API Plan 53B

API Plan 53B systems are employed with double mechanical seals containing a barrier fluid chamber between the inboard and outboard seal faces.

In API Plan 53B systems, when the pump is running, the pressurised barrier fluid is circulated in the barrier fluid chamber and pipework to the seal support system, by an integral seal pumping ring. When the pump is not running and at stand still, the barrier fluid is circulated by fluid convection.

Within the Plan 53B system, the pressurised barrier fluid is circulated to a heat exchanger containing a significant reservoir of barrier fluid. The heat exchanger is also referred as a seal support system.

The pressure within the Plan 53B system comprising of the seal support system, mechanical seal barrier chamber and connecting pipework is maintained by a pressurised bladder accumulator.

Importantly, in the API Plan 53B system, the pressurised gas is separated from contacting the barrier fluid.


  • The barrier fluid and the pressurised gas (inert gas) are separated by the bladder accumulator. This seperation avoids the gas contacting the barrier fluid thereby preventing fluid absorption, airation, mixing and/or deterioation of the barrier fluid from the pressurised gas.
  • In-direct pressurisation allows for use in high pressure applications.
  • The bladder accumulator is typically pressurised by a hand pump, therefore it is considered as a stand-alone system which does not rely upon a central pressure source.
  • Heat is removed from the Plan 53B system by an air-cooled or water-cooled heat exchanger or seal support system.
  • The pressure of the barrier fluid is set at a higher pressure than the process media. Typically 2 barg (30 psi) as a minimum.


  • As the pressurisation of the Plan 53B system doesnt need to rely on an external gas supply, Plan 53B systems are considered much more reliable than Plan 53A systems.
  • The Plan 53B system can be deployed in remote applications as it is a stand alone system
  • As the inert gas doesn't mix or deteriorate the barrier fluid, the barrier fluid is clean and provides a better fluid film condition between the counter-rotational mechanical seal faces gives. This improves seal and rotating equipment life.


  • As with all double mechanical seals using a higher barrier fluid pressure compared to the process fluid pressurise, process media quality can deteriorate due to barrier fluid contamination. In process media sensitive applications where barrier fluid contamination is not permitted, dry running sealing technology is employed, where the barrier media is gas.


  • Used in applications where direct gas pressurisation (as found in Plan 53A systems) is not acceptable.
  • Plan 53B systems are high-end systems which are typically used in hazardous applications where no leakage to atmosphere can be tolerated.
  • Typical applications of Plan 53B systems are found in the Oil and Gas industry, which process toxic, environmentally dangerous and inflammable media.
  • Plan 53B systems are excellent for dirty, abrasive and slurry applications, where the process media is not suitable to be used for the fluid film between the counter-rotational seal faces.
  • Plan 53B systems are also used in polymerizing products.

Warning Notes

  • The bladder accumulator is typically manufactured from a flexible, elastomeric material. Periodic inspection the bladder accumulator is therefore mandatory. Inspect for wear and deterioation of the bladder and/or surface finish contact/wear of the internal surfaces of the seal support system.
  • Recharge the bladder to x0.9 the working pressure.
  • As with any pipework system, always vent the Plan 53B system properly before equipment start up.
  • As with all pressurised double seals, always check the barrier fluid compatibility with process media. There will always be some leakage of barrier fluid into the process media.
  • Significant environmental temperature changes and heat load can cause fluctuations on the system pressure.

Related Technical Papers

For further reading on API Plan 53B seals and systems, please click the following links to related Technical Blogs;

Applicable Products

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