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API Plan 52

External buffer liquid reservoir supplying clean fluid to the outboard seal faces.

API Plan 52


This plan is used in Arrangement 2 seals and requires the use of an unpressurized external pump seal pot and forced circulation via a pumping ring.

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Application Notes

  • Used with Arrangement 2 seals.
  • Buffer system pressure is typically maintained close to atmospheric pressure.
  • Used in applications where product contamination from a barrier fluid is unacceptable.
  • Circulation of buffer liquid to and from the reservoir is dependent on thermal siphoning and/or an internal circulating device (pumping ring) inside the seal.
  • Used with clean, non-polymerizing, pure products that have a vapor pressure higher than the buffer system pressure.
  • Provides redundancy in the event of a seal failure.
  • Leakage of higher vapor pressure liquids into the buffer system will flash in the seal reservoir and the vapor can escape into the collection system.
  • Near zero process emissions.
  • Cooling system is typically built into the reservoir.
  • Buffer liquid must be compatible with the process liquid.
  • Best piping practices must be followed to avoid vapor entrapment and stalled flow in the tubing.


  • No process fluid contamination
  • Near-zero process emissions
  • Integral cooling system


  • Buffer fluid is easily contaminated over time
  • Requires strict adherence to best piping practices
  • Seal pot maintenance high with process fluids that solidify

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