If a valve is supposed to be say Class 800,  what does it mean?  I am sure it is related to pressure and temperature but do not know the exact meaning.

Also if I have a valve with known capability to handle a known pressure and temperature,  how do I know what Class it is?

I guess both questions are related .....

flanges and valves are rated by class.  most people will say that a flange or valve is a 600#, 800#, 1500# etc.  this is not quite right.  the word 'pound' sould not be used in the description.  the proper term is 'class' as you mentioned in your original post.  one source for the temp/pressure limits for flanges and valves is B16.5 and B16.34 respectively.  a pressure/temp example for a class 600 A105 flange from B16.5 show that at 200 degrees, the allowable working pressure is 1360 psig, but at 650 degrees the allowable working pressure drops to 1100 psig.

API Spec 6D 2002 refers to ASME B16.34 for rating tables by material groups.  No table 2.1 exists.  

API STD 602 indicates that "Class 800 is not a listed class designation, but is an intermediate class number widely used for socketwelding and threaded end compact valves."  Also "This International Standard is applicable to valve(ball valve) end flanges in accordance with ASME B 16.5 and valve body ends having tapered pipe threads to ISO 7-1 or ASME B 1.20.1. It is applicable to extended body construction in sizes and pressure designations of Class 800 and Class 1500, and to bellows and bellows assembly construction as may be adaptable to gate or globe valves in sizes. It
covers bellows stem seal type testing requirements."

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