Instrument and controller symbols

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. 

The symbols used to show the equipment, valves, instruments and control loops will depend on the practice of the particular design office. The equipment symbols are usually more detailed than those used for the process flow-sheet. A typical example of a P and I diagram is shown in Figure 5.25. 

This example illustrates how the techniques used in an operability study can be used to decide the instrumentation required for safe operation. Figure 9.6a shows the basic instrumentation and control systems required for the steady-state operation of the reactor section of the nitric acid process introduced in Figure 4.2. Figure 9.6b shows the additional instrumentation and safety trips added after making the operability study set out in this section. The instrument symbols used are explained in Chapter 5. 

BASIC, or Beginners Algebraic Symbolic Instruction Code, was developed by Kemeny63 as a "baby FORTRAN" for simple computers (e.g., minicomputers). BASIC does not wait for the whole user-written program to be finished, but compiled each typed line as soon as typed. It was ideally suited for a simple learning environment. Microsoft VISUAL BASIC is a GUI-interfaced version. Microsoft QUICK BASIC 4.5 is much better than FORTRAN embodiments in accessing instruments for real-time data acquisition and control. 

Instrummiaiion flow sheets are useful in determining the requirements for process control and instrumentation. The Instrument Society of America has pioneering standardization of instrumentation flow plan symbols. These systems of nomenclature are illustrated in Tables 3-1... 

Piping and Instrumentation Symbols Used in Flow Schemes (Table 6) Symbols Used in Measuring and Control Technology... 

Table 24. ISO codes and miscellaneous symbols for process measurement control functions and instrumentation... 

A selector is a practical solution for choosing the appropriate signal from among a number of available measurements. Selectors can be based on multiple measurement points, multiple final control elements, or multiple controllers, as discussed below. Selectors are used to improve the control system performance as well as to protect equipment from unsafe operating conditions. On instrumentation diagrams, the symbol (>) denotes a high selector and the symbol (<) a low selector. 

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. 

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. 

Standard symbols for instruments, controllers and valves are given in the British Standard BS 1646. 

Rather less freedom is allowed in the construction of mechanical flowsheets. The relative elevations and sizes of equipment are preserved as much as possible, but all pumps usually arc shown at the same level near the bottom of the drawing. Tabulations of instrumentation symbols or of control valve sizes or of relief valve sizes also often appear on P I diagrams. Engineering offices have elaborate checklists of information that should be included on the flowsheet, but such information is beyond the scope here. 

Main project tasks should only be considered as completed when their objective has been achieved. As a control instrument, these objectives must be included in the project plan and must be defined in a measurable way. Figure 2 depicts these only as a symbol ( ) followed by the reference number as given in Figure 1 to maintain the clarity of the example. Because of their importance, the main objectives may be accentuated and shown with their descriptions. They indicate the assumptions on which the rest of the plan is dependent. They also serve... 

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