Pump problem 1

[saeedplc]

Dear Experts

The photo is related to one of AHUs on the riser. The problem is that only
This circuit has problem of pump cavitation on the riser and other ahu pumps are working fine
When pressure independent control valve is open more the cavitation sound
Willl be higher and vice versa so could you please let me know which part of circuit should i
Check? I my opinion the check valve is faulty but i am not sure and need your idea.


Regards,

 

[LittleInch]

You really need to find some pressure readings.

Does the AHU give out cold air? so is there flow through the AHU?

Is there anything else different about this circuit than the others?
different AHU?
different pump?

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

 

[pierreick]

Hi,
The only way is to inspect the check valve and replace it if necessary . Can you use a portable (external) flow meter , let say ultrasonic flow meter, in the different parts of your system to confirm the issue.
By the way we don't have any indication about your system ( pipe size, flow ,...)
My 2 cents
Pierre

 

[1503-44]

saeedplc,

It would be great if you could post pictures as pictures, rather than attaching them as files. See the [CAMERA "Image"] button? Try it. That saves all of us a lot of time. We could just look at the picture, like this, and not have to waste time downloading, saving your pictures to my disk and then opening the file.

A black swan to a turkey is a white swan to the butcher ... and to Boeing.

 

[saeedplc]

Thank you for the replies

@LittleInch which components should i read pressure from? And whats your suspicion?

No the ahus dont have the same pump capacity but one of the Ahus has coil that is bigger than the one i mentioned but it's pump has no abnormal sound.
The is no obvious sign of not cooling or weak cooling in the AHU.
I want to solve the problem before the pump
become defective.
The coil capacity in the photo is 18 m3/h and duty point of pump is 1.2 bar/ 18m3/h
Pipe diameter is 2 1/2 inch

In higher percent of opening picv valve the cavitation sound become more.

@pierreick there no flowmeter available but could you please let me know how flow value can reveal the problem?

@1503-44 you are absolutely right it was my mistake.sorry.

 

[pierreick]

Hi,
Portable Flow meter should help you to find whether your check valve is faulty and will help you to assess your system.
Pierre

 

[LittleInch]

Ideally get pressures at pump inlet, pump outlet, node before the PICv, node after PICv.

Then you might be able to figure it out.

Currently I don't have clue as to why your pump might be cavitating. If your check valve is leaking then the pressure local to the pump might be reduced, but I'm just guessing.

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

 

[EdStainless]

If it is cavitating (and not just failing for some other reason) then either you are operating way too far to the right of the pump curve (more than intended flow) or you are choking the inlet side (blockage or valve not opening).
edit - Or you are trying to pump air + water.

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P.E. Metallurgy, consulting work welcomed

 

[bimr]

Not much to go on with the limited description.

Are all of the AHU on the same elevation?

The check valve is probably less expensive than the cost of your investigation, why not just replace it and make sure that it is oriented correctly.

Have you tried substituting the pump with one that is woring correctly, the pump may be bad.

Is this a new installation? Was the system flushed before use?

 

[LittleInch]

That's where knowing the pressure will help a lot.

And a bit of history would help - Is it new / old?
Did anything happen to change the system?
Is the expansion tank still working?
Have you tried just pumping the primary system up in pressure a bit more?

What is the pressure drop of the AHU at 18m3/hr??

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

 

[saeedplc]

Thank you all for the responses

The system is working almost 3 years
By increasing thr system pressure nkthing hapoen to improve the situation
@bimr before replacing the check valve or pump we should at least know which part is more probably defective.
I also thunk best way to work out what is happening is measuring pressure at the inlet and outlet of the pump to see low pressure at inlet cause cavitation(low NPSH or low dp across pump) and so on.
But something that is very bizarre for me is that the pump is in close loop and changing percent of picv valve should not have any effect on the pump operation but in higher percent the cavitation sound become much more!!!!!!!!!

 

[LittleInch]

Like bimr says - make sure they fitted the check valve the right way around for starters...

but if opening the control valve make sit worse, that sounds like even more flow through the pump so is there a mismatch between pump delivery flow / pressure versus the pressure drop through the AHU.

At that point is there a big difference in pressure between the two headers?

Where in the system is this particular AHU - Can you sketch the whole system and show us where it is reference to the circulating pump of the main header system.

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

 

[saeedplc]

The check valve is right mounted
You are right i need to measure pressure of different point to see if the pressure is within NPSHr and if the load curve is intersecting the pump curve on the right point or not.
unfirtunately it take time because there is no point to connect guages or pressure transmitters.

Thanks.

 

[LittleInch]

I don't really understand what the pump is there for. The pressure drop through the ahu is far too small for the pump so it is almost certainly operating beyond the curve and has a lot more than 18m3/hr flow.

Try removing the pump and see if it still works...

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

 

[saeedplc]

LittleInch I am not as good as you at mechanic but i think we should take into account the crank pressure of the check valve that i mentioned about 1 bar so head of the pump will be almost 1+0.2 bar that is in middle of the pump curve.
By bypassing the pump temperature is fluctuating and this pump is also as freeze protection pump as far as i know.

 

[LittleInch]

Yes, but as soon as you open the control valve the flow through the pump will increase.

Also maybe this "cracking pressure" of the NRV is not functioning properly?? If the spring has broken the pressure drop might be <0.1 bar.

Can you just replace the NRV with a new one to see if this is the real problem?

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

 

[saeedplc]

I dont think so by opening the picv valve the flow through the pump is increased.
Pump and coil and check valve together creates a close loop and basically the flow through coil should be always 18 m3/h and only flow at point between picv valve and check valve is changed.
Whatever the picv valve open that amount is taken from the supply line to the loop.
In normal situation pump operating point should has nothing to do with the picv valve position but failing check valve can cause low drop in ghe loop and pump point shift to right side of its curve and can cause cavitation.
If you have any other idea or think i am not correct your comment will be highly appreciated.

Anyway without measuring pressure nothing can be said definitely.

 

[LittleInch]

You can get a situation where the flow going out from the PICV and incoming from the header is added to the circulation flow. Especially if the NRV has a lower cracking pressure than it should.

All depends on the actual pressures.

But try a new check valve first. It might solve your problem and is easy to check.

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

 

[btrueblood]

I would also check the strainer - a clogged strainer will reduce the pump inlet pressure, such that it is always starved, and runs closed loop - drawing from the check valve bypass. Opening the PICV then starves the pump (lowers the pump inlet pressure coming from the check valve).

 

[LittleInch]

Agreed - Just as likely and fits the description of symptoms as well.

So either try them or fit some pressure guages...

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