Why are UNF threads being used in this high temperature Boiler Code application?

[Pkruse]

I saw the assembly of a high temperature Boiler Code application the other day. They used nickel and Chrome alloys for the high temperatures. I found the flanges to be particularly interesting. They were using UNF threads, which is not common in boilers, and they were using threaded rod with nuts on both ends.

So I talked with the design engineer. He had called out hex bolts on the drawing, but approved the change because they had a long lead time and were much more expensive. He also said that he likes the threaded rods better anyway because they are stronger, because they eliminate a KT between the shank and the head.

He used the UNF threads only because his customer was in love with them.

Have any of you ever considered this KT in one of your designs?

 

[Mech_Engineer]

In critical applications where safety or performance are concerns, it's best practice to use certified fasteners (e.g. fastener lots that have had a statistical sample tested for strength). I could see how an engineer might be interested in the stress concentrator between the shank & head of a bolt if they're designing a bolt from scratch, but in the design of a large system (like a boiler) which uses off-the-shelf fasteners, I doubt you'd take this into account. More likely you'll find certified fasteners which are strong enough for your application.

Using a stud with threads on both ends doesn't strike me as particurally unusual, as long as you're able to find a stud with certified strength equal to that of the equivalent bolt.

 

[Pkruse]

I'm assuming that all material in this project is certified. They used the same nickel super alloys you see in a jet engine from the same suppliers.