Liquid flow rate and pressure are the basic measures of pump performance. Flow rate is commonly measured in litres per second, per minute or per hour or sometimes in cubic metres per hour. Pressure is commonly measured in bar (1 bar is equal to standard atmospheric pressure at sea level, or 1kg per square centimetre, or 14.6lb per square inch (psi).
Head is sometimes used as a convenient alternative to pressure. Commonly expressed in metres or feet, head is the vertical height difference between the liquid supply and discharge levels. A vertical column of water 10 metres high has a pressure of 1 bar (above atmospheric pressure) at it’s base. A pump able to deliver water from ground level to an elevation of 10m is working against a ‘static head’ of 10m.
Water flowing though a pipe loses energy through friction at the pipe wall. The loss of energy appears as a pressure loss in the pipe. The pressure loss can be expressed as an equivalent head (‘dynamic head’).
Pipe friction losses may be negliable, or they may be substantial, depending on flow rate, pipe length and internal diameter (bore), and configuration. To minimise pipe friction losses, keep inlet and discharge pipework as short and straight as possible. Use the pump manufacturer’s recommended pipe size. As a general rule, the pipework bore should be at least as large as the diameter of the ports on the pump. If the pipework is very long or convoluted, use pipe of larger bore.