API Plan 52

API Plan 52

Reservoir providing buffer liquid for the outer seal of an arrangement 2 unpressurized dual seal. The buffer liquid shall be maintained at a pressure less than seal chamber pressure and less than 2.8 bar (40 PSI).

A  From mechanical seal
B  To mechanical seal
C  Filling connection
D  Cooling water IN
E  Cooling water OUT
F  Drain
G  Connection to flare

Seal chamber details

4  Flush (F)
5  Liquid buffer OUT (LBO)
6  Liquid buffer IN (LBI)
11  Seal chamber

Example seal: EagleBurgmann H75A4-T dual pusher mechanical seal.

P&ID according to API


1  To collection system
2  Reservoir
3  Make-up buffer liquid
7  Cooling water IN
8  Reservoir buffer fluid drain
9  Cooling water OUT
10  Buffer fluid drain

LI  Level indicator
LIT  Level transmitter with local indicator



  • Provides lubrication and cooling to the contacting wet outboard safety backup seal.
  • The pumped fluid will not become contaminated.
  • The pumped media does not leak directly to atmosphere.
  • No pressurization required for the reservoir as is case of a Plan 53A.
  • Provision of cooling coil inside the reservoir or an external water or air cooler takes care of the heat removal.
  • Uses an internal circulating device to ensure buffer liquid circulation.


  • For flashing services – for example light hydrocarbons, high vapor pressure fluids. Can also be used for non-flashing services.
  • For services where product contamination by a barrier fluid is not acceptable

Remarks, checkpoints

  • Do not use for contaminated and polymerizing media.
  • Use a restriction orifice plate in the vent line to maintain a back pressure in the reservoir.
  • Vent the system before start-up. Later ensure that the vent line always remains open and is connected to the flare.
  • The piping from the seal to the reservoir should be self-venting.
  • Sense the seal inlet and outlet line temperatures. Difference in the temperatures is an indication of proper flow.
  • An increase in either the reservoir pressure or reservoir level is an indication of inboard seal leakage.
  • A decrease in the reservoir level is an indication of outboard seal leakage.
  • Ensure the compatibility of buffer liquid with the pump media.
  • The buffer liquid shall be maintained at a pressure less than the seal chamber pressure and less than 2.8 bar (40 PSI).
  • Check buffer liquid quality at regular intervals and replace if required.

Pump media characteristics

  • 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)
  • Lubrication and flushing for the outboard seal and cooling for inboard as well as outboard seals.