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?

 

[1gibson]

Did manufacturer perform an NPSH test to confirm it met published values?
Is it a semi-open impeller, is lift set too high?
Is the vane eye tip speed too high? (ask manufacturer to confirm)
Is min submergence met, is it in a sump, do you have vortices?
Is there suction blockage?

 

[kalamarhss]

1gibson thanks for the interest.
No the manufacturer did not do a test.
It is a centrifugal pump single stage closed impeler.
How do u find the tip speed? The pump runs at 2900 1/min.
The pump sucks from a tank which is 10 meters higher. The tank is closed and there is 0.2 bar pressure in the tank.
There is a flow straightener installed before the pump.
There is a filter and a buterfly valve before the pump.
But as I said the manometer shows 0.2 bars before suction when NPSHr is 4-5 meters.

 

[bimr]

Actually 0.2 bar = 2.04 meter of water head. How did you come up with 12m?

Do you actually have a filter or a strainer on the inlet side of the pump. There is probably not enough head available for a filter.

Note that pump cavitation starts gradually. There is not a sharp break where cavitation starts to occure at 4-5 meters.

 

[kalamarhss]

I think that manometres show deferential pressure.
but when calculating NPSH we need absolute pressure.
Am I wrong?
You are right its a strainer.
Excuse my poor English.

 

[1gibson]

If it is an open sump, and that is 0.2barg, then it's about 7 ft head plus 32 ft (depending on elevation) for atmospheric pressure, which is about 12m. This is what I assumed, because I assumed it was a vertical pump.

Bimr, you asssumed it was a horizontal, closed system, so yes about 2 m.

Now more info, seems like it is leaning towards being a horizontal pump, with tank and 10m static head??? I'm out of guesses for today.

To find the tip speed, as I indicated, you ask the manufacturer. Sometimes 1800 rpm pumps are misapplied at 3000 rpm and impeller design is not appropriate. Is that pump available at 3600 RPM? If not, look into this furhter. If it is available at 3600, then nevermind. To estimate, take the impeller eye diameter (should be available in some for or another in the literature, if not then just take the impeller eye side wear ring diameter (check your spare parts list) and reduce it by about 1/2".) With diameter and speed, convert it to a velocity. Typically do not want to exceed 85 ft/s but this is more of a vertical pump rule.

 

[bimr]

What do you mean by the "The tank is closed"?

You should have enough NPSH if the tank is open to the atmosphere. If the tank is closed, you will be pumping from a vacuum and you will not have enough NPSH.

See the diagrams in this link:

http://www.mcnallyinstitute.com/11-html/11-12.html

 

[clay87]

Are you operating at a flow rate that is considerably lower than design?

 

[kalamarhss]

Ok sorry for the misunderstanding.
We have a closed tank wih water at 45 Celcius. The tank is elevated 10 meters. It is in -0.8bar vacuum.
In the suction line there is a strainer and a buterfly valve.
A pressure gauge at the suction of the pump shows 0.2 bar.
Dos this mean I have NPSHa = 2 - vapor pressure?
Or NPSHa = 12 - vapor pressure?
To make things more clear. I think NPSH is absolute pressure. But manometers show diferential.
If you place a manometer in the suction of a pump that pumps from a n open tank at 1m elevation it will show 0.1 bar.
But the NPSHa will be 11m.

 

[kalamarhss]

Yes clay but according to the manufacturers curve we should be OK.

 

[TD2K]

NPSH is the additional pressure at the suction of the pump in excess of the vapor pressure or o.2 bar(a) minus the vapor pressure at 45C. I'm sitting here in a hospital waiting room so I have limited resources but you don't have close to 12m NPSHA. What is the NPSHR?

 

[TD2K]

Please specify your pressures as gauge or absolute. I assumed from your various posts the pressure at the pump suction was absolute but in looking at your numbers again it could be gauge and that makes a huge difference.

Don't be vague in your units.

 

[kalamarhss]

Ok I will open a new thread about the NPSH.
Lets asume that I am sure that NPSHa > NPSHr.
What else should I check if there is cavitation?

 

[kalamarhss]

Sorry TD2K.
Its just my English. I will be more careful.

 

[TD2K]

Okay but it's a key piece of information to your problem. What is the suction pressure? Are you measuring that on the pump side of the suction strainer?

 

[bimr]

NPSH calculation:

Atmospheric pressure = 10.2 m
Static head = 10 meters
Tank Gauge pressure = -0.8 bar = -8.2 m
Estimate pf vapor pressure of water = 0.18 m
NPSHA (net positive suction head available) = 10.2 + 10 - 8.2 - 0.18 = 11.8 m.

It looks like you have adequate NPSH, but this NPSH calculation does not include the headloss through the suction piping. The pipe headloss should be subtracted.

What is the diameter of your piping? You should be using 200 mm dia. pipe for this water flow.

If you only have 0.2 bar (2 meter water head) at the suction of the pump, you may have too much headloss in the suction piping. Is the strainer clogged?

 

[stanier]

Is the pump running in the right direction?

IS it cavitation or perhaps vibration? Vibration can occur if you are running too ar left of BEP.

Is the pump running at duty point? NPSHr increases exponentially past BEP.

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

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

 

[clay87]

How do you know you are 'cavitating'?

Is it noise? If so, what exactly does it sound like?

Is it vibes? If so, what does the vibration signature look like?

Is it pressure/flow instability? If so, please explain.

Is it impeller pitting damage? If so, where on the impeller?

 

[BigInch]

Straight suction piping?

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

 

[kalamarhss]

TD2K I am measuring after the strainer before the pump suction. The gauge pressure is 0.2 bar.
bimr The suction line is DN 450. does the matter that i am measuring gauge pressure change anything?
stanier yes the pump is running at the right direction. It is definitely cavitation due to the damaged impeller at the pressure side of the vanes. The pump runs at half the design flow .
clay87 It sound like there are rocks in the pump. Lote of vibration. What do u mean vibration signature. What is the vibratio signature of cavitation? There is no great instability. A little fluctuation. As I said the impeller is damaged at the pressure side of the vanes. There is pitting of the impeller.
Big inch. After the strainer there are more than5X D of straight suction piping meaning aproximately 5m.
I am pretty sure it is cavitation but the question is why when the curve says that there shouldnt be any. The NPSHr at that point is 4-5 m.
Thanks everyone for the interest.