Tie bar welded in staggered arrangement to boiler tubes 3

[ashaari]

Dear all,

I have a gas-fired water tube D-type package boiler, manufactured by Babcock & Wilcox. My boiler is a B&W FM model boiler. On the rear of the furnace, the tubes are flat-studded tubes, with refractory on the outside with inner hot casing and channel tie bar.

The channel tie bar is welded to the tubes, in a staggered manner (one tube on the top of the channel, the next on the bottom of the channel, etc). I don't know the technical term of this welding pattern. If you refer to the photo I attached to this post, you can see what I mean as welding to the tubes in staggered manner. Just a disclaimer, the photo is not the photo of my furnace arrangement - I just attach it to illustrate my point of the staggered welding.

My question is, why must the welding between the tie channel and the tubes be in staggered manner? Is it to cater for expansion of the tie channel? If it is, I can't imagine how.

Thanks!

 

[Mr168]

It reduces the total amount of weld necessary while keeping the tie bar flat and straight.

 

[chicopee]

I agree with Mr168, it keep the tie bar flat and straight against the tube bank. If the welds were only on one side of the tie bar, the tie bar could rotate away or toward the tubes.

 

[ashaari]

Thanks Mr168 & chicopee for your replies!

If that is the case, then there's nothing wrong if we weld top and bottom of the same tubes throughout the length of the tie bar, apart from welding more than necessary?

I am actually doing a repair of the tie bar, but the as-found condition is that in the previous repair, the tie bar welding was a mix of staggered welding, plus two tubes both welded on top and bottom.

Thanks.

 

[chicopee]

I would not weld top and bottom of each tube as the welds may become too rigid for the tubes. I would stay with the "tried and proven" welding design.

 

[ashaari]

Ok got it. Thanks again for your help chicopee.

 

[boilerone]

I designed boilers for years. I can safely say that the staggering is for making sure that all tubes get welded to the tie bars, not just ever other ones and the loads get evenly distributed.

There is a slight concern, if not staggered but having two welds on every other tubes, of thermal stress (because of the tie bar being hotter) like you and chicopee mentioned but it is typically not a problem for the width of the tie bar in your picture otherwise, I would only weld tops or bottoms and use holding lugs with gaps on the other sides.

By the way, this kind of design is no longer used in the recent boilers that I was dealing with.

 

[ashaari]

Deaer boilerone,

Thanks for your reply. I'm interested in your statement that the reason for the staggering "is for making sure that all tubes get welded to the tie bars". Why do we need the tubes to be welded to the tie bar?

You are right about the design no longer used in recent boilers. The photo I've attached, which is a tangent tube wall arrangement, is merely an illustration I came across which had a clear photo of the staggered welding arrangement. In the description of the tangent tube wall, the literature did mention that the design is considered old, with 2 different designs now in preference to the photo. The first design is a round bar which is seal welded between each tube, and the second design is a flat bar being seal welded between the tubes just behind the flat studs (for widely spaced tubes).

Thanks.

 

[boilerone]

It is not the tubes that need welding but the tie bars need to count on the welds to each tube to carry the horizontal loads plus this design has the concern of possible tearing the tubes open through the tiny bits of welds if have localized and concentrated loads. It is also easy to burn holes on the tubes while welding the tie bars to the tubes during installations. The later designs like you mentioned are much better. I was using horizontal scallop bars recently so as to get better weldings to the tubes and the tie bars.