an Engineering & Service Bulletin

Technical Topics

J.H. WRIGHT & ASSOCIATES

CENTRIFUGAL PUMPS AND FOUR QUADRANT CURVES

In some process applications the liquid remains at a high pressure after it has completed it's cycle. It is therefore often desirable to recover this energy rather than just dissipate it. In other pumping applications, failure of a power source could cause back flow through the pump and extreme reverse rotation.

The two above cases bring me to a rather esoteric, but interesting centrifugal pump subject: four quadrant or Karman-Knapp pump curves.

Consider the above, a simplified four quadrant curve of a typical centrifugal pump.

You will note there are three performance curves shown. One for zero pump RPM, one for +1800RPM (in the correct direction of rotation) and one for -1800RPM (backward rotation).

Also, note that the ordinate and abscissa represent GPM and TDH. However, in Quadrant B & C they also represent negative values. The intersection of the two represents zero flow at zero head.

Quadrant A: represents the normal operation of a centrifugal pump, at least as far as +1800RPM is concerned. The curve labeled -1800RPM is the performance of the same pump with the impeller turning backwards!

Quadrant C: the -1800RPM line, represents the performance of a hydraulic turbine. You would use one of these to recover the energy remaining in a process fluid. High-pressure fluid would enter the discharge of the "pump" and exit through the suction - driving the "pump" (now a turbine) backward. Energy can now be recovered from the "pump" shaft. The flow in this quadrant is therefore described as negative.

Quadrant B: liquid has to be forced through the pump with a suction head pressure higher than discharge. This head is therefore described as negative. The -1800RPM curve in Quadrant B therefore describes a reverse flow turbine. +1800RPM and zero flow in the Quadrant B are simply energy dissipation. So is +1800RPM in Quadrant C.

Your eyes are probably in a fully glassed over condition at this point so let me pose a few practical questions that can be answered using these curves. 1. If a pump is spinning fully reversed due to discharge water column after shutdown, will the motor be able to restart it in the correct direction? 2. What will the pump speed be if the motor is shut off and a high suction pressure forces liquid through the pump in a forward direction? 3. What will the performance be of the pump running backward? 4. What is the maximum reverse speed?

A complete set of four Quadrant curves is more complex (torque is included) than my simplified version, but hopefully you now have a basic understanding of their usage.

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