Symbols, flowsheet

To reduce detailed written descriptions on flowsheets, it is usual practice to develop or adopt a set of s) mbols and codes which suit the purpose. Flowsheet symbol standardization has been developed by various professional and technical organizations for their particular fields. Most of these have also been adopted by the American National Standards Institute (ANSI). The following symbol references are related and useful for many chemical and mechanical processes ... 

Figure 1-20A. Special types of descriptive flowsheet symbols.

American Gas Association method, 121 Complex pipe systems, 122 Low pressure air, steam, 131 Panhandle formula, 120, 121 Panhandle-A formula, 121 Parallel system, 122 Series system, 122 Transmission factors, 120 Weymouth formula, 120 Flowsheet symbols, 17... 

Hill, R.G., Drawing Effective Flowsheet Symbols, Chem Eng., 75,1,... 

When drawing up the process flowsheets it is often not yet clear which type of pump or fitting or which type of agitator will finally be apphed. Therefore, the standards mentioned above contain general flowsheet symbols. 

To aid in readability by plant management as well as engineering and operating personnel, it is important that a set of symbols be developed as somewhat standard for a particular plant or company. Of course, these can be improved and modified with time and as needed, but with the basic forms and letters established, the sheets can be quite valuable. Many companies consider their flowsheets quite confidential since they contain the majority of key processing information, even if in summary form. 

Figure 1-20B. Commonly used instruments for process instrumentation flowsheets. Adapted by permission, ISA Std. ANSI Y32.20—1975, ISA S5.1—1973, Instrumentation Symbols and Identification," Latest edition, 1984.

Many symbols are pictorial w hich is helpful in representing process as well as control and mechanical operations. In general, experience indicates that the better the representation including relative locating of connections, key controls and even udlity connections, and service systems, the more useful will be the flowsheets for detailed project engineering and plant design. 

Since a symbol does not usually speak entirely for itself but also carries a name and a letter-number identification, the flowsheet can be made clear even with the roughest of equipment symbols. The... 

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. 

Appendix 2.1 provides the reader with material for the construction of flowsheets with the symbols of this chapter and possibly with some reference to Chapter 3. 

A gaseous product rich in ethylene and propylene is made by pyrolysis of crude oil fractions according to the following description. Construct a flowsheet for the process. Use standard symbols for equipment and operating conditions. Space the symbols and proportion them in such a way that the sketch will have a pleasing appearance. 

On working flowsheets the detectors, transmitters, and controllers are identified individually by appropriate letters and serial numbers in circles. Control valves are identified by the letters CV- followed by a serial number. When the intent is to show only in general the kind of control system, no special symbol is used for detectors, but simply a point of contact of the signal line with the equipment or process line. Transmitters are devices that convert the measured variable into air pressure for pneumatic controllers or units appropriate for electrical controllers. Temperature, for instance, may be detected with thermocouples or electrical resistance or height of a liquid column or radiant flux, etc., but the controller can accept only pneumatic or electrical signals depending on its type. When the nature of the transmitter is clear, it may be represented by an encircled cross or left out entirely. For clarity, the flowsheet can include only the most essential information. In an actual design... 

Figure 3.4. Symbols for control elements to be used on flowsheets. Instrument Society of America (ISA) publication no. S51.5 is devoted to process instrumentation terminology.

Linear. Since mass and energy are linearly related between modules, purely linear flowsheet calculations can be formulated as a solution to a set of linear equations once linear models for the modules can be constructed. Linear systems, especially for material balance calculations can be very useful (16). Two general systems, based on linear models, SYMBOL (77) and MPB II (7 ) are indicated in Table 1. MPB II is based on a thesis by Kniele (79). If Y is the vector of stream outputs and the module stream inputs are X, then as discussed by Mahalec, Kluzik and Evans (80)... 

Figure 1. Flowsheet of the deacidification unit of olive oil by supercritical carbon dioxide. Symbols are defined in Table 2 other, PC pressure control, LC flow control.

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