Control system split-range

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 differential pressure controller acts in split range on the inlet control valve and the bypass valves. The differential pressure governor is retained as the standby and backup system. 

The fix for the erratic reflux drum pressure problem was to provide for separate pressure control of the fractionator column and the reflux drum. A new pressure control valve was installed upstream of the condenser and the old condenser outlet control valve was removed. A hot gas bypass, designed for 20% vapor flow, was installed around the pressure control valve and condenser. A control valve was installed in the hot gas bypass line. The column pressure was then maintained by throttling the new control valve upstream of the condenser. The reflux drum pressure w as controlled by the hot gas bypass control valve and the psv saver working in split range. The new system is shown in the figure below. 

Turbine speed is controlled by two split-range valves, one on the 10 psig inlet to the turbine and the other on the 100 psig steam that can also be used to drive the turbine. Your instrumentation system should be designed so that the valve on the 10 psig steam is wide open before any 100 psig steam is used. 

To illustrate some of the design and control issues, a vessel size (DR = 2 m, VR = 12.57 m3, jacket heat transfer area Aj = 25.13 m2) and a maximum reactor temperature (7j) ax = 340 K) are selected. The vessel is initially heated with a hot fluid until the reaction begins to generate heat. Then a cold fluid is used. A split-range-heating/ cooling system is used that adds hot or cold water to a circulating-water system, which is assumed to be perfectly mixed at temperature Tj. The setpoint of a reactor temperature controller is ramped up from 300 K to the maximum temperature over some time period. 

All of these systems have some common control loops. The system pressure is controlled by manipulating the fresh feed of A (F0A). The concentration controller with ratio control is used to control reactor inlet gas composition by manipulating the fresh feed of B (F0B). Bypassing (Fhy) around the FEHE is used to control gas mixture temperature Tmix. Reactor inlet temperature (Tin or T ) is controlled by manipulating the furnace heat input QF. The setpoints of these two temperature controllers are the same, and the controller output signals are split-ranged so that bypassing and furnace heat input cannot occur simultaneously. 

The limitations of such a split-range operation include that near 50%, the system can be unstable and cycling, and when the signal is between 50% and 100%, the shell side of the exchanger can become a reservoir of cold heat transfer fluid. This upsets the control system twice once, when the system just begins to heat, and once when the cold fluid has been completely displaced and the outlet temperature suddenly rises. Finally, most of these sys-... 

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. 

In the modified setup (see colored lines in Fig. 2), Valve A feeds the makeup gas directly into the condenser the makeup also functions as the purge gas, to dilute and sweep out residual trapped corrosives (as well as noncondensible light components). The split-range system is converted to a dual-purpose one that regulates the pressure and controls corrosion, without any noticeable effect on the controlled pressure. 

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

Part V (Chapters 19 through 22) deals with the description, analysis, and design of more complex control systems, with one controlled output. In particular, Chapter 19 introduces the concept of feedback compensation with Smith s predictor, to cope with systems possessing large dead times or inverse response. Chapter 20 describes and analyzes a variety of multiloop control systems (with one controlled output) often encountered in chemical processes, such as cascade, selective, and split-range. Chapter 21 is devoted exclusively to the analysis and design of feedforward and ratio control systems, while Chapter 22 makes a rather descriptive presentation of adaptive and inferential control schemes why they are needed and how they can be used. 

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. 

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