Cavitation or deadhead?
Cavitation or deadhead?
(OP)
I have a sump pump pumping from a pit below the suction line with a center to center height difference of 10'. The pump is a Gould's 3796 MTX 3x3-13 with a 12 1/8" impeller running at 1765 RPM on a 20 hp motor. The service is water at 70 deg F. The discharge pressure directly downstream of the pump is 55 psig. According to the pump curve, the pump is deadheaded with a differential head of 138 ft (not including minimal line losses). However, the pump sounds like it is severely cavitating. The pump was designed for a differential head of 105 ft. If there was a blockage in the suction line, this should not increase the discharge head. My first guess is air ingestion, but the suction piping is new. Has anyone experienced something similar to this problem?





RE: Cavitation or deadhead?
Something is blocked. If your suction is blocked, or for that matter, if discharge is blocked, and you still have fluid in the pump, you will see deadhead pressure.
If you had air, you would get almost no head. Pumps don't work well with air, which is why they invented compressors.
BTW, is it getting hot around there?
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"Pumping accounts for 20% of the world's energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies) http://virtualpipeline.spaces.live.com/
RE: Cavitation or deadhead?
34 - VP of water@70 F - 10' ~ 23'. The NPSHr of the pump is 10'.
To get to 10', my water temperature would have to be 170 def F. That is the only temperature I see it being able to cavitate. Can deadheading a pump create this much of a temperature spike? I'm thinking not, but I'm not sure.
Can the suction and discharge both be blocked, leading to cavitation and deadheading at the same time?
RE: Cavitation or deadhead?
RE: Cavitation or deadhead?
Is the discharge into a pressurized line?
RE: Cavitation or deadhead?
RE: Cavitation or deadhead?
If you're deadheading, the pump will never be able to evacuate the air, because it can't go anywhere.
Is the pump actually pumping anything at all?
RE: Cavitation or deadhead?
the noise will disappeare
RE: Cavitation or deadhead?
I let the level in the sump climb up about 1 foot by shutting off the pump, then turned the pump on and it discharged at 40 psig (which it was designed for) and wasn't cavitating. Once the level was pumped down to normal after 30 seconds later, the pressure climbed back up to 55 psig started cavitating. The discharge valve of the pump is controlled by the level in the sump. I was thinking of just readjusting the settings, but the previous issue still isn't answered.
RE: Cavitation or deadhead?
The needed dimensions are sump water level to intake and intake to pump centerline.
**********************
"Pumping accounts for 20% of the world's energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies) http://virtualpipeline.spaces.live.com/
RE: Cavitation or deadhead?
RE: Cavitation or deadhead?
What do you think the flowrate was before the problem started? Estimate that by the sump level differences.
Are the distances from intake to bottom of sump and intake to side of sump sufficent to prevent interference with intake streams?
**********************
"Pumping accounts for 20% of the world's energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies) http://virtualpipeline.spaces.live.com/
RE: Cavitation or deadhead?
I wouldn't expect the intake streams to interfere with each other.
RE: Cavitation or deadhead?
I think its possible that the impurity with a vapor pressure of 7 psig may affect the NPSHA. I don't think the resulting vapor pressure is so very dependent on what the exact quantity is. Its more like, if it can boil off, it will. Maybe a chemE can give us an idea about that.
**********************
"Pumping accounts for 20% of the world's energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies) http://virtualpipeline.spaces.live.com/
RE: Cavitation or deadhead?
RE: Cavitation or deadhead?
Usually throttling a discharge valve will lower the required NPSH, but if NPSHa < NPSHr, then you can get cavitation.
RE: Cavitation or deadhead?
RE: Cavitation or deadhead?
RE: Cavitation or deadhead?
**********************
"Pumping accounts for 20% of the world's energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies) http://virtualpipeline.spaces.live.com/
RE: Cavitation or deadhead?
-Mike
RE: Cavitation or deadhead?
-Mike
RE: Cavitation or deadhead?
Not sure but 8" seems too close. Might be picking up junk off the bottom too.
**********************
"Pumping accounts for 20% of the world's energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies) http://virtualpipeline.spaces.live.com/
RE: Cavitation or deadhead?
This pump is capable of a much greater flow at 55psi than 80 gpm.
The inlet pipe 8" from the bottom of the sump should be Ok.
How does the flow enter into the sump - piped into the sump below the water level of the sump or does it "free fall" onto the surface of the sump?
RE: Cavitation or deadhead?
RE: Cavitation or deadhead?
RE: Cavitation or deadhead?
Discharge Cavitation
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Discharge Cavitation occurs when the pump discharge head is too high where the pump runs at or near shutoff.
Symptoms
1. The pump sounds like it is pumping rocks!
2. High Discharge Gauge reading
3. Low flow
Causes
1. Clogged discharge pipe
2. Discharge line too long
3. Discharge line diameter too small
4. Discharge static head too high
5. Discharge line valve only partially open
Remedies
1. Remove debris from discharge line
2. Decrease discharge line length
3. Increase discharge line diameter
4. Decrease discharge static head requirement
5. Install larger pump which will maintain the required flow without discharge cavitating
6. Fully open discharge line valve
-Mike
RE: Cavitation or deadhead?
RE: Cavitation or deadhead?