API Plan 21

API Plan 21

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

A  Flow control orifice
B  Cooler
C  Temperature indicator

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


TI  Temperature indicator



  • Provides a cool flush to the seal.
  • Plan 21 is self-venting.
  • Improves the product temperature margin to avoid vapor formation.
  • Helps reduce coking.
  • Helps improve lubricity (e.g. hot water).
  • Utilizes a close clearance throat bushing for better efficiency.


  • Fluids with low vapor pressure margin in seal chamber.

Remarks, checkpoints

  • Minimum size of the orifice should be 3 mm (1/8").
  • High cooler duty is leading to fouling and plugging on the water side.
  • Plan 21 consumes more energy than Plan 23 because the fluid needs to be pumped back continuously.
  • Potential plugging on the process side if the fluid viscosity gets high quickly.

Pump media characteristics

  • Close to vaporization point.
  • Hot, non-polymerizing clean fluids typically less than 176 °C (349 °F).
  • Bad lubricity fluids (like hot water).

Location, function

  • Process side
  • Flushing and cooling
  • Increases vapor pressure margin



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