Split range control

Other types of selective systems employ multiple final control elements or multiple controllers. In some applications, several manipulated variables are used to control a single process variable (also called split-range control). Typical examples include the adjustment of both inflow and outflow from a chemic reactor in order to control reactor pressure or the use of both acid and base to control pH in waste-water treatment. In this approach, the selector chooses from several controller outputs which final control element should be adjusted (Marlin, Process Control, McGraw-Hill, New York, 1995). 

A split-range control-system is normally used for an overhead low-pressure condenser (see Fig. 2). In this setup. Valve A controls makeup gas to the accumulator, and Valve B releases excess pressure. Valve A opens or closes according to a preset system pressure, delivering makeup gas whenever needed. 

Pressure control of a tank at atmospheric conditions can be achieved with a simple vent. However, often air cannot be allowed to come into contact with the process, or volatile material cannot be allowed to escape to the atmosphere. In these cases, an inert gas is used to blanket the material in the tank at a pressure slightly above atmospheric. Pressure control is achieved with split range control valves, as shown in Figure 3.7(C). If liquid is withdrawn from the tank, the pressure will decrease and the controller will open valve PV-1, allowing nitrogen to restore the pressure to set-point. If the tank fills with liquid, the pressure will increase and the controller will close valve PV-1, and then open PV-2 to let excess nitrogen out of the tank. 

For a condenser operating at atmospheric pressure, an adequate vent is all that is necessary. However, air is often not suitable for contact with the process due to concern about contamination or flammability. In these cases, the vent may be connected to a source of low pressure nitrogen, or other inert gas. For vacuum operation, the vent must also be connected to a vacuum pump or steam jet (eductor), as shown in Figure 3.11(B). The pressure controller adjusts the split range control valves such that as its output decreases, first PV-2 closes then PV-1 opens. Normal operation would... 

Figure 20.10 (a) Reactor system with split-range control (b) action of two valves. 

What is split-range control In Example 20.6 we have a situation with split-range control. To control the pressure in the reactor we could use valve V, or valve V2 with simple control configurations or both valves in a split-range control configuration. Which of the three is better Why ... 

Unlike the cascade and selective control schemes examined in Sections 20.1 and 20.2, the split-range control configuration has one measurement only (controlled output) and more than one manipulated variable. 

Example 20.6 Split-Range Control of a Chemical Reactor... 

Figure 20.11 Steam header with split-range control.

Example 20.7 Split-Range Control of the Pressure in a Steam Header... 

Split-range control discusses several interesting practical systems used in chemical processes, such as several boilers discharging into a common steam header, and parallel compressors discharging into a common header. 

Recall Examples 20.6 and 20.7 on split-range control. Notice that the number of manipulated variables used for control is larger than the number of controlled outputs. Therefore, eq. (23.4) determines the minimum number of required manipulations. 

Split range control. Often, a split-range pressure control is used. With this method, pressure may be normally controlled by venting a small vapor product stream. If the pressure falls and vent closure is insufficient to reinstate it, one of the other control methods automatically cuts in. 

Fig. 2.8-17 Control strategies, a) Cascade control for the example level controls quantity , b) Ratio control for the example quantity of B controls quantity in a constant ratio , c) Split-range control for the example of temperature controls cold water In the range of 0.2—0.6 bar, and hot water in the range of 0.6—1.0 bar .

Pressure control before and after hydrogen compressors is by standard techniques. The consumption rate is not always matched to the production rate, and some of the gas may be vented from time to time. A split-range controller managing both product hydrogen and vent valves can handle this situation. Vent system design requires caiefiil consideration of the hazards of hydrogen (Section 9.2.6). 

The control strategy, presented in Fig. 11.35, provides smooth transfer with precise control. It must allow controlled venting as well as controlled transfer to compression, and so it requires two control valves and split-range control. The cell room header pressure is measured by a d/p cell with a Monel diaphragm. If desired, a flush-type cell can be used. It is located on top of the header as it leaves the cell room. It is important that connections to the d/p cell be freely draining. 

Figure 16.14 Split range control a) control loop configuration, b) valve position-controller output relationship.

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