SVCN 7 INSTALLATION, OPERATION, & MAINTENANCE
3. 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.
3.1.1 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.
3.1.2 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.
3.1.3 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.
3.1.4 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 vapor 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 section - 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 vapor 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.
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DO2604-13
Summary of Contents for SVCN 7
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