Your comment, "Think 125 pound steam running at 350 degrees F or higher, where the pipe dope would flow." got me thinking about a time back in the late 50's when I was working for the water department and we had to work around some guys doing some steam pipe work. They were using some nasty black sealant
on the pipe threads that I'm sure were NPT. So I decided to look up steam heat sealant, here's one of the products I found.
Copaltite Threaded joints holding 10,000 PSI 1,200 ° F
I also checked to see if NPT threads are used on with steam heat. It is.
I'm happy to be corrected on this issue, but it's my understanding that NPTF (Dryseal) threads, a variation of the NPT thread, were introduced to overcome the problem of spiral leaking, in particular, the gap between the root (valley) and the crest (peak) of the threads, that gap appears to be part of the NPT standard. This seems to coincide with my reading of the ASTM thread standard, specifically for thermoplastic fittings.
The following is from National Pipe Thread vs. National Pipe Thread Fuel
. I've also included a few similar articles that seem to reinforce this position.
"Both NPT and NPTF have the same threads-per-inch, pitch
diameters, and taper-per-inch. The differences come in the major and minor diameters, the root and crest of the threads."
"The NPTF thread crests fall with in the parameters of the NPT requirements, but have a smaller range than the NPT."
"The NPTF thread roots are different than the NPT thread roots. NPTF thread roots are designed to interfere with the crest of the mating thread with the intention of creating a mechanical seal through thread form deformation at assembly. NPTF threads have two classes
identified: Class 1 and Class 2. NPT thread roots are designed to allow clearance with the mating thread crests on assembly."
"NPT threads are designed to screw together. In most cases there will be no interference
between the root and crest of the threads at assembly. The thread is designed to be assembled with some form of sealant to assure a leak free joint."
"NPTF threads will screw together with NPT threads and should have no noticeable assembly problem. There will most likely be an interference
fit between the root and crest on either the major or minor diameter of the thread, depending on which part is NPTF. To accomplish a seal on the joint, a sealant will be required."
NPT Vs. NPTF Taper Pipe Threads
Even if we can't agree on this more esoteric issue I think we can agree on the more important issue, when joining NPT threads always use teflon tape or paste.