I can't speak to the EN Standard or provide a comparison to ASME Section I or IV for you.
If your boiler is a steam boiler and has a MAWP of greater than 15 psi, it requires the "S" Stamp from ASME. If the design pressure is 15 psi or below, it would require the "H" stamp for heating boilers...
To determine RV minimum requirements look at HG400.2(e). It is the greater of the two cases specified. With regard to sizing the inlet/outlet of the RV, you probably don't need to do that since almost all relief valves in this service are stock items from the valve manufacturer.
Within the US not only do you have state law to deal with but also municipal regulation as well. In PA, as an example, there is no state wide requirement to license boiler operators. The City of Philadelphia does though. In New Jersey they have very specific rules for licensing.
The National...
UL,
Amerian Assoc of Railroads writes rules for rail car tankers, ASME does not.
Over the road tankers that carry hazardous materials and have MAWP's in excess of 35 psi are supposed to be U Stamped. ASME in conjunction with US DOT has developed Section XII for transport tanks. It is based on...
