What S means on a P&ID.
First slot S means speed. Second slot S means switch. SI is a speed indicator. PS is a pressure switch. The same character, two different jobs, decided entirely by where it sits in the tag. Read the first letter first or you'll wire a tachometer to a discrete input card and wonder why the loop doesn't behave.
Also stands for.
- Modifier. Switch, when used as a second letter
Common second letters.
| Tag | Meaning |
|---|---|
| ST | Transmitter, ST. Speed transmitter |
| SI | Indicator, SI. Tachometer |
| SAH | Alarm high, SAH. High-speed alarm |
| SSH | Switch high, SSH. Overspeed switch, often a trip on rotating equipment |
Example tags.
| Tag | Meaning |
|---|---|
| ST-205 | Speed transmitter, often on a compressor or turbine shaft |
| SI-205 | Speed indicator, tachometer reading |
| SSH-210 | Overspeed switch tripping a turbine or compressor on high speed |
Typical placement.
On rotating equipment with safety-critical speed monitoring, turbines, compressors, large pumps. Speed measurement uses S as a first letter. The more common second-letter use of S is for switches, PS, FS, LS.
Common questions
What does the ISA 5.1 first letter S stand for?
In ISA 5.1, the first letter S stands for Speed, frequency. It can also stand for Modifier. Switch, when used as a second letter.
How is letter S typically used on a P&ID?
On rotating equipment with safety-critical speed monitoring, turbines, compressors, large pumps. Speed measurement uses S as a first letter. The more common second-letter use of S is for switches, PS, FS, LS.
What second letters commonly follow S in ISA 5.1 tags?
Common combinations include ST (Transmitter, ST. Speed transmitter), SI (Indicator, SI. Tachometer), SAH (Alarm high, SAH. High-speed alarm), SSH (Switch high, SSH. Overspeed switch, often a trip on rotating equipment).
What are example ISA 5.1 tags starting with S?
ST-205 (Speed transmitter, often on a compressor or turbine shaft); SI-205 (Speed indicator, tachometer reading); SSH-210 (Overspeed switch tripping a turbine or compressor on high speed).
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