Operational constraints examples

The mechanism by which nonkey components affect a given separation is more complex in practice than the broad arguments presented here. There are complex interrelationships between the volatility of the key and nonkey components, and so on. Also, it is often the case that the distillations system has constraints to prevent certain heat integration opportunities. Such constraints will often present themselves as constraints over which the pressure of the distillation columns will operate. For example, it is often the case that the maximum pressure of a distillation column is restricted to avoid decomposition of material in the reboiler. This is especially the case when reboiling high molar mass material. Distillation of high molar mass material is often constrained to operate under vacuum conditions. Clearly, if the pressure of the distillation column is constrained, then this restricts the heat integration opportunities. Another factor that can create... 

Operational constraints The various types of equipments used in a chemical plant have constraints inherent to their operation. Such constraints should be satisfied throughout the operation of a plant. For example, pumps must maintain a certain net positive suction head tanks should not overflow or go dry distillation columns should not be flooded the temperature in a catalytic reactor should not exceed an upper limit since the catalyst will be destroyed. Control systems are needed to satisfy all these operational constraints. 

Figure 12.1 shows the interactions between development and operations. At the end of the development process, the safety constraints, the results of the hazard analyses, as well as documentation of the safety-related design features and design rationale, should be passed on to those responsible for the maintenance and evolution of the system. This information forms the baseline for safe operations. For example, the identification of safety-critical items in the hazard analysis should be used as input to the maintenance process for prioritization of effort. 

Inequality constraints, g x, are expressions that involve any or all of the set of variables, i, and are used to bound the feasible region of operation. For example, when operating a centrifugal pump, the head developed decreases with increasing flow rate according to a pump characteristic curve. Hence, if the flow rate is varied when optimizing the process, care must be taken to make sure that the required pressure increase (head) does not exceed that available from the pump. The expression might be of the form. 

Step 5. Control product quality and meet safety, environmental, and operational constraints. A conventional pressure and temperature control system is set up for the flash vessel, as in the previous example. 

Specific applications of advanced composites in spacecraft are too numerous to be comprehensively reviewed in the present paper. The several examples discussed here will serve to illustrate how the material selection and structural design are related to operational constraints. 

At times it is not possible to reveal all of the undetected failure modes at the proof test interval assumed in the PFD calculation, for example, on account of operational constraints. In such cases, a partial proof testing can he applied as long as the overall integrity requirements of the SIS are met. 

CSTEP supports interface timing constraints which specify minimum and maximum times between operations. Constraint declarations in CSTEP describe a constraint name, a time interval between operators, and an inequality with a constant that is expressed in either nanoseconds or clock periods. For example, the following maximum-time constraint specifies that operator xl must execute at least 80ns before operator x2 ... 

Timing constraints and control flow. Timing constraints may be placed on any pair of operators in a behavioral specification. Evaluation of timing constraints must consider the possibility that two operations may be separated by conditionals and loops. In the case of conditionals, there may be more than one path of execution between the two operators. For example. Figure 5-4 shows two operations separated by a single IF-THEN-ELSE conditional construct. Since the two parts of the conditional may take different amounts of time to execute, any timing constraint on the two operators must be satisfied when either of the two paths is executed. When conditionals are nested, additional paths of execution are possible. CSTEP deals with this possibility in a preprocessing step that enumerates the paths between operators and creates a separate constraint for each path. 

SACs often mean that during the application of the considered item, special expenditures or special restrictions are necessary (examples for special expenditures maintenance expenditures or development expenditures in the project of the superior item examples for special restrictions project planning restrictions and operation constraints). If such application expenditures or restrictions are to be avoided, then on the one hand higher development expenditures can be implicated for the own item. For example, there might be application cases which are not required by the customer but which are safety critical and must be excluded by certain SACs (e.g. the use of an interface for a safety related purpose). On the other hand also the benefits have to be considered, which SACs can have in the total Product Life Cycle. 

A system mode is the functional configuration of the system, which established the system manner of operation. A mode is a set of functional capabilities that allow the operational system to accomplish tasks or activities. Modes tend to establish operational segments within the system mission. A system can have primary modes and sub-modes of operation. The system can only be in one mode at any one time. Correctly defining the mode established all of the modal constraints, which impact both design and operation. For example, a stopwatch is a system that typically has modes such as on, off, timing, and reset. The on mode may go into the initialization state, which requires the initialization software package. 

The minimal information needed in carrying out this type of RtR control is a static model relating the manipulated variable to the quality variables at the end of a batch. It can be as simple as a steady-state (constant) gain relationship, or as complicated as a nonhnear model that includes the effects of different initial conditions and the batch time. In contrast, a time-dependent profile for the manipulated variable during the batch can be adjusted from run-to-run to meet the end-of-the-batch quality requirements as well as operating constraints, for example, upper and lower bounds on the manipulated variables (Bonvin et al., 2002). Other variations of RtR controllers in different applications have been reported by Zafiriou et al. (1995), Clarke-Pringle and MacGregor (1998), and Zisser et al., (2005). 

An example can be given of just how much the plantwide control system design changes if a slightly different set of assumptions is made. What if the short-term operating constraint on production rate was tighter than specified above ( 1% instead of 3%), or the plant involved more than just three process units In such a situation, the decision to control the production rate by a cascade loop that extends back to the A feed stream flow rate (ivi) may not be practical. The intervening... 

Some processes require certain operating constraints to be set. These are referred to as artificial measurements. These operating constraints can be set through the use of selectors. For example, consider a distillation column whose feed versus steam characteristic is shown in Figure 6.18. 

Find a way to overcome the constraint while still maintaining the areas. This is often possible by using indirect heat transfer between the two areas. The simplest option is via the existing utility system. For example, rather than have a direct match between two streams, one can perhaps generate steam to be fed into the steam mains and the other use steam from the same mains. The utility system then acts as a buffer between the two areas. Another possibility might be to use a heat transfer medium such as a hot oil which circulates between the two streams being matched. To maintain operational independence, a standby heater and cooler supplied by utilities is needed in the hot oil circuit such that if either area is not operational, utilities could substitute heat recovery for short periods. 

Fig. 20. Example of constraint control operating window for a distillation column, where (-----) is the vessel pressure limit, and ( ) represents constant...

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