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,

 

[bimr]

LittleInch

Pumps are used to create heating zones in a hydronic heating system.

 

[saeedplc]

Thank you for the replies

@bimr could you please explain in detail?
When is the pump in this type of pattern used? If we ignore the freeze protection function does it have any benefit to be used in AHUs coils?
These pumps are in both heating and cooling coils.

 

[LittleInch]

fair they even out flow, but it was just that with the pressure drop already there from the differenc ebetween the headers it just seemed a bit odd.

But with the cracking pressur eof the NRV it makes more sense.

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

 

[TugboatEng]

While checking the strainer pull one from a working system to compare mesh/perforation size.

 

[georgeverghese]

Alternatively, temporarily remove the strainer mesh element from this pump, and see if this problem disappears when the PCV opens.

 

[bimr]

My first thought when you posted is that you are using zones to heat, maybe not. You can use multiple pumps or multiple valves to zone HVAC:

Zones

Zoning

 

[saeedplc]

Thank you so much for all your replies

I will perform the mentioned tasks but because of bad designing the components and troubles for fitting some guages it will take time but i will inform you all once it is done.

But @littleInch regarding your comment on pump location and it's function please have a look at the thread403-268639 that discussed before.

Thanks a lot again for your valuable responses.

 

[Snickster]

I am not sure how the system is controlled. Appears the PICV is temperature controlled to throttle when there is a reduced demand for heating causing a high back pressure on the upstream side of the valve allowing more recirculation through the check valve? If this is the case then I don't think a faulty check valve would cause cavitation if it is stuck more open since that will supply more fluid to the pump inlet through recirculation and not starve pump. If the check is stuck closed then that would just reduce flow to pump but also pressure drop in main supply line and also reduce NPSHR. When PICV is fully open then this is when maximum flow and pressure drop in suction line and highest NPSHR occurs. I suspect there is something in the main suction line causing too high pressure drop like a faulty manufactured strainer or something else causing abnormal pressure drop in main suction line.

 

[saeedplc]

@snickster thank you for the reply
Although measuting pressure will reveal all
Things and cause of problem but sometimes analysing a circuit is helpful for us to increase our knowledge in my opinion and i really enjoy from others' technical standpoint

I dont agree with you on case check valve stuck fully open. In this case there is no Crank pressure and the only pressure that the pump see is pressure drop of the coil( about 0.2 bar) and that will be pump head so in this pressure pump is working in minimum head and rightest side of it's curve that can cause cavitation however in this point the NPSHR needs to be checked from the pump curve.
I am going to run the pump when the picv valve is 0% and see the pump behavior if the problem exist that means there is somdthing wrong in the check valve if not the problem is result of the strainer or the gate valve.

 

[pierreick]

saeedplc,
Is that so difficult to check a strainer and a check valve ? I really don't understand !
my 2 cents
Pierre

 

[saeedplc]

I will check the components and inform you.

 

[Snickster]

Saeedplc

But you say that cavitation happens when the PICV is fully open. If the check valve is fully or partially open due to malfunction when the PCIV is also fully open then the pump should be less starved, or overall suction pressure drop should be less than if the check valve was closed when PCIV is fully open. Therefore a faulty check valve can only help reduce cavitation with PICV fully open under full flow conditions?

Also are you sure that the cavitation noise is not comming from the PICV rather than the pump?

 

[saeedplc]

Snickster I said when the PICV is opened more(in higher %) the cavitation worsen.

Yes the sound is like a tiny stone inside the pump and not from picv.

Please let me install some pressure guages and inspect the chack valve and strainer and inform you about the result.

Thank you so much.