Force fit tubes vs welded tubes

[ScottEIT]

Per ASME code, at what pressure do the tubes have to be welded connections? I don't have a copy of the code available. I am trying to understand why there is a discrepency with a vendor.
Thanks.

 

[metengr]

Please specify your component - is this for an unfired heat exchanger or a firetube boiler?

 

[ScottEIT]

Sorry, firetube boiler.

 

[metengr]

Ok. If you refer to ASME Section I, Part UFT, you have several options under PFT 12 for tube to tubesheet attachments. The options are;

1. Expanded and beaded

2. Expanded, beaded and seal welded

3. Expanded and seal welded

4. Welded

The welded in 4) uses partial penetration groove welds that provide the necessary pull out strength versus expanded and seal welded.

All four methods of attachment are not related to pressure. They are options for the designer to consider. I would obtain a copy of PFT and review it especially if you are in the position of evaluating design options of vendors.

 

[ScottEIT]

Thanks. I am new on the scene and don't have boiler experience. Our existing boiler is new, running off and on for 3-4 months. We've had problems with the tubes leaking at the steam drum, vendor re-seated and hydro tested, still leaks badly. They are going to weld all tubes starting next week. In researching the problem, I've been told that the ASME code specified welded tubes over 150psi, our spec was 200psi. I'm looking for confirmation that is was true. I'll see about getting a copy of PFT.

 

[deanc]

Hmmm

You say firetube in your second post but refer to the steam drum in the third. You could fall under PWT-11 or PFT-12.

There may be a case of driven ferrules or simple tube expansion. The tubes may be over expanded or not expanded enough. You may have tubehole cracks. The tubes may have been dirty when they were expanded.

My suggestion would be to have a good "R" stamp repair firm to have a look at this for another opinion.

Seal welding is not the answer for leaky tubes. You have other problems.

 

[metengr]

I would agree with deanc, this sounds more like a watertube boiler and, as such, would fall under PWT-11. I don't believe that ASME singles out any particular method of tube attachment based on operating pressure.

If this boiler is only 3-4 months old and you already have persistent tube/tubesheet leaks from cycling this is a significant problem.

If this is a warranty issue being only 3-4 months old, and the vendor is proposing seal welding you better sit down and confront them as to why they intend to seal weld. Were the tubes over-expanded into the drum? How were the tubes expanded into the drum?

The reason for my asking these questions is to show how detailed you need to be. If you have an inspection agency that insures the boiler for your company or your Jurisdiction provides this service, you might want to hire a Boiler Inspector to review your problem with you so that you can gain some OTJ experience (in a hurry).

 

[ScottEIT]

Thanks for clarifying. I've been in touch with the inspection agency and they too are looking into it also. I wanted to do my own research to get a better handle on this problem.

 

[InTheField]

I would also stay away from seal welding the tubes if this is a water tube boiler. You will likely only postpone the problem of changing the tubes in the future.

For small package tube boiler there generally have rolled connections - WITH NO SEAL WELDING. Tubes are rolled between 8%-10% wall reduction - basically strain. You can go higher without problems. I have seen tubes rolled to 34% that have been in service for 10 years, although I would not recommend going over 15% - it all depends on the final hardness of the rolled material - I believe NACE has guidlens for max hardess to avoid SCC.

Remember the words seal weld imply just that - it is a seal not a strength weld. The tube is held into the tube sheet by the mechanical rolling process. If the tubes are seal welded, the welds will likely fail from fatigue. Another thing to look at is the tubes on the OD of the drum - it is common for tube failures to occur there with boiler that are cycled.

Cheers.

 

[chicopee]

So! what type of boiler is it?

 

[davefitz]

Seal welding the rolled tube in a steam drum would normally require passivating the weld surface with some weak acid ( acidic acid?), then flusing with demin water, to remove all traces of caustics. Welding over caustic deposits would leave a brittle and porous weld, whcih would not only leak, but cause britle cracks to extend into the drum cylinder parent metal.

 

[davefitz]

oops.
citric acid, not acidic acid. Duh.