What is making this sound? (in a Rotary lobe pump)

[Minghan]

What is the reason making this sound?? ( Rotary lobe pump)

 

[anorlunda]

Cavitation is my first guess.

On Youtube you can find numerous audio examples of cavitation noise. It is described as a sound like rocks going through the pipes.

You said lube pump. Is it pumping oil or water? Cavitation is most often associated with water pumps, but it is not impossible in oil pumps, such as in this video.

 

[Asymptotic]

Cavitation is my first guess as well. What are the pump specs, and what type of fluid, temperature, motor speed, pump input shaft speed, pump outlet pressure, and (if known) motor power?

I've encountered similar issues with gear pumps used for gearbox lubrication (ISO 320 oil) that were prone to cavitation until the oil warmed up. If you have one handy, as a troubleshooting step run the motor from a variable frequency drive to determine if it quiets down when operated at reduced speeds.

Is this the suction run? If so, overall length and fittings conspire to create a decent amount of flow resistance, and could be starving the pump suction inlet (especially so if the fluid has high viscosity).

 

[Minghan]

Cavitation is my first guess.

On Youtube you can find numerous audio examples of cavitation noise. It is described as a sound like rocks going through the pipes.

You said lube pump. Is it pumping oil or water? Cavitation is most often associated with water pumps, but it is not impossible in oil pumps, such as in this video.

Thank you!
And I just wondered if this setting could cause "Aeration" and make bubbles into pipe which make "Cavitation" easier to happen??

 

[Minghan]

Cavitation is my first guess as well. What are the pump specs, and what type of fluid, temperature, motor speed, pump input shaft speed, pump outlet pressure, and (if known) motor power?

I've encountered similar issues with gear pumps used for gearbox lubrication (ISO 320 oil) that were prone to cavitation until the oil warmed up. If you have one handy, as a troubleshooting step run the motor from a variable frequency drive to determine if it quiets down when operated at reduced speeds.
View attachment 223925
Is this the suction run? If so, overall length and fittings conspire to create a decent amount of flow resistance, and could be starving the pump suction inlet (especially so if the fluid has high viscosity).

Thank you!
And I just wondered if this setting could cause "Aeration" and make bubbles into pipe which make "Cavitation" easier to happen??

 

[Asymptotic]

And I just wondered if this setting could cause "Aeration" and make bubbles into pipe which make "Cavitation" easier to happen??

Air entrained into the pump suction both causes symptoms akin to cavitation, and makes true cavitation more likely. The course of action required is installation dependent, but often involves the strategic placement of baffles and increasing liquid level height.

Do a Google search for "difference between entrained air and cavitation". This return hits the high points. http://www.pumpscout.com/articles-expert-advice/am-i-experiencing-cavitation-or-not?-aid102.html

I'll inquire again, what are the pump specs, and what type of fluid, temperature, motor speed, pump input shaft speed, pump outlet pressure, and is that the suction run outlined in yellow in the photo in post #3?

 

[Minghan]

Thank you!
And I just wondered if this setting could cause "Aeration" and make bubbles into pipe which make "Cavitation" easier to happen??

Air entrained into the pump suction both causes symptoms akin to cavitation, and makes true cavitation more likely. The course of action required is installation dependent, but often involves the strategic placement of baffles and increasing liquid level height.

Do a Google search for "difference between entrained air and cavitation". This return hits the high points. http://www.pumpscout.com/articles-expert-advice/am-i-experiencing-cavitation-or-not?-aid102.html

I'll inquire again, what are the pump specs, and what type of fluid, temperature, motor speed, pump input shaft speed, pump outlet pressure, and is that the suction run outlined in yellow in the photo in post #3?

"Air entrainment and recirculation will also cause the same rumbling/rattling noise and high vibration as cavitation, as well as the recognizable impeller pitting damage."
I understand, thank so much!

Pump specs: 120 L/min
Type of fluid: Water
Temperature: 25°C
Pump input shaft speed: 400 rpm
Pump outlet pressure: 0.5 kg/cm^2
Is that the suction run outlined in yellow in the photo in post #3? : Yes!

Is increasing pressure of pump outlet a factor to reduce cavitation in rotary lobe pump??

