The arrangement of pipework, the liquid to be pumped and its purpose will result in certain system requirements or characteristics that must be met by the pump. A pumping system on a ship will consist of suction piping, a pump and discharge piping. The system is arranged to provide a positive pressure or head at some point and discharge the liquid.
The pump provides the energy to develop the head and overcome any losses in the system. Losses are mainly due to friction within the pipes and the difference between the initial and final liquid levels. The total system losses, HTOTAL are found as follows:
HTOTAL — HFRSUCT + HFRDIS + HDISTANK + HSUCTTANK
where HFRSUCT = friction head loss in suction piping
HFRDIS = friction head loss in discharge piping
HDISTANK — height of discharge tank level above pump
HsucrrTANK= height of suction tank level above pump
(negative when tank level is below pump suction)
All values are in metres of liquid. The system flow rate or capacity will be known and the pump manufacturer will provide a head—flow characteristic for his equipment which must be matched to the system curve. To obtain the best operating conditions for the pump it should operate over its range of maximum efficiency.
An important consideration, particularly when drawing liquids from below the pump, is the suction-side conditions of the system. The determination of Net Positive Suction Head (NPSH) is undertaken for both the system and the pump.
Net Positive Suction Head is the difference between the absolute pump inlet pressure and the vapour pressure of the liquid, and is expressed in metres of liquid. Vapour pressure is temperature dependent and therefore NPSH should be given for the operating temperature of the liquid. The NPSH available in the system is found as follows:
The above values are usually expressed in metres head of sea water. The pump manufacturer provides a NPSH required characteristic for the pump which is also in metres head of sea water. The pump and system must be matched in terms of NPSH such that PPSH required is always greater than NPSH available.
An insufficient value of NPSH required will result in cavitation, i.e. the forming and collapsing of bubbles in the liquid, which will affect the pumping operation and may damage the pump.
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