"When a boiler is fitted with two or more pressure relief valves on one connection, this connection to the boiler shall have a cross-sectional area not less than the combined areas of inlet connections of all the pressure relief valves with which it connects and shall also meet the requirements of PG-71.3"
PG-71.3 also says:
"The opening or connection between the boiler and the pressure relief valve shall have at least the area of the valve inlet".
What do they define as the combined area of the PSVs? Only nominal pipe size, or do we consider the ID/OD instead?
Let's say I have two identical 1.5x2" PSVs attached by a 2.5" nozzle on a boiler. Using the NPS areas, this means that the combined areas of inlet connections is 2*PI/4*1.5^2 = 1.125PI in^2 and the nozzle connection is PI/4*2.5^2 = 1.5625PIin^2 and is acceptable.