Flowserve Twin Screw Rotary - Manual (Page 35)

TWIN SCREW PUMPS. ORIGINAL USER INSTRUCTIONS. ENGLISH. 71569243 – 07/10
Page 32 of 53
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5.11.3 Net positive suction head (NPSH
A
)
NPSH available (NPSH
A
) is a measure of the head or
energy available in the pumped liquid, above its
vapour pressure, at the pump suction branch.
NPSH required (NPSH
R
) is a measure of the head
required in the pumped liquid, above its vapour
pressure, to prevent vaporization of the liquid. This
phenomenon, called cavitation, can cause serious
damage to the pump and adversely affect
performance. It is important that NPSH
A > NPSH
R
to
avoid vaporization of the liquid. The margin between
NPSH
A
> NPSH
R
should be as large as possible.
NPSH
A
is affected by changes in the liquid
temperature and vapour pressure, the altitude of the
installation, entrained gasses in the liquid, and the
viscosity of the liquid.
If any change in NPSH
A
is proposed, ensure these
margins are not significantly eroded. Refer to the
pump performance curve to determine exact
requirements particularly if flow has changed. If in
doubt please consult your nearest Flowserve office
for advice and details of the minimum allowable
margin for your application.
5.11.4 Pumped flow
Pump displacement varies directly with speed. The
slip, or leakage from discharge to suction through the
internal clearances varies with differential pressure
and viscosity. The pumped flow is the displacement
less the slip.
5.11.5 Pressure surges
The pump must not be subjected to pressure surges
such as may be caused by waterhammer or sudden
check valve closure.