There are two items required to size a pump;
· Fluid flow rate
· Pressure to be developed.
This is usually found from pipe sizing tables or from other methods.
See Science section of these notes - Fluids section; Head Loss due to friction in a pipe.
The flow rate of fluid is also found from pipe sizing tables or given in other data.
Add 20% margin to pump pressure to allow for future extensions and the system getting less efficient.
The designer must be careful when adding a margin to pump pressure since too much pressure can lead to ‘pumping over’ in open systems and other problems.
Some pump catalogues have units of head instead of pressure.
Pressure (Pa) = density of water x acceleration due to gravity x head (m)
Or P = r x g x H
Where, P = Pump pressure (Pa)
r = Density of water approx.1000 kg/m3.
g = Acceleration due to gravity 9.81 m/s2.
H = Head (m)
H = P / r x g
H = P / 1000 x 9.81
H = P / 9810
The flow rate of water that the pump delivers will be the flow rate in the section in which the pump is installed.
A 20% margin may be added to this flow rate to allow for future extensions to the system.
A pump catalogue may be consulted to choose a suitable unit.
The operating point can be super-imposed on the pump graph for pressure (head) against flow rate in kg/s or l/s.
It is best to choose a pump with the operating point near the lower speeds or the bottom end of the performance curve so that the pump will not be operating at its maximum capacity, thus allowing little room for error or margin.
A typical pump sizing curve is shown below with a system operating point superimposed on the curve.
Not all system operating points are directly on top of a pump graph or curve as shown below.
It would be best to choose a pump on the curve above the operating point, i.e. Pump B since the output of both pressure and flow rate will be slightly above that required and not below.
Similarly if a pump has three speeds then three curves will be shown.
It would be best to operate a pump at a lower speed if possible to prolong the life of the pump and bearings.
The diagram below shows a 3-speed pump with the operating point between speed No.1 and No.2.
The pump would then be installed to run at speed No.2; this means that if the system is extended at a later date the pump speed may be increased to accommodate this increase in flow rate and pressure.