Some good source of Relief Valves Problems/Design.
I have one recommendation for Relief Systems Handbook from Cyril F. Parry.
Know somebody if it is possible to download it from net ?
There is a Safety-relief Valve engineering subforum in the Chemical Engineering section.  Perhaps you should move the thread there. 
Crosby/Farris/Consolidated all print engineering manuals that offer a wealth of information.  Jump on their websites to download the document or to request a printed copy. 
from ASME, section 1 :
PG-67.2.6 Any economizer lhal may be shut off
from the boiler, thereby permitting the economizer to
become a fired pressure ves sel, shall have Dne or more
safety relief valves with a total discharge capacity, in
lb/br (kg/hr), calculated from the maximum expected
heat absorption in Btu/br (W), as determined by the
Manufacturer, divided by 1,000 (1.6).

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