One of the most common ways of addressing multiple process inputs is split range control. A splitter block, which appears as one valve to the PID block, can be used in the control strategy to define a fixed relationship between the controller output and each manipulated process input. In Advanced Control Foundation – Tools, Techniques, and Applications, Chapter 3 we used the following example in the discussion of split range control. To achieve consistent control behavior, the splitter block setup must account for the gain associated with each process input. Unfortunately, in many installations the split range setup was arbitrarily defined during commissioning as an equal split among the process inputs. Such a setting can result in sluggish or unstable operation when the controller output transitions from one input to another input.
The manner in which the splitter block setup accounts for the gain associated with each process input may be illustrated by considering a steam header pressure control application. One or more power boilers may be used to meet the process steam requirements of a plant. Also, this steam may be used in turbine generators to meet some or all of the plant requirements for steam. Steam turbines used to generate electricity are powered by high pressure steam. For example, in newer installations, the supply header may be at 1475 psi. The pressure of the steam is reduced as it flows through the turbine, allowing lower pressure steam to be extracted at various points. This lower pressure steam may be used to meet the plant’s process steam demands. For example, at one point, the extraction may be regulated to meet the steam demands of a 400 psi header. Extraction valves associated with the turbine are automatically regulated to maintain the lower header pressure constant.
To allow the plant to continue operation if a turbine or generator fails and must be shut down, pressure reducing valves (PRVs) between the high pressure header and the lower pressure header can be adjusted to meet the lower pressure header steam demand and to maintain the lower header pressure constant. This can be accomplished by using a splitter block in conjunction with a PID block to adjust the pressure-reducing valves as illustrated below.
If the valve sizes or operating conditions differ from loop to loop it is necessary to characterize the splitter to compensate for these differences through the setup of the characterizer block. When a loop that includes a split range output has low utilization, examine the assumptions made in the split range setup and, if necessary, modify the setup to provide a constant process gain as seen by the controller.