air in the water supply stream
air in the water supply stream
(OP)
Hello group!
I have a vertical booster pump (Grundfos CRN 3-9) which’s been driven on up to 6000 rpm to get 120 l/min on 22bar g. The pressure sensor is installed on the pump top position. The pump is driven by Lenze inverter. The media is water of up to 60 degrees of Celsius. This system is proven on few installation places.
Now the problem:
It seems that water inlet stream has air bubbles inside. The outlet pressure is varying significantly, and the system has a problem to stabilize outlet pressure. All is followed with motor heating and winding temperature protection alarms.
I am interested to hear some of your opinions and some suggestions how to determine the presence of the air bubbles in supply water stream?
With kind regards,
Ivan.
I have a vertical booster pump (Grundfos CRN 3-9) which’s been driven on up to 6000 rpm to get 120 l/min on 22bar g. The pressure sensor is installed on the pump top position. The pump is driven by Lenze inverter. The media is water of up to 60 degrees of Celsius. This system is proven on few installation places.
Now the problem:
It seems that water inlet stream has air bubbles inside. The outlet pressure is varying significantly, and the system has a problem to stabilize outlet pressure. All is followed with motor heating and winding temperature protection alarms.
I am interested to hear some of your opinions and some suggestions how to determine the presence of the air bubbles in supply water stream?
With kind regards,
Ivan.





RE: air in the water supply stream
Are you certain that the outlet pressure is varying or could it be a measurement problem due to compressible gases collecting below the pressure transmitter?
RE: air in the water supply stream
To get rid of your entrained air , fit a high point auto vent, ball and float arrangement.
Offshore Engineering&Design
RE: air in the water supply stream
RE: air in the water supply stream
http://virtualpipeline.spaces.msn.com
"We can't solve problems by using the same kind of thinking we used when we created them." -Albert Einstein
RE: air in the water supply stream
6000rpm @60C, like you I would think the unit would be cavitating but wouldn't expect it to result in "air" in the discharge, the gas would go back into solution due to the increase in pressure as part of the cavitation cycle.
I think it is more likely free air coming out of solution due to very low suction pressure, this could well be beneficial anyway as it would reduce the cavitation effect.
RE: air in the water supply stream
http://virtualpipeline.spaces.msn.com
"We can't solve problems by using the same kind of thinking we used when we created them." -Albert Einstein
RE: air in the water supply stream
Thank you for your effort. Honestly, I didn’t expect that so many of your answer so fast.
Now update on topic. Mentioned system is designed to maintain certain amount of pressure (16-25 bar g). That is adjustable. In order to provide that, the system is equipped with pressure sensor located on the top of the pump. It is not matter if I set the value of 18 bar or 25 bar or any between, the system cannot hold it steady, e.g. for 18 bars, the system will vary from 13 to 22. Of course, if I increase the desired pressure on 25 bar, the amount of current will rise and the thermal protection of the motor windings will shut the system down.
My first guess was PID parameters, but it is not the matter.
One last thing is to describe how the pressure changes through time. For 18 bar, the system hold 18 bar for a few secs, then drops on 13(14), then the system increases Hz and/or voltage from inverter to motor, then it comes close to 18, sometimes less, then falls again on 13(14), etc…
So my presumption was that air bubbles cause the pressure drop due their compressibility, than system speeds up in order to reach the bars…
Any idea or suggestion?
Regards,
Ivan.
RE: air in the water supply stream
Regarding water supply pressure:
Both, dry-run sensor and my pressure gauge showed that inlet pressure never drops below 2 bar.
RE: air in the water supply stream
http://virtualpipeline.spaces.msn.com
"We can't solve problems by using the same kind of thinking we used when we created them." -Albert Einstein
RE: air in the water supply stream
Measure your pressure in a different location to help determine if it is a measurement problem. Also measure the amps or power if you can to see if it is varying with the pressure. If it is not then you probably have a measurement problem.
RE: air in the water supply stream
Ensure you have a check valve on the discharge of the pump, to hold system pressure.
Offshore Engineering&Design