API Plan 12

API Plan 12

Recirculation from the pump discharge through a strainer and a flow control orifice into the seal chamber.

A  Strainer
B  Flow control orifice

Seal chamber details

1  From high pressure region of pump (pump discharge or pump discharge piping)
3  Flush (F)
4  Quench (Q)
5  Drain
6  Seal chamber

Example seal: EagleBurgmann LL9UC single pusher mechanical seal.

P&ID according to API


2  Strainer



  • Similar to Plan 11 but with the addition of a "Y" strainer to remove occasional solid particles.
  • Acts as a flushing as well as a self-venting plan for horizontal pumps.
  • Uses a flow control orifice to restrict the flow of flush fluid going to the mechanical seal.


  • Slightly contaminated (fluids with suspended solids), non-polymerizing fluids with low temperatures.

Remarks, checkpoints

  • The strainer must always be installed before the orifice.
  • Strainers often have a tendency to choke and thus lead to starvation and, eventually, failure of the mechanical seal. The strainer should be maintained and serviced at regular intervals.
  • This plan does not function reliably enough to achieve a three-year operating life and is therefore generally discouraged.
  • For the same reason Plan 22 (12 + cooler) is also discouraged.
  • A Plan 32 or a dual pressurized seal is a recommended option for such applications.

Pump media characteristics

  • Clean, non-polymerizing fluids with moderate temperatures.

Location, function

  • Process side
  • Flushing and cooling



3CW-FF*, 3CW-BB*, 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.


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