(2) Net Positive Suction Head (NPSH). To enable acceptable
performance under all operating conditions of the boiler water feed pumps, the
pressure at the pump suction must be sufficiently above the vapor pressure of
the fluid being handled (corresponding to the saturation pressure of the fluid
at the suction temperature) to prevent steam binding and cavitation which will
destroy the pump impeller. Proper operation requires maintenance of a "net
positive suction head above vapor pressure" (NPSH) at all times. NPSH is a
function of suction head or lift, friction head and vapor pressure of liquid
at suction temperature.
2.31 (Ps - Pv)
NPSH =S)))))))))))))))+ Hs - Hf
Pressure in suction vessel in psia at the prevailing altitudes.
Vapor pressure of pumped liquid in psia.
Static elevation in feet of the liquid surface above (+) or
below (-) the centerline of the impeller.
Friction loss, in feet of liquid, in the suction line from the
vessel to the pump suction flange.
Specific gravity of the liquid at suction temperature.
(3) Conditions resulting in undue fluctuations in heater pressure
and/or other sudden reductions in pump suction pressure may require injection
of cold water into the pump suction to prevent flashing with resulting
(4) Boiler feed pumps operating at or near shut-off are subject to
damage due to overheating and possible seizure between the rotating and
stationary parts, imbalance due to cavitation and other miscellaneous
hydraulic forces. At complete shut-off the only fluid protecting the parts
from damage is the amount contained in the casing which may be raised to a
boiling temperature in minutes or even seconds. Provision must be made for
adequate flow through the pump to prevent pump damage. This is accomplished
by discharging a constant flow through an orifice or discharging of a
controlled flow through an automatically controlled discharge bypass system or
automatic flow control valve; this valve is located after the pump and
discharges to a bypass recirculating line which discharges to the dearator
heater or tank.