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 | |
