TABLE OF CONTENTS This chapter discusses a number of useful control techniques that are not covered in previous chapters. The topics included are:
Split-range control
Cross-limiting control
Floating control
Techniques for increasing effective valve rangeability
Time proportioning control
A common application for split-range control is when a temperature control loop must at times apply heat to a process and at other times must apply cooling. This is normally accomplished by using two valves, one called the "heating" valve, the other the "cooling" valve. In the usual installation, each valve operates through one-half of the controller's output range, applying the maximum heating or cooling at the extremes of the controller's output range. At the midpoint of the range, neither heating nor cooling is applied.
Figure 16-1 shows a traditional installation using pneumatic instrumentation. The 3 15 psi controller output goes to both valves in parallel. From 3 9 psi (0 50%), the cooling valve strokes from open to closed. From 9 15 psi (50-100%), the heating valve strokes from closed to open. If valve-spring ranges alone are used to achieve this schedule, then the heating valve is selected to be fail-closed and the cooling valve to be fail-open.
Alternatively, valve positioners can be used, rather than relying on the valve-spring ranges to achieve the schedule. Not only will this provide more accurate and precise range splitting, but it will also make the overall design of the installation more flexible. For instance, if the application requires that both valves be...
