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SECTION III
OPERATION
3.1
Technical Data
These pumps are furnished for a particular service condition. Changes in the hydraulic system
may affect the pump's performance adversely. This is 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.
3.2
Effects of Specific Gravity
The capacity and total head in feet of liquid developed by a centrifugal pump are fixed for every
point on the curve and are always the same speed. Neither capacity not total head will be affected
by a change in the specific gravity of the liquid pumped. However, since the developed gauge
pressure and the power required 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 the specific gravity. Therefore, a
change in specific gravity will affect the discharge gauge pressure. Any changes should be noted,
in that they may overload the pump's driver.
3.3
Effects of Viscosity
The pump is designed to deliver rated capacity and rated head for a liquid with a particular
viscosity. When contemplating operation at some viscosity other than for which the pump was
originally designed and/or applied, the changed conditions should be referred to FLOWSERVE for
our recommendations.
3.4
Changing the Pump Speed
Changing the speed of a centrifugal pump changes the capacity, total head and brake
horsepower. The capacity will vary in a direct ratio with the speed, whereas, the total head will vary
as the square of the speed The brake horsepower will vary in the ratio of the speed cubed except
in cases where the speed change also changes the efficiency of the pump. When contemplating
speeds other than of the original condition, refer to FLOWSERVE for recommendations.
3.5
Net Positive Suction Head (NPSH)
Any liquid, hot or cold, must be pushed into the impeller of the pump by absolute pressure, such
as the atmospheric or vessel pressure from which the pump takes its suction.
The head in feet of liquid necessary to push the required flow into the pump is called the Net
Positive Suction Head. This value, more commonly called NPSH, is measured above the vapour
pressure of the liquid at the pumping temperature.
There are two kinds of NPSH: the NPSHR is the head required by the pump to cover the losses in
the pump suction - that is shown on the pump characteristic curve. The second, NPSHA, is the
head available in the system, taking into account friction loss in suction piping, valves, fittings etc.
In all cases the NPSHA, measured above vapour pressure, must exceed the NPSHR in order to
push the liquid into the pump. Failure to have this will result in both bad performance and
mechanical damage to the pump, and in certain cases actual pump failure.
