Superheat Header-Tube Socket Fitting

[1464649]

I have a lot more information to share on this, but in the interest of brevity: A boiler repair (B&W A-style Cyclone) we're working on has some socket style fittings that fit into the intermediate superheat header and connect the tubes to the header. They are similar to a sock-o-let except that they slide into the header rather than over it. Both the socket fitting and the tubes are fillet welded. Both are two inch OD. What is the purpose of this socket fitting (roughly 1.5 to 2 inches long)? Why are the tubes not welded right into the header? The socket is only located on twelve of more than sixty tube connections. The remaining tubes are welded directly to the header.

Thanks,
Brian

 

[metengr]

I would agree this does seem rather strange. Normally, stub tubes are welded to a header where a shoulder or recess is machined into the header tube hole and the tube is placed on the shoulder and fillet welded (see ASME B&PV Code, Section I, Part UW).

The use of fittings on only 12 tubes implies to me that previous weld repairs were made and the contractor or owner decided to use weld-o-lets to make the tube to header attachment weld. Not typical.

 

[boilerone]

There are basically three different types of tube or pipe fittings could you tell which ones are likely what have?

1. Through type: the hole is slightly larger than the OD of the stub, the stub can (but does not have to) penetrate the entire thickness of the header. This one has the largest cut-out on the header so is tough for header thickness compensation calculation also requires relatively much larger header size than that of the stub. It is lest frequently used but can still be seen on such like boiler steam drums.

2. Tube seat type: the stub sits on top of a machined flat surface with groove around it like described by metengr, the hole has the same or larger diameter of the stub. This type is most frequently used especially for shop-manufactured headers. The flat shoulder surface and groove help to get good dimensional and welding qualities. These are normally made by digitally-controlled machining and welding equipment. The stub for this type also has to be small so that there is room for making the shoulder and the groove.

3. Set-on type: the hole diameter is the same as the ID of the stub, the stub sits right on the header. This type has the smallest opening on the header so requires least strength compensation and you can also use commercially-available self-reinfored sock-o-lets (for small stubs) or weld-o-lets (for larger stubs) to totally avoid compensation calculation so it is widely used for on-site repairs and modifications. It also does not limit the size of the stub because a fish-mouth shape can be used. Due to the higher cost of the stubs, this type should not be seen very to have many on, such like, your superheater headers.

All of these types work as long as they satisfy code requirements. Which type to use is normally decided by economical and feasibility situations.

 

[rmw]

Sounds to me like someone had some tubing that wasn't dimensionally correct and they used the stub ends, excuse me, socket ends to make up the difference.

rmw

 

[1464649]

The stubs in question are similar to scenario #1 by boilerone above. I have since learned from B&W that since we are using thicker tubes (0.250" wall versus 0.165" as originally designed) the stubs are not required and we can weld the tubes directly into the header.

The rows the stubs were used on had the greatest bend and I'm told by B&W that the stubs were there for reinforcement on those tubes. This doesn't make sense to me as the shear and bending stress would be (almost) the same on the tube whether the socket fitting were there or not. The only change I see is a slight reduction in bending stress due to moving the weld about 1" (from a 4" leg to a 3" leg) closer to the vertical run of the tube. The lowest tube is bent at 60 deg.

Thanks for everyone's feedback!