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API Plan 53C

External barrier liquid system pressurized by a piston accumulator supplying clean fluid to the inboard and outboard seal faces.

API Plan 53C


This plan works using an extraneous barrier fluid system that is pressurized by a piston accumulator supplying clean fluid to both inboard and onboard seal faces.

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Application Notes

  • Used with Arrangement 3 seals.
  • Piston accumulator senses pressure from a reference source (normally the seal chamber) and maintains a higher barrier system pressure through differential areas in the piston.
  • Barrier liquid pressure should always exceed the maximum seal chamber pressure by at least 20 psi [1.4 bar].
  • Barrier liquid is circulated through the fluid circuit by means of an internal circulating device (pumping ring).
  • Materials used in the piston accumulator must be compatible with the process fluid.
  • Process fluid never leaks to the atmosphere unless system pressure is lost.
  • Heat is removed using a heat exchanger.
  • Barrier liquid must be compatible with the process liquid because it will leak into the product.


  • Piston system provides continuous positive pressure differential between barrier fluid and seal faces
  • Heat exchanger efficiently removes generated heat


  • Process fluids containing solids may accumulate on metal surfaces
  • Regular maintenance is required
  • Barrier fluid and process fluids must be compatible due to the possibility of mixing

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