Split-range valves

Both poor calibration situations will severely impact controller performance. Further, if the gas, being used to pressurise the column through valve A, is valuable and depressur-isation through valve B is to some waste gas system, then the overlap would mean that both valves are open simultaneously and incurring an unnecessary cost. 

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 

In many cases split-ranging is a perfectly satisfactory way of meeting the control objectives. However, poor performance may go unnoticed if the controller spends the majority of the time in one part of the range. It is also possible to split the controller output into more than two parts if there are more MVs available to extend the operating range. With more splits, accurate valve calibration becomes more important and it is more likely fliat the there will a greater variation in process dynamics. 

With pneumatic control systems one advantage of split-ranging is that both valves can be connected to the same pneumatic signal line, thus saving the cost of the second line. With 

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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. 

Decanter Models in Aspen. Up to this point, we have been using the Decanter model in the simulations. It assumes that there are only two phases, both liquid, and that the decanter pressure is fixed at the specified design value (pressure does not change with temperature or composition). These assumptions do not reflect reality but appear to suffice for steady-state design. In a real decanter, there is a vapor space above the two liquid phases, and pressure will vary with compositions and temperatures unless an inert gas is used to keep a constant pressure via a vent-bleed split-ranged valve setup. 

FIGURE S.11 2 Sdiemes for control of column pressure (a) pressure control for an atmospheric column (vent bleed to atmosphere) (b) split range valves in a block and bleed anangement (vent Weed to vacuum) . (c) hot vapor bypass pressure control and (flooded condenser pressure control. 

PI control Simple proportional-integral SISO loops provide effective control of the vast majority of all chemical plants. These systems require process understanding to set up, rational tuning methods, the use of overrides to handle constraints and split-ranged valves to handle the case where several manipulated variables can be used to control a single controlled variable. 

Determining minimum required flow is even more difficult. Fortunately most wet chemical processes seldom have a turndown (maximum required flow/minimum required flow) greater than two or three to one for normal operation. Startups and shutdowns, however, often require very low flows for awhile. A suggested typical process turndown figure for ve sizing and selection is therefore five to one. Valve-flow turndown is defined here as the ratio of flow not Cp) at 95 percent lift to that at 10 percent lift. If, for a given application, calculated valve-flow turndown is less than process turndown, two or more split-ranged valves may be required. 

Water-cooled condensers and coolers are widely used in the chemical and petroleum industries. Smdies have shown that cooling-water turndown requirements are usually much higher than process turndown. In fact, cooling-water turndown may be proportional to the square or cube of process turndown. Many cooling-water valves are nearly closed in the winter and wide open in summer because of the difference between minimum winter load and maximum summer load. In many applications, dual, split-ranged valves should be used. 

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