Thank you for all the replies, to clarify it is 9' diameter, carbon steel and large enough to be considered a Code vessel. According to the Owners Agent, the failure condition is for a dust explosion (which would produce about 150 PSI). I recommended a burst plate or other means of safely designing this pressure into the system, they however said they didn't want to do this.
I guess my question stems from the definition of design pressure in the BPVC which is the "most severe condition of coincident pressure and temperature expected in normal operation", and according to the Agent this is not in normal operation but an unlikely event (that would result in the vessel being taken out of service if it were to occur).
Since we will not be building this vessel and the Agent is responsible for telling the designer/fabricator the design conditions I wasn't sure if I was interpreting the rules of the Code correctly. If in fact the design pressure is meant to cover the pressures in 'normal operation', and it is the owners responsibility to inform us of the design conditions it seems to me that this vessel could be considered non-Code.
Also, the vessel is designed at the 5PSI with the appropriate SF, but the explosion condition they want designed to SF=1.5 at UTS.
I told them that I thought it should be Code Stamped (which if it is the SF for the 150PSI would be 3.5), but ultimately they are the ones signing off on the design so I'm not sure if I've covered my bases or if I should be doing something else, which I guess would be declining to do these calcs if they insist on it being non-Code and not introducing a burst plate or other means of dealing with the overpressure condition. What do you think?