API Plan 55

API Plan 55

Unpressurized external buffer fluid system supplying clean buffer liquid for the outer seal of an arrangement 2 unpressurized dual seal. Buffer liquid is circulated by an external pump or pressure system.

A  From external source
B  To external source

Seal chamber details

1  From external source
2  To external source
3  Flush (F)
4  Liquid buffer OUT (LBO)
5  Liquid buffer IN (LBI)
6  Seal chamber

Example seal: EagleBurgmann LY9DZTT dual pusher mechanical seal.

P&ID according to API


1  From external source
2  To external source


  • Provides lubrication and cooling to the outboard safety backup seal.
  • The pumped fluid does not become contaminated.
  • The pumped media does not leak directly to the atmosphere.
  • Uses an external pump or pressure system to ensure buffer liquid circulation.
  • Provides buffer liquid circulation and cooling even though the process pump is in stand-by mode.


  • For flashing services – for example light hydrocarbons, high vapor pressure fluids.
  • Can also be used for non-flashing service.
  • For services where product dilution is not acceptable.
  • Where a single system is to be utilized for multiple seals.

Remarks, checkpoints

  • Not to be used for contaminated and polymerizing media.
  • Sense the seal inlet and outlet line temperatures. Difference in the temperatures is an indication of proper flow.
  • Provide proper instruments like a flowmeter which will indicate outboard seal leakage when the flow increases. Also provide suitable pressure reduction as well as non-return valves in the flushing pipeline.
  • The buffer liquid shall be maintained at a pressure less than seal chamber pressure and less than 2.8 bar (40 PSI).
  • Ensure the compatibility of buffer liquid with the pump media.
  • These systems may need additional engineering depending on application.

Pump media characteristics

  • High temperature
  • High vapor pressure, flashing media
  • Clean, non-polymerizing
  • Hazardous / environmentally harmful if leaked directly to the atmosphere.

Location, function

  • Between seals
  • Buffer system (psealchamber > pbetweenseals)
  • Flushing and cooling




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