Cavitation or deadhead?

[timmyc00]

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?

 

[mjpetrag]

sorry posted under my other computer's logon (timmyc00 is my laptop)

-Mike

 

[BigInch]

The recirculation line wouldn't normally have to be open until the 3" is probably around 10-20% open. The 3/4" line could have a maximum capacity of around 50% of BEP flow, if it was only needed for starting up and stopping the pump. If its there for extended recirculation when the 3" might be closed for long times while the pump continues to run, then it would have to be sized for full BEP flow. Undoubtedly too small for that. Probably just intended for starting.

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/

 

[Artisi]

Do you have a performance curve - seems something is amiss with your 55psi and the 80 GPM shown on your sketch.
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?

 

[timmyc00]

the water freefalls onto the surface on the sump, but it falls about 3' in front of the suction line. The perforance curve i have is for the goulds 3796 3x3-13 with a 12 1/8" impeller and 1770 RPM. Looking at the curve it seems like i am right on the pump curve at 137' head at 80 gpm.

 

[Artisi]

If the water free-falls into the sump you are probably driving a lot of air into the water which aerates the sump, this puts the pump "off-prime" meaning it is constantly re-priming which puts it off performance and making it noisy in operation. Can you re-arrange the inflow and pipe it in below the water level.

 

[mjpetrag]

I found this bit of advice online...looks to be just what I have because I opened the discharge valve wide open, discharge pressure dropped and cavitation went away...weird right?


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

 

[Artisi]

No - not weird. The reason why a curve was requested to see what was going on with the pump.

 

[flexibox]

Is the pumps motor wired up corectly? Is pump rotation correct direction?