Control valves elimination, variable-speed pump

For variable-speed pumps, such as steam turbine-driven pumps, control valves should not be used. The facility shown in Fig. 38.2 is a control scheme that is used not infrequently at older plants. It s a fine piece of technology, which has been lost with time. No control valve is used. The turbine speed is altered to directly maintain the vessel level. With the increasing use of variable-speed alternating-current motors, elimination of parasitic control valves should become more common. 

Figure 38.2 Variable-speed pump eliminates control valve.

Turbine A turbine uses steam pressure or burning gas to drive pumps and compressors at variable speeds. Motor drives are usually fixed-speed machines. Variable speed is an energy-efficient way to control flows by eliminating the downstream control valve. 

Pumps are driven by either fixed-speed or variable-speed motors. Variable-speed motors are becoming increasingly common. Flow can be controlled by varying the pump speed with the motor and thus eliminating parasitic energy losses across a flow control valve. However, because of the extra electronic components needed for a variable-speed AC motor, 95 percent of the motors we work with are fixed-speed alternating-current motors. I will only discuss fixed-speed, alternating-current (AC) motors in this chapter. 

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