Comment

September 2009

Revised symbols standards approved

In industries worldwide, the Piping and Instrument Diagram (P&ID) is a document that sees use to communicate mechanical equipment, piping, and instrumentation and control involved in an industrial plant. Everyone from maintenance to senior management reads this document. As businesses become more global, and industrial processes and plants involve companies from all over the world, it is important to use a standard methodology to depict the measurements and controls on these drawings. ISA-5.1 provides that vehicle to facilitate common understanding of the automation involved in an industrial process.

The ISA5.1, Instrumentation Symbols and Identification standard has received final approval and is slated for publication this month. “The standard is suitable for use whenever any reference to an instrument is required in the chemical, petroleum, power generation, air conditioning, metal refining, and numerous other industries,” said Jim Federlein, a voting member on the ISA5.1 committee and a principal at Federlein & Associates in McMurray, Penn.

The primary reason for standards related to symbols used to designate instruments and functions is to ensure anyone picking up a P&ID will be able to understand the symbols being used, confident the definition of those symbols is the same regardless of the industry in which they are seeing use, or regardless of which engineering firm did the process design, said Don Frey, a voting member of ISA5.1 and a construction manager at Reliatech in Saint Cloud, Minn.

“Another important reason for using standard symbols is exceptions to standards, usually-understood definitions, can then be shown on a legend sheet and will be understood by all users to be a deviation from industry-accepted usage,” Frey said. “Symbols drawn by Brown and Root will be understood by engineers from Bechtel, Fluor, Jacobs, Mitsubishi, John Brown, Alstom, Dow Chemical, BASF, Bayer, etc. Standard symbols become a universal language of automation,” Frey said.

ISA-5.1 provides a means to show all measurement and control points in the process, operator interface functionality, detailed information about the instrument types (by the use of specific symbols), and the basic process control system as well as the safety instrumented system.

Process applications

The standard primarily sees use with respect to P&IDs in a wide variety of industries to show the measurement and control involved in the industrial process. The P&ID serves useful purposes throughout the life cycle of an industrial plant—from initial process conceptualization through engineering and design, construction, startup, and operation and maintenance. It is even involved in all process and control changes made during the life of the plant. “That’s why it’s important to use standard symbology for the measurement and control shown on the drawing to facilitate its designed use and understanding,” Federlein said.

The chemical and petroleum industries, in particular, have very high numbers of field sensors associated with their processes, Frey said. “The 5.1 standard provides a uniform means of not only depicting those field devices but also, through the tagging portion of the standard, a uniform method of describing the function of each field device.  Again, consistency here is the goal, ensuring that anyone who reads a device tag can easily understand the function of that device. The power industry, on the other hand, doesn’t have nearly as many field devices, but quite often employs significant control integration and interlocking,” Frey said.

Manufacturers and suppliers to the power industry for years used control logic symbols developed by the Scientific Apparatus Manufacturers Association (SAMA) to describe the functionality contained within control schemes shown on P&IDs and logic diagrams.  Those SAMA symbols have now been incorporated in 5.1 as part of the control functionality section, Frey said.

Users see commonality

Understanding the standard symbols in ISA-5.1 does require some plant knowledge and experience though. Probably the most common symbols would be the circle (or “bubble”) with a device tag inside, showing a field sensor; a circle within a square containing a device tag, showing shared display and shared control functions; and the control valve symbol, showing the most common form of final control element. These symbols are all shown in the illustrations in the 5.1 standard, Frey said.

“All companies that use flow diagrams, whether they are manufacturers or suppliers, can benefit from the use of standard symbology for flow diagrams and related drawings,” Federlein said. “It promotes better understanding of the drawings by the wide range of people reading them. It provides an efficient, standard means of communicating the vast technical information involved in the measurement and control of industrial processes today.”

“Without the 5.1 standard, there would not likely be any uniformity in the way automation devices and functions are portrayed,” Frey said. “Unless a user had his or her entire plant designed and built by the same firm or group of engineers and contractors, there would be significant chance for error when trying to determine the functionality of a particular device. Similarly, a firm trying to undertake a modernization or expansion of an existing facility would have to develop their own ‘magic decoder ring’ to ensure understanding of the symbols and functions. Neither of these scenarios is desirable, and the likelihood of errors would be greatly increased, with possibly catastrophic results,” he said.

Document changes

The last issue of the standard was in 1992 when ANSI/ISA-S5.1-1984 (R1992) was reaffirmed after the last major revision in 1984. Important changes made in the new ISA-5.1 standard include incorporating ISA-5.2 and ISA-5.3 into the standard; designating symbols and tags for safety instrumented systems; adding symbols for signal communication (such as HART, fieldbus, and wireless); and adding new primary measurement element symbols.

Updates in this standard include symbology to handle new technology such as fieldbus or wireless signal communications and safety instrumented systems. It also incorporates and replaces ISA-5.2 Binary Logic Diagrams and ISA-5.3 Graphic Symbols for Distributed Controls/Shared Display Instrumentation, Logic and Computer Systems, making it the primary symbol/identification source for control and automation.

“The standard has been divided into mandatory clauses and informative annexes. The major changes are the meanings of identification letters and graphic symbols, which are covered in the mandatory clauses. And identification and graphic usage examples are covered in the informative annexes,” said Jim Carew, committee chairman. “Some reasons for making the standard mandatory include government and industry demands for life cycle documentation as well as uniform graphic symbols. If someone unfamiliar with instrument drawings is trying to read a document that includes instrument diagrams, ISA-5.1 would aid in understanding, provided they follow ISA-5.1 as written,” Carew said.

Ellen Fussell Policastro (efussellpolicastro@isa.org) writes and edits Standards.