Valves, control split-ranging

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. 

Operators generally interpret a controller output of 0% as a signal to close the valve and 100% to open a valve. With split-ranging it may not be possible to display the signal in this way so requiring special attention to operator training. This can be complicated by some... 

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. 

Fig. 1. Batch reactor with temperature control, adding steam and water via a split-range valve to the reactor jacket.

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. 

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

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. 

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. 

Control of the steam valve is also quite simple. It is normally handled in the conventional split-range manner, using the upper half of the PID output range. That is, the output to the steam valve is twice the controller output minus 100%, with a practical lower limit of 0%. Of course, if the jacket drain timer has not expired, the steam output must be forced to 0%. The steam valve logic will also normally incorporate several other interlocks. 

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 ... 

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