A Fast Estimation Method of Clean Water Pump Power

Author of this article: Moexfish

The power of the motor equipped with the water pump is an important parameter of the water pump, and its calculation method is also relatively mature.

1. Conventional method

 The classic calculation method of motor power N(KW) is generally as follows:

N=KP=KPe/n=KpgQH/1000n(KW)

The meaning of each symbol in the formula is as follows:

P: The shaft power of the pump (KW), also called the input power, that is, the power transmitted from the motor to the pump shaft;

Pe: The effective power of the pump (KW). Also called output power, that is, the effective energy obtained from the pump by the output medium per unit time;

p: The density of the medium delivered by the pump (Kg/m3). Generally, the density of water is 1000Kg/m3 (specific gravity is 1)

Q: The flow rate of the pump (m3/s). When the flow unit is m3/h, it should be converted into m3/s;

H: the head of the pump (m); g: the acceleration of gravity (m/s2), which is 9.8m/s2;

K: The safety factor of the motor, generally 1.1～1.3;

n: The efficiency of the pump.

That is: flow × head × 9.81 × medium specific gravity ÷ 3600 ÷ pump efficiency

2. Simplification method (graphic method)

   The author often needs to estimate the motor power in his work, and found that although the algorithm of motor power is simple, the numbers in the formula are actually relatively long. There are 5-6 data that need to be entered in the actual calculation. Sometimes when pressing the calculator, there will be errors. The situation happened. Therefore, the author often calculates the result according to the formula first, and then presses it again as a review. In view of this situation,

 The author considered another idea and wrote it for your reference.

 Because the author's main project is a clean water pump, the specific gravity of the liquid is relatively fixed, so the flow and head of the clean water are made into an A4 size curve table (other liquids can be converted proportionally)

   The abscissa is the flow and the ordinate is the head (because the calculation is flow × head, it is also possible to change the horizontal axis to indicate the head and the vertical axis to indicate the flow). This chart uses logarithmic coordinates.

[Calculation example] The flow rate is 50m3/h, the head is 40 meters, the pump efficiency is 0.78, and the motor power is estimated

This chart can be used like this.

1. The flow rate is 50m3/h and the efficiency is 0.78. Find the point 50÷0.78≈65 on the horizontal axis;

2. Find the point 40 on the vertical axis;

3. Find the intersection of (65,40) and find that the intersection is below the line "7.5", which indicates that the shaft power at this operating point is slightly less than 7.5kw;

4. Therefore, it is reliable to use the 11kw motor according to this working condition point.

In this way, a certain calculation accuracy is sacrificed to estimate the motor power by means of graphs, but fortunately, the estimation speed is relatively fast, and the plan can be quickly planned without the help of electronic calculation tools, which can be used as a supplement to computerized tools.