What Y means on a P&ID.
First-letter Y is rare in modern drawings. State and event detection migrated to Z-prefix position switches and service-letter switches, FS, PS, LS decades ago. The Y tags you do see are usually relics on legacy drawings that haven't been redrafted since the 1980s. Second-letter Y, by contrast, is everywhere as the computing-relay marker, FY, PY, TY.
Also stands for.
- Modifier. Computing relay, when used as a second letter, e.g. FY, PY
Common second letters.
| Tag | Meaning |
|---|---|
| YS | Switch, YS. State switch |
| YAH | Alarm high, YAH. State alarm |
Example tags.
| Tag | Meaning |
|---|---|
| YS-201 | State switch, often a presence detector or event indicator |
Typical placement.
On event detection scope. Flame failure detectors, often combined with B-prefix, end-of-stroke switches on actuators, often combined with Z-prefix. First-letter Y is rare. Second-letter Y, computing relay is much more common.
Common questions
What does the ISA 5.1 first letter Y stand for?
In ISA 5.1, the first letter Y stands for Event, state, presence. It can also stand for Modifier. Computing relay, when used as a second letter, e.g. FY, PY.
How is letter Y typically used on a P&ID?
On event detection scope. Flame failure detectors, often combined with B-prefix, end-of-stroke switches on actuators, often combined with Z-prefix. First-letter Y is rare. Second-letter Y, computing relay is much more common.
What second letters commonly follow Y in ISA 5.1 tags?
Common combinations include YS (Switch, YS. State switch), YAH (Alarm high, YAH. State alarm).
What are example ISA 5.1 tags starting with Y?
YS-201 (State switch, often a presence detector or event indicator).
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