 

[Asymptotic]

Is that the suction run outlined in yellow in the photo in post #3? : Yes!

This is a major contributor to your problem. I recommend thoroughly documenting your installation (a drawing with pump size/model number, pipe diameter(s), run lengths, fitting details, etc.) and consulting with the pump manufacturer's engineering department.

Read the pump installation manual - it ought to have a section on desired suction piping - but generally speaking, anything that contributes to turbulent flow (fittings closer than 6 to 20 pipe diameters to suction inlet, depending upon the particulars), and pressure drop in the suction line (long runs to the sump, small pipe diameter) must be avoided. A common practice is to oversize inlet piping one to two trade sizes larger than the pump suction port.

Is increasing pressure of pump outlet a factor to reduce cavitation in rotary lobe pump??

Do you mean by throttling the pump outlet valve as is common to reduce flow in a (non-positive displacement) centrifugal pump installation? A rotary lobe pump is positive displacement, and although to some small extent outlet throttling reduces flow (mostly, by increasing slip), the increase in outlet pressure at the operating point where flow rate has been reduced enough to prevent cavitation may exceed pump and/or pipe pressure ratings, it puts more wear and tear on the pump, and is a very energy inefficient way to reduce flow.

PD pump flow control is usually done by varying pump speed. Another possibility (one that isn't applicable in all situations) is to bypass part of the output flow back to sump, but this is also energy inefficient.

 

[Minghan]

Do you mean by throttling the pump outlet valve as is common to reduce flow in a (non-positive displacement) centrifugal pump installation? A rotary lobe pump is positive displacement, and although to some small extent outlet throttling reduces flow (mostly, by increasing slip), the increase in outlet pressure at the operating point where flow rate has been reduced enough to prevent cavitation may exceed pump and/or pipe pressure ratings, it puts more wear and tear on the pump, and is a very energy inefficient way to reduce flow.

1.Could the slip which is caused by higher pressure at the outlet side add pressure to the inlet side and minimize the cavitation ??
2.And I have read pump troubleshooting and it said that
" Noisy operation=> Air or gas in fluid => Dissolved gas or naturally aerated products => Minimize discharge pressure."
What this means?? Minimizing discharge pressure to ensure bubble won't implode at the outlet side??

 

[Asymptotic]

Minimizing discharge pressure to ensure bubble won't implode at the outlet side??

When outlet pressure is too high, discharge cavitation can occur.

1.Could the slip which is caused by higher pressure at the outlet side add pressure to the inlet side and minimize the cavitation ??

No. Unless the pump is badly worn only a small percentage of flow is expressed as slip.

The solution for that part of the problem caused by air entrainment resides in reducing or eliminating air making it back to the pump, and for suction cavitation it is eliminating restrictions in the suction line. After all that can be done to reduce inlet piping restrictions has been done it may be necessary to reduce pump speed (flow rate) to the point where enough pressure remains at the suction to prevent cavitation from occurring.

There are two other things that contribute to noisy pump operation that may or may not be in play here, but are worth looking into.

1. Pump body distortion caused by unsupported and/or out-of-plane ("cockeyed") piping. Excessive forces coupled by the pipe system into the normally tight clearances within a gear pump/rotary lobe pump aren't a marriage made in heaven, can lead to interference fits from lobe-to-lobe, and lobe-to-body, and a noisy pump as it smashes itself into oblivion.
2. Motor-to-pump shaft alignment.

 

[Minghan]

No. Unless the pump is badly worn only a small percentage of flow is expressed as slip.

1. Increasing clearance could minimize cavitation because of adding more slips and more pressure to inlet side ??
2. And another question making me confused : If the liquid viscosity is too high and rotary PD pump's speed still do not slow down ,then severe noise and vibration will happen. Why?? How to explain this phenomena??

 

[ChemAir]

1. Increasing clearance could minimize cavitation because of adding more slips and more pressure to inlet side ??

I would discourage casually increasing clearances in a positive displacement pump to reduce cavitation. Trivial changes in clearance can result in huge performance changes and losses in efficiency. If the pump manufacturer has specific lobe designs that address specific physical property issues (viscosity/Sp.Gr./Vapor pressure), they may be a resource. I would ask them.

