Page 89 - Subyek Teknik Mesin - Forsthoffers Best Practice Handbook for Rotating Machinery by William E Forsthoffer

Pump Best Practices Best Practice 2 .12
show that the fluid type (latent heat of vaporization for a given
temperature) and degree of vaporization determine the energy
released when the vapor is recompressed to a liquid. Note that
the abcissa is BTU/lb. When cavitation occurs, a given amount
of energy (BTU/lb) will be transferred from the fluid to the
impeller. Energy can also be expressed in ft-lb F /lb M by multi-
plying as follows:
BLU 778 ft lb F

lb M BLU
It can therefore be seen that during cavitation, the energy
transferred by the fluid to the vanes can greatly exceed the head
produced by the impeller.
In general, single component liquids produce the highest Fig 2.12.16 Efficiency as a function of pump specific speed and
energy values during cavitation and are therefore the most flow
damaging fluids. Hydrocarbon mixtures produce less energy,
and have higher viscosities which reduce damage. Regardless of
composition, all liquids produce high noise levels during cavi- Impeller discharge flow velocity
tation, which typically sounds as if solid ‘rocks’ are passing Impeller tip speed
through the pump. Always remember that there are different Impeller inlet and discharge blade angles
causes of vaporization which result in cavitation. Discharge throat velocity

Figure 2.12.16 presents a plot of pump specific speed (US
Preventing hydraulic disturbances units) vs. efficiency for various flow rates. This is a generic chart,
and its precise form will vary slightly from pump vendor to
In the previous sections, the causes of liquid disturbances in pump vendor, based on the specifics of pump design. It can be
centrifugal pumps were discussed. In this section, practical used for estimating purposes in determining pump efficiency to
advice on how to prevent purchasing troublesome pumps in the obtain pump required horsepower.
design phase, and practical solutions on how to solve existing
field problems caused by liquid disturbances, will be presented. Net Positive Suction Head Available
NPSH A has been previously discussed. As we have learned, it
The project design phase must be greater than the NPSH R in order to prevent cavi-
tation. Methods for determining NPSH A have been
Action taken during the early stages of a project can significantly presented.
increase pump reliability and safety by eliminating all sources of
vaporization within a pump. Sources of vaporization exist both Net Positive Suction Head Required
in the process and in the pump. Before presenting solutions, Figure 2.12.17 shows the NPSH required within a typical cen-
a number of important concepts must be reviewed and trifugal pump. It can be seen that this value is actually the
presented. pressure drop from the suction flange to the impeller vane inlet
expressed in energy terms (head).
Concepts Perhaps now, we can truly understand why NPSH AVAILABLE
must be NPSH REQUIRED . As we have learned,
Specific speed
If NPSH A NPSH R
Specific speed is a non-dimensional value that is a function of Then; the fluid will not vaporize
pump speed, flow and head:
Therefore; no vaporization; no cavitation; no damage
p
N Q
ffiffiffiffiffi
N S ¼ However, it must be remembered that NPSH A NPSH R is
HD 3=4 only one of the requirements that must be met to prevent
Where : N S ¼ specific speed vaporization. The following causes of vaporization must also be
N ¼ pump speed rpm prevented:
Q ¼ pump flow gpm
ft=lb Low velocity stall
Hd ¼ pump produced head F
lb M Low flow temperature rise
Note: For double suction impellers, Q ¼ Q/2 Low flow temperature rise and fluid vaporization can be de-
Specific speed is used extensively in both pump and com- termined by the relationship shown in Figure 2.12.11, and the
pressor design, to optimize stage efficiency for a given value of pumped fluid characteristics. However, the determination of
flow and head required. In pump design, specific speed is used low velocity stall or recirculation requires an understanding of
to optimize the following design parameters: the concept of suction specific speed.

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