Set point changes

A real-time optimization (RTO) system determines set point changes and implements them via the computer control system without intervention from unit operators. The RTO system completes all data transfer, optimization c culations, and set point implementation before unit conditions change and invahdate the computed optimum. In addition, the RTO system should perform all tasks without upsetting plant operations. Several steps are necessaiy for implementation of RTO, including determination of the plant steady state, data gathering and vahdation, updating of model parameters (if necessaiy) to match current operations, calculation of the new (optimized) set points, and the implementation of these set points. 

The reactor should be connected to a power source for heating and running the motor. Wall thermocouples should give a signal to a temperature controller. Inside thermocouples should be read and the set point changed on the wall temperature controller to get the desired result... 

The applicability of these findings for actual operational tasks was evaluated by considering the incidence of data entry errors recorded by the on-line plant computer system over the 24-hour shift cycles. It was judged that the data entry task, which involved evaluating the set point changes needed for... 

Action Right action on wrong object Set point changed on wrong controller Same as above Same as above Clearly label controllers to distinguish among set point controls... 

Fig. 6. Performance of control system with set point change in H2 production rate by 10% at 15 sec...

To reduce derivative kick (the sudden jolt in response to set point changes), the derivative action can be based on the rate of change of the measured (controlled) variable instead of the rate of change of the error. One possible implementation of this idea is in Fig. 5.3. This way, the derivative control action ignores changes in the reference and just tries to keep the measured variable constant.2... 

Ideally (meaning unlikely to be encountered in reality), we would like to achieve perfect tracking of set point changes C = R. Reminder, we are working with deviation variables. 

In terms of controller design, we can take an entirely analytical approach when the system is simple enough. Of course, such circumstances are not common in real life. Furthermore, we often have to compromise between conflicting criteria. For example, we cannot require a system to have both a very fast rise time and a very short settling time. If we want to provide a smooth response to a set point change without excessive overshoot, we cannot also expect a fast and snappy initial response. As engineers, it is our job to decide. 

By and large, a quarter decay ratio response is acceptable for disturbances but not desirable for set point changes. Theoretically, we can pick any decay ratio of our liking. Recall Section 2.7 (p. 2-17) that the position of the closed-loop pole lies on a line governed by 0 = cos C In the next chapter, we will locate the pole position on a root locus plot based on a given damping ratio. 

The slave loop will have a 10% offset with respect to desired set point changes in the secondary controller. 

We want the controlled variable to track set point changes (R) precisely, so we substitute the ideal scenario C = R, and rearrange Eq. (10-4) to... 

Since we do not have the precise model function Gp embedded in the feedforward controller function in Eq. (10-8), we cannot expect perfect rejection of disturbances. In fact, feedforward control is never used by itself it is implemented in conjunction with a feedback loop to provide the so-called feedback trim (Fig. 10.4a). The feedback loop handles (1) measurement errors, (2) errors in the feedforward function, (3) changes in unmeasured load variables, such as the inlet process stream temperature in the furnace that one single feedforward loop cannot handle, and of course, (4) set point changes. 

Proportional gain, integral and derivative time constants to PI and PID controllers. Cohen-Coon was designed to handle disturbances by preventing a large initial deviation from the set point. The one-quarter decay ratio response is generally too underdamped for set point changes. 

If there are any differences it then adjusts the steam valve. If the downstream pressure changes, a correction in the control valve is made immediately, instead of waiting for a product temperature change. Should the output temperature of the process stream rise, this would cause a set point change of the steam-pressure controller, which would cause a decrease in the steam pressure in the heat exchanger. Cascade control is very useful when the variation in the quality of a utility or other manipulable stream can cause deviations from the desired output. 

The closed loop transfer function for a set-point change is given by ... 

Plantwide real-time optimization Set point changes... 

Set point changes, 20 667 Setter tile, 21 481 Setting operations... 

Set the initial concentration profiles in the two phases equal to the steady state open loop response (Kc= 0), and study the response of the system to another set point change or flow rate change. 

Dynamic Behavior of the Heavy Component (Set Point Change)... 

When a set point change from 0.986 to 0.99 for the heavy component (C) was considered, the responses shown in Figure 5 were obtained. The three options show good d mamic responses, with relatively low values of settling times, lower than 0.1 hours. The systems with unidirectional flows show some overshooting, while the Petlyuk system offers a more sluggish response. 

Fig. 5. Closed loop responses for a set point change in the composition of the heavy component...

Table 2. Set-point changes induced along the experiment...

Control can be exercised by the operator in the control roam on the well chokes and manifold divert valves (although this is normally handled by the HOC), start/atop of the oil shipping pumps, utility equipment, switching valves, gas plant feed valves, gas compressor start/stop and emergency shutdown of any production train or the entire GC. Since the closed loop process control is performed by local pneumatic instruments, all set point changes and controller troubleshooting must be accomplished locally. 

This is employed when the process is not well-known. The Model Reference Adaptive Controller contains a reference model to which the command signal or set point change is applied as well as to the process itself (Fig. 7.98)<4 ). The output of the reference model is postulated as the desired controlled process output and this is compared with the actual process output. The difference (or error) e , between the two outputs is used to adjust the controller parameters so as to minimise the relevant integral criterion. For example, if the ISE criterion is employed then the quantity... 

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