2. And another question making me confused : If the liquid viscosity is too high and rotary PD pump's speed still do not slow down ,then severe noise and vibration will happen. Why?? How to explain this phenomena??

I'm not sure what you mean by "slow down". If it is powered by an AC motor with no Variable Frequency Drive, the pump will always be at the same (motor) speed. A power controlled VFD might be used to reduce cavitation, but flow will also be reduced, so it may or may not flow the amount needed with or without cavitation. A larger, lower RPM pump "might" be a solution, but I'd hate to suggest it, without knowing more about this specific application, and how it is operated.

Positive displacement pumps have their place, which is to move a specific volume at a specific rate. Normally, if you are experiencing cavitation, you have the wrong pump for the job, or the system it is in may need some redesign. PD pump systems need to be evaluated pretty carefully, there is usually a pressure relief system integrated to address the discharge pressure spikes they can generate. I have seen pumps cavitate (or at least exhibit cavitation like symptoms) when relief or minimum flow control systems weren't working properly.

Regarding your original question: "What is making this noise?" Many have a relief valve that can dump excess discharge pressure back to the inlet. It may be internal or external. If flow usage is below what the pump can deliver, the relief valve will vent the excess back to the suction. A relief valve operating at just slightly above its set point can chatter on its seat and make a nice rattle. If it has one, I would discourage operating the pump and only relying on the internal protection on its own. It can get hot.

 

[Asymptotic]

1. Increasing clearance could minimize cavitation because of adding more slips and more pressure to inlet side ??

No. Cavitation occurs when NPSHa (Net Positive Suction Head Available) in the system a pump is a part of is lower than NPSHr (Net Positive Suction Head Required) by the pump. One of the contributors to pressure drop in the suction line is flow rate (more flow >> more pressure drop).

As pump clearance increases, pumping efficiency (the amount of fluid displaced per input shaft rotation) decreases, so too does the flow rate, and in this narrow way affects the propensity for cavitation to occur. This isn't a road leading toward a solution to your noisy pump problem.

2. And another question making me confused : If the liquid viscosity is too high and rotary PD pump's speed still do not slow down ,then severe noise and vibration will happen. Why?? How to explain this phenomena??

I'm not certain what you are asking here. You've indicated this is a water pumping application, and water viscosity doesn't change very much (from about 1.9 to 0.32 cSt) between 0°C or 100°C.

Viscosity is a big deal, however, when it comes to oils. For example, let's say I have a gearpump capable of moving 55 LPM of ISO 320 oil at 65°C without suction cavitation, but the onset of cavitation is heard when oil has cooled to 40°C, and the lowest temperature likely to be encountered during a machine startup is 10°C. ISO 320 oil viscosity is about 105 cSt at 65°C, 325 cSt at 40°C, and 2000 cSt at 10°C - quite a wide range! - but it is a problem easily handled in our age of inexpensive VFDs.

I'm still of a mind that the way forward is to have the pump manufacturer's engineering department, local rep, system designer, etc. get involved, but @ChemAir makes an excellent point.

PD pump systems need to be evaluated pretty carefully, there is usually a pressure relief system integrated to address the discharge pressure spikes they can generate. I have seen pumps cavitate (or at least exhibit cavitation like symptoms) when relief or minimum flow control systems weren't working properly.

Regarding your original question: "What is making this noise?" Many have a relief valve that can dump excess discharge pressure back to the inlet. It may be internal or external. If flow usage is below what the pump can deliver, the relief valve will vent the excess back to the suction. A relief valve operating at just slightly above its set point can chatter on its seat and make a nice rattle.

Pump specs: 120 L/min
Type of fluid: Water
Temperature: 25°C
Pump input shaft speed: 400 rpm
Pump outlet pressure: 0.5 kg/cm^2

Is this pressure from post #7 your nominal operating pressure, or is it the pump's internal relief setting? 0.5 kg/cm² isn't much pressure (about 7.1 PSI) and the pump could very likely be tripping into relief if the latter.

