Characteristics, control valve

This is a simple way to define control valve characteristics, including their capacity. Using this relationship to the K value derivation herein, the following equations derived by numerous vendors may be applied. 

The most common control valve characteristics (top) and the recommended valve characteristics selections (bottom) for a variety of applications. 

A flow control valve will be installed in a process system in which the flow may vary from 100 to 20 percent while the pressure drop in the system rises from 5 to 80 percent. What is the required rangeability of the control valve What type of control-valve characteristic should be used Show how the effective characteristic is related to the pressure drop the valve should handle. 

BHP break horse power Cp, Cp fluid heat capacity of hot (cold) stream Cv Control valve characteristic factor E electricity... 

The functional relationship between the mass flow rate of coolant and the controller output is usually given by the controller manufacturer, or can be determined experimentally. Figure 4.7 gives the control valve characteristic curve determined for this system. The fraction controller output is related to the proportional gain through... 

An approximate curve fit to the control valve characteristic curve of Figure 4.7 is... 

WATER-SOFTENER SELECTION AND ANALYSIS 19.20 COMPLETE DEIONIZATION OF WATER 19.22 COOLING-POND SIZE FOR A KNOWN HEAT LOAD 19.24 PROCESS TEMPERATURE-CONTROL ANALYSIS 19.26 CONTROL-VALVE SELECTION FOR PROCESS CONTROL 19.27 CONTROL-VALVE CHARACTERISTICS AND RANGEABILITY 19.29 CAVITATION, SUBCRITICAL, AND CRITICAL-FLOW CONSIDERATIONS IN CONTROLLER SELECTION 19.30 INDIRECT DRYING OF SOLIDS 19.34 VACUUM DRYING OF SOLIDS 19.36 ESTIMATING THERMODYNAMIC AND TRANSPORT PROPERTIES OF WATER 19.37... 

Many difficult process control problems have two distinguishing characteristics (i) significant interactions occur among key process variables, and (ii) inequality constraints exist for manipulated and controlled variables. The inequality constraints include upper and lower limits. For example, each manipulated flow rate has an upper limit determined by the pump and control valve characteristics. The lower limit may be zero, or a small positive value, based on safety considerations. Limits on controlled variables reflect equipment constraints (for example, metallurgical limits) and the operating objectives for the process. For example, a reactor temperature may have an upper limit to avoid undesired side reactions or catalyst degradation, and a lower limit to ensure that the reaction(s) proceed. 

Specification of the valve size is dependent on the valve characteristic /. Three control valve characteristics are mainly used. For a fixed pressure drop across the valve, the valve characteristic/(O < / < 1) is related to the lift (0 < 1) that is, the extent of valve opening, by one of the following relations ... 

Cascaded flow controllers can reduce the effect of any upstream pressure variations or changes in control valve characteristics resulting from nonlinearities or from fouling, as noted in Step II.A.3. Ratio control between wi and W2 can maintain the desired stoichiometric ratio of reactants approximately constant, despite changes in production rate or feed composition. Finally, cascade control can help deal with disturbances introduced by intentional changes in production rate W4, as is discussed next. 

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