I am trying to determine in which standard the p/T ratings of PN designated valves(Forged Steel Valves) is given. Or in other words, when a valve is stated to be PN16 Rated - to which standard is is rated as such?

I am aware of BS EN standards that cover the face to face dimensions etc. but I cannot seem to find anything that allows you to look up the material, class, temperature and the corresponding pressure rating for any given PN designation. (Like one does with the tables in ASME B16.5)

I am aware that EN1092-1 covers the p/T ratings of the flanges. Do these extend automatically to the valve as the valve body and components are assumed to be always stronger than the flanges?

Or is there some other theory behind the determination of a PN designated valves p/T rating?

You don't say where you are from, but from the nomenclature, it appears you are talking about European (CE)specs?  The CE Pressure Equipment Directive is 97/23/EN (google on those terms, you should be able to find a copy of this from the CE servers, in the language of your choice).  The PED has tables that specify the pressure and temperature limits for a part to fall within a particular classification.  Moreover, the manufacturer is supposed to maintain the certification, which should include the details of the calculations done to show certification, as well as the temperature and pressure limits defined for the part, in order to comply with the PED.

Hope that helps.  Though, being from the wrong side of the pond, it probably doesn't. 

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