Blowdown piping to B31.1

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.

Thank you for your post, but what I am most interested in is where in B31.1 do you find any reference to blowdown and blowoff piping rated at below 100 psi.

MAWP on nameplate is 75 psi

Then you take the value of "P" as 75, and run the required pipe wall thickness calculations. Unless it states otherwise, you can probably use sch 40 steel pipe. At a value of less than 100, you can use a variety of material for fittings and valves, brass, bronze, malleable iron, etc., if I remember correctly.

try B31.1 paragraph 122.1.4.

Please note there is a big difference between blowdown and blowoff- so make sure you have the right nomenclature.

min. thickness covered in B31.1 and clearly defined. what year issue of B31 are you using?

Agree with the previous suggestions and strongly suggest that you clarify the service that your considering. Blowdown/blowoff are too ambiguous.

are you replacing existing pipe are designing new?

how i select pipe diameter for given mass flowrate and inlet pressure.i want to know the technique to select pipe diameter.

economic pipe sizing issues should be posted as a new question to get the best response