Centrifugal Pump Loss of suction 1

[NickParker]

What happens if the centrifugal pump suddenly loss the suction due to some blockage in the suction pipe? I understand that impeller gets heated up and cavitation etc. What I would like to know is this sudden loss of load causes the shaft to rotate at a speed greater than the speed before the blockage? Because it can cause regeneration of power in the VFD.

 

[mk3223]

yes, when the pump lost the suction, the VFD is to jump up the speed as it tries to make up the loss of the flow and/or pressure in the discharge.

 

[LittleInch]

"Regeneration of power" ????

Please explain this phrase.

Depends what the VFD.is controlling on but I would imagine the speed could increase.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[itsmoked]

It completely depends on what mode the VFD is running in. If you're running in Vector mode it won't speed up at all. If you're running in Scalar mode it will only speed up a little, something in the range of the difference between the slip speed and synchronous speed.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[ScottyUK]

For regeneration to occur there has to be torque taking the motor above synchronous speed. Where would this torque come from?

As the load reduces due to cavitation the slip may reduce slightly resulting in an increase in motor speed, but the speed will still be sub-synchronous. You can't get regeneration until the motor is at a super-synchronous speed.

The situation is slightly complicated in a VFD because synchronous speed itself is a variable, so depending on how the VFD control loop behaves it is possible that the motor could briefly go super-synchronous if the VFD demanded a forced deceleration, but this would only be a transient condition while the supply absorbed kinetic energy stored in the rotating masses of the pump and motor; it is unsustainable in steady-state unless the process is capable of driving the pump in reverse.

 

[jraef]

I agree with ScottyUK 100%. Even if your VFD did not trip from that momentary supersynchronous spike, the energy just isn't there to sustain that or to do anything useful like make the motor generate. Think in terms of the first law of thermodynamics; energy cannot be created or destroyed, only transferred. Where is the energy in the pump with no suction?


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" -- W. H. Auden

 

[georgeverghese]

There may be a momentary increase in speed due to the loss in load, but the VFD would soon bring the speed back to the required setting. But, if for example, level in an upstream vessel is configured in the plant DCS to reset the speed of this VFD, the level controller, on sensing rising level due to the sudden loss in outflow to this pump, may force the VFD speed controller to raise its speed setpoint in an attempt to bring level back to normal. This is usually within the normal expected envelope of operation of the VFD - pump set. Speed will continue to rise with increasing level till it hits the max permissible speed and then stay flat at that max speed till level drops.