Engineering Symbology, Prints and Drawings - Volume 1.pdf (original) (raw)
A vast amount of documentation is required for the design and construction of a process facility, which are front-end and detailed engineering drawings. The main engineering documents used on a regular basis by the engineering staff for smooth and efficient running, maintenance, and upgrading of the facility are Alarm and Trip Systems , PLC documentation, and Pipe and Instrumentation Diagrams (P&ID). As in all engineering disciplines, the initial accuracy of these documents, and the regular updating of them when changes are made, is critical, and one of the most important aspects of engineering. For this reason, documentation is discussed in this chapter. Documentation standards and symbols for all aspects of process control have been set up and standardized by the ISA, in conjunction with the ANSI [1]. 17.2 Alarm and Trip Systems The purpose of an alarm system is to bring a malfunction to the attention of operators and maintenance personnel, whereas the purpose of a trip system is to shut down a system in an orderly fashion when a malfunction occurs, or to switch failed units over to standby units. The elements used in the process control system are the first warnings of a failure. This could show up as an inconsistency in a process parameter, or as a parameter going out of its set limits. The sensors and instruments used in the alarm and trip system are the second line of defense, and must be totally separate from those used in the process control system. Alarm and trip system information and its implementation are given in ANSI/ISA-84.01-1996—Application of Safety Instrumented Systems for the Process Control Industry. 17.2.1 Safety Instrumented Systems The alarm and trip system, or Safety Instrumented System (SIS), has its own sensors, logic, and control elements, so that under failure conditions, it will take the process to a safe state to protect the personnel, facility, and environment. To ensure full functionality of the SIS, it must be regularly tested. In an extreme situation, such as with deadly chemicals, a second or third SIS system with redundancy can be used in conjunction with the first SIS system, to ensure as close to 100% protection as possible. The sensors in the SIS usually will be of a different type than those used for process control. The control devices are used to accurately sense varying levels in the 297