I'm a newcomer here. I work in a bio-ethanol plant.  The valves in following services will be considered critical (in the opinion of our Contractor):  
  1)      Safety and relief valves.  
  2)  Control valves in the severe corrosive/erosive service if specified by Licensor / Owner.
  3)   Valves for which Licensor has required specific performance testing like helium testing etc.  
  4)    ESD valves in inventory isolation service
  5)  Valves with gearbox or actuator on extended bonnet  
  6)  Main isolation valves for HHP Boilers in HHP steam service. The valves in vent, drain, instrument isolation and other small diameter valves in HHP steam service are excluded from the critical valves list.
  7)  Two slide valves in Regenerator Flue gas service.

More details:
"Criteria's for the identification of the critical valves have to take into account the following general considerations -
4.1.1 Valves of "Large" quarter turn type (For plug valve>10", for ball valve>24"and for butterfly valve>60')  
4.1.2 Valves that are in service requiring specific performance testing like Helium testing etc.
4.1.3 Valves which have critical functions (eg. frequent operational isolation duties, or emergency shutdown isolation, or venting duties).  
4.1.4 Valves which have gear box or motor actuator mounted on extended bonnet. (Refer Instrument summary for Instrument valves)  
4.1.5 Valves which are otherwise novel or in extreme or unusual   applications.
4.1.6 All relief valves and safety valves (Refer Instrument summary for Instrument valves).
4.1.7  All high pressure valves (1500# and above)."

More details:
"Criteria's for the identification of the critical valves have to take into account the following general considerations -
4.1.1 Valves of "Large" quarter turn type (For plug valve>10", for ball valve>24"and for butterfly valve>60')  
4.1.2 Valves that are in service requiring specific performance testing like Helium testing etc.
4.1.3 Valves which have critical functions (eg. frequent operational isolation duties, or emergency shutdown isolation, or venting duties).  
4.1.4 Valves which have gear box or motor actuator mounted on extended bonnet. (Refer Instrument summary for Instrument valves)  
4.1.5 Valves which are otherwise novel or in extreme or unusual   applications.
4.1.6 All relief valves and safety valves (Refer Instrument summary for Instrument valves).
4.1.7  All high pressure valves (1500# and above)."

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