Page 19 - Centrifugal Pumps 2E

Introduction 7

mate responsibility and must confirm that the desired speed is compatible
with NPSHA and satisfies optimum efficiency selection.

Liquid Characteristics
To have reasonable life expectancy, pump materials must be compati-
ble with the liquid. Having intimate knowledge of the liquid to be
pumped, the user will often specify materials to the pump manufacturer.
When the pump manufacturer is required to specify materials, it is essen-
tial that the user supply all relevant information. Since liquids range from
clear to those that contain gases, vapors, and solid material, essential in-
formation includes temperature, specific gravity, pH level, solid content,
amount of entrained air and/or dissolved gas, and whether the liquid is
corrosive. In determining final material selection the pump manufacturer
must also consider operating stresses and effects of corrosion, erosion,
and abrasion,

Viscosity
As liquid flows through a pump, the hydrodynamic losses are influ-
enced by viscosity and any increase results in a reduction in head gener-
ated and efficiency, with an increase in power absorbed (Figure 1-5).
Centrifugal pumps are routinely applied in services having viscosities be-
low 3,000 SSU and have been used in applications with product viscosi-
ties up to 15,(XX) SSU. It is important to realize that the size of the inter-
nal flow passages has a significant effect on the losses, thus the smaller
the pump is, the greater are the effects of viscosity. As the physical size
of a pump increases, the maximum viscosity it can handle increases. A
pump with a 3-in. discharge nozzle can handle 500 SSU, while a pump
with a 6-in. discharge nozzle can handle 1,700 SSU. Centrifugal pumps
can handle much higher viscosities, but beyond these limits, there is an
increasing penalty loss. When viscosity is too high for a particular size
pump, it will be necessary to go to a larger pump. A reasonable operating
range of viscosity versus pump size is shown in Figure 1-6. Methods to
predict pump performance with viscous liquids are clearly defined in the
Hydraulic Institute Standards.

Specific Gravity
When pumping a nonviscous liquid, pumps will generate the same
head uninfluenced by the specific gravity of the liquid. Pressure will
change with specific gravity and can be calculated from:

~. ff . , .. . Differential head (ft) x sp gr
Differential pressure (psi) = — —¥~