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resizing of the impeller

resizing of the impeller

resizing of the impeller

Hi All
I'm changing impeller from 8 inches to 8.5 inches and I want to confirm the motor size of 20 HP is adequate or not?
I have an existing pump with an 8" impeller with a differential head of 242 ft and a flow of 220 gpm read from the curve with an efficiency of 64% and a motor eff of 91%, I get a 15.4 BHP
To double check the calculations, I went and took amps reading and read 13.4 amps. The full load amps is 18.4 amps
I used the actual amps reading of 13.4 and calculated the power by using the formula VxIxPFx1.73xEff/746 = 15.2 hp. The two numbers are pretty darn close..
Ok having done all that, I still don't know the system resistance when I can put the new impeller in service. With the new impeller in service the pump will try to put more flow and head and the system will be hydraulically balanced and the pump will ride the curve. supposedly I maintain the same flow of 220 gpm, the new head on 8.5 inch will be 290 feet. The new calculated power will be 18.5 hP
I've talked to the pump vendor and he confirmed that 20 Hp will be enough. The motor has a SF of 1.15
I want an expert opinion in this matter whether to change the motor or not?
Thanks in advance for your help

RE: resizing of the impeller

You missed out what the SG is of the fluid you're transporting.
From the pump curve your current operating point should be about 21 BHP in water. You state 15.4 so an SG of about 0.75 ( gasoline?)

Anyway your key problem assuming your fluid stays the same is an inability to estimate or calculate increased flow due to this increased differential head.
If you can't generate extra flow, why are you increasing the head?

So looking at the chart, you could operate your 8 1/2 " impellor up to say 300 gpm. The curves say this is 30 bhp and not far off max power at end of curve. At SG of 0.75 that's 22.5 bhp for your 20 hp motor.

Long term operation not good, but depending on the temperature of the cooling air maybe enough.
All depends what this SF actually means. I can only imagine it is some sort of short term limit otherwise he would sell you a 23 hp motor and not a 20.
Using your calculation the full load current ( the hint is in the name "FULL") comes to 20.8 bhp.
So you could be 10% short.

So you need to explain a bit more why the flow doesn't change or what the flow might be in the future.

From extrapolation of the 8 inch curve, using 0.75SG, the max pump power is approx. 20 bhp
It is generally good practice to fit a motor that has enough power to operate at any point of the curve as the vendor doesn't know what system the unit will be fitted to.

So in short, you might "get away" with not changing the motor if you can keep the flow rate to 260 gpm or less.

If not then I would change the motor.
Or buy a bigger pump and motor (might be cheaper)

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: resizing of the impeller

Assuming your SG is 1 or approximately so, I'd recommend at least a 25 if not a 30 hp motor. If I were your vendor, I'd tell you to use a 30 (assuming a 1.0 SG).

If your SG is actually .75, then 25 hp.

I never suggest running into the SF of a motor, the electrical guys do not like it.

RE: resizing of the impeller


I back calculated your SG as 0.73.

Assuming no static head on your system, your system will fit the curve H = 0.0068493 F^2
Where H is head in ft and F is gpm.

On the pump curve, you are just over 200 gpm and about 285 ft of head. Horse power requirements according to your pump curve are a hair under 25 HP. I agree with TenPenny, go with a 30 HP motor.

I used to count sand. Now I don't count at all.

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