Konrad;
In the US, the selection of safety or safety relief valves for steam service and relief valves for fluid service are based on set pressure (which is based on some percentage of operating or maximum allowable working pressure) and steaming or fluid capacity of the component (depending on the function of the valve). ASME code doesn't specify a particular valve it is up to the owner of the vessel to select appropriate valves and assure proper installation based on code requirements.

I am not familiar with European codes or standards so I included the attached web site which provides information on standards for safety valves and safety devices for protection against over-pressure for Europe and other countries. I would suggest looking into obtaining a copy of this standard for Europe and obtaining ISO 4126;

http://www.steam-toolbox.com/41_439.html

Konrad,

I'm curious about your comment that "almost all valves I could find available, are rated in MBH."  Where were you looking?  I'm not familiar with the rating units of MBH.

In the US, the typical units used for manufacturer certified relief valve capacities are
air, standard cubic feet per minute (scfm)
water, gallons per minute (gpm)
steam, pounds per hour (lb/hr)

A given valve with a given set pressure will be stamped with a value designating its capacity at 10% overpressure in terms of one of the above fluids.  Otherwise, a manufacturer's catalog will show a valve's capacity at other set pressures as well.

To select a relief valve, you should contact a valve manufacturer, they can provide you with additional assistance.  You may want to visit some of the relief valve manufacturer websites

Farris
http://www.cwfc.com/index.html

Consolidated
http://www.consolidatedvalve.com/

Anderson Greenwood
http://www.andersongreenwood.com/default.asp
The Pressure Relief Valve Engineering Handbook found at this site is a reasonably good reference to have.

Konrad,

After taking a little time to think about your request, I believe my last post is not  the information you need.  Those links and the valve capacity information I indicated previously are primarily for equipment built to ASME Section VIII.  You are interested in relief valves that might be used on equipment built to ASME Section IV.

That is a little different and I'm not that versed in ASME Section IV relief systems.  Maybe someone else can offer a little more help.  But from what I've seen, typically, relief valves used for Section IV service may show a rating as either btu/hr or lb/hr steam.  It is just a matter of knowing the steam latent heat to convert between the two.  So I suspect that MBH may actually be the units of Million Btus per Hour.  I don't have a complete and up-to-date copy of ASME Section IV but I think part HG-402 will explain about Section IV relief valve capacities.

Again, I would suggest contacting a relief valve manufacturer.  The following are some manufacturers of Section IV valves that I have seen.

Kunkle
http://www.kunklevalve.com/catalog/KUKMC-0388-US.pdf

Bell & Gossett
http://www.bellgossett.com/literature/bulletin/bghyd/a415.PDF

Watts
http://www.watts-regulator.com/pdf-files/productguides/PG-SSRV.pdf

Thank you guys, really. Yes, I didn't dig deep enough, maybe because I was kinda forced by our customer to use Conbraco (www.conbraco.com), which looks like a piece of el cheapo equipment. Now I know at least how to convince him to get rid of his valve.

Regards
Konrad

PS. MBH=1000 btu/hr