Flowserve DMX - Manual (Page 33)

DMX/DMXD/DMXH/DMXDH USER INSTRUCTIONS ENGLISH 85392728 - 10/09
Page 33 of 60
especially true if the changes reduce the pressure at
the suction flange or if the liquid temperature is
increased. In case of doubt, contact the nearest
Flowserve office.
5.10.1 Specific gravity (SG)
The capacity and total head in m (ft) of liquid
developed by a centrifugal pump are fixed for every
point on the curve and are always at the same speed.
Neither capacity nor total head will be affected by a
change in the specific gravity of the liquid pumped.
However, since the developed pressure in bar (psi)
and the power to drive the pump are a function of the
specific gravity of the liquid, both will be affected in
direct proportion by any change in specific gravity.
Therefore, a change in specific gravity will affect the
discharge gauge pressure and power. Any changes
should be noted, in that they may overload the
pump’s driver.
5.10.2 Viscosity
The pump is designed to deliver rated capacity at
rated head for a liquid with a particular viscosity.
When contemplating operation at some viscosity
other than that for which the pump was originally
designed and/or applied, the changed conditions
should be referred to Flowserve for
recommendations.
5.10.3 Changing the Pump speed
Changing the speed of a centrifugal pump changes
the capacity, total head and brake Horse Power. The
capacity will vary in a direct ratio with the speed,
whereas the total head will vary as the ratio of the
speed squared. The brake Horse Power will vary as
the ratio of the speed cubed except in cases where
the speed change also affects the efficiency of the
pump.
When contemplating speeds other than of the original
condition, refer to Flowserve for recommendations.
5.10.4 Net positive suction head (NPSH)
Any liquid, hot or cold, must be forced into the
impeller of the pump without vaporization by the
pressure of the vessel from which the pump takes its
suction.
The head of liquid necessary to force the required
flow into the pump is called the Net Positive Suction
Head. This value, more commonly called NPSH, is
the head above the vapor pressure of the liquid at the
pumping temperature.
There are two kinds of NPSH: The NPSHR by the
pump, as shown on the pump curve, is the head
needed to cover the losses in the pump suction. The
second NPSH is that available in the system, taking
into account the friction loss in suction piping, valves,
fittings etc. In all cases the NPSHA (measured above
vapor pressure) must exceed the NPSHR in order to
push the liquid into the pump.
Failure to have an adequate margin of NPSHA over
NPSHR will cause a reduction of pump performance
and internal damage to the pump.