API Plan 53C

API Plan 53C

Barrier fluid system pressurized by a piston accumulator supplying clean liquid for an arrangement 3 pressurized dual seal. The barrier pressure is generated from the seal chamber pressure. The system is self-energizing and reacts to fluctuations in the seal chamber fluid pressure.

A  Cooler
B  Piston accumulator
C  Pressure reference
D  Drain
E  Vent

Seal chamber details

3  Pressure reference
4  Flush (F)
5  Liquid barrier OUT (LBO)
6  Liquid barrier IN (LBI)
7  Seal chamber

Example seal: EagleBurgmann MBS682-PTA dual metal bellows mechanical seal.

P&ID according to API


1  Make-up barrier liquid
2  Piston accumulator
5  Liquid barrier OUT (LBO)
6  Liquid barrier IN (LBI)
8  Vent
9  Barrier fluid drain

LI  Level indicator
LT  Level transmitter
PRV  Pressure relief valve*
PDIT  Differential pressure transmitter with local indicator
TI  Temperature indicator*

* If specified
** Vertically oriented, finned air cooler may be provided if specified


  • A piston accumulator dynamically adjusts the barrier pressure which is based on the seal chamber pressure, thus maintaining a fixed, steady differential pressure across the inboard seal faces.
  • Barrier liquid enters the process medium through the inner seal faces.
  • Uses the seal chamber pressure as a reference pressure and accordingly magnifies the barrier pressure by a pre-defined ratio.
  • Highly reliable since there is no need for an external pressure source.
  • Heat is removed with the help of a water cooler or an air cooler.
  • Uses an internal circulating device to ensure barrier liquid circulation.


  • For services where product dilution is acceptable.
  • For services where media is unsuitable for flushing the inner seal faces.
  • For services where Plan 53A cannot be used due to non-availability of a permanent, reliable nitrogen supply.
  • Ideal plan for applications with varying seal chamber pressures. Pressure ratio depending on stuffing box pressure range:
    • 4 bar (58 PSI) ... <10 bar (145 PSI) = piston ratio 1:1.5
    • 10 bar (145 PSI) ... <22 bar (319 PSI) = piston ratio 1:1.2
    • 22 bar (319 PSI) ... 40 bar (580 PSI) = piston ratio 1:1.1;
      For stuffing box pressures less than 4 bar (58 PSI) or above 40 bar (580 PSI) consult EagleBurgmann.
  • Check with the process engineer if product contamination is acceptable.

Remarks, checkpoints

  • Check barrier fluid compatibility with the media.
  • Vent the system before start-up. Later ensure that the vent line always remains closed.
  • Sense the seal inlet and outlet line temperatures. Difference in the temperatures is an indication of proper flow.
  • Ensure that the barrier pressure is always higher than the seal chamber pressure by at least 1.4 bar (20 PSI) (for piston ratio selection).
  • Since there is less barrier fluid volume in the accumulator, heat dissipation is very much a function of the cooler efficiency.
  • Ensure that the seal chamber reference line is not choked and that it is connected to the downstream of the accumulator.
  • Ensure proper heating or cooling of the reference pressure line if necessary.
  • Check the compatibility of the accumulator materials with the pump media.

Pump media characteristics

  • Non-polymerizing and clean media
  • Poor lubrication properties
  • Hazardous / environmentally harmful
  • Close to vaporization point
  • Can tolerate dilution with barrier media

Location, function

  • Between seals
  • Barrier system (psealchamber < pbetweenseals)
  • Lubrication, flushing and cooling for both inboard as well as outboard seals