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It depend on the application and your requirements.
You must know the pumping medium, and your system design.
calculate flowrate, required head and friction losses.
In plumbing field there are two types of application; one is a lift pump transferring water from the underground storage tank to overhead tank on the roof top, or to the day tank in ground level.
Another is the booster pump used for distributing water to all the toilets if a single dwelling villas or to all toilets in flats in case of high rise buildings.
The pump capacity is determined by two parameters; one is the water flow rate in GPM (gallons per minute) and the pressure required to transfer the medium from the source (where medium is drawn)to the destination (where it should be discharged), the technical term shall be Head in meters.
The water flow rate for the lift pump is calculated by summing up the required water per day for each toilets and total summing for the flats. The Head to be calculated from two things : one is the static head, the vertical distance in meters from the surface of the water where water is drawn to the discharge point two is the friction loss of the pipes and fittings calculated in equivalent head all the length of the pipes and fittings used to transfer water. The pump shall be used only an hour or two a day depending on the flow rate of the pump and the total required water a day to be pumped at the discharge point.
In case of booster pump, the flow rate shall be calculated in GPM to be referred to the demand load versus Fixture units characteristic curve and graph, and fixture units schedule available in Uniform plumbing code or any International code published by recognized institutions around the world.
Estimating the water flow rate of a booster pump for a building, there are standard fixture unit values for each of the sanitary fixtures used in all toilets can be found in the code books. In a spread sheet enter the total fixtures units and the fixtures units value referring to the code. For example Water closet - 5, Lavatory - 2, Shower - 3 etc, multiply the values with corresponding fixture unit quantities we get subtotals.
We get an overall fixture unit value by summing up the subtotal, this value in the X axis to be intersected to the GPM value using the curve in the Demand load vs Fixture unit graph as said above.
Hence we get the total GPM which shall be capacity of the pump, and for the head calculation the static head and the friction loss of pipes and fittings are calculated same as lift pump procedure, except a small consideration of the length of the pipe. Since water is distributed to multiple outlets and multiple discharge points rather than a single discharge point as in case of lift pump, by other words termed as a piping network. The longest length of the pipe or the total length of pipe to be considered shall be from the surface of water drawn to the longest or the last discharge point in the network, wherever necessary the static height in meters to be taken care.
For both the cases a 25 % Factor of safety allowance to be added for the gross capacity of the pump.
