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.

 

[TenPenny]

If you go down to 1750 rpm, you will never make the 200 ft head required. You will have to look at the pump curve to see what sort of operation you'll have, somewhere between 70- and 150 gpm. You could simply pop the pump apart and trim the impeller to suit. What model is the 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.

 

[pmover]

so the revised flow design condition is half of original design and what pump will deliver, yet maintaining the required head.

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

 

[BigInch]

There several trouble spots that come to mind.

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.

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[moltenmetal]

Wow BigInch, you really baby your pumps! Bet you get superb performance and lifetime out of them. But I know of plenty of applications where pumps are running far to the left of the BEP and have been for a long, long time. BEP is just that: the best efficiency point. If a pump's bearings/bushings/seal design is so fragile that running somewhere above min flow and below BEP will wreck the pump, I'd suggest buying another brand. To my knowledge, Goulds still makes good stuff. Remember this is a 2x1.5x 8, not a giant by any stretch.

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.

 

[BigInch]

Moltenmetal, I said he could use the same pump, if he wants to. BTW you make more of a case for buying a new pump than I do. That aside, I think if you had multi-million dollar babies installed at the End-of-the-World, you'd want to keep them as healthy as possible too. In fact, depending on location, a small pump can be almost as difficult and costly to fix as a large one. Big or small, principals are the same. No need to operate something poorly just cause its tiny.

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

 

[ScottyUK]

Running an over-sized motor means the motor starter will need to be oversized as the majority of starting surrent is reactive and dependant on the motor: bigger motors take more starting current. An over-sized motor will also run at a lower power factor because it will draw more magnetising current than a smaller motor. This will likely result in a power factor penalty from the utility if not corrected.

Depends if the downsides are outweighed by the benefits!


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If we learn from our mistakes I'm getting a great education!

 

[BigInch]

He's not using a bigger motor, just thinking about running it at a lighter load.

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

 

[TenPenny]

I'd like to know what model pump it is. As far as I know, it's not going to be an ANSI pump, so it's probably a Goulds commercial pump.

I would expect that min flow won't be a problem, but I'm not sure without the curve.

 

[21121956]

Hello everybody:

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.

 

[TenPenny]

Pop the pump apart, trim the impeller down to 6 7/8 dia. You can safely take min flow to be 50 gpm. Your motor load will be less than 7.5 hp, but that's no problem for a 15 hp motor.

Pump curve is on p7 of this

http://www.goulds.com/pdf/36-3756S.pdf

 

[21121956]

Hello everybody:

Well, I forgot to say that here and the surroundings the frequency is 60 Hz.

 

[ScottyUK]

He's not using a bigger motor, just thinking about running it at a lighter load.

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!

 

[BigInch]

21121956,

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.

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

 

[MortenA]

I was going to say what tenpenny says - he even supplied a pump curve (gave him a star)!

Best regards

Morten

 

[21121956]

Hello everybody:

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.

 

[BigInch]

WEll if that's true, 93 meters = 305 ft head and that's nowhere near the 200 ft you mentioned above. I thought something wasn't right. Look I'm a bit busy right now, so I'll get back to you on this a little later.

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

 

[21121956]

Hello everybody:

I made a mistake, the calculated total head on the pump is 472 ft = 144 m.

 

[BigInch]

OK, come back if you want to try again.

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:

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.