I've run across a comprehensive PD pump troubleshooting guide by Alfa Laval you may be interested in.
http://www.stuartjohnsonco.com/images/uploads/documents/Rotary-Lobe-Problem_Solving-RLPs.pdf

 

[Minghan]

I'm not certain what you are asking here. You've indicated this is a water pumping application, and water viscosity doesn't change very much (from about 1.9 to 0.32 cSt) between 0°C or 100°C.

Sorry to make you confused.
I made this rotary PD pump to suction another high viscosity liquid and I used frequency inverter to change the motor speed.
At the full speed, severe noise and vibration happened.
Troubleshooting said "The liquid viscosity is too high. The pump is starving. "
After I tune the frequency inverter, motor speed slows down and transporting liquid become smooth.No noisy and vibration!
Why?? How to explain this phenomenon??

 

[Tom.G]

At the full speed, severe noise and vibration happened.
Troubleshooting said "The liquid viscosity is too high. The pump is starving. "
After I tune the frequency inverter, motor speed slows down and transporting liquid become smooth.No noisy and vibration!
Why?? How to explain this phenomenon??

The pump is starving.
The pump is trying to move more liquid than can be pulled in from the source. No more liquid can be pulled in because the high viscosity does not let the fluid flow very fast. It is like trying to drink a very thick milkshake thru a straw, no matter how hard you suck you can not get very much of the milkshake. When this happens, a partial vacuum is made in the input pipes.

Maybe you remember that the boiling temperature of a liquid depends on the pressure. When the pressure goes down the boiling temperature also goes down. In school or college science classes there is shown an experiment where there is a glass of water at room temperature. Then it is put in a vacuum chamber and the vacuum started. The water starts to boil, but it is still at room temperature!

That is what is happening with your pump. It is sucking so hard that there are bubbles in the liquid in the input pipes. Since the bubbles are not viscous, when they get to the pump the pump speeds up a little bit and the bubbles get even bigger. When the bubbles get to the high pressure side of the pump they collapse suddenly making shock waves; just like when you clap your hands.

It is the speeding up and the shock waves from the bubbles that make the sound and vibration you are getting.

The making of bubbles in the input pipes is called 'Cavitation'. The word means "making an empty space". It is from the Latin, then French, to English.

If you want to get more of the science of Cavitation, try looking up "Vapor Pressure" and "Cavitation" on the Internet.

Cheers,
Tom

 

[Minghan]

Thank you so much !

 

[Asymptotic]

Sorry to make you confused.
I made this rotary PD pump to suction another high viscosity liquid and I used frequency inverter to change the motor speed.
At the full speed, severe noise and vibration happened.
Troubleshooting said "The liquid viscosity is too high. The pump is starving. "
After I tune the frequency inverter, motor speed slows down and transporting liquid become smooth.No noisy and vibration!
Why?? How to explain this phenomenon??

@Tom.G has covered the phenomenon better than I, but allow me to add that cavitation in your case appears to be due to restriction in the suction line caused by a combination of a long run in smallish diameter pipe, and fitting placement.

Does this high viscosity fluid have Newtonian or non-Newtonian viscosity characteristics?
Water and mineral oils are Newtonian. Many fluids aren't, and these would complicate the troubleshooting picture.
https://en.wikipedia.org/wiki/Viscosity#Newtonian_and_non-Newtonian_fluids

Is flow rate sufficient for your process at the reduced speed necessary to prevent cavitation?
If yes, declare victory. If not, then address the suction restrictions.

As speed is increased, at what speed does the pump begin to cavitate? Note this value before making suction line modifications, and re-test afterwards to determine how effective the modifications were. Troubleshooting will be aided by adding a pressure gauge at the pump discharge and a combination vacuum/pressure gauge at the pump suction during the plumbing modifications.

If cavitation still occurs before reaching the pump speed required to provide the desired flow rate then @ChemAir 's suggestion of a larger pump size (with a correspondingly larger suction port) may be necessary. This is why I'm advising bringing in a pump manufacturer's representative for evaluation and consultation - the cost to redesign is directly proportional to the amount of guesswork involved in the decision-making process.

 

[Minghan]

Thank you so much! I 'm very grateful for your help !