Cavitation in a closed loop pumping system

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Discussion Overview

The discussion revolves around cavitation in a closed loop pumping system, particularly focusing on the identification of sounds from centrifugal pumps and the conditions under which cavitation may occur. Participants explore various factors such as suction pressure, discharge pressure, and potential causes of noise in the system.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant suggests that attributing noise from the pump to cavitation is difficult if the discharge pressure is normal, citing a previous incident where a closed suction valve led to no flow.
  • Another participant notes that they have experienced cavitation in closed loop systems, describing the sound as distinct from bearing noise and suggesting temperature checks for the bearing housing.
  • Several participants propose using a stethoscope to differentiate between bearing noise and rotor noise.
  • A participant questions whether cavitation could occur with a suction pressure of 5 Psig and a temperature of 100 F, specifically for demineralized water.
  • Concerns are raised about the measurement location of the suction pressure and the potential impact of elevation and flow losses on cavitation risk.
  • One participant suggests temporarily slowing the pump to determine if the noise is related to cavitation or bearing issues.
  • Another participant emphasizes the need to inspect or replace the pump if issues persist, mentioning that cavitation in closed loops could be due to air leaks.
  • A participant expresses skepticism about air leaks in a pressurized system, arguing that such leaks should not occur if the system is maintained above atmospheric pressure.

Areas of Agreement / Disagreement

Participants express differing views on the causes of noise in the pump and the likelihood of cavitation occurring under specific conditions. There is no consensus on the exact nature of the problem or the best approach to diagnose it.

Contextual Notes

Participants mention the absence of pump curves showing required NPSH vs flow rate, which may limit the analysis of cavitation risk. Additionally, the discussion includes various assumptions about system conditions and the interpretation of sounds.

Aptx4869
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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
 
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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?
 
Using a stethoscope, it should be possible to tell if noise is coming from a bearing or the rotor.
 
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gmax137 said:
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 ?
 
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?

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

This is good advice.
 
Can you temporarily slow the pump down? If the noise stops it could be cavitation. If it just reduces, probably more likely a bearing?
 
Aptx4869 said:
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.
 
gmax137 said:
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.
 
CWatters said:
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 ?
 
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Outhouse said:
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.
 

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