Page 127 - Introduction to Marine Engineering
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114 Pumps and pumping systems
Pump characteristic
Pump efficiency
NPSH required
N P S H available
3
Flow {m /h)
Figure 6.2 Overall pump system characteristics
— HATM HSUCTTANK HFRSUCT HVAPPRESS
absolute pump inlet pressure
where HATM = atmospheric pressure
HSUCTTANK = tank level from pump (negative when tank level
is below pump)
HFRSUCT = friction head loss in suction piping
=
HVAPPRKSS liquid vapour pressure
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 (Figure 6.2). 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.
