Setpoint specification

Construction and equipment meet the design specifications. Obtaining field verification or performing document review for the new or modified process can validate design specifications for construction and equipment. If a change is not physical (such as a setpoint for an interlock shutdown), the method for the change and its anticipated effects should be reviewed. 

The very low storage temperature and low operating temperature specifications (see Section 4.3 below) also required dedicated design features because many of the components, such as the pumps and processors, cannot operate at very low temperatures. Because of this limitation there were added to several components cold start heaters controlled by snap switches. Further, an industrial temperature grade microprocessor was used in the SDU. With the snap switches the cold start heaters can come on when the power is applied at less than about 32°F/0°C. The snap switches cut off the cold start heaters and apply power to the full CBMS II system once their setpoint temperature is reached. 

The mode of control is the manner in which a control system makes corrections relative to an error that exists between the desired value (setpoint) of a controlled variable and its actual value. The mode of control used for a specific application depends on the characteristics of the process being controlled. For example, some processes can be operated over a wide band, while others must be maintained very close to the setpoint. Also, some processes change relatively slowly, while others change almost immediately. 

In MPC a dynamic model is used to predict the future output over the prediction horizon based on a set of control changes. The desired output is generated as a set-point that may vary as a function of time the prediction error is the difference between the setpoint trajectory and the model prediction. A model predictive controller is based on minimizing a quadratic objective function over a specific time horizon based on the sum of the square of the prediction errors plus a penalty... 

There are a number of time-domain specifications. A few of the most frequently used dynamic specifications are listed below (see also Prob. 6.11). The traditional test input signal is a step change in setpoint. 

The steadystate error is another time-domain specification. It is not a dynamic specification, but it is an important performance criterion. In many loops (but not all) a steadystate error of zero is desired, i.e, the value of the controlled variable should eventually level out at the setpoint. 

With these specifications for JffJ and Jf, we can calculate the minimal-prototype controller for ramp setpoint changes using Eq. (20.15). 

Automatic flow controllers have recently become available which operate in the 1-100 cm3/min flow range. These are voltage settable such that thumb wheels switches or potentiometers can be used to set the flowrate. These are specified at a repeatability of setpoint of about 0.2% and an accuracy of 1-2% of full scale. These flow controllers can easily be adapted to give a flow readout. The price of these units is still quite high at about ( 500 - 600) per channel but all indications are that they will be more widely used in the future. Typical specifications for three such devices are given in Table 6.4. 

In specific extraordinary situations it may be necessary to temporarily increase the setpoint or bypass or deactivate some type of instrumentation or other equipment safeguard. However, this should only be done in an established manner, according to appropriate Management of Change procedures with proper approval. Furthermore, any special defeat to a safeguard should be properly signaled for the operators and documented in a special way. Operators and supervisors must regularly review that out-of-compliance log or alarm point summary so that the defeated system can be properly restored at the earliest possible time. 

Engineering process control involves measurement of a product property and comparison to a desired value. The process operation is then immediately adjusted to reduce the deviation from the specification. This feedback procedure adjusts the process whenever the product deviates from setpoint and is used to change operating points and to reject the effect of outside disturbances. 

Control engineers know that it takes one manipulated variable for each measured variable we wish to control to setpoint. When the number of controlled variables equals the number of manipulated variables we pair up the different variables and use PI controllers for regulation. Sometimes we are fortunate to have more manipulated variables than control specifications. We can then optimize the use of the manipulators while controlling to setpoint (e.g., valve position control). Sometimes, however, the number of control objectives exceeds the number of available manipulators and we cannot control all variables to setpoint. This is when the concept of partial control is useful. 

We have used the reactor cooling water valve to stabilize the system by controlling reactor temperature. However there is no specific temperature at which the reactor must operate. The best way to manage the reactor temperature setpoint is not immediately obvious. It might be used in conjunction with the production rate controller, i.e., higher temperatures may be needed to increase throughputs. It might be adjusted to maximize yields and suppress undesired by-products. 

Figure 15.64 shows a distillation column that reaches an upper limit on the reboiler duty. When the remote setpoint for the steam flow rate to the reboiler is consistently greater than the measured steam flow, an override controller switches to using the column feed rate as a manipulated variable to keep the bottom product purity on specification. When the column feed rate is adjusted back to its normal level and the control valve on the steam to the reboiler is no longer saturated (i.e., fully open), the control configuration is changed so that the reboiler duty is manipulated to control the bottom product purity. 

Secondly, candidates for manipulated inputs can be found. When they are flow rates connecting BFS s, the choice affect the control of both upstream and downstream. As an example, it was proposed to keep reactant recycle on flow control and to change the setpoint of this loop when production changes are required. This implies that the inventory control of the upstream unit is in direction opposite to flow, while the inventory control of the downstream unit is in the direction of flow. Manipulated flows should be chosen with care to avoid over-specification with respect to plant mass balance. The set-points of the BFS s form another category of plantwide manipulated variables. Examples are reaction conversion, separation performance, etc. 

Allows operator changes of selected parameters(i.e. orifice size, run sizes, default value for BTU and Specific gravity, override setpoints, etc) through the console unit. 

The C R analysis in the steady state predicts the superior performance of the modified HEN, which allows all three target temperatures to be controlled at their setpoints in the face of disturbances in the feed flow rate and temperature of the hot stream. More specifically, the steady-state RGA indicates that a decentralized control system can be configured for the modified HEN in which 02 — Fj, 64 — Fj and 73 - ( ) are paired, and in which the first loop is almost perfectly decoupled, with moderate coupling between the other two loops. Finally, aided by DC analysis, the nominal bypass fraction is selected to be 0.25, providing the best trade-oif between increased plant costs and adequate resiliency. 

Ageing effects can be detected by checking the performance of a system, structure or component (e.g. drifting of setpoints or deterioration of electronic or mechanical components of valves and valve actuators or control rod drive mechanisms, may cause changes in the performance of a control system). For this reason, the results of the performance test programme, which is dependent on the specific design and operation of the facility, should be examined for evidence of trends which may indicate ageing problems. 

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