exchanger
Description: Plan 21 is a cooled version of Plan 11. The product from pump discharge is directed to a heat exchanger to lower the temperature before being introduced into the seal chamber.
Advantages: Process fluid cools and lubricates the seal, therefore no dilution of process stream. Cooling improves lubricity and reduces the possibility of vaporization in the seal chamber.
General: Plan 21 is not a preferred plan, either by API or many users, due to the high heat load put on the heat exchanger. A Plan 23 is preferred.