Selective Control Loops

Frequently a situation is encountered where two or more variables must not be allowed to pass specified limits for reasons of economy, efficiency, or safety. If the number of controlled variables exceeds the number of manipulated variables, whichever ones are in most need must logically be selected for control. (This is the case of the squeaky wheel getting the grease.) Automatic selector units are available for this type of service. They are employed in four basic areas of application  

As an example of how equipment might be protected by a selective control system, eonsider a compressor whose discharge is ordinarily on 

Motor speed is manipulated by whichever controller has the lower ouput. 

A high selector is used to permit control of the peak reactor temperature. 

To protect against an instrument failure placing the plant in a hazardous condition, key instruments can be duplicated. Often, duplicated 

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It should be noted that interactions between control loops is not just limited to interacting units but will also occur within single units. A typical example is an exothermic reactor where a change in the control loop which controls the level in the reactor will have an effect on the amount of material in the reactor. This in turn will affect the heat removal requirements and, therefore, the cooling water control loop. Many strategies for reducing loop interactions, and for selecting control loops so as to minimise interactions, can be found in most standard control textbooks.1-11... 

Chapter 20. Chapter 6 of Shinskey s book [Ref. 3] is an excellent reference for multiple-loop control systems. It treats cascade, selective control loops, and adaptive systems. Besides the general treatment of each control configuration, it discusses the practical considerations guiding the selection and design of such control systems. In particular, it covers the following items which could attract the interest of the reader ... 

Selective control loops covers a large number of diversified selective control systems for protection of equipment, auctioneering, redundant instrumentation, variable structuring, and so on. 

Discuss again the arguments that were used in the text to select control loop configuration 3 for the benzene distillation column. 

Shows how to use quantitative analysis with steady-state and dynamic relative gain arrays (RGA and DRGA) to select control loop pairings reliably and to use the IMC model-based approach to provide preliminary tuning of single-loop PI controllers. 

If ever the number of variables desired to be controlled exceeds the number available to be manipulated, the latter must be shared among the former on a logical baas. (Several of these situations were described in the previous chapter under Selective Control Loops.)... 

Figure 16.16 Block diagram for the selective control loop with two measurements and two controllers.

Installed control valve perfonnance is refeired to control valve performance including associated process system and control loop elements. Flow through control valve depends on the control valve selected (such as valve type valve trim, actuator, etc.), associated process system (such a.s equipment/instrument and piping in the system), and selected control loop elements (such as controller, positioner, etc.). It is the real performance measurement of a control loop. A go control loop should be able to control the process parameter within an acceptable range. 

Feedback Control In a feedback control loop, the controlled variable is compared to the set point R, with the difference, deviation, or error e acted upon by the controller to move m in such a way as to minimize the error. This ac tion is specifically negative feedback, in that an increase in deviation moves m so as to decrease the deviation. (Positive feedback would cause the deviation to expand rather than diminish and therefore does not regulate.) The action of the controller is selectable to allow use on process gains of both signs. 

The selection of controlled and manipulated variables is of crucial importance in designing a control system. In particular, a judicious choice may significantly reduce control loop interactions. For the blending process in Fig. 8-40(d ), a straightforward control strategy would be to control x by adjusting w, and w by adjusting Wg. But... 

Identify the key process variables that need to be controlled to achieve the specified product quality. Include control loops using direct measurement of the controlled variable, where possible if not practicable, select a suitable dependent variable. 

Although many pressure relief devices are called SRVs, not every SRV has the same characteristics or operational precision. Only the choice of the correct pressure safety device for the right application will assure the safety of the system and allow the user to maximize process output and minimize downtime for maintenance purposes. Making the correct choice also means avoiding interference between the process instrumentation set points in the control loop and the pressure relief device limits selected. These SRV operational limits can vary greatly even when all are complying with the codes. 

Each instrument in this control loop has its tolerances, for instance +/ — 5%. To ensure smooth operation, tolerances should never interfere with each other. Also, the SRV should be selected so that it does not start to open under the highest pressure switch setting plus its tolerance. Therefore, it is important to know the tolerance of the pressure relief device, or in this case the SRV, and the same applies for the SRV closure. In short, tolerances should never interfere... 

Figures 7. Simulated start-up of vinyl acetate polymerization at low emulsifier level (0.01 mol/L H20) under closed-loop control with arbitrarily selected controller tuning constants and manipulation of initiator flow rate at 50°C conversion in R1—STD feedback (--------------------------) vs. DTC (----)...

In a similar way, a control signal is moved to the flowsheet and positioned on the arrow pointing out of the LC controller. The window at the bottom left of Figure 3.64 opens on which we select LC.OP (the OP signal from this controller). Then the cursor is clicked on the control valve V3 on the flowsheet in the product line. Figure 3.65 shows the level control loop installed on the flowsheet. 

In selective and cascade control loops, external feedback is the most-often-applied solution. Here, instead of looking at its own output, which can be blocked, the integral mode of the controller looks at an external feedback signal (such as the opening of the valve), which cannot be blocked. In surge control or reactor heat-up applications, the chosen solution usually is to use the slave measurement as the external reset signal to prevent saturation. 

The cause of control loop cycling can be that they interact with other loops. When loops interact, it is necessary to make sure that their response speeds are not even similar. To avoid oscillation, one should select response speeds that differ by a factor of 3 to 10, depending on the degree of interaction. In tuning interacting loops, one would place the downstream loops in manual while tuning the "upstream loop," and once the upstream loop s speed of response is determined, use a multiple of that to set the downstream controllers. 

An addition to the noted advantages is that the set point of the secondary controller can be limited. In addition, by speeding up the overall cascade loop response, the sensitivity of the primary process variable to process upsets is also reduced, whereas the secondary loop can reduce the effect of control valve sticking or actuator nonlinearity. The primary or outer control loop of a cascade system is usually a PI or PID controller. A properly selected secondary will reduce the proportional band of the primary controller. 

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