Page 53 - Subyek Teknik Mesin - Forsthoffers Best Practice Handbook for Rotating Machinery by William E Forsthoffer

Pump Best Practices Be st Practice 2.2
Completely define the operating conditions resistance of the suction and discharge systems. It is also used as
a guideline to determine whether a pump casing design should
Correctly stated operating conditions are essential for proper be of the vertical (radial) split or horizontal (axial) configuration
definition, and for subsequent selection of a specific type and (refer to Figure 2.2.3 for some guidelines).
configuration of pump.
Once it is decided to install a pumping system, a sketch
should be drawn to define all of the components that are re- Use radial split casing for:
quired to be included into that system. Some of the factors S.G. 0.7 @ pumping temperature
which need to be considered in completing the sketch and Pumping temperature 200 C (400 F)
system design are as follows: Flammable or toxic liquids at rated discharge pressures above
6,896 Kpa (1000 psig)
Flow rate e All flow rates, including minimum, normal and
rated should be listed in the data sheet. Normal flow is usually
that required to achieve a specific process operation. The rated Fig 2.2.3 Casing configuration guidelines
flow is normally a set percentage increase over the normal flow,
and it usually includes consideration for pump wear, and the NPSH available e Net positive suction head available is
type of operation within the process system. It can be up to ten a characteristic of the process suction system. It is the energy
(10) percent, depending upon specific company practice. The above the vapor pressure of the liquid, measured at the suction
minimum flow is important to identify in order to establish if flange of the pump, which is required to maintain the fluid in
a minimum flow bypass line is required for process or me- a liquid state. In a centrifugal pump it is usually measured in feet
chanical design considerations. of liquid (refer to Figure 2.2.4 for a typical method for calcu-
Head required e The required head that the pump must lating NPSH available). It is important to note that the pressure
develop is based on the static pressure difference between the at the suction source cannot be considered equal to the NPSH.
discharge terminal point and the suction source, the elevation In Figure 2.2.4, it can be seen that the source pressure is the
difference and the friction losses through system components same as the vapor pressure, indicating that the liquid is at its
including suction and discharge side piping, pressure drop boiling point. When the vapor pressure is subtracted from the
through heat exchangers, furnaces, control valves and other suction pressure, the resulting NPSH available is 2.1 psi or ten
equipment. It is represented by the equation in Figure 2.2.2. (10) feet. When calculating NPSH available, it is prudent to
incorporate a margin of safety to protect the pump from po-
tential cavitation damage resulting from unexpected upsets. The
H = 10.2 P (at pump flanges) actual margin will vary from company to company. Some use
S.G. the normal liquid level as the datum point, while others use the
P = Total pressure difference between the discharge system and vessel tangent or the bottom of the vessel. Typical suggested
suction system, measured at the pump flanges in barg margins are two (2) feet for hydrocarbon liquids (including low
S.G. = Specific gravity of the liquid at pump temperature S.G.), and ten (10) feet for boiling water.
H = Pump required head in meter-kg force/kg mass
Notes: If pressure is measured in kPa, constant = 0.102
2
If pressure is measured in kg/cm , constant = 10.003 Defining the pump rated point for efficient
If pressure is measured in PSIG, constant = 2.311 and head
required is measured in ft-lb force/lb mass operation
Since centrifugal pumps are not normally custom designed
Fig 2.2.2 Required head equation
items of equipment, it is important to ensure that each vendor
will quote similar pump configurations for the specific operating
conditions set forth on each application data sheet. When
Liquid properties e Viscosity, vapor pressure and specific
gravity each play an important role in achieving the required establishing which pump characteristic and impeller pattern to
select for a specific application, certain guidelines should be
level of pump reliability within the operating system. Vis-
cosity canimpactpumpperformance to the extentthatit may followed (refer to Figure 2.2.5).
not be justified to even use a centrifugal pump when the
viscosity values are greater than 7.5 centistokes (50 SSU). Carefully define critical component
The hydraulic institute has published curves which can be
used to calculate the performance effects resulting from requirements
pumping viscous liquid.
Vapor pressure and specific gravity influence the type of The reliability of a pump can be improved through proper spec-
ification, selection and operation of components such as bearings,
pump to use and its mechanical design configuration. Vapor
pressure is an important property when determining whether mechanical seals and drivers. Industry standards such as API
there is adequate net energy available from the pump suction to Standard 610 for centrifugal pumps, and Standard 682 for
avoid vaporization of the liquid e which can lead to performance mechanical seals, contain minimum requirements which, if
deterioration and possible lower life expectancy of the pump. implemented, should result in improved reliability and extended
Specific gravity is the liquid property used to calculate the on-stream operating time. Some salient points about each of
amount of head a pump has to produce to overcome the these components are highlighted in Figures 2.2.8e2.2.10.

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