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API Plan 53A

Pressurized external barrier liquid reservoir supplying clean fluid to the inboard and outboard seal faces.

API Plan 53A


This plan is used in Arrangement 3 seals and requires the use of a pressurized barrier fluid seal reservoir and forced circulation via a pumping ring.

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Application Notes

  • Used with Arrangement 3 seals.
  • Typically chosen for dirty, abrasive, or polymerizing products not compatible with a Plan 32.
  • Barrier liquid is maintained at a pressure greater than the seal chamber pressure. It is recommended that the barrier liquid pressure always exceeds the maximum seal chamber pressure by at least 20 psi [1.4 bar].
  • Circulation of barrier liquid to and from the reservoir is dependent on thermal siphoning and/or an internal circulating device (pumping ring) inside the seal.
  • Used when no process leakage to the atmosphere can be tolerated.
  • Process fluid never leaks to the atmosphere unless reservoir pressure is lost.
  • Cooling system is typically built into the reservoir.
  • Barrier liquid must be compatible with the process liquid because it will leak into the product.
  • Best piping practices must be followed to avoid vapor entrapment and stalled flow in the tubing.


  • No process fluid leaks
  • Integral cooling system


  • Barrier fluids might undergo gas entrapment
  • Requires strict adherence to best piping practices
  • Barrier fluid levels must be maintained
  • Barrier fluid and process fluids must be compatible due to the possibility of mixing

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