Non-self-regulating

This is called an integrating (also capacitive or non-self-regulating) process. We can associate the name with charging a capacitor or filling up a tank. 

Dynamic responses can be divided into the categories of selfregulating and non-self-regulating. A self-regulating response has inherent negative feedback and will always reach a new steady-state in response to an input change. Self-regulating response dynamics can be approximated with a combination of a deadtime and a first-order lag with an appropriate time constant. 

If a process settles at a new steady state after an input change, the process is referred to as self-regulating. Levels in tanks, accumulators, and reboilers, and many pressure systems, behave as integrating processes. Consider the level in a tank for which both the flow in and the flow out are set independently. Initially, the flow out, is equal to the flow in, and the level is constant. Figure 15.8 shows the level as a function of time for a step change in the flow out at time equal to 10 s. Note that the level in the tank begins to decrease at a constant rate. This is an example of a non-self-regulating process, since the process does not move to a new steady state. 

A pure capacitive process will cause serious control problems, because it cannot balance itself. In the tank of Example 10.3, we can adjust manually the speed of the constant-displacement pump, so as to balance the flow coming in and thus keep the level constant. But any small change in the flow rate of the inlet stream will make the tank flood or run dry (empty). This attribute is known as non-self-regulation. 

Selection of controlled variables. Ca should be selected because it affects the product quahty directly (Guideline 3). T should be selected because it must be regulated properly to avoid safety problems (Guideline 2) and because it interacts with Q (Guideline 4). Finally, h must be selected as a controlled output because it is non-self-regulating (Guideline 1). 

Step 6. Fix a flow rate in every recycle loop and control vapor and liquid inventories (vessel pressures and levels). Process unit inventories, such as liquid holdups and vessel pressures (measures of vapor holdups), are relatively easy to control. While vessel holdups are usually non-self-regulating (Guideline 1), the dynamic performance of their controllers is less important. In fact, level controllers are usually detuned to allow the vessel accumulations to dampen disturbances in the same way that shock absorbers cushion... 

Step 6. Fix recycle flow rates and vapor and liquid inventories. The liquid inventories in the flash vessel and reactor are non-self-regulating, and therefore, need to be controlled (Guideline 1). Since the liquid product valve from the flash vessel has been assigned to control the product flow rate, the inventory control must be in the reverse direction to the process flow. Thus, the reactor effluent valve, V-4, controls the flash vessel liquid level, and the feed valve, V-1, controls the reactor liquid level. Both of these valves have rapid, direct effects on the liquid holdups (Guidelines 6, 7, and 8). The vapor product valve, V-5, which has been assigned to control the pressure in V-100, thereby controls the vapor inventory. 

Variables must be measured that might quickly deviate from the acceptable range such as (i) non-self-regulating variables, example levd, (ii) unstable variables, example some temperatures in reactors and (iii) sensitive variables that vary quickly in response to small disturbances, example pressure in a closed vessel. 

The fired heater that we have worked with is an example of a self-regulating process. Following the disturbance to the fuel valve the temperature will reach a new steady state without any manual intervention. Not all processes behave this way. For example, if we trying to obtain the dynamics for a future level controller we would make a step change to the manipulated flow. The level would not reach a new steady state unless some intervention is made. This non-self-regulating process can also be described as an integrating process. 

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. 

Rice, R. and Cooper, D.J. (2002) Design and tuning of PID controllers for integrating (non-self regulating) processes. Procedures of the ISA 2002 Annual Meeting. 

The process behaviour is different. It is the most common example of a non-self-regulating (or integrating) process. It wiU not, after a change is made to the manipulated flow, reach a new equilibrium. The level wiU continue moving until either the process operator or a trip system intervenes. This affects the way that we execute plant tests and the way that we analyse the results. 

Suppose the situation of the tank in Figure 12.17 is that the effluent flow restrictor is replaced by a fixed fiow rate. This is a non-self-regulating or integrating process. The mass balance is Eq. (69), as before [22]. 

With dynamic systems that are non-self-regulating one finds, likewise, step responses with and without dead time. An integrator plus dead time process would be modeled like Eq. (77). 

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