Combined Cycle HRSG's - - Tube-to-Header Stick-Through Designs

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Posted by W. R. (Bill) Clements on February 20, 2000 at 12:58:10:

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" ? ?

I further clarified my question: When a tube-to-header stick-thru design is based on a radial entry ("straight-on") or a tangential ("eccentric", or "sidehill") tube to header "stick-through" connection, there is a "J"-groove or a "V"-bevel milled into the surface of the header around each bored tube insertion hole. As the buyer, we expected to see a full 360 degree bevel weld (or, a "J"-groove weld) around each tube, and that weld would then be "capped" (reinforced) with a fillet weld.

The HRSG manufacturer's 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. In fact on very small (3.5" O.D.) headers, our inspectors found that in the HRSG fabrication shop that there was actually no bevel at all on the low 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 the manufacturer may use, it may or may not be possible to have a uniform groove depth. If the depth cannot be uniformly milled, then we would prefer the "T"-sub-"W" to be = 0.25" at the two lowest points, and therefore the "T"-sub-"W" would 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 Figure PW-16.1-(z) 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?

Alternately, even if you accept the current "state-of-the-art" designs from the HRSG vendors, and if you accept that the weld bevel may be less than 0.25" at some points around the circle, shouldn't there be at least some measurable weld bevel on the low side? Again, our inspectors found that in the HRSG fabrication shop that there was actually no bevel at all on the low side on fairly small diameter headers. I had always been taught that the intent of the Code was that for boiler internal ("fired") pressure-retaining welds, that the weld had to be a combination weld, never just a fillet-weld alone.

Comments from those knowledgeable of ASME Section One would be appreciated.

W. R. ("Bill") Clements
Technical Lead, Fabrication and Construction
Siemens-Westinghouse Power Corporation
Orlando, FL

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