Self-Regulating Process Response

As discussed in Chapter 9, the process dynamics of most process variables can be characterized as a self-regulating process response. When the controller output changes the automatic valve position of the manipulated variable, the process variable moves to a new steady-state value. 

The most common model of a self-regulating process response is a first-order plus dead time (FOPDT) response. 

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Know the difference between a self-regulating process response and an integrating process response... 

Measure the response time for a self-regulating process variable to reach the first peak after a step-up change in setpoint when tuning a self-regulating process response... 

The process dynamics of most process variables can be characterized as a self-regulating process response. One example is the response of a liquid flow rate when a valve position is opened. The liquid flow rate will increase from the initial flow rate to a new steady-state flow rate. Another example is the response of the temperature of a liquid flowing through a heat exchanger that is heated with steam. When the steam valve position is increased, the temperature of the liquid outlet will increase to a new steady-state temperature. 

This chapter described the concept of a self-regulating process response and an integrating process response. Several methods were described for tuning a control loop while the controller is in automatic output mode. These include the trial-and-error method, the Zlegler-Nlchols ultimate gain method, and the Robbins pattern recognition methods. 

The open-loop process gain, K, for a self-regulating process response is calculated from Equation 10.1. 

Recognize the desired response pattern when a step change in setpoint is introduced to the controller for a self-regulating process variable with optimum... 

The recommended method for tuning level control loops is given in Section 9.8. It is different from the method for tuning controllers with a self-regulating process variable response. 

The term open-loop unstable is also used to describe process behaviour. Some would apply it to any integrating process. But others would reserve it to describe inherently unstable processes such as exothermic reactors. Figure 2.21 shows the impact that increasing the reactor inlet temperature has on reactor outlet temperature. The additional conversion caused by the temperature increase generates additional heat which increases conversion further. It differs from most non-self-regulating processes in that the rate of change of PV increases over time. It often described as a runaway response. Of course, the outlet temperature will eventually reach a new steady state when aU the reactants are consumed however this may be well above the maximum permitted. 

At the beginning of the step response, the self-regulating process resembles the non-self-regulating or integrating process. But after sufficient time, it resembles a process without dynamics. The first-order lag is thus made up of two components, one responsive to a fast-changing input, the other responsive to a steady input. This is apparent from examining the differential equation... 

The Federal Drug Administration (FDA) describes the biopharmaceutical industries as self-regulated, retaining for itself the responsibility of assuring and checking on that self-regulatory process. Not surprisingly, then. 

Write the time response solutions to the integrating process in (3-14) when the input is (a) a unit step and (b) an impulse. How are they different from the solutions to a self-regulating... 

The dynamics of four different tubular reactor systems have been explored. The cooled reactor has the best dynamic response because of the internal heat transfer helps to provide some self-regulation. As discussed in Chapter 5, the cooled reactor is also the best from the standpoint of steady-state economics. This combination of both superior steady-state and dynamic performance is quite unusual in chemical processes. The typical situation is that there is a conflict between economics and dynamics. 

Self-regulation through an appropriate QA review allows your company to take control of the process and to assume appropriate responsibility. 

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