When May "T"-sub-"W" Be < 0.25" ?

This question is in reference to the header design in HRSG harps and bundles where thin-wall boiler tubes are inserted into headers in a "stick-through" design, and fillet-welded all the way around.

When I asked this question of one boiler manufacturer - -

Can you tell me what code case allows the "T"-sub-"W" to be < 0.25" ? ?

Your design seems to be based on straight-on (radial entry, not "sidehill") tube
to header "stick-through" connections. There is a "J"-groove or a "V"-bevel milled into the surface of the header around each bored tube insertion hole.

Your drawings show the "T"-sub-"W" to be < 0.25" on
the low side of the tube nozzle hole boring in the header, and equal to
0.25" really only at two points on the circle, each on the "high" side.

The entire "J"-Groove (or "V"-Bevel) does not seem to be
greater than or equal to 0.25" depth at all points along the circle.
Depending on the style of milling machine (straight bore, two-axis VMB,
etc.) that you use, it may or may not be possible to have a
uniform groove depth. If the depth cannot be uniformly milled, then we
would expect the "T"-sub-"W" to be = 0.25" at the two lowest points, and
therefore the "T"-sub-"W" to be > 0.25" at the two high points.

Can you please refer me to either the addendum and/or the Code Case that
allows for this, since the "bare code" states that "T"-sub-"W" will be a
minimum of 0.25"?

The Boiler Manufacturer's answer:

We are following exactly what is required by ASME Section I. The only code case that pertains to the tube welding and paths of failure calculation is CC 2191. We use Section I Figure PW-16.1 (z) for attachment of tubes to the header using the "Stick Through" construction. In figure PW-16.1 (z) it show the attachment of the tube to the header where the bevel on the high side is minimum as defined by the rules and drops off on the low side. In this case only "Tc" is required on the low side. If there is some bevel left all the better but it is not required. In addition per CC 2191 Reply (c) exempts this detail for paths of failure calculations of PG-37.2 and PW-15, provided the header has been designed to the provisions of PG-52 for ligaments and this detail meets the requirements of PW-16.1, which it does.

Question for the Group - - The Intent of the Code - -

When you look at Figure PW-16.1-(z), it appears that the intent was to cover heavy-wall pipe branch nozzles that are inserted into a sort of milled socket in an equally heavy-wall header. There is no illustration in the code of a very thin-wall boiler tube (for + 2,000 psig applications ! !) being completely "stuck-through" a heavy-wall header. So, my question - - when you look at that illustration in the Code, what was the real intent? Are the boiler manufacturers "stretching" the legitimate application of this detail to include very thin-wall boiler tubes at very high pressures that should probably be connected to the headers with a set-on full-penetration weld design?

W. R. ("Bill") Clements
Technical Lead, Fabrication and Construction
Siemens-Westinghouse Power Corporation
Orlando, FL
407-736-6814
William.Clements@swpc.siemens.com