Split-range

This is pressure drop (including friction loss) between run and branch, based on velocity in the mainstream before branching. Actual value depends on the flow split, ranging from 0.5 to 1.3 if mainstream enters run and from 0.7 to 1.5 if mainstream enters branch. 

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). 

The eontrol of the proeess is based on the reaetor-to-regenerator pressure differential. The pressure differential signal will be transmitted to the expander inlet butterfly eontrol valve and expander bypass eontrol valve, whieh will operate on split range eontrol. 

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. 

Figure 5.21. Batch reactor with temperature control, adding steam and water via a split-range...

The data from all of these spectra are compared in Table II, which lists the quadrupole splitting and the isomer shift (relative to iron in palladium) in millimeters per second and the ferric-ferrous ratio (as obtained from the Mossbauer data, by comparing the areas of the two sets of lines). The variations in the quadrupole splitting, ranging from 1.84 to 2.08 mm./sec., are certainly outside the range of experimental error and must be attributed to actual variations in the different tektites studied. The only type of tektite for which we measured many diflFerent samples was the indochinite, and the quadrupole splitting and isomer shift of these samples were the same for all samples within experimental error. Hence, these two parameters would be useful in verifying the classification of different types of tektites. The difficulties involved in... 

Valve positioners are little feedback controllers that sense the actual position of the stem, compare it with the desired position as given by the signal from the controller and adjust the air pressure on the diaphragm to drive the stem to its correct position. Valve positioners can also be used to make valves open and close over various ranges (split-range valves). 

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. 

Split-range system is used so that steam to the auxiliary leboiler is only used when insulGcient heat is available from the vapor from the first column. 

Level indicator, controller High-pressure source Split range Relief valve... 

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. 

Figure 4.1 Batch reactor split-range temperature control.

The effect of heat transfer area is illustrated in Figure 4.3. Three different areas are used. The temperature controller is proportional with a gain of 0.1 (dimensionless using a 50-K temperature transmitter span and split-range hows shown in Figure 2.1). The set-point is ramped to 340 K in 60 min. Clearly in the numerical example, a jacket-cooled batch reactor of the size selected (2 m diameter) and with the given heat of reaction would produce runaway reactions. An external heat exchanger with 4 times the jacket area would be required to catch the reaction. 

The split-range hot-water valve is set up in this example to be wide open when the controller output is 100% and closed when the output is 75%. This gives a gap between the two control actions that seems to work better in this example. 

So it is important to vent off some vapor to get rid of the ethane, but not too much. The vent valve (AC) is split-range, so that it is closed when the pressure controller output signal is 83% of full scale. It is wide open when the controller output is at 0%. The sizing of the steam, chilled water, and vent valves is critical to the safe and efficient operation of this batch reactor. Figure 4.42 gives a sketch of the reactor, the controller, the setpoint generator, and the three control valves. 

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. 

ZEB control schemes include the single set point split-range approach (shown on left) and the dual set point approach (shown on right). 

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-... 

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