Page 403 - Subyek Teknik Mesin - Forsthoffers Best Practice Handbook for Rotating Machinery by William E Forsthoffer

Be st Practice 7 .7 Lube, Seal and Control Oil System Best Practices
As a first move, for selecting a properly sized pump, one can
use the corrected valves of Q and head above to determine the Positive displacement Dynamic
approximate size pump impeller required.
Rated flow
Qrated 60 SSU (Normal) 77 GPM 93.3
Q selection ¼ 1000 SSU (Max.) 83.3 GPM 75 GPM
Q corr: factor
Discharge pressure
75 GPM 6 0 S S U N ( r o m ) l a 1 6 0 2 0 0
Q selection ¼ ¼ 93:75 GPM 1 0 0 0 S S U (Max.) 2 5 0 2 0 0
0:8
Pump horsepower
HD rated 60 SSU (Normal) 14.8 BHP 23 BHP
HD selection ¼ 1000 SSU (Max.) 19.66 BHP 46 BHP
HD: corr factor
540:5
HD selection ¼ ¼ 540:5 feet Fig 7.7.14 Pump performance positive displacement vs. dynamic
1:0
lube oil application
A selection for a single stage centrifugal pump operating at
3,550 RPM with a 2" inlet, 1" discharge and 11" diameter im- A comparison of positive displacement vs. dynamic pump
peller is made. The curve is noted in Figure 7.7.8 and shows the performance for this application is noted in Figure 7.7.14 for
following pump operating points. normal and rated points at normal and maximum viscosities.
The following facts should be noted:
- Rated flow @ maximum viscosity (1,000 SSU)
A. The positive displacement pump meets the required
- Normal flow @ maximum viscosity (1,000 SSU)
- Rated flow @ normal viscosity (60 SSU) discharge pressure at greater flows than specified.
- Normal flow @ normal viscosity (60 SSU) B. The dynamic pump meets the required flow at greater
discharge pressures than specified.
The following facts can be observed from Figure 7.7.10: C. The efficiency for a centrifugal pump at high viscosities and
this relatively low flow rate is significantly lower than the
A. All operating points other than the rated point at 1,000 SSU positive displacement pump. Note that for high flow rate
viscosity exceed the head required of 540.0 feet. applications the efficiency of a dynamic pump can actually
B. A specific differential head value exists for each pump flow exceed a P.D. pump.
and liquid viscosity. D. The required horsepower of a centrifugal pump for this
C. Pump efficiency is low for this relatively small centrifugal application is far greater than the positive displacement pump.
pump and is reduced approximately 50% for the high
viscosity case. The preceding example demonstrates why positive dis-
placement pumps are usually favored over dynamic pumps in
The pump horsepower is calculated by (if a viscosity cor- relatively low flow systems using viscous liquids. However, if the
rection is not required):
liquid viscosity does not vary significantly, and the system flow
requirements are large, centrifugal pumps provide efficient, low
HeadðftÞ flowðGPMÞ specific gravity
BHP ¼ maintenance service and are usually used.
3960 Pump effciencyð%Þ
It must be remembered that dynamic pumps, like positive
If a viscosity correction is required: displacement pumps, must be provided with the correct suction
conditions to ensure reliable operation. Cavitation, entrained

ðheadÞ ðhead viscosity corr:Þ ðflowÞ ðflow viscosity corr:Þ specific gravity
BHP ¼
ð3960Þ ðPump efficiencyÞ ðPump efficiency vicosity corr:Þ

For the maximum viscosity condition: gases and low flow rates can cause impeller damage and wear that
will reduce pressure output and result in lower system flow rates.
ð540:5Þ ð93:75Þ ð0:8Þ ð:85Þ The types of pumps used for our example are only two of the
BHP ¼
ð3960Þð:38Þ ð:5Þ types used for auxiliary system service. Other types of pumps
¼ 46:0 BHP that can be used for auxiliary systems are:
- Gear type (external or internal gear)
For the normal viscosity condition:
- Twin screw
- Lobe type
ð540:5Þ ð75Þð:85Þ
BHP ¼ - Sliding vane
ð3960Þð0:38Þ
Before concluding this section on auxiliary system pumps,
¼ 23:0 BHP
the subject of component redundancy must be discussed. In
The pump NPSH required is obtained from the pump order to satisfy the design objective of ‘continuously supplying.’
curve. the pumps, as well as all the major components of the system
374