API Plan 31

API Plan 31

Recirculation from the pump discharge through a cyclone separator delivering the clean fluid to the seal chamber. The solids are delivered to the pump suction line.

A  Cyclone separator
B  Contaminated liquid IN
C  Clean liquid OUT
D  Separated liquid OUT

Seal chamber details

1  From clean discharge connection of cyclone separator
2  Flush (F)
3  Quench (Q)
4  Drain (D)
5  Seal chamber

Example seal: EagleBurgmann SH single pusher mechanical seal.

P&ID according to API


  • Plan 31 is a variant of Plan 11 (Plan 11 + cyclone separator).
  • The cyclone separator removes solids and prevents all but a few minor particles from reaching the seal chamber.
  • The media properties retained as solids are routed back to pump suction.


  • Horizontal pumps conveying media like water with sand particles.
  • Horizontal pumps conveying media with heavy suspended solids.

Remarks, checkpoints

  • Specific gravity of the solids should be at least twice that of the pumped media. In other cases Plan 32 is recommended.
  • The installation of the pump throat bushing must be ensured.
  • Do not use for extremely contaminated media. Plan 32 is recommended in such cases.
  • Can lead to reduced reliability due to abrasive wear inside the cyclone.
  • Routine inspection of the cyclone for abrasive wear is recommended.

Pump media characteristics

  • Suspended fluids

Location, function

  • Process side
  • Flushing and cooling



3NC-FB*, 3NC-BB*, 3NC-FF*

* Remark: The configurations listed for each individual piping plan are to be understood as recommendations including possible utilizations which may also be applied.