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Cavitation or deadhead?

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?

55 psi = 127 ft total discharge head, including suction head.  Differential head is less than that, unless suction head is negative, which it apparently is by the 10 ft draw up from the pit.  So lets call that 137 ft and, you are right, its deadheading.

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?

**********************
"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?

(OP)
I shot an IR thermometer on the pump casing and it was around 72 F.  The NPSHa if the water is 70 F is...

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?

(OP)
I found an impurity in the water whose vapor pressure is about 7 psig at these conditions.  However, it is only in the range of about 0.1% of the fluid.  Can this boil out and cause cavitation?

RE: Cavitation or deadhead?

Has anything changed on this pump?  Any parts been replaced?

Is the discharge into a pressurized line?   

RE: Cavitation or deadhead?

(OP)
The problem I'm worried about isn't the deadheading, it's the cavitation.  I will worry about the deadheading later.  Apparently these problems have existed since the pump was installed

RE: Cavitation or deadhead?

The 3796 primes by mixing the air in the suction with the liquid that's in the casing, and pumping that liquid with entrained air downstream.

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?

choke the discharge valve.
the noise will disappeare
 

RE: Cavitation or deadhead?

(OP)
It is pumping something because the level stays constant even though there is water being dumped into the pit.  

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?

You said the center to center distance was 10 ft.  What's a center of a sump anyway?

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?

(OP)
it is 10' from the pump intake centerline to the water level

RE: Cavitation or deadhead?

When you let the level climb 1 ft and it was at disch 40 psig, was the level then at 11 feet from water surface to intake?

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?

(OP)
This is not a submerged sump pump.  When the level climbed 1 ft the difference between the level of the pump and level of the water was 9' instead of 10'.  Amps also went up on the motor as expected.

I wouldn't expect the intake streams to interfere with each other.

RE: Cavitation or deadhead?

Nearby walls and sump bottom can affect NPSHA, which is why I asked.

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?

(OP)
BigInch, I think the suction pipe might be too close to the bottom of the pit.  Do you have any resource with info about this?  This could be the source of cavitation, but can a pump cavitate with a severely throttled discharge valve?

RE: Cavitation or deadhead?

If the suction pipe is too close to the bottom of the pit, then that creates an additional resistance in the suction line, and will lower the available NPSH. It depends on how close as to how much resistance.

Usually throttling a discharge valve will lower the required NPSH, but if NPSHa < NPSHr, then you can get cavitation.

RE: Cavitation or deadhead?

How close is the inlet to the bottom of the sump?  

RE: Cavitation or deadhead?

Can you post a copy of the curve and a sketch of the layout - this might help get to the bottom of the problem
 

RE: Cavitation or deadhead?

I think the "bottom" of the problem is too close.

**********************
"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?

Here's an attached sketch.  The suction piping is 8" from the sump floor.  I noticed also that there is a 20' long 3/4" bypass line with a control valve down stream of the pump that goes back into the sump.  This was installed so the pump doesn't deadhead when the 3" main control valve (shown in picture inline of the pump discharge) closes.  However, even when the 3/4" control valve is full open and the main control valve is 40% open (doesn't close much more than this) no flow goes out of the 3/4" line.  It just trickles.  This is another reason why it is deadheading.  I'm thinking that the 3/4" line is too small and has too high of a resistance to flow compared to the main control valve with a 4" diameter, instead of the 3/4" line being plugged up with junk.

-Mike

RE: Cavitation or deadhead?

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

-Mike

RE: Cavitation or deadhead?

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/

RE: Cavitation or deadhead?

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?    

RE: Cavitation or deadhead?

(OP)
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.

RE: Cavitation or deadhead?

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.

RE: Cavitation or deadhead?

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


--------------------------------------------------------------------------------

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?

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

RE: Cavitation or deadhead?

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

 

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