1969grad and piping designer get a star. ISA 5.1 includes the Piping and Instrument Diagram symbols and the identification letters. The loop number is largely client preference. A process unit may be cloned to use the same tag numbers. Often a plant or unit may use prefix or suffix associated with the devices. PCV suggets a self container pressure control valve or regulator. PV is normally used it the actuator receives a signal from a controller. LCV could suggest that a float operates a lever that opens and closes a valve directly. LV would normally be used if a level transmitter and controller generate the output signal to the valve.
The first alph character pertains to the measured variable. A analysis, F flow, L level, P pressure, T temperature and others. Sometimes there is a modifier like PDT for pressure differential transmitter. Subsequent letters pertain to indication functions, output functions, etc. The V in LV pertains to valve, T for transmitter. Other examples include C for control, A for alarm, S for switch and a confusing on Y for relay or other computing elements or converters, etc. Additional letters often modify these letters such as LAHH for level alarm high-high at the shutdown setting that is set higher than the LAH that pre-alarms the high level. LC is for level controller; or LIC for level indicating controller. Many people use LIT for level indicating transmitter with a local indication.
ISA
Attn: Standards Department
67 Alexander Drive
P.O. Box 12277
Research Triangle Park, NC 27709
John