Page 400 - Subyek Teknik Mesin - Forsthoffers Best Practice Handbook for Rotating Machinery by William E Forsthoffer

Lube, Seal and Control Oil System Best Practices Be st Practice 7.7

Fig 7.7.10 Screw pump sizing data (Courtesy of IMO Industries)

and the corresponding pump performance curve is shown in the rated flow case (maximum flow). Normal flow operation
Figure 7.7.11. will be attained by bypassing excess flow.
From this data, the following values of pump performance are - Maximum pump horsepower occurs at the ‘cold’ condition,
obtained: where oil viscosity and system pressure drop will be
maximum. Therefore the pump relief valve setting plus relief
Item Temperature valve accumulation is used.
- The NPSH required is relatively constant but varies with
Condition Min Normal Maximum
(40 F) (150 F) (160 F) viscosity and is less than the NPSH available in the system.

Before proceeding to a centrifugal pump selection, the fol-
Corresponding viscosity (SSU) 1,000 60 57
Actual pump flow (GPM) 86.3 77 77 lowing comments are made concerning positive displacement
@ normal pressure of 160 PSIG pump reliability.
Actual pump flow
@ maximum - Rotor Contact e The majority of screw and gear pumps used
pressure of 250 PSIG (pump in lubrication service rely on the pumped fluid to separate the
R.V. setting þ accumulation) 83.3 73 73 pump rotating elements and lubricate the pump bearings. As
Pump horsepower 15.0 9 9 a result, attention must be paid to selecting these pumps
Normal pressure (160 PSIG) within the specified pump vendor limits of pumped fluid
Normal viscosity (60 SSU) pressure and temperature, and to designing a system that
Maximum pressure 255 PSIG 19.66 14.8 14.8 ensures these conditions are maintained. Some users use
Maximum viscosity a safety factor of 80e90% from the vendors maximum
NPSH required (PSIA) 11.6 8.5 8.5 pressure for a given pump and viscosity (240e270 PSI in
Figure 7.7.10) to ensure reliable operation. Pumps equipped
The following facts can be observed from the above selection:
with external timing gears will not require this consideration
- Since the positive displacement pump (screw pump) is since the timing gears prevent the rotors from coming into
essentially a constant volume device, the pump is selected for contact.

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