Centrifugal Pump Suction Problem 1

[rarmour]

Hello all,

What types of suction problems could you see in a pump design as follows:

1. Underground sump containing water approx. 13-14 feet down.
2. Pump 4-5 feet away from hole going into sump (operating around 2250 rpm).
3. Motor is 15 hp.
4. NPSH (required)= 9.3 feet.

Attached is a photo of the pump. Let me know if more information is needed. Thanks!

 

[pennpiper]

It appears that the pump has been modified from a direct drive to a reduced speed configuration. Is that correct?

Was that modification discussed with and approved by the pump vendor?

Does the new modified RPM of the pump still allow it to perform properly for the service and installation?

Have you had the Vendor Service Representative out to the plant to evaluate the situation?

 

[BigInch]

and what's the fluid and temperature?

**********************
"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]

To give you a definitive answer to your question a pump curve will be needed which give all performance data at 2250rpm operating. This information will allow a closer study of the "suction" lift capabilities and the likelyhood of successful operation under the conditions you have.

Don't forget BigInch's request - this could be very important info.

Appears to be a somewhat neglected and haphazard installation - what is the condition of the pump internals -also an important consideration for a "wet-selfpriming" pump.

 

[wimple]

Hi

Just question, why you say speed reducer and not speed increaser?
How would you recognize the position of pinion if it is on top or bottom?

Thanks
Rgds
Wimple

 

[MartinSr00]

This looks like a self priming pump

If the piping isn't gas-tight above the water line, you could have trouble, since you have negative guage pressure in the line.

Maybe they slowed it down to get better NPSH performance. Remember, NPSH also follows the affinity laws.

 

[pennpiper]

wimple,
You wrote:
"Just question, why you say speed reducer and not speed increaser?
How would you recognize the position of pinion if it is on top or bottom?"

Well of course I do not have "X-Ray" vision but I have some smarts.
When you look at the posted picture you see that there is a motor mount that is not now being used. You see that the new motor position is off to the left side of the pump base (viewed from the rear) and at a much lower level relative to the pump center line. Therefore this pump is no-longer a direct drive pump.
So how is this pump connected to the motor?
There are two simple methods to do it and both are consistent with the pictorial evidence.
The two methods are Spur Gear Drive and Sprocket and Chain Drive.
If you look at the picture you see that the "New" coupling cover has very little distance from the motor shaft hole to the edge/bottom indication a small wheel/gear/sprocket. And the cover has a much larger distance from the apparent center of the pump shaft to the edge/top indicating a much larger wheel/gear/sprocket.
The small wheel will turn multiple times for each one turn of the larger wheel.
Thus, in my opinion I think the pump is is running at a much slower speed that when originally manufactured.

 

[rarmour]

Pennpiper - Excellent information, thank you!! I did not even notice the original motor mount; this reduced speed configuration should account for the suction problems if the new RPM value was not meant for the pump to perform properly. Thanks again.

BigInch - The fluid is water and the temperature is 70 degrees F.

 

[Artisi]

pennpiper - Ok. maybe a reduction drive but what speed is the motor 3600 /1800 rpm?

Still need a curve for the operating speed to decide if the pump is capable of the duty you require.

 

[MartinSr00]

But if it's a centrifugal unit, reduced speed should improve suction performance, all other things being equal.

NPSH should reduce by the square of the speed.

Of course, so will pump todal dynamic head. If that's too low, you don't have an NPSH problem, you have a pump curve/system curve problem.

The original post was about suction performance, though.

 

[TenPenny]

I don't know why the discussion about reduced speed is coming up - that looks like a direct drive converted to a belt drive, which may well have been done to get the 2250 rpm speed (OP mentioned 2250 rpm, which isn't going to be a direct drive speed).

That looks like an old AC pump, what is that, a Trash Hog?

Someone needs to look at the belt drive, and check what the operating speed will be, before basing any decisions on that.

 

[Artisi]

TenPenny- correct, all guess work at the moment and going round in circles without definite data.
It's now up to the OP - if he / she can't get us info. how can we help?

 

[sunloco]

we'd better check the performance curve,aslo suction lift,acutally many small size self-priming pump can't achieve good suction lift at low speed,especially you mentioned 13-14 feet down and 4-5 feet away.from pictorial pump it looks like " trash hog" pump, is there any solids in the water liquid?

 

[pennpiper]

Rarmour, have you had the time to check on the original pump speed and the current pump speed with the different motor and the speed reduction configuration?