Your boiler operates at 75 PSIG, or the safety valves are set for 75? What's the max allowable pressure on the boiler nameplate?
You can run into local spins on code stuff like this. To make sure that you're doing things that will pass in your jurisdiction, ask the inspection authority. I like using email for this, as it has the exact wording of the question posed, along with a time & date. I been involved in several situations where two people from the same inspection authority have given totally different answers to the same question.
Having said that, in a recent discussion with my local code-types on a similar issue, I was told that the value of 'P' is not the safety valve setting of the boiler, but rather the name-plate pressure rating. It also depends upon where the last valve is, in the pipe run. If it's just after the boiler, then the rest of the pipe, on the way to the blowdown tank with an open vent, and overflow drain didn't have to be sch 80. However, if a valve was ever cut-in, then the valve, and upstream piping would now fall under the requirements for blowdown/off piping outlined in B31.1.
The pipe won't be less than sch 40 - that's the thinnest wall you can cut threads in. You've got a boiler at 75 PSIG, how big can this piping be? I'd run sch 80 & 3000# fittings anyway, the increased cost will be pretty small, and if there's ever a change like someone cutting in a valve, you're very likely covered.
