I Cavitation in a closed loop pumping system

[Aptx4869]

Dears

Kindly, I want to hear from you regarding this topic. The system is closed cooling system.

I believe it is difficult to attribute the sound you hear from centrifugal pump to be a cavitation especially if the discharge pressure is normal.

The last time we had a problem with this system is that the the suction valve was fully closed (we thought it was open).The discharge pressure was normal and there is no sound from the pump. This happens because the suction valve is like a discharge valve that is far away from pump (closed loop). So if the suction valve is closed, there will be no flow and liquid will just recirculate in the pump casing.

If there is a strainer upstream of the pump that is nearly clogged, it may cause cavitation, but only if the discharge pressure is low. then you can say there is a possibility of cavitation.

Now, one of our pumps has a sound (difficult to tell what it is), but since the discharge pressure reads normal, I would say maybe the sound is from the pump bearing.

Please I will be happy to hear from you to know If I have some misconceptions here. Please comment

 

[gmax137]

I have heard pumps in closed loop systems cavitating. It sounded much different than a bearing noise. To my ear anyway. Can you check the temperature of the bearing housing? Do you have baseline temp readings to see if it is running hotter than before?

 

[anorlunda]

Using a stethoscope, it should be possible to tell if noise is coming from a bearing or the rotor.

 

[Aptx4869]

I have heard pumps in closed loop systems cavitating. It sounded much different than a bearing noise. To my ear anyway. Can you check the temperature of the bearing housing? Do you have baseline temp readings to see if it is running hotter than before?

What if the suction pressure is 5 Psig and temperature is 100 F. The liquid is demineralized water

Do you think cavitation will happen to such a system in that case ?

 

[gmax137]

How close to the eye of the impeller are you measuring the 5 psig suction? What elevation and flow losses are there between the 5 psig and the eye?

Do you have pump curves showing required NPSH vs flowrate?

Using a stethoscope, it should be possible to tell if noise is coming from a bearing or the rotor.

This is good advice.

 

[CWatters]

Can you temporarily slow the pump down? If the noise stops it could be cavitation. If it just reduces, probably more likely a bearing?

 

[Outhouse]

Now, one of our pumps has a sound (difficult to tell what it is), but since the discharge pressure reads normal, I would say maybe the sound is from the pump bearing.

Best bet is to take pump offline and inspect/replace as needed

You know there is issue, so now when you work on it is your choice, soon you will have no choice.

Cavitation in a closed loop would generally be due to a air leak. It seems you have found Hydrodynamic cavitation and know what that sounds like.

Without a stethoscope, a broom handle to your ear placed om each side of pump if possible can pinpoint bearing failure. With noise though, you really need to go into pinpoint exact cause/s.

 

[Aptx4869]

How close to the eye of the impeller are you measuring the 5 psig suction? What elevation and flow losses are there between the 5 psig and the eye?

Do you have pump curves showing required NPSH vs flowrate?
This is good advice.

Dear gmax137,

The suction pressure gauge is near the eye of the impeller (around 2-3 meters only). The only thing between the pressure gauge and the suction of the pump is the rubber expansion joint.

The manual does't provide the NPSH vs flow rate. I have to ask the vendor to provide it.

 

[Aptx4869]

Can you temporarily slow the pump down? If the noise stops it could be cavitation. If it just reduces, probably more likely a bearing?

Dear CWatters

how can we decrease the pump speed ? can this be achieved by changing the motor delta connection to Y connection or vice versa ?

 

[Aptx4869]

Best bet is to take pump offline and inspect/replace as needed

You know there is issue, so now when you work on it is your choice, soon you will have no choice.

Cavitation in a closed loop would generally be due to a air leak. It seems you have found Hydrodynamic cavitation and know what that sounds like.

Without a stethoscope, a broom handle to your ear placed om each side of pump if possible can pinpoint bearing failure. With noise though, you really need to go into pinpoint exact cause/s.

Thank you for the advice.

one comment regarding the air leaks:

I believe for a system that is pressurized above atmospheric at all points, will not have air leaks in.