Use of an oversized pump 3

[21121956]

Hello everybody:

For a particular project, somebody makes the calculations and selects a centrifugal pump with these characteristics: diameter of the impeller = 7¼, inlet Ø = 2", outlet Ø = 1½, speed and power of the electrical motor = 3500 RPM and 15 HP.

The BEP for this Goulds pump is around 150 GPM for a head of around 200 ft. The pump is already stored in the warehouse.

At present, some conditions have changed in this project and the new water flow is 70 GPM pumped to the same head.
The calculated power for the pump to work for this new condition is 12 HP.

The questions are: can we use the existing pump? I think there is no problem with an oversized power of the motor but, how about the speed? or do we need to change to an electrical motor for 12 HP and 1750 RPM?

Thanks in advance for your advise.

 

[BigInch]

I rechecked this setup with 300 m of 2", the original pump and your outlet pressures, etc. and it looks like the operation point will settle in on the system curve at around 100 gpm, pipe losses according to the Churchill equation. There may be some variation there, as I didn't actually plot the pump curve on the system curve and used no pipe fittings.

You could restrict the flow to 70 gpm by increasing the outlet pressure somehow, maybe by adding a valve or perhaps adding a nozzle will do the trick and get you a good outlet velocity. Pipe flow velocity is about 6 ft/sec.

No VFD, as you will lose the head you need if you try to reduced the speed.

I also get a Brake Horsepower consumed of 7 to 7.5 HP at the 100 gpm flowrate.

http://virtualpipeline.spaces.msn.com

"What gets us into trouble is not what we don't know, its what we know for sure" - Mark Twain

 

[21121956]

Hello everybody:

Thanks BigInch for your help or better I say "muchas gracias". For you and TenPenny, a star.

Nuevamente, gracias.

 

[Artisi]

I haven't checked the curve but doubt that this pump will achieve 144m head.

 

[BigInch]

Artisi you're right. It won't. But 143 m doesn't look to me like it is needed.

Rechecking pump curves, I also see I was looking at the 8" impeller to get the 100 gpm. (I shouldn't try to do two things at the same time).

A 7" impeller will get right about 65 gpm with a system discharge of 220 ft of head. The suction pressure adds enough head to the differential to reach it.

http://virtualpipeline.spaces.msn.com

"What gets us into trouble is not what we don't know, its what we know for sure" - Mark Twain

 

[quark]

I got about 136ft, by 2-K and 3-K methods, including 12.5m (41ft) elevation with the revised conditions. Still, have a chance to reduce the speed by 17% and then allow the pump to run on its curve, by controlling the flow (it is 124gpm at 83% speed).

The reduced speed curve can be considered equivalent to a 6" dia pump and the head capacity at 70gpm is about 150ft. Got a BHP of (about) 5.