What P means on a P&ID.
Pressure carries the most SIS instruments in petrochemicals because most process safety hazards manifest as overpressure. Watch the PSHH, PSLL, PALL family. These prefixes signal SIS scope and should be filtered into a separate I/O list workbook with their own SIL allocation.
Also stands for.
- Pressure differential, PDT
- Vacuum, when service is sub-atmospheric
Common second letters.
| Tag | Meaning |
|---|---|
| PT | Transmitter, PT. Pressure transmitter |
| PDT | Differential pressure transmitter, PDT |
| PI | Indicator, PI. Pressure gauge or indicator |
| PIC | Indicator plus controller, PIC |
| PSV | Safety valve, PSV. Pressure safety valve |
| PRV | Relief valve, PRV |
| PSHH | Switch high-high, PSHH. SIS trip |
| PALL | Alarm low-low, PALL. SIS trip |
Example tags.
| Tag | Meaning |
|---|---|
| PT-501 | Pressure transmitter on loop 501 |
| PDT-105 | Differential pressure transmitter, e.g. Across an orifice, loop 105 |
| PSV-2210 | Pressure safety valve, train 22, loop 10 |
| PSHH-7250-1oo2 | Pressure switch high-high, voted 1-out-of-2, SIS service |
| PI-405 | Pressure indicator, often a mechanical gauge, loop 405 |
Typical placement.
On vessel nozzles, pipe taps, and pump discharges. PSV, PRV instruments draw with the standard valve symbology plus the spring-loaded actuator. PDT is the trickiest because it spans two physical taps. The P&ID shows both impulse lines connecting to a common transmitter body.
Common questions
What does the ISA 5.1 first letter P stand for?
In ISA 5.1, the first letter P stands for Pressure. It can also stand for Pressure differential, PDT, Vacuum, when service is sub-atmospheric.
How is letter P typically used on a P&ID?
On vessel nozzles, pipe taps, and pump discharges. PSV, PRV instruments draw with the standard valve symbology plus the spring-loaded actuator. PDT is the trickiest because it spans two physical taps. The P&ID shows both impulse lines connecting to a common transmitter body.
What second letters commonly follow P in ISA 5.1 tags?
Common combinations include PT (Transmitter, PT. Pressure transmitter), PDT (Differential pressure transmitter, PDT), PI (Indicator, PI. Pressure gauge or indicator), PIC (Indicator plus controller, PIC), PSV (Safety valve, PSV. Pressure safety valve), PRV (Relief valve, PRV).
What are example ISA 5.1 tags starting with P?
PT-501 (Pressure transmitter on loop 501); PDT-105 (Differential pressure transmitter, e.g. Across an orifice, loop 105); PSV-2210 (Pressure safety valve, train 22, loop 10).
Run it on your drawings.
Upload a P&ID set and see every tag read in the convention the drawing already uses.