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Wednesday, February 3, 2010

Combined Displacement and Centrifugal Pumps

Flow Control in Combined Displacement - Centrifugal Pumps

Systems that involve both centrifugal and reciprocating positive displacement pumps deserve some special consideration. Centrifugal pumps are often used as suction boosters to overcome acceleration head requirements peculiar to reciprocating pumps but are rarely used to supplement flow. Some unique characteristics of each type of pump that affect the other type must be considered in the design, operation, and control or the interrelated system.

Comparison of several capacity control schemes for positive displacement pump

Control method
Degree of Modulation
First Cost
Operating Cost
Comments
Directing Acting Pumps
Steam throttle
Full zero to 100%
low
low
Steam pressure required to balance liquid piston force and overcome breakaway friction. Steam volume throttled to produce desired capacity
Power Pumps
Start-stop
Zero or 100%
low
low
Limited in frequency of starts because of temperature rise from inrush.
Multispeed motors
Steps dependent on motor winding
medium
low
Cost of motor controller switch gear motors must be assessed
Variable frequency
Full zero to 100%+
high
low
Limited by current handling capacity of solid state controller
Direct current
Full zero to 100%
High
Medium
Drive is torque speed sensitive, pump is torque pressure sensitive at all speeds. Check drive for required
Wound rotor motors
Full zero to 100%
High
low
Drive is torque speed sensitive. Pump is torque pressure sensitive at all speeds. Check drive for required torque at minimum and maximum speeds
Combustion engine
Variable for limited range
high
low
Torsional analysis required to avoid high torsional stresses.
Steam or Gas turbine
Variable for limited range
medium
medium
Drive is torque speed sensitive at all speeds. Check drive for required torque at minimum and maximum speeds.
Hydraulic torque converter
Full zero to 100%
high
high
Low full speed efficiency

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