API Plan 31

API Plan 31 takes process fluid from discharge and recirculates it through a cyclone separator and back to the seal chamber or mechanical seal flush port.

The Plan 31 system directs substantially clean fluid to the mechanical seal faces to help change the sealing environment. Instead of sealing a process fluid full of seal harming contaminants, the Plan 31 system provides clean, non-slurry fluid for cooling and lubrication.

API Plan 31 is similar to Plan 11 but solids are seperated via cyclone separator.


- Integral cyclone separator (vortex separator) removes contaminants, debris and solids from the process fluid.

- Particles and debris from cyclone separator are returned to suction.


  • Simple, efficient elimination of solid particles
  • The Plan 31 system provides a substantially clean flush for the mechanical seal.
  • The system is optimised, when the mechanical seal or pump seal chamber, has an integral Vortex Breaker gland plate feature. The vortex breaker helps to prevent seal and component erosion from any particles that the vortex separator doesn't remove.


  • Periodic cyclone separator checks to determine internal wear


1. Used in process fluid applications containing suspended solids.

2. Mineral and mining applications

3. Offshore, oil and gas applications

4. Used in applications containing solids with specific gravity twice or more than that of process fluid.

Warning Notes

1. The Plan 31 cyclone separator ideally requires at least a 1.7 bar (25 psi) differential between pump discharge and the seal chamber for optimum performance.

2. A throat (restriction) bushing is recommended at the bottom of the seal chamber.

3. Cyclone separator must be periodically inspected for abrasive wear. If left unchecked, and wear reduces the seperator wall thickness, catasphoic failure can result.

3. The efficiency of a cyclone separator is proportional to it's diameter. A large cyclone diameter the less efficient the separation process.

4. The differential pressure between the inlet and outlet of the cyclone separator should be as close to zero as possible.

5. The maximum process fluid particle size should be less than 25% of the cyclone separator inlet orifice.

6. Avoid using Plan 31 on very dirty or slurry applications

Related Technical Papers

For further reading on API Plan 31, please click the following links to related Technical Blogs;

- Enhancing API Plan 31 Systems: Best Practices for Reliability

Applicable Products

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