Pump & motor size selection

Given the flow and head requirements, it really should be a simple matter of reviewing pump curves from pump manufacturers. Based on your design criteria, you can identify a pump with a best operating point which is where your flow and head intersect at the pump curve where pumping efficiency is maximum. The pump curve would state pump efficiency and pumping break horsepower. You would then select a motor sized to handle the pump load. I was looking at some pump curves and from what I was seeing, you would need approxmiately a 75 hp or 56 kW (at least) motor. Motor efficiency is a function of the motor design and material. However, at that size, a premium efficiency motor should be at least 95%.

Pump manufacturers have a ton of information and should help you select a best sized pump. Below is a link to a U.S. manufacturer of pumps. It goes directly to their page where you can download their pump curves. They also have software and engineering support. Pump data is provided in both U.S. and metric units.

http://fhaspapp.ittind.com/BG-Literature-Detail.asp?LiteratureID=492&CatalogNum=46

Also, if you can not find the perfect pump size exactly for where you need it, I would suggest slightly over sizing it and installing a VFD. Then balance the system to determine the VFD speed to serve your requirements.

Hope this helps.

Michael

You also have to decide whether you want a non-overloading motor.

for a pump,

bhp = (gpm * tdh * G)/(3900 * eff.)

A rule of thumb for efficiency (if unknown) is 70% and the eqn. reduces to:

bhp = gpm * tdh/2800

Of course this is IP. I leave it to someone else to convert to SI.

bph = brake horsepower

gpm = gallons per minute

tdh = total dynamic head in feet

G = specific gravity for water (1)

eff = efficiency

If efficiency is unknown (you haven't selected a pump or pump curve is unavailable), then 70% can be assumed for an estimate of power requirement. The second equation (estimate) is rounded off (it would correspond to an efficiency of 70.7%).

Your question is a simple one and as a guideline, this is the process for arriving to an answer.

First of all arrive at Specific Speed nš = 3.65n × √Q / H¾

Q = Flow rate in m³/sec.

H = Head in m.

n = Speed in RPM.

Specific Speed of a pump impeller is the RPM to produce in Head at unit BHP under best efficiency conditions (i.e. capable of raising 75 kg of water to a height of 1m).

Specific Speed /Effy. range /Output range (lpm) /Remarks /Type

30 - 100 /59 - 60 /upto 12000 /Heads possible are upto 600m in multistage horiz. volute type./Centrifugal

100 - 160 /60 - 65 / / /Centrifugal

160 - 300 /65 - 70 / / /Centrifugal

300 - 400 /70 - 75 /upto 25000 /Medium heads mostly vert. turbine francis /Centrifug

300 - 600 /75 - 80 /2000 - 450000 /Lift upto 60m but in max. disch. range only upto 25m / Mixed flow

400 - 1800 /80 - 90 /50000 - 45000 /Low lift /Axial flow.

Once the specific speed is calculated, the type of pump is established, basis the above guideline, and water horsepower calculated by applying a suitable formula.(♥)

Finally apply the drive effy. 0.95 and motor effy. 0.85-0.95 to arrive at the motor horsepower.

(♥)Or you can get pump supplier's performance curve for a pump which meets your requirement, to arrive at motor horsepower for Best Effy. Point (BEP) plus some safety margin.

Pressure Head, Brake Horsepower, and Efficiency can only be determined experimentally. Manufacturers supply characteristic curves for these variables and even NSPH for every pump. It's then necessary for you to contact the manufacturers for proper sizing. All pumps have their own efficiencies built into them!

I agree fully with your in put here. However, to get an idea on type of pump that one is looking for, the guideline given, will help to narrow down the selection process. And hence, it is high-lighted here.

Ofcourse, the final pump performance data/curves comes from the manufacturers themselves, as you have rightly indicated. The perfomance of pump is established in their test-bed, after a trial and error procedure followed.

My experience is also, that even though the pump performance is established, but when one makes the technology transfer to any other place or country, a similar trial and error procedure needs to be followed again without any exception.

Calculating the hydraulic horsepower based on your flow and pressure I come up with a requirement of about 55hp. Different type of pumps have different head capabilities. If you are using impeller type pumps typically closed impellers are capable of higher pressures. Usually pump companies catalog their pumps by flow rate and head pressure. Actual horsepower may be a little higher because of the inefficiency of the pump.

Don't forget the material to be pumped.......this could change the type and build of the pump you need!

Why have PD pumps been ignored?

Are fluid characteristics and application description available?

You better tried a software from a company like B&G and Taco to name a few, try to select Pump curves just left from BEP (best efficiency point), if you like to know how to interpret CUrves from different pumps that depends on pump speed and impeller diameter (to name a few), if you want sent me a mail @ ffallas@climaideal.com and I'll show you different curves....

We are now in a 21st century......... just send your inquiry to the manufacturers or dealers of any branded pumps. just mentioned what is the services conditions.

or if you have a catalogue, anybody can size-up the pumps as the info are all mentioned.