Pump caviation again?

[daviddsims]

Glad to see so many people on the ammonia side of things here. I have a question on the design of our recirculation vessels. We are a new plant that has been going for a little over a year. We have a single stage system with 5 Frick 270 screw compressors, three being low temp and two medium temp. We operate the Medium temp at 38 psi and low temp at 2.5hg. We use an economizer that feeds these vessels 5 degree liquid. On the vessel skids we have two teikoku each with the medium temp at about 60gpm and low temp at 120 gpm going to our freeze tunnel. The pumps have Danfoss differential pressure switches set to about 10 psi. I am getting a lot of cavitation alarms throughout the day at random times. The pumps attempt to restart three times with 30 seconds in between restarts until it cuts the pump out if not satisfied. Coming off the liquid pump discharge header there is a Danfoss regulator that goes back into the vessel. Is this regulator for overpressurization? Most of the vessel packages I have dealt with did not have these. Should these be set at a certain pressure?

[MikeHolm]

Is the lock out only on the low pressure system or both systems? 2.5" is really low pressure. What are the pumps? I suspect they are well below atmospheric on the suction.

[Segei]

We had several posts related to this issue. Read them.

[mbc]

30 second for restart is not enough I think you should go for more than 2 minutes ,
What is the deference pressure for starting of compressor (goes on & off) , if that is too much it happen ,
Check the size of piping and slope of that to inlet pump.

[Magoo]

Hi David.
possibly the danfoss bypass reg vav is there for minimum flow through pump. Do you have any gauges after the pump. If differential switch is faulting system at 10 psig, either short of liquid in vessel, or rapid loading of compressors causing boil up in vessel, and causing cavitaion. You need a minimum 2 metre liquid head above pump.

[Sandro Baptista]

Hi David.
You need a minimum 2 metre liquid head above pump.

It depends on the NPSH of the pump for the operating flow...nevertheless 2 meters is a good high.