Cavitation in a closed loop pumping system

In summary,Dears,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. However, if the discharge pressure is low, there is a possibility of cavitation. We also have a pump with a sound, but since the discharge pressure reads normal, I would say maybe the sound is from the pump bearing. I would like to hear from you regarding this topic.
  • #1
Aptx4869
14
1
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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  • #2
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?
 
  • #3
Using a stethoscope, it should be possible to tell if noise is coming from a bearing or the rotor.
 
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  • #4
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 ?
 
  • #5
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.
 
  • #6
Can you temporarily slow the pump down? If the noise stops it could be cavitation. If it just reduces, probably more likely a bearing?
 
  • #7
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.
 
  • #8
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.
 
  • #9
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 ?
 
  • #10
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.
 

1. What is cavitation in a closed loop pumping system?

Cavitation is a phenomenon that occurs when the pressure in a liquid drops below its vapor pressure, causing the formation of vapor bubbles. In a closed loop pumping system, this can happen when the liquid is flowing through a pump, causing the formation and collapse of these bubbles, which can lead to damage and inefficiencies in the system.

2. What causes cavitation in a closed loop pumping system?

Cavitation can be caused by a number of factors, including high flow rates, high temperatures, and low pressure conditions. It can also be caused by obstructions or rough surfaces in the system that create areas of low pressure.

3. What are the effects of cavitation on a closed loop pumping system?

Cavitation can lead to decreased efficiency and performance of the pumping system, as well as damage to the pump and other components. It can also create noise and vibrations, which can be disruptive and potentially harmful.

4. How can cavitation be prevented in a closed loop pumping system?

To prevent cavitation, it is important to properly design and maintain the system. This may include using a pump with a higher capacity, ensuring proper flow rates and pressures, and smoothing out any rough surfaces in the system. Proper maintenance and regular inspections can also help prevent cavitation.

5. How is cavitation diagnosed in a closed loop pumping system?

Cavitation can be diagnosed through visual inspection, as well as by listening for noise or vibrations in the system. Monitoring pressure and flow rates can also help identify areas of low pressure that may be causing cavitation. In some cases, specialized equipment may be needed to diagnose and measure cavitation in a closed loop pumping system.

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