If you work with instrumentation, automation or process plants, you will recognise tags such as PT, FT, LT and TT. Much of that notation comes from the ISA system for identifying instruments and control functions.
What is ISA-5.1?
ANSI/ISA-5.1 is a widely used standard for showing and identifying instrumentation and control functions. The aim is simple: someone reading the drawing should see what is measured, how it is controlled and how the instruments connect to the process.
ISA uses letter identification to describe what an instrument measures and what function it performs.
For example, PT is commonly used for a pressure transmitter, PI for a pressure indicator, PIC for a pressure indicating controller and PSH for a high pressure switch. This allows an engineer to understand the purpose of an instrument from its identification without needing a long description beside every symbol.
What does ISA affect on a P&ID?
Mostly the instrumentation layer:
- instruments and their functions
- instrument identification letters
- field and panel locations
- control functions
- signal connections
- actuators and final control elements
That is why two instruments measuring the same pressure can be drawn differently: one is mounted in the field, the other is a function available to the operator in the control room.
ISA and ISO are not competitors
Real P&IDs often combine both. ISA matters most for instrumentation and control. ISO standards cover graphical symbols and rules for process equipment. A typical drawing uses ISA-style instruments together with ISO-based pumps, valves and vessels. See also: ISO symbols for P&IDs, or see InduDraft's ISA P&ID editor.
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Open a new P&ID with the ISA library already selected.
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