Suction Lift with non Self-priming Centrifugal Pumps
When handling water at temperatures below 100 degrees Celsius i.e. chilled or cold water services the available inlet pressure at the pump suction port will become negative in value. That means that the local atmospheric pressure is higher than the sum of the net positive suction head and the vapour pressure. Thus the inlet pressure is no longer required. This interrelation is based on the greatly reduced vapour pressure of cold water compared with hot water. In practice this means:
- Pumps operating at a negative minimum inlet pressure are capable of creating a suction lift (non-self priming).
- The suction capacity is approximately equal to the level of the negative minimum inlet pressure minus a 1m safety factor.
- As pumps normally used in conjunction with building services are generally not of self priming characteristics the following conditions for suction lift operation must be met.
- Filling and venting of the suction-side pipework including the pump before commissioning.
- Avoiding air entrainment during pump operation (aeration will result in break down of suction capability)
- Avoiding drainage of suction-side piping whilst pump is off by providing a foot valve. Foot valves are susceptible to seepage due to build up of dirt particles on the valve faces.
- Reliance on non-return valves in the discharge pipe is not sufficient in itself, as air can still be entrained by way of the shaft seal (mechanical or packed gland) whilst the pump is off.
- The suction capability of non-self priming pumps is, on account of their design, is generally limited to a maximum of 2 to 4m. Higher suction lifts (max 8 to 9m) and self-priming operation require the use of special pumps.
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