In some cases, a step change in a process input will result in the process output changing continuously. A process that exhibits such a response is known as an integrating process. As part of the chapter on process characterization in Chapter 9 of Control Loop Foundation, we address integrating process response. In the process industry, there are many examples of integrating processes such as level control of storage tanks and pressure in a gas or steam header.
When an input to an integrating process changes, the output may not begin to respond for a period of time because of measurement or transport delay. Thus, one parameter that may be used to characterize an integrating process is the process deadtime. Once the process output begins to change in response to a change in input, then the observed rate of change in the output is used to characterize an integrating process. Specifically, the rate of change in the output per percent change in input per second is defined as the integrating gain. Thus, the response of an integrating process in general is described by the process integrating gain and deadtime but in many cases, the deadtime is zero. The response of an integrating process and the calculation of the integrating gain are shown below.
Integrating processes are also described as non-self-regulating processes. An example of an integrating process is a tank where the flow into the tank is the manipulated input, the tank level is the controlled output, and the discharge from the tank is a gear pump as illustrated below and thus is not affected by the tank level.
In this tank level control example, if the inlet flow doesn’t match the outlet flow, then the level will constantly change until the tank overflows or goes dry.
The third workshop for the chapter on process characterization is designed to give the user experience working with a process that may characterized as exhibiting an integrating process response. By accessing the book’s web site, you may complete this third exercise on characterization using your web browser. The viewer below may be used to see the solution to this exercise.