Page 91 - Subyek Teknik Mesin - Forsthoffers Best Practice Handbook for Rotating Machinery by William E Forsthoffer

Pump Best Practices Best Practice 2 .12

Fig 2.12.20 Centrifugal pump control options

Define operating range of application
Accurately define liquid characteristics:
Vapor pressure
Pumping temperature
Viscosity
Perform hydraulic calculations for all required flow rates to
determine:
Head required
NPSH AVAILABLE

Fig 2.12.21 Preventing liquid disturbances by accurately defining
process requirements

1. NPSH margin at maximum operating flow
2. Approximate recirculation margin at minimum operating flow
3. NPSH margin at minimum operating flow

Fig 2.12.22 Hydraulic disturbances e areas of concern Fig 2.12.23 Hydraulic disturbance e areas of concern

Before proceeding, an important question regarding the
typical pump performance curve needs to be asked. Why is Hopefully your answer took the following form:
the NPSH R curve not drawn to zero flow like the head curve? ‘Liquid disturbances can occur at low flows if the vapor
Based on the information presented in this course, you
should be able to answer this question. Consider the fol- pressure of the pumped liquid exceeds the surrounding pressure
of the liquid’.
lowing facts:
‘Flow separation and/or liquid temperature rise which can
The standard shop test fluid for all pumps is water occur at low flows will either reduce the surrounding pressure
The causes of vaporization at low flows on a liquid or increase its vapor pressure’.

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