air in the water supply stream 1

[cikiste]

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.

 

[gepman]

Do you just want to determine when there are air bubbles in the suction, do you want to eliminate the air bubbles, or do you just want to be able to control your pump discharge pressure better?

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?

 

[chief]

To get rid of your entrained air , fit a high point auto vent, ball and float arrangement.

Offshore Engineering&Design

 

[khardy]

Get a copy of Goulds Pump's "GPM" manual. It is avaiable at thier web site for free i believe. The back of the manual has several pages devoted to keeping air out of a pump.

 

[BigInch]

Since you say "driven up to 6000 rpm", it might mean that when running at lower rpm you didn't have this problem. Please explain. In other words, are you sure your air is entering from the supply, or is it that your cavitating at this increased suction speed?

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"We can't solve problems by using the same kind of thinking we used when we created them." -Albert Einstein

 

[Artisi]

BigInch -
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.

 

[BigInch]

Artisi, ya you're right, but just making sure. Going to 6000 rpm has undoubtedly increased the suction specific speed a lot and consequently also increased the NPSHR. Cavitation is certainly possible if no adjustment to suction pressure was made from the previous rpm. If you cavitate so much you lose constant discharge pressure, you can still see a lot of bubbles. In any case something's funny since any small quantity of entrained air bubbles should be pretty much compressed into nonexistance at 22 Barg. He's either drawing in big gulps of it with the higher suction velocity than before, or making it somehow. If its entrained, its probably way more than 5% and consequently losing power and constant discharge pressure... also possible with cavitation, so just asking.

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

 

[cikiste]

Hello guys!

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.

 

[cikiste]

Sorry:

Regarding water supply pressure:

Both, dry-run sensor and my pressure gauge showed that inlet pressure never drops below 2 bar.

 

[BigInch]

If you're not sure that the air is coming from suction, that might be borderline for hot water at high suction specific speed. Have you checked the estimated NPSHR at that new SSS?

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

 

[gepman]

It is the lowest pressure at the eye of the pump that determines cavitation, not the pressure at the suction. That is why you need to determine the NPSHR as others have suggested. Usually any cavitation severe enough to affect pump performance will sound like it is pumping rocks.

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.

 

[chief]

Ensure you have a check valve on the discharge of the pump, to hold system pressure.

Offshore Engineering&Design