why pump cavitates despite enough NPSH 2

[kalamarhss]

Hi to all fellow engineers.
We have a pump that cavitates despite there is enough NPSH.
The working fluid is water at 45 Celcius. Pressure just before the pump is 0.2 bar that means NPSH=12m, well above the NPSHr.
What are the possible reasons for this except wrong curve from the manufacturer?

 

[bimr]

Is the pump running at half the design flow because of poor pump system performance or by operational requirements?

You may have air ingestion. Air ingestion is not really cavitation, but acts like it.

Air ingestion has an adverse affect on the pump. Air ingestion seldom causes damage to the impeller or casing. The main effect of air ingestion is loss of capacity.

 

[Artisi]

Running at half its design flow is probably where the problem stems from, poor entry conditions of the flow onto the impeller blade - this usually results in noise which can sound like cavitation.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[nfinit]

just a question, have you verified that the suction pressure gauge is reading the correct pressure?

 

[BigInch]

Perhaps for a large diameter pipe, you may have prerotation eddies that do not disipate in such a large space within only 5 diameters. Remember that 5D is an absolute minimum.

If you are controlling the flow to 1/2 BEP with the butterfly valve, I would suspect the problem therein is, especially if the filter and valve are between the straightner and the pump.

From "BigInch's Extremely simple theory of everything."

 

[BigInch]

It also seems that (I don't pump hot water myself) hot water can require even more NPSH than what would be indicated from adjusting the cold water curve for hot water's vapor pressure alone, but I'll leave that topic open for those hot water pumpers out there. I'll bet on the partially closed butterfly valve.

From "BigInch's Extremely simple theory of everything."

 

[TD2K]

"TD2K I am measuring after the strainer before the pump suction. The gauge pressure is 0.2 bar."

Thanks. If you have 0.2 barg at the pump suction and you are pumping 45C water, you have about 11.3m NPSHA per my calc, pretty close to your estimate, that's a fair bit. What is the vendor saying the pump requirement should be for NPSHR?

Where is the operational point sitting on the pump curve? Is the NPSHR rapidly increasing at this flow? Can you reduce the flow and see what happens?

Lots of other good suggestions also from other people.

 

[TD2K]

Sorry, I missed seeing your earlier post where you provided the NPSHR, you should have ample.

I looked at a troubleshooting boot by Norm Leiberman, his comments are:

1. The pump discharge pressure is erratic, it varies from normal to low
2. the pump flow is erratic
3. The pump suction pressure is drawn down slowly but evenly and then jumps back up
4. The pump amps are erratically low
5. The pump may (but not always) make a rattling sound

He then says "Unfortunately reality is often more complex than this conventional definition of cavitation"

If you raise or lower the level in the suction vessel is there any change in what you are seeing?

Have you looked at the manufacturer's standard curves for your style of pump and seen if the NPSHR for those impellers are in the same area as your impeller?

Maybe you've provided this and I'm missing it also but what is the operational flow rate and design head. Is the pump's current operational point agree with the pump curve?

 

[clay87]

I hate to keep pestering but I don't quite understand:

When you say 'pressure side' does that mean the damage is on the underside of the vanes at the suction (in other words, do you need a mirror to see the pitting)? If so, suction recirculation as Artisi suggests sounds like your problem. This is consistant with low flow rate, pumping rocks noise, and high vibrations.

Out of curiosity, is this a double suction impeller?

 

[bimr]

I believe you may be experiencing recirculation cavitation because you are operating the pump at far below the flow range of the pump.

In centrifugal pumps, there is typically a small flow from the impeller discharge back to the suction through the clearance between the impeller hub and front wear ring. When some pumps run at flows well below the design range, water can recirculate within the pump.

A characteristic damage from suction recirculation is that most of the damage occurs on the high pressure side of the vane. The noise from cavitation also tends to be higher than that cause by low NPSH cavitation.

 

[bimr]

Detailed explanation of suction recirculation.

 

[kalamarhss]

bimr the pump runs at half the flow due to operational reasons. Air ingestion is a possibility but there is severe damage of the impeler. The damage actually is in both sides but it is more serious in the obvious side ( the one you dont need a mirror to see). Recirculation is one of the possibilities. I have to check with the diagramms you uploaded. Very useful thanks.
Big inch the valve is completely open. Hot water pumpers come out please. And the distance is more than 10X 0.4 which is the ID of the pipe.
TD2K thanks for the input. I wouldnt say the pressure is "erratic". It fluctuates from 28-30 bar.
clay as I said the damage is in both sides but more severe in the obvious side.
If it is not cavitation shouldnt the manufacturer point out the operational limits of the pump and to prohibit operation under a certain flow?
Thanks everyone for the valuable info and your time.

 

[bimr]

Operation of pumps at less than optimum conditions is a major problem in industry. Most of the pump vendors that I have worked with provide minimal input in the pump selection process.

Here is a link to some reprints from Igor Karassik articles discussing the problems associated with off BEP operation.

http://www.pumped101.com/#karassik

 

[clay87]

Good luck, kalamar. Maybe there is some way to increase flow rate, perhaps you can open a recirc line? I suppose this would make the condition worse if you are experiencing NPSH cavitation but at least you'd know .

 

[stanier]

Perhaps you should consider reducing the diamter of the impeller so that it is on point.

"Sharing knowledge is the way to immortality"
His Holiness the Dalai Lama.

http://waterhammer.hopout.com.au/

 

[Artisi]

kalamarhss,
we all working in the dark, therefore our crystal balls don't function all that well - how about some meaningful operating and pump data, this might go a long way to help in solving the problem.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[BigInch]

And tell the truth. You're throttling with the butterfly on suction arn't you.

From "BigInch's Extremely simple theory of everything."

 

[kalamarhss]

bimr the manufacturer gives a min operational Q of 90 qmph. But I guess this is for cold water . Is there a way to translate this to my operational temp?
clay it is a possibility
stanier there would not be enpugh head
Artisi you are right i will come back with moore data I promise.
Big inch I wish I was (sigh)things would be much simpler.

 

[BigInch]

If you're not throttling with that butterfly, then that should not be a butterfly valve. Remove it and replace it with a fully opening ball valve at your earliest opportunity and leave it fully open unless the pump is off.

From "BigInch's Extremely simple theory of everything."

 

[bimr]

My opinion based on the information provided is that you have suction recirculation. Have you tried operating the pump at close to design flow rate and observed the cavitation?

Regarding "the manufacturer gives a min operational Q of 90 qmph. But I guess this is for cold water. Is there a way to translate this to my operational temp?"

Is that 90 cubic meters per hour? That would represent about 25% of the design flow if 2900 liters per minute is half.

Have you contacted the manufacturer about this problem and given the manufacturer your as-built operating parameters?

 

[BigInch]

I agree. Just trying to emphisize that the butterfly valve there isn't helping things.

From "BigInch's Extremely simple theory of everything."