Control valve rangeability

Control valves have orifices that can be adjusted to regulate the flow of fluids through them. Four features important to their use are capacity, characteristic, rangeability and recoveiy. 

This cycling can be eliminated by mounting the control valve in the condensate pipe, but this creates new problems, because when the load decreases, the process is slow steam has to condense before the condensate level is affected, and when the load increases, the process is fast, because blowing out liquid condensate is fast. With such "nonsymmetrical" process dynamics, control is bound to be poor. A better option is to use lifting traps to prevent condensate accumulation. These pumping traps will make temperature control possible even when the heater is under vacuum, but will not improve the problem of low rangeability, and the possible use of two control valves in parallel can still be necessary. 

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. 

Select the type of valve characteristic to use. Table 19.1 lists the typical characteristics of various control valves. Study of Table 19.1 shows that an equal-percentage valve must be used if a rangeability of 20 is required. Such a valve has equal stem movements for equal-percentage changes... 

We want tight control of these important quantities for economic and operational reasons. Hence we should select manipulated variables such that the dynamic relationships between the controlled and manipulated variables feature small time constants and deadtimes and large steady-state gains. The former gives small closed-loop time constants and the latter prevents problems with the rangeability of the manipulated variable (control valve saturation t... 

The drop in bottoms flow requires a large increase in the flowrate of fresh acetic acid makeup so that the total acetic acid flowrate stays constant. Fresh acetic acid increases from about 750 up to 2200 mol/h in 30 minutes. This illustrates that the pumps and control valves used in a fresh feed makeup system must be designed to provide much wider rangeability than we would predict from just steady-state conditions alone. 

Flow ranges of control valves frequently are characterized as rangeability, which is the ratio of maximum to minimum flow rates, and turndown, which is the ratio of normal maximum operating flow rate to minimum flow rates. The minimum flow rate for control valves is usually not zero to preclude hysteresis effects that might result from total closing. 

However, the control engineer wants to take a large pressure drop over the valve. Why Basically, it is a question of rangeability the larger the pressure drop, the larger the changes that can be made in the flow rate (in both directions-increase and decrease). Let s examine two different designs to show why it is desirable from a dynamic viewpoint to have more pressure drop over the control valve. 

In preparation for exporting the steady-state flowsheet into Aspen Dynamics, aU equipment is sized. Column diameters are calculated by Aspen Tray Sizing. Reflux drums and column bases are sized to provide 5 min of holdup when 50% full, based on the total liquid entering the surge capacity. Pumps and control valves are specified to give adequate dynamic rangeability. Typical valve pressure drops are 2 atm. 

Control valve sizing Smaller design pressure drops save pumping cost but reduce rangeability so control valve saturation is more frequent. 

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... 

The flow range required of the reagents is moderated by whatever tolerance is set on final pH. The scheme shown in Fig. 10.14 is wholly unsuited to such an application, because the differential meters are accurate to 1 percent of span only from 25 to 100 percent, a 4 1 range. Linear control valves are limited to the vicinity of 25 1 rangeability, while... 

Rangeability is the ratio of maximum to minimum flows over which the valve can give good control. This concept is difficult to quantify and is not used much for valve selection. A valve generally can be designed properly for a suitably wide flow range. 

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