Required Motor Size (KW) 2

[fadi123]

I always depend on the following equation in calculating the required KW for the motor of the pump (Pumped Fluid : clod water):

KW (Absorbed power) = (H x Flow)/ Pump Eff. x 367

Motor power = P abs. X Motor Eff.

Then I check with the suppliers the availabe motor size that close to my calculation and I CHOSE one with higher rated power. Inaddition I ask for 1.15 SF ,

Do I need to be more strict in the calculation ? is there are any other points I forget to consider in may calculation to the KW?

 

[Artisi]

A couple of points you need to clarify,
1. what are the units used for Q and H
2. are you using the H and Q at duty point (operating point) for you calculations or some arbitrary point on the pump H/Q curve or the end of curve H/Q.

The correct motor selection is really an engineering exercise based on a number of factors which are dependant on the pump operating conditions and any upset conditions likely to be encountered.

 

[fadi123]

Thanks Artisi,

1. The units that I used are : Q = m3/hr & H in meter to get the power in KW.

2. I use the H & Q at the duty point

 

[25362]

The general formula for the power input to a liquid is expressed by the formula:

P = [ρ].Q.g.H/[η]​

When Q is in m³/h and H in m, g = 9.81 m/s², the liquid power in kW, is

P = [ρ].Q.H/(367.[η])​

where [ρ] is the density in kg/L (=1 for water).
367 = 3600 (s/h) [÷] 9.81 (m/s²).

Thus your formula is OK.

In all respects proceed according to Artisi's message.

 

[timbones]

The duty point is almost never where the maximum power point is. What happens if your head rise is much lower than expected under some conditions? Is the power required for the particular pump you have higher at the higher flow condition? Is the motor suitable for operation at this higher flow condition?

 

[pumpking]

When we select pumps/motors we use 25% SF for motors up to 11kw, 15% SF from 15kw to 30kw, and 10% thereafter.

It is better to have motors sized in this was as their optimum efficiency point is between 75-90% of their rated power.

Some Engineers still like to go end of curve on the selection, this is OK, but leads to inefficient motors costing more to run than correctly sized motors, and also much more expensive as larger motors are required to cover a theoretical end of curve situation. Also, the pump will only operate to the system curve, and i have come across instances where it will be physically impossible to run end of curve, but still massive motors are used "just in case"

It once again underlines the importance to have a pressure gauge fitted at the pump discharge to check where exactly the pump is operating on its curve - once you know this, you can make further tweeks to ensure you get optimum pump life from the pump assembly.

 

[PumpSmart]

Fadi123,
You must calculate: Rating Power at Rated point, Run Out
and see Pump Performance for Max Rating Power.
Standard as API610 10th for centrifugal pumps talks about oversizing Power ratings as follows KW< 22 add 125%, 22 to 55 add 115% > 55 add 110%.
There are two more options:
-In case you will expand your capacity in near future, you should consider future parameters to sizing motor.
-In case you need a operation flexibility, you should consider to sizing motor to cover all recomendend operation region in performance curve for the same trimmed impeller diameter.