Use of an oversized pump
Use of an oversized pump
(OP)
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.
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.





RE: Use of an oversized pump
You can't buy a 12 hp motor, typically, so keep the 15. However, keep in mind the motor puts out the power required by the pump - therefore, using a 15 hp motor when 12 hp is required means that the motor will only draw enough electricity to put out 12 hp, you aren't wasting 3 hp.
RE: Use of an oversized pump
i doubt very much the existing pump will satisfactorily operate at the revised design conditions, but you need to investigate the performance curves. you need to determine minimum flow conditions and determine whether the pump can continuously operate at these conditions.
trimming the impeller will result in a reduction of the head delivered by the pump.
fyi, if you provide the goulds model, the performance curves can be viewed online by other participants. thus provide you with a better answer.
good luck!
-pmover
RE: Use of an oversized pump
With the new flowrate at 50% of the rated flow of the pump, this would normally be considered borderline for continuous operation at constant speed. Unbalanced forces at that flow will undoubtedly cause maintenance issues to arise with seals and bearings. Normally you would want to keep pump flows within 70-100% of the BEP.
Using a 12 HP motor will not help the situation. A 15 HP motor will be running at 80% load, which isn't all bad (it will probably even run cooler than before), and it won't draw any more power than the 12 HP needed to move the fluid to 200 foot head anyway, so no energy savings there and consequently no overpowering reason to change the motor that I can see.
Using a VFD is a problem, since you still have to lift the fluid to 200 ft head. This pump running at half speed will only generate about 140 ft of head, so that option seems to be out the window too, as I doubt you can fit a larger diameter impeller in the same casing that would be needed to generate 200 ft @ 1/2 speed.
I'd suggest you buy a new pump, but if you must use this same pump, use it as is with a control valve to cut the flow, just be prepared and buy some spare seals and bearings to keep in the warehouse. If its not continuous duty, you may be able to get enough calendar time out of them that it won't be too much of an inconvenience.
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
RE: Use of an oversized pump
Then again, you're going to waste a few horsepower, which is going to amount to some money in operating cost over the years if indeed the duty is continuous. A couple horsepower isn't chump change unless your plant has a cogen plant and gets its electricity from waste heat etc. A pump this size isn't too expensive capital-wise nor is it very long delivery. What do you pay for electricity? What's the payback on a new pump that is sized/trimmed to suit the new duty, operating closer to its BEP? Get out the Goulds catalog, select a "right sized" model, get a price on it, and do the arithmetic.
RE: Use of an oversized pump
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
RE: Use of an oversized pump
Depends if the downsides are outweighed by the benefits!
----------------------------------
If we learn from our mistakes I'm getting a great education!
RE: Use of an oversized pump
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
RE: Use of an oversized pump
I would expect that min flow won't be a problem, but I'm not sure without the curve.
RE: Use of an oversized pump
Thanks to all for your valuable comments. The pump is a Goulds, model 3656 5BF. The motor is 15 HP, 3500 RPM, V = 230/460, eff = 87,2.
RE: Use of an oversized pump
Pump curve is on p7 of this
http://www.goulds.com/pdf/36-3756S.pdf
RE: Use of an oversized pump
Well, I forgot to say that here and the surroundings the frequency is 60 Hz.
RE: Use of an oversized pump
I'm addressing your comment that there is nothing gained by using a right-sized motor (12HP) instead an over-sized one (15HP), when in fact there are benefits and downsides.
----------------------------------
If we learn from our mistakes I'm getting a great education!
RE: Use of an oversized pump
How'd you arrive at 12 HP needed? If its only 200 ft elevation change, without appreciable friction loss from piping, that's 3.5 hydraulic HP and at this pump's flowrate you have 50% efficiency, so it is as TenPenny says, its only about 7 Brake HP, not 12. Are there frictional flow losses too? How long a 1.5" diameter pipe do you have? Assuming the frictional losses go down with the lesser flow, you're probably ok, but still curious about the 12.
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
RE: Use of an oversized pump
Best regards
Morten
RE: Use of an oversized pump
BigInch,
The details of the project are as follows:
Static suction head = 7,55 m
Static discharge head = 12.5 m
Total static head = 4,95 m
Flow = 70 GPM
Suction pipe length, Ø 2½ = 28 m
Discharge pipe length, Ø 2" = 564 m
The fittings, Ø 2" are:
Elbow 90º, standard = 56
Tee = 14
Swing check valve = 1
Shut off valves = 18
Universal joint = 10
The purpose of this project is to obtain a water jet for wash cooling radiators.
The calculated Dynamic total head on the pump is around 93 m, the pressure at the outlet of the discharge pipe is 4,25 bar, creating a pressure drop of 4,64 bar.
RE: Use of an oversized pump
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
RE: Use of an oversized pump
I made a mistake, the calculated total head on the pump is 472 ft = 144 m.
RE: Use of an oversized pump
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
RE: Use of an oversized pump
I beg your pardon for the lack of information given to you in my query.
All that accesories and fittings, length of discharge pipe are for the entire installation constituted by several branches but, these branches will not be in service simultaneously but one at a time.
The longest branch from the pump has a length of 300 m, 10 elbows 90º standard, 4 Tees, 2 shut off valves.
RE: Use of an oversized pump
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
RE: Use of an oversized pump
Thanks BigInch for your help or better I say "muchas gracias". For you and TenPenny, a star.
Nuevamente, gracias.
RE: Use of an oversized pump
RE: Use of an oversized pump
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
RE: Use of an oversized pump
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.