Speed of control

An auxiliary controller can be used to increase the accuracy and speed of control. Often... 

Control rods are used primarily for power distribution shaping and for shim control of long-term reactivity changes, which occur as a result of fuel irradiation. The flow control function, which is used to follow rapid load changes, reduces requirements on speed of control rod response and thus improves plant safety. Every 2-3 months, the control rod patterns are altered to provide more uniform fuel and control rod burnup. In normal daily operation, little control rod movement is required for depletion of reactivity. The resulting low frequency of control rod changes reduces the possibility of operator error. 

Speed of control depends on response time of sensor, location, time for transmission of error signal (i.e deviation from set point), force available to operate the... 

The primary system must have speed of control insertion adequate to (a) limit a power excursion from credible accidents which would In themselves result In damage, (loss of coolant, for example,) to one of a magnitude such that the ultimate consequences of the excursion are no worse than those which would have resulted from the causative accident alone, or (b) limit a power excturslon from other credible accidents (such as fast rod withdrawal) to one of a magnitude such that no damage beyond mild fuel stressing will result. 

The second safety control criterion is a restatement of the Hanford Speed-of-Control Criterion. Since the water-loss accident in N Reactor does not result in a rapid increase in reactivity as is the case in the present reactors, the accident... 

In order to assess the degree to which the safety systems meet both these speed-of-control requirements, it is first necessary to consider the sources and mag nltudes of the reactor excursions Then, peurtlcular exciursions are examined to determine whether the criteria are indeed satisfied. 

Inoperable Ball Column Limit 1 6 Total Control Considerations 1 7 Speed of Control Considerations... 

The second safety control criterion (Section 1.5.1.2) is a reatatsnent of the Hanford Speed-of Control Criterion. Since the water-loss accident in N Reactor does not result in a rapid increase in reactivity as is the case in the present reactors, the accident governing the response of the safety systems is one associated with rod-withdrawal. Since the horisontal rods combine both operational and safety functions, the limitation on withdrawal rate specified in the operational control criteria can alao be included in this discussion. 

The safety philosophy requires that the primary safety control system has a response which is fast enough to limit any credible power excursion to one of a magnitude that no damage beyond mild fisel stressing will result. Since the magnitudes of the credible reactivity ramps in N Reactor are considerably smaller than in the older Hanford reactors, speed of control in H Reactor is not so severe a problem as it is in the present reactors, nevertheless, the scram... 

The environmental model is the product of a problem analysis. It forms the basis for the development of behavioral models of the process. This holds for physical models, in which the internal dynamic mechanism is described by pltysical laws, as well as empirical (black box) models, in which only overall dynamic relationships between process inputs and outputs are formulated. The environmental model can also be used for the development of a process control scheme. By evaluating the static (power of control) and dynamic relationships (speed of control) between process inputs and outputs, the process control scheme can be selected. In general no dynamic model of the process is required yet at this stage. This is the starting point that is used in the chapters on process control. 

The transfer function of the process Gp can, to a certain extent, be affected by the choice of the control scheme. It is important to choose a high gain and fast response. This will result in maximum power and speed of control, as will be explained further in section 33.1.8. However, the possibilities to achieve this are limited. 

In this chapter, the design of a control scheme for an entire plant will be discussed. On the basis of the relationship between process outputs and inputs, the control scheme will be developed. The first part of the procedure is similar to the procedure for the development of an environmental model, which is identifying the inputs and outputs of the process. Measurement problems and costs of the correcting devices, however, should now also be taken into consideration. The result of this procedure is a table with interactions, in which the relationships between the manipulated and controlled variables is shown. The static relationship determines the power of control the dynamic relationship determines the speed of control. The design procedure is illustrated by an example. Subsequently, methods for optimization and extension of the control scheme are discussed. 

Sometimes a weighting between power of control and speed of control is necessary. An example of such a situation is shown in Fig. 33.4. 

Fig. 33.4. Difference in process dynamics leading to difference in power and speed of control.

When the temperature sensor is located in the top of the column, the speed of control for the reflux is high. However, owing to small temperature fluctuations, often within 0.1 °C, the power of control is limited. If the temperature sensor is located in the middle of the column, the power of control is at its maximum and the speed of control is lower. 

Table 33.5. Interaction table showing power and speed of control for the desulphurization process.

For every pair of comectmg and controlled variables, the power and speed of control should be investigated. This results in a table, which can serve as a guide to develop a basic control scheme. The relationships shown in the sequel were developed by Rademaker et al. (1975) and discussed in Luyben et at. (1992). 

The conclusion is that C and H are suited for pressure control (the speed of control is determined by small time constants ), unless, in the case of H, the composition selfregulation in the condenser is too strong. 

The name material balance control was introduced by Shinkey (1984). The different control schemes that the author developed were based on the concept of relative gains (= power of control) of the different input-output combinations. Speed of control was only considered as a secondary factor. A simple explanation is given by Ryskamp (1980). Also Van der Grinten (1970) presented a nrrmber of common control schemes for distillation colnmns. The latter author used behavioral models in the eontrol scheme selection procedure. None of the mentioned references takes inverse responses into accoimt when X > 0.5. In the case of the more traditional approach, the energy balanee eontrol, the reflux ratio and/or vapor flow is used to eontrol the top product qrrality, while the distillate and bottom flow are nsed to maintain the mass balanee. In the ease of the material balance control, one of the prodnct flows is used to control product qrrahty, while the other product flow maintains the material balance. 

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