info required on a vsd drive for condenser water pumps

[smvasakam]

at one of our client site, we have a centrifugal pump of capacity 800 gpm and 35 mtrs head for pumping condenser water. condenser water is supplied to package units and returned back to the cooling tower. The package units (20 no's) are provided only with gate valves.
Lately vsd was installed to reduce the pump speed to save energy. whenever the vsd reduces the pump speed say even at 45 hz there was airlock in the piping system and knocking sound in the condenser water pipe lines.
so currently the system is operating at full speed to avoid the air lock and knocking.

why there is a air lock in the pipeline and how to avoid it.

 

[katmar]

What parameter are you measuring to control the set point on the VSD? Is it flow, or pressure? How much of your 35 m of head is contributed by static head, and how much by friction? If the head is mainly static head, as soon as you slow the pump down there may not be sufficient head to get the water up to the condensers.

Katmar Software
Engineering & Risk Analysis Software

http://katmarsoftware.com

 

[Artisi]

It is unlikely that slowing the pump is causing air lock and vibration - as katmar has pointed out it seems more likely a hydraulic problem. Are you slowing from 60 or 50 to 45 Hz.
Remember, head changes as the square of the speed change.

Phitsanulok
Thailand

 

[BigInch]

As Artisi and Harvy point out, it appears that the slower rpm is not generating sufficient head (that's probably quite a large drop in discharge head) to keep flow moving and possibly any entrapped air from clearing the system. The trapped air may be expanding and contracting causing local velocity and flowrate changes as the water moves past the vapor space and resulting in the noise in the pipe you are hearing.

BigInch-born in the trenches.

http://virtualpipeline.spaces.msn.com