Whenever possible the lowest centrifugal pump speed should be selected, as this will save wear and tear on the rotating parts.
Centrifugal pumps always pump somewhere on their operating curve, but should be selected to pump as close to the best efficiency point (B.E.P.) as possible. The B.E.P. will fall some where between 80% and 85% of the shut off head (maximum head).
Once the pump manufacturer has a clear idea as to the shape of your system curve, and the head and capacity numbers needed, he can then select the proper centrifugal pump. The shape of his curve will be pretty much determined by the specific speed number of the impeller.
The net positive suction head required by the pump (NPSHR) varies by the square of the number.
• Example: If the NPSHR at 1450rpm is 3m, what would be the NPSHR for the given pump if its speed was increased to 2900rpm
• A 3 metre NPSHR x 4 (22) = 12 metre NPSHR at 2900rpm
Fortunately, or unfortunately, as the case may be, high speed pumps are smaller and cheaper in initial cost. Besides taking up less space, in the case bimr mentioned above, the higher speed pumps would probably decrease in size by half, roughly costing anywhere from 40% - 60% less, although running at twice the speed, maintenance expense should probably be estimated to be twice as much, although the smaller parts for some components might lower that factor a bit.
What would you be doing, if you knew that you could not fail?
You can calculate the pump suction specific speed with the following equation:
Nss = ( N * (GPM)^0.5 ) / ( NPSH3^(3/4) )
Nss is generally constant regardless of the pump speed.
With this relationship, you can predict the effect on NSPH3.
NOTE:
1) GPM and NPSH3 are values determined at the pump's "BEP."
2) After a brutal H.I. meeting, NPSHr was renamed to NPSH3. The 3, in this case, is identifying the NPSH required based on a 3% head loss. Different sectors of the pump industry use different values. (i.e. NPSH3, NPSH1, etc)
gibson, That's true for a pump that is already purchased and installed.
bimr, At same flowrate, differential head and efficiency, power required is equal for pumps of all speeds, so I believe that point doesn't enter into the speed selection decision.
What would you be doing, if you knew that you could not fail?
It does fit in because it shows that since most of the life cycle cost of a pump is in power usage, it is important to procure the pump that has the lowest life cycle cost for your application.
The optimum pump would not necessarily be the pump that is smaller and cheaper in initial cost.
Also agree with bimr. I'm approaching this question in terms of "what can I put into a pump specification about running speed." My answer is, "absolutely nothing."
Needs to be looked at case by case, if you eliminate higher speed pumps then you will at some point miss out on a good selection for no other reason.
I forgot to respond to the NPSH question. The effect of pump selection on NPSH required is look at the potential curves, and pick one that has NPSHR characteristics that are suitable.
BigInch, you are coming at this with a "pumps already in the field" angle and I promise I read between all the lines in the first post, didn't get that scent.
Speed doesn't enter into the equation for power usage *IF* there are near optimal selections at various speeds, exactly at the BEP of the pump. I'd make the font larger on the *IF*, if I could.
Things like allowable operating region, end of curve considerations (max hp, NPSH), shutoff considerations (could be max hp if high specific speed, MAWP) and all of the maintenance requirements, should factor into each decision. Not just design point power consumption.
I just get the distinct impressions that he's trying to write a speed limit into a very generic pump specification. Inevitably, some bidders will play by the rules and offer slow/big/expensive machines, while a few will take exception and offer fast/small/cheaper machines. Then procurement will decide the speed clause in the spec wasn't really that important after all, when they can save 40% up front.
Realistically, that leaves you selecting pumps from only the vendors who like to cheat... Or, potentially selecting a pump with design point at 60% of BEP because that is the only thing available for the service at slow speed.
You know all this I'm sure. Nothing like a vauge question to spark an interesting discussion, right?
The answer is not as simple as selecting the highest pump speed so you can get the cheapest pump. Sorry, life is never simple.
A pump salesman may provide that answer. He would also be happy to sell you a replacement pump when the pump wears out quickly as well as repair parts.
The end user of the pump would not. Speed affects pump component wear and NPSH requirements, along with the head, capacity, and the pump size. High speed pumps cost less initially, but the maintenance costs can be staggering.
I seldom get a pump with a BEP exactly equal to my Q and changing its speed to match, if within +/- 5%, usually isn't going to make that much difference.
Yes looking at the question again it is actually too vague to waste so much time with it. At the time, I just thought there were not enough + in the high speed column and wanted to add some balance.
What would you be doing, if you knew that you could not fail?
