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?

 

[BigInch]

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/

 

[timmyc00]

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?

 

[timmyc00]

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?

 

[TenPenny]

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

Is the discharge into a pressurized line?

 

[timmyc00]

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

 

[TenPenny]

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?

 

[CH5OH]

choke the discharge valve.
the noise will disappeare

 

[timmyc00]

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.

 

[BigInch]

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/

 

[timmyc00]

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

 

[BigInch]

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/

 

[timmyc00]

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.

 

[BigInch]

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/

 

[timmyc00]

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?

 

[JohnGP]

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.

 

[Artisi]

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

 

[Artisi]

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

 

[BigInch]

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/

 

[mjpetrag]

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