By definition, NPSH means: “net
positive suction head” above the
vapor pressure of the pumped
liquid available at the centerline
of the pump. It should always be
given in feet of pumped liquid. The
NPSH is actually a measurement
of the amount of energy available
in the pumped liquid to produce the
required absolute entrance velocity in
the pump. If a pump requires more
energy (or NPSH) than is available at
a given capacity, the pressure at the
inlet will fall below the vapor pres-
sure of the pumped liquid and loss of
performance will result.
P
s
= Pressure in the suction vessel
in PSIA.
P
vp
= Vapor pressure of the pumped
fluid in PSIA.
H
s
= Static height of the pumped fluid
above (+) or below (-) the centerline
of the pump in feet.
H
f
= All friction losses from the vessel
to the pump in feet.
NPSH = 2.31(
)+H
s
-H
f
For boiling liquids, P
s
and P
vp
are
equal. This item then becomes zero
and can be omitted from the equa-
tion.
1D6 Noise
Regenerative turbine pumps typically
produce a high pitched whine that
increases in intensity as the differen-
tial pressure produced in the pump
increases. While high frequency
sound is attenuated more easily than
lower frequencies, piping structures
and the fluids in them readily transmit
noise. Motors, bearings, and other
rotating components add to the noise
and sometimes create objectionable
harmonics.
Careful pump installation can al-
leviate noise problems. Proper
alignment of the pump and driver
is essential. Adequate supports
for the inlet and discharge piping is
equally important. A degree of noise
reduction may be obtained when
the pumping unit is supported free
of building structures by the use of
vibration isolators and flexible piping
and conduit connections. Elastomer
type couplings are the best choice to
separate motor noises from the fluid
and piping structure.
Section
T51 • E51
Page 503
Dated
February 2014
P
s
- P
vp
sp. gr.
pump inlet is even slightly higher than
the fluid source. It should be the
same size as the pump inlet or sized
based on reasonable fluid friction
losses.
A foot valve is recommended when
lifting fluid from a sump. This will
save wear and tear on any pump,
even those equipped with self prim-
ing capability.
A Y-Strainer is recommended im-
mediately ahead of the pump on any
newly constructed system. This is
advisable due to the probability that
foreign material large enough to dam-
age pump clearances may remain
even though the piping has been
flushed.
Valves in the outlet piping of a regen-
erative turbine pump should always
be open as far as possible when the
pump is started. This will reduce
the start-up load on the pump and
motor. Never start the pump with the
discharge valve closed.
The inlet valving should be open
when starting any pumping system.
Without some fluid in the pump, it can
gall and lock up impellers. Violent
pump failure will result from contin-
ued operation with the inlet valve
closed.
1D4 Priming
Regardless of whether self-priming
equipment is used or not, always fill
the pump and vent it of air before
starting for best seal and pump life.
Under most circumstances, regener-
ative turbine pumps can be made to
self-prime as long as a small amount
of fluid can be recirculated through
the impeller and the fluid doesn’t heat
up noticeably.
1D5 NPSH (Net Positive Suction
Head)
The NPSH required varies with every
size and capacity of pump. The
NPSH required by your unit can
be obtained from the performance
curves or from your MTH representa-
tive.
If the NPSH available is not equal
to or greater than that required by
the pump, it must be increased or a
different pump selected. The usual
method for increasing NPSH is to
raise the static head on the pump
inlet, H
s
.
1D7 Freezing
When ambient temperatures drop
below the freezing point of the fluid
in a pump, consideration should
be given to heating, insulating, or
draining the pump. If you choose to
drain the pump, and it will only be for
a short period, first remove the drain
plugs, and then drain the inlet and
outlet lines. Carefully blow out the
pump with compressed air to clear all
internal cavities of fluid.
1E Recommended Spare Parts
FOR CRITICAL SERVICES - a
duplex installation, with two identical
pumping units in parallel, is the safest
and many times the most cost effec-
tive choice.
FOR IMPORTANT SERVICES - a
standby pump, ready for installation
is advised. Special pricing and new
pump warranty is offered for factory
rebuilding. Turn around time can
be as short as one or two days for
standard models.
FOR ROUTINE MAINTENANCE
- only the mechanical seals and a
complete set of “O” ring gaskets are
recommended. Should additional
components show wear, they are
available from stock at the factory.
FOR SERVICING A PUMP THAT
DOES NOT PRODUCE RATED
HEAD - mechanical seals, “O” ring
gaskets, impeller, motor bracket, and
cover.
FOR REBUILDING A PUMP - all the
components required for servicing,
plus bearings, shaft, and drive keys
for flexible coupled pumps, should be
obtained. A factory rebuild should be
considered whenever your disassem-
bly indicates rebuilding is necessary
as this is usually more economical.
The factory recommendation for
spare parts are all of those needed
for rebuilding a pump and are shown
on the exploded view drawings for
each individual type of pump.
