How the flange rating of the valve must be mentioned with "CLASS" or "LBS"
Example: CL 150 or 150 LBS
Until the publication of B16.5-1973, valve pressure ratings were determined by B16.5, "Steel Pipe Flanges and Flanged Fittings". When "classes" of fittings were originally established, the pressure class of the valve was the allowable pressure [in psig] at 850F [except the 150 class that used 500F] and carbon steel was assumed. Using the material properties for carbon steel at 850F, the minimum wall was established. Using the minimum wall,  the process was reversed for other materials to give allowable pressures. If you go to the 1968 edition of B16.5, you will find the last use of "lb" to describe the pressure ratings, as well as the last time you will find the 850F "rule."
In 1973 B16.34 was first issued to cover butt-welding end valves. Concurrently B16.5 was also issued and covered just flanged valves. [The title of 16.5-1973 explicitly has the word "flanged valve." Earlier editions considered valves as "fittings." Butt-welding end valve were covered although not explicitly.] Both B16.5 and B16.34 presented a revised rating basis and both contained a completely revised set of rating tables. The use of "lb" was completely eliminated. Wall thickness was established using the class designation in psi, i.e., for Class 150, Pc = 150 psi, and a stress factor of 7000 psi. The pressure temperature tables were then generated based on material properties and temperature. In 1977 flanged valves were moved from B16.5 to B16.34.
So the proper term is Class 150, Class 300, etc. "Lb" is not proper, but is used frequently, usually as "300 lb Class," etc, by the old-timers and thus it is perpetuated.

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