Flow Control In Combined Displacement And 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.

TABLE 1 Comparison of several capacity control schemes for positive displacement pumps

Control Method

Degree of Modulation

First Cost

Operating Cost

Comments

Directing-acting pumps

Steam Full: zero Low Low Steam pressure required to throttle to 100% balance liquid piston force and overcome breakaway friction. Steam volume throttled to produce desired capacity.

Power pumps

Start-stop

Multispeed motors

Variable frequency

Direct current

Wound rotor motors

Combustion engines

Steam or gas turbines

Hydraulic torque converter

Zero or 100% Low

Steps dependent on motor winding Full: zero to 100% +

Full: zero High Medium to 100%

Full: zero to 100%

Variable for limited range Variable for limited range

Full: zero to 100%

Low Limited in frequency of starts because of temperature rise from inrush.

Medium Low Cost of motor controller switch gear motors must be assessed.

High Low Limited by current-handling capacity of solid-state controller. Drive is torque-speed sensitive, pump is torque-pressure sensitive at all speeds. Check drive for required torque at minimum and maximum speeds.

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.

High Low Torsional analysis required to avoid high torsional stresses.

Medium Medium Drive is torque-speed sensitive. Pump is torque-pressure sensitive at all speeds. Check drive for required torque at minimum and maximum speeds.

High High Low full speed efficiency.

TABLE 1 Comparison of several capacity control schemes for positive displacement pumps (Continued)

Control Method

Degree of First Modulation Cost

Operating Cost

Comments

Power pumps

Hydroviscous speed control Fluid coupling

Magnetic (eddy) coupling Suction valve unloaders

Variable-pitch belt

Full: zero to 100%

Full: zero to 100%

Bypass valve Limited

Limited

High High at part load Medium High

Full: zero High to 100%

Zero or 100% Low

High

High

Efficiency proportional to capacity. Loss to heat exchanger.

Full-speed slip loss greater than hydroviscous. Loss to heat exchanger.

Full-speed slip loss.

Synchronization with suction stroke avoids startstop and shock problems. uses full power at zero capacity with full valve wear. Limited to belt drive horsepower.

FIGURE 3 Relative efficiencies of speed control options.

FIGURE 3 Relative efficiencies of speed control options.

Survival Treasure

Survival Treasure

This is a collection of 3 guides all about survival. Within this collection you find the following titles: Outdoor Survival Skills, Survival Basics and The Wilderness Survival Guide.

Get My Free Ebook


Post a comment