API Plan 11

API Plan 11

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

A  Flow control

Seal chamber details

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

Example seal: EagleBurgmann H75VA4-S single pusher mechanical seal.

P&ID according to API


  • Default flush plan for all single seals.
  • Acts as a flushing as well as a self-venting plan for horizontal pumps.
  • Helps in creating additional vapor pressure margin in the seal chamber.
  • Uses a flow control orifice(s) to restrict the flow of flush fluid going to the mechanical seal.
  • Use of a distributed flush makes the cooling and lubrication more effective.


  • Normally applicable for all general duties except pumps with less differential between discharge and seal chamber pressures.

Remarks, checkpoints

  • The orifice size and/or the number of orifices for high head applications must be calculated very carefully. Minimum orifice size should be 3 mm (1/8").
  • Throat bush clearance and orifice sizing together both ensure the proper flow of flush fluid to the seal.
  • Always check the difference between the discharge and the seal chamber pressure. If the difference is too low, Plan 13 must be preferred.
  • Use for media with solids or abrasives or polymerizing media to be avoided.
  • Orifice clogging can be confirmed by checking pipe surface temperatures upstream and downstream of the orifice.

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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