Page 162 - Centrifugal Pumps Design and Application

140 Centrifugal Pumps: Design and Application

Variable capacity requirements or horsepower limitations may dictate a
need for multiple pumps. In this event it must be decided if the pump
should operate in series or in parallel. With series operation, each pump
delivers full throughput and generates part of the total station pressure.
Once pump conditions and mode of operation are clearly determined,
pumps can be selected. With pumps currently evaluated competitively in
excess of $1,000 per horsepower per year and pipelines operating up to
40,000 horsepower per station, it is essential that pumps be selected for
optimum efficiency. This requires an understanding of the losses that oc-
cur inside a pump. These are:
« Friction losses.
• Shock losses at inlet to the impeller.
• Shock losses leaving the impeller.
• Shock losses during the conversion of mechanical power to velocity en-
ergy then to potential energy.
• Mechanical losses.
• Leakage losses at impeller rings and interstage bushings.
» Disc friction losses at the impeller shrouds.

Figure 10-1. Typical analysis of pump losses.