Process control adjustable-speed pumps

The process controller is the master of the process-control system. It accepts a set point and other inputs and generates an output or outputs that it computes from a rule or set of rules that are part of its internal configuration. The controller output seiwes as an input to another controller or, more often, as an input to a final control element. The final control element is the device that affects the flow in the piping system of the process. The final control element seiwes as an interface between the process controller and the process. Control valves and adjustable speed pumps are the principal types discussed. 

The centrifugal pump directly driven by a variable-speed electric motor is the most commonly used hardware comoination for adjustable speed pumping. The motor is operated by an electronic-motor speed controller whose function is to generate the voltage or current waveform required by the motor to make the speed of the motor track the input command input signal from the process controller. 

From a dynamic-response standpoint, the adjustable speed pump has a dynamic characteristic that is more suitable in process-control apphcations than those characteristics of control valves. The small amphtude response of an adjustable speed pump does not contain the dead baud or the dead time commonly found in the small amphtude response of the control valve. Nonhnearities associated with frictions in the valve and discontinuities in the pneumatic portion of the control-valve instrumentation are not present with electronic... 

FIG. 8-85 Pressure, flow, and power for a throttling process using (1) a control valve and a constant speed pump and (2) an adjustable speed pump. 

Most induction ac motors are fixed-speed. However, a large number of motor applications would benefit if the motor speed could be adjusted to match process requirements. Motor speed controls are the devices which, when properly applied, can tap most of the potential energy savings in motor systems. Motor speed controls are particularly attractive in applications where there is variable fluid flow. In many centrifugal pump, fan, and compressor applications mechanical power grows roughly with the cube of the fluid flow. To move 80 percent of the nominal flow only half of the power is required. Centrifugal loads are therefore excellent candidates for motor speed control. Other loads that may benefit from the use of motor speed controls include conveyers, traction drives, winders, machine tools and robotics. 

Input variables are classified as either manipulated variables (MVs) or disturbance variables (DVs). Manipulated variables are used to adjust the rates of material and energy that enter or leave a process. The MVs are often flow rates adjusted by control valves, variable-speed pumps or compressors, or conveyor belts (for solid materials). An energy input, such as the power to an electrical heater, can also be an MV. If an MV is a flow rate, there must be some place for the material to accumulate. For example, it is not feasible to place two control valves at different locations on the same pipe. Manipulated variables are often inlet flow rates. However, an exit flow rate can also be an MV, for example, when the liquid level in a tank is controlled by manipulating an exit flow rate. 

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