Plan 31S

Plan 31S is Reliability innovative adaption of a conventional Plan 31 system.

As per API Plan 31, process fluid from discharge is recirculated through a cyclone separator and back to the seal chamber or mechanical seal flush port. However the Plan 31S system includes a sample collector which allows in-progress inspection of the process media entering the mechanical seal environment.

The Plan 31 system is designed to provide substantially clean fluid to the mechanical seal faces and the sample collector helps the engineer to condition monitor the performance of the system.

Instead of sealing a process fluid full of seal harming contaminants, the Plan 31S system provides clean fluid for cooling and lubrication of the mechanical seal.


  • Integral cyclone separator (vortex separator) removes contaminants, debris and solids from the process fluid.
  • Particles and debris from cyclone separator are returned to suction.
  • Integral Sample Collector Pot with ball valve.


  • 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.
  • In-cycle process fluid collection for condition monitoring of the fluid quality entering the mechanical seal flush orifice.


  • 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 systems and seals, please click the following links to related Technical Blogs;

- Enhancing API Plan 31 Systems: Best Practices for Reliability

Applicable Products

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