Pump speed selection
Pump speed selection
(OP)
How is the pump speed selected?What is the effect of selection on NPSH required?
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS Come Join Us!Are you an
Engineering professional? Join Eng-Tips Forums!
*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail. Posting GuidelinesJobs |
|
RE: Pump speed selection
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.
http://www.mcnallyinstitute.com/05-html/5-12.html
http:
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
RE: Pump speed selection
What would you be doing, if you knew that you could not fail?
RE: Pump speed selection
http://www.youtube.com/watch?v=GwS6SS55ZnE
RE: Pump speed selection
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)
RE: Pump speed selection
RE: Pump speed selection
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?
RE: Pump speed selection
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.
RE: Pump speed selection
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.
RE: Pump speed selection
Speed does not enter into the equation for power usage, for one hour, or for the entire life cycle.
Torque is another story. A lower speed pump will have more available torque.
What would you be doing, if you knew that you could not fail?
RE: Pump speed selection
I simply listed the general advantages of choosing a smaller pump running at 3600 RPM vs a pump appx 2 X the size running at 1800.
What would you be doing, if you knew that you could not fail?
RE: Pump speed selection
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?
RE: Pump speed selection
"How is the pump speed selected?"
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.
RE: Pump speed selection
RE: Pump speed selection
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?