Control symbols

The ISA symbology provides different symbols for different types of actuators. Furthermore, variations for the controller symbol distinguish control algorithms implemented in DCS technology from panel-mounted single-loop controllers. 

Figure 3 Schematic drawing of the cell designed to work In the limit of chemical control. Symbols are defined In text except 2 three-way valve I Inlet 0 outlet.

All process information that can be measured in the plant is shown on the P ID by circular flags. This includes the information to be recorded and used in process control loops. The circular flags on the diagram indicate where the information is obtained in the process and identify the measurements taken and how the information is dealt with. Table 1.10 summarizes the conventions used to identify information related to instrumentation and control. Exanple 1.9 illustrates the interpretation of instrumentation and control symbols. 

Figure 4. Correlation between retinoid-induced decreases in saturation density and induction of gap junctional communication. Confluent cultures of 10 Tl/2 cells (panel A) or methylcholantlirene transformed 10 Tl/2 cells (panel B) were treated with various concentrations of retinoids for five days. At this time cells were probed by microinjection of Lucifer yellow and transfer of dye to adjacent cells was quantitated. Total cell counts were also performed at this time. Results are expressed as a percent of the untreated controls. Symbols retinol (A) retinyl acetate (A) diW-trans retinoic acid ( ) 13-cw retinoic acid ( ) TTNPB (O). The correlations were statistically highly significant, for 10 Tl/2 cells and for transformed cells, P < 0.001. Data from reference [28] with permission.

A trademark may be Hcensed, but special rules apply to such licensing because of the nature and function of the trademark right. Thus the pubHc has an iaterest ia the mark as a symbol of origin of the goods or services ia the owaer and should be able to rely on the mark as iadicating the character and quaHty of the goods or services that the owner controls. Essential, therefore, to a vaHd Hcense is provision for the Hcensor to control the nature and quaHty... 

The symbol for the control valve in Fig. 8-47 is for a pneumatic positioning valve without a valve positioner. 

Fig. 4.1 Block diagram of a closed-loop control system. R s) = Laplace transform of reference input r(t) C(s) = Laplace transform of controlled output c(t) B s) = Primary feedback signal, of value H(s)C(s) E s) = Actuating or error signal, of value R s) - B s), G s) = Product of all transfer functions along the forward path H s) = Product of all transfer functions along the feedback path G s)H s) = Open-loop transfer function = summing point symbol, used to denote algebraic summation = Signal take-off point Direction of information flow.

Instrumentation normally is denoted by a circle in which the variable being measured or controlled is denoted by an appropriate letter symbol inside the circle. When the control device is to be located remotely, the circle is divided in half with a horizontal line. Table 1.3 gives various instrumentation symbols and corresponding letter codes. The specific operating details and selection criteria for various process instrumentation are not discussed in this book. The reader is referred to earlier works by Cheremisinoff [1,2] for discussions on essential control and measurement instrumentation. 

The standard requires the supplier to apply the appropriate methods to identify special characteristics, to include these characteristics in the control plan, and to comply with any specific definitions and symbols the customer may use. 

The standard also requires documents such as FMEA, control plans, etc. to be marked with the customer s specific symbols to indicate those process steps that affect special characteristics. As the characteristics in question will be specified within documents, the required symbols should be applied where the characteristic is mentioned rather than on the face of the document. For drawings, the symbol should be applied close to the appropriate dimension or item. Alternatively, where a document specifies processes that affect a special characteristic, the appropriate symbol should be denoted against the particular stage in the process that affects that characteristic. The symbols therefore need to be applied during document preparation and not to copies of the document. The instructions to apply these symbols should be included within the procedures that govern the preparation of the documents concerned. 

This clause requires the designation of special characteristics that should have been accomplished during product realization (as required by clause 4.2.4.7). As for the documentation of special characteristics, the symbols should have been applied both when establishing the process controls and preparing the control plan (also clause 4.2.4.7) and associated documentation during the planning phase. Therefore the requirements not previously addressed are for the control of special characteristics and evidence of compliance i.e. quality records. 

As is stated in the standard, all characteristics are important and need to be controlled. However, some need special attention as excessive variation may affect product safety, compliance with government regulations, fit, form, function, appearance, or the quality of subsequent operations. Designating such characteristics with special symbols alerts planners and operators to take particular care. It also alerts those responsible for dispo-sitioning nonconforming product to exercise due care when reaching their decisions. 

Operational sequence diagrams are flcw-charting techniques that represent any sequence of control movements and information collection activities that are executed in order to perform a task. Various activities in the diagram are represented with a symbolic notation, supported where necessary by a text description. For the majority of simple applications, OSDs assume a linear flow drawn from top to bottom with a limited degree of branching and looping. The symbols used are usually tailored to fit the type of task being studied and its level of analysis. 

Figure 4.8 shows a specific example of this type of diagram which includes some symbols. The diagram shows the tasks that the operator and the computer must perform in a computer controlled reactor. The central column is used to show any functional requirements of the human-computer interface. 

Data on spare and parallel equipment are often omitted. Valving is also generally omitted. A alve is shown only where its specification can aid in understanding intermittent or alternate flows. Instrumentation is indicated to show the location of variables being controlled and the location of the actuating device, usually a control valve. To help the reader better understand the process flow sheet, a list of commonly used symbols is presented in Fig. 5.9.1. 

Bums and Hazzan demonstrated tlie use of event tree and fault tree analysis in tlie study of a potential accident sequence leading to a toxic vapor release at an industrial chemical process plant. The initiator of tlie accident sequence studied is event P, the failure of a plant programmable automatic controller. Tliis event, in conjunction willi the success or failure of a process water system (a glycol cooling system) mid an operator-manual shutdown of tlie distillation system produced minor, moderate, or major release of toxic material as indicated in Fig. 21.4.1. The symbols W, G, O represent tlie events listed ... 

Figure 12-61D. Centrifugal compressor surge control schematic diagram shows instrumentation required when primary flow-measuring device is located in centrifugal compressor discharge line. Symbols T = temperature P = pressure A = differential across compressor outlet to inlet. See Reference 89 for a detailed discussion. (Used by permission White, M. H. Chemical Engineering, p. 54, Dec. 25,1972. McGraw-Hill, Inc. All rights reserved.)...

In Equation 2-114, is the rate of entropy production within the control volume symbols with dots refer to the time rate of change of the quantity in question. The second law requires that the rate of entropy production be positive. 

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