Pump Formulas
In order to make the most common pump formulas, we have created an overview of pump calculations with a brief explanation on each. All mentioned formulas are based on theoretical pumping technology and are given as a simple aid for making pump formulas. In practice, additional factors can have an influence and may lead to deviations from theoretical values. If you have any questions or are unable to solve your issue, please contact us for personal pump advice.
Centrifugal Pump Power Formula
The pump power is shown in the pump curve chart or in the cutsheet. The required pump power, also called shaft power, is given in brake horsepower – bhp – and can be easily calculated using the following pump power formula: P = (Q x H x SG) ÷ (3960 x η)P = pump power bhp
Q = pump flow gpm
H = pump head feet
SG = specific gravity of the fluid kg/dm3
η = pump efficiency %
3960 = constant factor
Pump Power Formula Example
We are using a BA150E D285 for pumping water at an operating point of 1600 gpm @ 90 feet. The pump efficiency at that point is 80 % according to the curve chart. The specific gravity of water is 1 kg/dm3 (62.4 lb/ft3).| BBA Pumps Model | BA150E D285 |
| Operating point example | 1600 gpm @ 90 feet |
| Specific gravity of water | 1 kg/dm3 |
| Pump efficiency at operating point | η is 80 % |
| Q x H x SG = 1600 x 90 x 1 | 144,000 |
| Constant factor x η = 3960 x 80% | 3168 |
| Pump power = 144,000 ÷ 3168 | P = 45.5 bhp |
Centrifugal Pump Affinity Laws
The affinity laws for centrifugal pumps express the relationship between the several variables involved in pump performance. The below calculation applies to centrifugal pumps and gives a good indication of the differences in pump capacity, pump head and absorbed pump power when changing the pump speed but with the impeller diameter held constant.- The capacity changes in proportion to the pump speed: Q1 ÷ Q2 = N1 ÷ N2
- The head is proportional to the square of the pump speed: H1 ÷ H2 = (N1 ÷ N2)2
- The power is proportional to the cube of the pump speed: P1 ÷ P2 = (N1 ÷ N2)3
Q = pump flow gpm
H = pump head feet
P = pump power bhp
N = pump speed rpm
Pump Affinity Laws Example
We are using a BA150E D285 for pumping water. The pump curve chart shows the maximum pump speed of 1900 rpm. With that reference pump speed we can easily calculate the performance at the proposed speed of e.g. 1500 rpm.| BBA Pumps model | BA150E D285 |
| Proposed operational speed N1 | 1500 rpm |
| Reference pump speed N2 | 1900 rpm |
| Calculated ratio | 1500 ÷ 1900 = 0.79 |
| Operating point Q1 @ H1 | 1600 gpm @ 90 feet at 1900 rpm |
| Q1 x 0.79 = Q2 | Q2 = 1600 gpm x 0.79 = 1264 gpm |
| H1 x (0.79)2 = H2 | H2 = 90 feet x 0.62 = 56 feet |
| P1 x (0.79)3 = P2 | P2 = 45.5 bhp x 0.49 = 22.3 bhp |
| Calculated point Q2 @ H2 | 1264 gpm @ 56 feet at 1500 rpm |
Pump Sizing Formula
In order to make the optimal formula of the size of the pump requires extensive knowledge about liquids and pump technology. One of the key considerations in optimization is to choose an operating point at Best Efficiency Point in the pump curve chart. Though, our portable pumps are supplied to operate over a wide range, even at the end of the rating curve. If the NPSH available is sufficient to prevent cavitation, the pump will give satisfactory operation.Please contact us and let our experienced pump experts advise you on the best pump for your application. Our experience can save you money and its given free of charge.