Cleghorn - About Pumps

General Information

Flow Rate is commonly measured in:
- litres per second, per minute or per hour.
or sometimes in
- cubic metres per hour.

1 litre of water weigh 1 kilogram.
1000 litres of water (1 cubic metre) weighs 1 tonne.

Pressure is commonly measured in:
- bar (1 bar is equal to standard atmospheric pressure at sea level, or 1 kg per square centimetre, or 14.6 lb 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 its 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 through 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').

For example: if water flowing through a pipe loses 0.5 bar pressure from one end of the pipe to the other, this is equivalent to:
0.5 x (equivalent of 1 bar pressure expressed in metres of head).
= 0.5 x 10 (see above)
= 5 metres

In this instance, an extra ('dynamic') head of 5m would need to be added to the static head against which the pump must work.

Pipe friction losses may be negligible, 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.

Self-Priming Pumps

A self-priming pump is capable of exhausting the air from its inlet pipe, drawing water from a lower level up into its inlet when it is started. JABSCO flexible impeller pumps will self-prime rapidly from dry, so that they can be installed well above the level of the water source. For efficient self-priming, it is essential that the inlet pipework is fully airtight. Air flows easily through the smallest leak at a pipe joint. An air leak can slow down or prevent priming, with the risk of dry-running damage to the pump.