changing speed of pump 1

[micy]

Hye,
I have a 40" centrifugal impeller in a slurry pump.
Sometimes, in case that lower duty required, we change to 38" impeller.
I want to install speed reducer, instead of impeller changing.
What should be the speed reducing, so pumping will be equivalent to the one with 38" impeller?

 

[insideman]

The two impellers cannot be exactly geometrically similar if they both must fit in the same casing. So similarity laws would not be exactly true.

But head should be proprtional to size squared. So the 38 inch impeller would give
(38/40)^2 times the original head at the same rpm.

 

[quark]

Micy,
Considering energy consumption you may not get much benifit but in operational point of view it will be of great use. The capacity of the pump is directly proportional to the diameter and speed of the pump. So the new speed of the pump will be (38/40)times speed.

But it's always better to check the actual duty point at which the pump is running. Then you may further reduce the speed and can conserve some energy.

 

[MortenA]

Before considering freq. control you should perhaps be made aware of the laws of affinty for pumps:

n: normal
x: new speed
Q=flow
H=head
P=power

figure out the units youself: Strick SI units will work with no "factors". If you use american/brithish unit you will need some factors)

Q_n/Q_1=n/x
(H_n/H_x)^2=(n/x)^2
and
(P_n/P_x)^3=(n/x)^3

This is with the assupmtion that the efficiency remain constant. In practise this is _NOT_ true but normally for small changes its not important.

From these formulas you can see that a different speed corresponds to parralel shifting of the pump curve. Since your sysm resistance curve remains constant you should be able to select the new speed grahpically.

Best Regards

Morten

 

[sprintcar]

Also remember that changing the running conditions of a slurry pump can change the wear life of the parts. You might want to check w/ your pump vendor for recommendation.

 

[macm]

since your application is working with both the 40" dia and the 38" dia, it would seem that you are generating sufficient head with the 38" impeller. Hence, as previously mentioned, you are looking at a volume reduction of 38/40 or about 5%, and that is what your desired speed change should be. Note, you could run with the 40" impeller and slow down, or with the 38" impeller and speed up. However, the impeller and suction plate wear will increase with the speed up option.
good luck
mac

 

[PacificSteve]

Are you thinking of a VFD or some mechanical speed reducing system, perhaps fixed? If VFD, you have complete flexibility of what % speed to run at, and you can easily tune your pump to system performance requirement at any point in the future. These are expensive, but compared to swapping impellers, should pay back quickly.

I wouldn't assume that the 38" impeller was perfectly chosen for your low demand condition, so trying to match this exactly with a calculated speed reduction may simply be an engineering exercise where application of infinity laws is imprecise.

Steve

 

[315636]

System and pump charecteristic determines the operating point of your slurry pump. If there is a significant static head in your system. Pump efficiency may decrease. I will not advice VFD because you will be loosing 4-5% to cover the VFD losses.

 

[PacificSteve]

To 315636: Thanks for the feedback. My rule of thumb, coming from VFD representatives is to expect 1 to 2% loss in the VFD. I would be real interested in seeing or hearing of any real world experience (from recent years, considering improvements in drives) that would show the rep's claims to be a little too optimistic.

What do you think about what I call "lack of a magic number" in terms of pumping capacity needed? i.e. neither 40" or 38" is a magic number, but puts the process somewhere close to where it needs to be, and always higher if the system works at all. If the real process demand were 5% less in each case, slowing down the pumping rate GPM would lessen the head losses associated with the flow, perhaps paying for the VFD losses. Just an idea. We don't pump too much slurry here in Hawaii, perhaps the viscosity would mean a limited effect on flow head losses, compared to flow rate....?

PacificSteve

 

[MortenA]

315636

Generally advicing against VFD becuase it uses 3-5% of when may installtions throttles 20-30% of the energy seems like a bad advice. The question deals with a situation where a 40" impellar can be replaced with a 38" implellar. A rough estimation of the saved power would then be: (1-(38/40)^3)*100%=14% For such a large installation this could be serious money.

Best Regards

Morten

 

[raj2001]

I have heard of the VS coupling and the KS coupling. It is an electrical componenet which is installed between the motor and the pump. I have seen it being used during start up of the slurry pump. They keep the speed minimum thus increasing the torque and reducing the power required for start up. I dont' exactly know whether it will be useful for your purpose. May be somenone can help you at this point.

all the best.

 

[Slagathor]

Pacific Steve,

Don't let the VFD guys pull the wool over your eyes. That was the efficiency for the switching portion of the drive only. The transformer losses to go from AC to DC are not included in the 1%-2% losses they talk about. The best drive effs I have seen are about 96%.

I worked on a project with a 2000 HP VFD where there were LD clauses for unit (pump/motor/drive string) efficiency. I had one VFD mfg claiming 96.8%. I told them that if the overall pump/motor/drive eff was off (at full speed), we would pull out the drive, run the unit across the line, and measure the eff again. If the efficiency rose by more than 1/96.8% I would hit him with the associated penalty. Suddenly the guaratee dropped by 1.1%!

 

[PacificSteve]

Slagathor,

Thanks for the tips. Its like pulling teeth to get good info from the manufacturers, but I like your strategy.

PacificSteve