Centrifugal pumps horsepower

[Echo82]

Hi everyone

How can I practically calculate centrifugal pumps horsepower and size the electric motor driver? including all mechanical losses of the pump and coupling and all other factors related to electric driver.

 

[rutherford703]

Brake horse power, KW=Q (m3/hr)* SG* H (m)*9.8/3600/Eo
Eo pump efficiency including mechanical loss.

Sizing the electric motor with a safety factor of 125

 

[bimr]

From Pump Handbook by Karassik et al. McGraw-Hill, Section 3.1, Power Pumps.

In USCS units:

bhp = (Q×Ptd)÷(1714×μ)

Q = delivered capacity (including gases and solids) in gpm
Ptd = developed (differential) pressure, psi
μ = mechanical efficiency.

http://www.mcnallyinstitute.com/12-html/12-11.html

online calculator:

http://www.pumpcalcs.com/calculators/view/73/

The rated horsepower of the motor necessary to produce the desired brake and water horsepower.

Motor HP = bhp/ Motor Efficiency

Where Motor efficiency is 85-90%

 

[BigInch]

1.25 is too big a factor for oversizing the motor.
Motor size should generally not be more than 10% higher than actually required, however the final value (maybe as high as 1.15) will depend on what you wind up with when you select what you actually need from the standard frame motor ratings.
85 to 90% efficiencies for a motor are pretty low these days. Motor efficiencies on new energy saver types built in the last 10 years can reach max values 92 to 95%.

What would you be doing, if you knew that you could not fail? Ans. Bonds and derivative brokering.

 

[bimr]

Standard electrical motors constructed according NEMA Design B meet the efficiencies below:

Power
(hp) Minimum Nominal Efficiency
1 - 4 78.8
5 - 9 84.0
10 - 19 85.5
20 - 49 88.5

so go with 90%....

The NEMA Design B is what you will get unless you pay extra for a premium motor.

 

[rmw]

I'd say you better look at your pump curve and your system resistance curve and figure out how far on the curve you pump is going to have to pump if the system resistance is less than what you design for or if something happens or changes and size for the maximum flow condition that the pump will see. Then build in what margin you want and pick from standard ratings as recommended above.

Unless, of course, you WANT your motor to trip on overload if it runs out past the design point.

rmw

 

[Artisi]

You have 2 things to consider, 1 - the power at the operating point and 2, the motor size required to cover any upsets to the system, ie running out on its curve.

If using metric units the motor size in kW is simply
Flow (L/s) x Head (metres)x SG (1 for water) / 102 / pump efficiency.
The motor size selection is an engineering consideration based on a number of considerations for which you need the system curve, the pump curve and the operating conditions of the overall system.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)