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API Plan 21

Recirculation from pump discharge through a flow control orifice and cooler to the seal chamber.

API Plan 21


Plan 21 is like Plan 11 but with the addition of a mechanical seal cooling system

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Application Notes

  • Plan 11 with cooler.
  • Throat bushing is used to isolate product in the seal chamber from product in the pump volute.
  • Provides a cool flush to the seal to improve lubricity, reduce coking, and/or avoid fluid flashing to vapor.
  • If the cooler duty is high, fouling and clogging on the water side can occur over time.
  • Potential clogging on the process side if the fluid becomes too viscous at reduced temperature.
  • Requires more energy usage than Plan 23 since the fluid must be continuously pumped back to discharge.


  • Dramatic seal life enhancement with proper design and operation
  • Renewable filters to ensure continuous supply of clean flush liquids
  • Cool flush improves seal lubricity and reduces coking


  • Clogging risk with elevated temperatures
  • Higher energy usage
  • Maintenance of the Seal Cooler

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