Valve Body Thickness

[JohnsonMadukayil]

I am currently working on a Design Report for a Threaded Pressure Relief Valve for a Nuclear application. ASME Section III recommends the use of ANSI B16.34 for body min wall thickness and pressure rating calculations. Table 3 of B16.34 is used for the body wall. While Table 4 is to be used for the threaded end connection. While Table 3 is more amenable, Table 4 seems to be extremely conservative and bodies which might deem to be a class 150 or 300 according to Table 3, fail the Table 4 test.

Under what circumstances is Table 4 to be used? The threaded valve has the body end connection as male end with NPT threads. It is 1/2" NPS. The outside diameter to the root of the thread is 0.726" while the inside diameter of the bore is 0.500". This gives a end wall thickness of 0.082" while the minimum required according to Table 4 is 0.130". Can somebody please help throw some light on this?

 

[JimCasey]

I took a look at table 4, and it only addresses female connnections. A female NPT connection would be expected to be under substantial tensile load due to the wedging effect of the 2-degree taper of the pipe thread. Similarly with socketweld, all the beef is there to give you a place for the weld-bead to be laid without causing meltback or other distortion of the body.

The diagonal measurement of the thickness shown in table 4 would be the only applicable dimension In My Humble Opinion (legal disclaimer).

Here's my question....How did you get agreement to use NPT threads on a Nuclear Safety Related application? I won't question the safety of the NPT threads, but the industry is so conservative and a threaded connection has a continuous leak path. Nukes I know want to weld everything. Also, TFE-based pipe tapes or thread dope would not be applicable in a radiation field..You'd have to put SOMETHING on the threads as a sealant/antiseize....

 

[JohnsonMadukayil]

Thanks for the repsonse Jim.

For class 1, 2 and 3 applications in the Nuclear Industry, we have been supplying threaded relief valves since the last 6 years or so.

You are correct in saying that Table 4 of ANSI B16.34 shows only the Female NPT thread ends. But then paragraph 6.2.4 of ANSI B16.34 talks about using Table 4 for 'threaded ends'. If it was only for female ends, it could be logical to assume that they would have stated it explicitly here.

It is indeed somewhat confusing.