Instrumentation and control objectives

The primary objectives of the designer when specifying instrumentation and control schemes are  

To maintain the product composition within the specified quality standards. 

To operate at the lowest production cost, commensurate with the other objectives. 

These are not separate objectives and must be considered together. The order in which they are listed is not meant to imply the precedence of any objective over another, other than that of putting safety first. Product quality, production rate and the cost of production will be dependent on sales requirements. For example, it may be a better strategy to produce a better-quality product at a higher cost. 

In a typical chemical processing plant these objectives are achieved by a combination of automatic control, manual monitoring and laboratory analysis. 

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Draw an instrumentation and control diagram that accomplishes the following objectives ... 

At such a point, an agonizing reappraiseLL of the whole instrumentation and control package Is generally Initiated to determine the proper course of action with respect to plant life and future plant operation The objective of this report is to begin this appraisal and outline a course of action ... 

Switchover from injection to recirculation phases involves realignment of several valves and may be accomplished by manual, automatic or semi-automatic operations. The logic has to be designed to deal with two contradictory objectives avoiding spurious actuation (in case of loss of power for example) and insuring a high level of protection. All these considerations involve a complicated instrumentation and control. 

The following Space Nuclear Reactor technologies shall be developed for Lunar and Mars surface power reactors 1) Nuclear fuel, 2) Reactor core materials and coolants, and 3) Instrumentation and Control. (This item was indicated as an objective - minimum requirement not yet defined.)... 

After ten years of reactor operation, the system has revealed some features of ageing and obsolescence. As its components and equipment -were produced in the 70 s, some integrated circuits (IC) and equipment were hard to find on the market and as a consequence maintenance work of the system became a big difficulty. To solve this problem the Dalat Nuclear Research Institute has asked for assistance from the IAEA through the TC Project, VIE/4A)I0 - "Renovation of the Dalat Reactor Instrumentation and Control System". The project has the following main objects ... 

An even greater pitfall into which many young process control engineers fall, particularly in recent years, is to get so involved in the fancy computer control hardware that is now available that they lose sight of the process control objectives. All the beautifiil CRT displays and the blue smoke and mirrors that computer control salespersons are notorious for using to sell hardware and software can easily seduce the unsuspecting control engineer. Keep in mind your main objective to come up with an effective control system. How you implement it, in a sophisticated computer or in simple pneumatic instruments, is of much less importance. 

Suppose the objective is to control the temperature of a bioreactor. The temperature of a bioreactor is measured by instrumentation and compared with a set-point value. Based on the difference between the measured and the set-point temperature, the flow rate of cooling water into a fermentor jacket is increased or decreased by manipulating a control valve of cooling water until the difference between the measured and the set-point temperature becomes zero. By repeating this operation, the temperature of a bioreactor can be kept constant regardless of changes in the outer temperature or from the internal generation of heat. 

Figure 13.10. Batch distillation McCabe-Thiele constructions and control modes, (a) Construction for constant overhead composition with continuously adjusted reflux rate, (b) Construction at constant reflux at a series of overhead compositions with an objective of specified average overhead composition, (c) Instrumentation for constant vaporization rate and constant overhead composition. For constant reflux rate, the temperature or composition controller is replaced by a flow controller.

A fundamental objective of a computer system applied to automate a pharmaceutical GMP operation is to ensure the quality attributes of the drug product are upheld throughout the manufacturing process. It is therefore important that quality-critical parameters are determined and approved early in the validation life cycle. The exercise should be undertaken to a written procedure with base information from the master product/production record file examined and quality-critical parameter values and limits documented and approved for the process and its operation. In addition, the process and instrument diagrams (P IDs) should be reviewed to confirm the measurement and control components that have a direct impact on the quality-critical parameters and data. This exercise should be carried out by an assessment team made up of user representatives with detailed knowledge of both the computer system application and process, and with responsibility for product quality, system operational use, maintenance, and project implementation. This exercise may be conducted as part of an initial hazard and operability study (HAZOP) and needs to confirm the quality-related critical parameters for use in (or referenced by) the computer control system URS. 

The computer system URS needs to describe the levels of functionality and operability required from the computer system, its application, and the location with regard to the process. Definition of approved and accurate manufacturing and process data is a key objective of the URS and is essential in order for the computer system supplier or integrator to fully understand and develop the computer application and to engineer the field instrumentation and electrical controls. This must include the quality-related critical parameters that are fundamental in determining and ensuring the satisfactory quality of a drug product. Parameters, data, and functions that are necessary to uphold GMP must always be considered as firm requirements and candidates for validation. 

The specification of a control scheme and the associated instrumentation for a chemical plant should satisfy several main objectives. First, the plant should operate at all times in a safe manner. Dangerous situations should be detected as early as possible and appropriate action initiated, also the process variables should be maintained within safe operating limits. Second, the plant should operate at the lowest cost of production. Finally, the production rate and the product quality must be maintained within specified operating limits. These objectives may be conflicting, and the final control scheme to be adopted is based upon a realistic and acceptable compromise between the various factors. The main conflict is between the need to design and operate as safe a plant as possible and the desire to produce the chemical at the lowest cost. Safe plant operation can be expensive, both in terms of the capital cost of instrumentation and the annual operating costs, e.g. maintenance. 

For instrumental evaluation spectrophotometers are preferred. The colorimetric values obtained depend on the instrument and its instantaneous state (chiefly the sample illumination conditions) and must be controlled by suitable methods [174], The influence of the instrument can be eliminated by conversion to the standard illumination D 65, representing daylight with a correlated color temperature of 6504 K [175]. The degree of whiteness and the tint values can then be calculated from these colorimetric data with appropriate formulas. A selection of currently used whiteness formulas can be found in [176,177], For recent attempts at standardizing the assessment methods for white objects, see [9,178-185],... 

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