Non-self-regulating process

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. 

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. 

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 level in the tank could be controlled by manually adjusting the valve position, thereby setting inflow. But if inflow varied in the slightest from outflow, the tank would eventually flood or run dry. This characteristic is called non-self-regulation. It, means that the integrating process cannot balance itself-it has no natural equilibrium or steady state. The non-self-regulating process cannot be left unattended for long periods of time without automatic control. 

Whereas the non-self-regulating process cycled uniformly with an integrating controller, the self-regulating will not. The phase shift of the self-regulating process only reaches -90 at a period of zero. As a result, the loop could only oscillate at zero period, where the gain of both process and controller are zero. The loop cannot, therefore, sustain oscillations. 

Notice that as ti approaches zero, P approaches infinity. This is much worse than having no capacity at all, i.e., dead time alone. The reason is that this expression holds only for a non-self-regulating process whose gain varies inversely with the time constant without limit. Fortunately, non-self-regulating processes dominated by dead time are virtually nonexistent. 

Loops containing two integrations are capable of a limit cycle, however. An example would be a non-self-regulating process such as liquid level, with a proportional-plus-reset controller. The gain product of the two integrating elements will vary as the square of the period, more than can be offset by the gain of hysteresis. Under these conditions, loop gain varies inversely with amplitude. 

As with other self-optimizers, this, too, is an integrating device. It therefore cannot be used on non self regulating processes. This class of processes has no equilibrium, no steady state, so the characteristic curve never comes to rest it is always floating. Unfortunately, floating control action cannot hold it. 

This example illustrates that integrating process units do not reach a new steady state when subjected to step changes in inputs, in contrast to first-order processes (cf. Eq. 5-18). Integrating systems represent an example of non-self-regulating processes. Closed pulse inputs, where the initial and final values of the input are equal, do lead... 

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. 

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. 

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). 

IMost liquid-level processes are non-self-regulating occasionally other processes will exhibit this characteristic. In general, it is not harmful as long as its peculiarities are taken into account. One of these peculiarities is its phase shift. Like the integrating controller, the non-selfregulating process exhibits a phase lag of 90° to any periodic wave. Consequently ... 

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... 

Adding another lag anywhere in the loop will change the previous level process to two-capacity, as shovm in Fig. 1.20. A chamber is attached to the tank although we wish to control tank level, chamber level is measured, which lags behind tank level. The time constant of the chamber is its volume divided by the maximum rate at which hquid can enter. This time constant will be designated n. Control of a two-capacity process is easiest to illustrate if one of the capacities is non-self-regulating. 

Fortunately we already Investigated this problem when we discussed Integral control of dead time. Figure 1.25 indicates the similarity of the loops. If the process Is non-self-regulating (integrating), the representation Is exact. Because the phase lag of the dead time is limited to 90°,... 

Reset, then, is necessary if offset is to be eliminated altogether. Whether proportional and derivative are useful modes depends on the nature of the process. If rapid load changes outside the forward loop may be encountered, proportional and derivative action could be advantageous. If the process Is fundamentally non-self-regulating, as in level control, proportional action Is essential. Finally, if the process is fairly easy to control because of the absence of dead time, derivative may be useful in Improving the dynamic load response-but this is unusual. 

Equation (10.24) shows that, because of the recycle loop, the process is non-self-regulating. Consequently an integral controller cannot be used to regulate composition. This rules out any kind of feedback-optimizing control system. But because of the lack of self-regulation, end-point... 

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