P - Pressure
Reference for the ISA 5.1 first letter P. 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.
Read one of your own drawings.
Drop a P&ID, instrument index, or schedule. Tagsight reads it to the tag and opens a workspace you keep when you sign in.
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Also stands for.
- Pressure differential, PDT
- Vacuum, when service is sub-atmospheric
Common second letters.
Transmitter, PT. Pressure transmitter
Differential pressure transmitter, PDT
Indicator, PI. Pressure gauge or indicator
Indicator plus controller, PIC
Safety valve, PSV. Pressure safety valve
Relief valve, PRV
Switch high-high, PSHH. SIS trip
Alarm low-low, PALL. SIS trip
Example tags.
Pressure transmitter on loop 501
Differential pressure transmitter, e.g. Across an orifice, loop 105
Pressure safety valve, train 22, loop 10
Pressure switch high-high, voted 1-out-of-2, SIS service
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.