Process-control systems

The hardware architecture must be flexible with respect to bus technology, expansions, network integration and modem access. The use of standard components and software and also a maintenance-friendly design will simplify qualification and validation for both the manufacturer and the user. It is advisable to design the machine control separately from the data management (i.e. recipe and data administration) to ensure that if there is a failure, loss of data does not result in losing a batch. Pro- 

SCADA Client 1 SCADS Client 2.SCADA Climl X 

At the PC level, the freeze-drying process and all secondary processes are visualized, controlled and documented. Such secondary processes include condenser defrosting, CIP/SIP, plant tests, failure statistics, calibration values, counter of operating hours, plant schematic, table of valves, actual temperature and pressure values, recipes, start parameters and machine parameters. 

A freeze-drying plant should have as a minimum the following measuring capabilities  

For production plants the following data of machines should be supervised  

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F. G. Shinskey, Process Control Systems, McGraw-HiU Book Co., Inc., New York, 1967. 

There are special numerical analysis techniques for solving such differential equations. New issues related to the stabiUty and convergence of a set of differential equations must be addressed. The differential equation models of unsteady-state process dynamics and a number of computer programs model such unsteady-state operations. They are of paramount importance in the design and analysis of process control systems (see Process control). 

SCHWEITZER Handbook of Separation Techniques for Chemical Engineers, Third Edition SHINSKEY Process Control Systems, Fourth Edition SHUGAR, BALLINGER Chemical Technicians Ready Reference Handbook, Fourth Edition SHUGAR, DEAN The Chemist s Ready Reference Handbook, Third Edition... 

Real-time clocks (RTCs). Real-time systems are required to respond to events, as they occur, in a timely manner. This is especially crucial in process control systems where control actions applied at the wrong time may amplify process deviations or destabilize the processes. The nodes in the systems are interrupted periodically by the real-time clocks to maintain the ac tual elapsed times. 

To allow flexibility, the database manager must also perform point addition or deletion. However, the abihty to create a point type or to add or delete attributes of a point type is not normally required because, unlike other data processing systems, a process control system normally involves a fixed number of point types and related attributes. For example, analog and binary input and output types are required for process I/O points. Related attributes for these point types include tag names, values, and hardware addresses. Different system manufacturers may define different point types using different data structures. We will discuss other commonly used point types and attributes as they appear. 

The process controller is the master of the process-control system. It accepts a set point and other inputs and generates an output or outputs that it computes from a rule or set of rules that are part of its internal configuration. The controller output seiwes as an input to another controller or, more often, as an input to a final control element. The final control element is the device that affects the flow in the piping system of the process. The final control element seiwes as an interface between the process controller and the process. Control valves and adjustable speed pumps are the principal types discussed. 

The difference in the nature of process controls and safety interlock systems leads to the conclusion that these two should be physically separated (see Fig. 8-89). That is, safety interlocks should not be piggy-backed onto a process-control system. Instead, the safety interlocks should be provided by equipment, either hard-wired or programmable, that is dedicated to the safety functions. As the process controls become more complex, faults are more likely. Separation means that faults within the process controls have no consequences in the safety interlock system. 

Implementation of process interlocks within process control systems is perfectly acceptable. Furthermore, it is also permissible (and probably advisable) that responsible operations personnel be authorized to bypass or ignore a process. Safety interlocks must be implemented within the separate safety interlock system. Bypassing or ignoring safety interlocks by operations personnel is simply not permitted. When this is necessary for ac tions such as verifying that the interlock continues to be func tional, such situations must be infrequent and incorporated into the design of the interlock. 

Basic process control system (BPCS) loops are needed to control operating parameters like reactor temperature and pressure. This involves monitoring and manipulation of process variables. The batch process, however, is discontinuous. This adds a new dimension to batch control because of frequent start-ups and shutdowns. During these transient states, control-tuning parameters such as controller gain may have to be adjusted for optimum dynamic response. 

Same sensor used for basic process control system and safety instrumented system. Failure of sensor leads to loss of control system and safety system functionality. 

Eailure of basic process control system (BPCS) resulting in loss of control. 

Part 2 Classification of Process Control Systems Realization, operation and testing of safety instrumented systems (December 1998)... 

Part 3 Building and installation requirements for functional safety of process control systems under emergency conditions (December 1998)... 

Gas-monitoring systems are more widely used than particulate monitoring systems. They can also be used for both emission compliance monitors and process control systems. Gas monitors may be of either the in situ or... 

Fig. 4.27 Block diagram for liquid-level process control system.

Example 4.5 (Liquid-Level Process Control System)... 

A distributed control system (DCS) normally uses input and output modules which contain eight, sixteen, or more inputs or outputs. Failure of the module will simultaneously disable a large number of control loops. Attention to the assignment of input/output points to the modules makes the plant more tolerant of a failure of an input or output module (CCPS, 1993a). For a more detailed discussion of process control systems, see the process control part of Section 4.4, and Sections 6.4 and 6.5. 

Basic Process Control System (BPCS) and Safety Interlock System (SIS)... 

Computerized process control systems add complexity to operating instructions. These operating instructions need to describe the logic of the software as well as the relationship between the equipment and the control system otherwise, it may not be apparent to the operator. 

The object of a process control system is to make economic and sound decisions about the actions affecting the process. Data concerning the variations in process performance are collected and analyzed and decisions taken as to whether action on the process is or is not necessary to maintain production of conforming product (see Figure 9.1). However, process control and process capability are not one and the same, as illustrated in Figure 9.5. 

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