Swaged Tubes in Boiler

[Shofa]

Dear Expertise,

We are in final process to order a water tube boiler with 125 T/h capacity. The vendor is insisting to use swaged tubes for the connection between the tubes and mud drum. Our specifications is not allowing for that. Can you please tell me the advantage and disadvantage to use the swaged tubes?

Thanks for your valuable support.

 

[metengr]

Are you referring to roll/expanded tubes into the drum or connecting furnace tubes to terminal tubes already on the drum by swaging?

Swaged boiler tube ends are commonly used by boiler OEM's to transition between tube- to-tube dimensions (OD variations). We have many of them in our Power boilers. They are reliable connections provided the material is carbon and low alloy steel material. When you get into austenitic stainless steel boiler tubes, the tubes connections that are swaged must be solution annealed (after swaging).

 

[Shofa]

Thank you metengr for your input. My enquiry regarding the roll/expanded tubes into the drum. My concern is the stress concentration due the change in the tube diameter and the local stress load . Also, ASME I is not referring to swaged connection in PWT-11.

 

[metengr]

Shofa;
Roll expanded tubes installed in lower drums is not a problem. We have several old B&W boilers which were designed using roll expanded tubes (the portion of the rolled tube exposed to the waterside surface in the drum is seal welded to assure leak tighness) in the lower drum. The reduction in tube wall thickness after roll expanding is in the range of 8 to 10%. I don't see any stress concentration issues.

The tube holes in the lower drum are typically grooved to provide a mechanical seal after the tubes are roll expanded. We have never had roll joint leaks or tube failures, even after 45 years of service.

 

[Shofa]

Thank you for your helpful.

What about connecting furnace tubes to terminal tubes already on the drum by swaging.

 

[metengr]

No problem.

 

[metengr]

Also, I should mention that the swage tube end should be engineered with a suitable transition or taper to assure no local stress concentration.

 

[Shofa]

metengr,

I really appreciate your support and prompt action.

Thank you and best regards.

 

[davefitz]

Problems with swaged tubes:

a) the swaging process generates thin slivers of metal , that accumulate in the mud drum and tend to choke off bottom blowdown drains

b)If the upper steam drum has a thick wall or if you startup too quickly, the upper steam drum will "bananna" and cause some boiler bank tubes to stretch, this tends to pull the swaged tubes out of their connection and they leak . If the steam drum is predicted to "bananna" during startups or shutdowns, then the tubes whould be welded into the drums.

c) later seal welding of swaged tubes is sometimes problematic, as embedded casutics in the swaged joint will cause weld porosity and brittleness.

 

[metengr]

davefitz;
I am curious, from the problems you indicated above these have got to be HRSG or newer unit construction problems. On the larger and older Power boilers (B&W, CE), we just have not experienced the problems you mentioned above, and either have my peers in the Power Generation sector on older Fossil units.

 

[unclesyd]

The only time I've seen any problem with weepers on the power boiler steam drum is when someone quenches the boiler. Our smaller boilers are mostly B & W. age 55.

We did have a little trouble with our thermal oil vaporizer "D" tube heaters at age 20-25. Both drums were given a slight reroll and no more problems. Most of the vaporizers are at age 50-55 now.

 

[davefitz]

Meteng-
I have seen the mud drum loaded with rolled shavings in the Georgetown fluid bed boiler ( startup 1980), and the rolled steam drum tubes pull loose in the Fort McMurray Suncor units ( related to drum bottom quenching when tripped due to lo lo water level and refilled with icy feedwater and no girth baffle plate to assuage the shock).

 

[aobjr]

We have been using swaged tubes for our small capacity bi-drum boilers. Based on experienced by using swaged tubes we can decrease the drum thickness due to increase in ligament efficiency, Take note we need to consider also the other drum holes configurations whichever is the least ligament efficiency value.

1.Please be sure when using swaged tube, the hardness should be the same as the mother tube before using. If the hardness after swaging process is greater than the mother tube, please conduct heat treatment to lower the hardness.
2. Please conduct also VT or as much as possible PT to be sure of its surface quality. You may waived this requirement if your swage maker is reliable.
3. Initial rolling should be 8~10, then final would be 12. IF there is leakage reroll up to 15

 

[deanc]

Roller expanding has been used for over 100 years. If "properly" done it is a excellent process. If you see metal shaving the tubes are grossly overrolled and will not seal. The only answer then is to replace the tube and check for hole damage,such as cracked ligaments. Seal welding is not the answer at this point. That is not the intent.

Pull outs....That is the reason the holes are grooved and the tubes flared or beaded. Once again it needs to be properly done. The "Code" is a good place to start,(hummm).

Swaging has also been performed for a like time. There should be no problem if properly done. Check the B&W site.

Rolling: Airetool,Elliot,Thomas Wilson for info.

http://www.cooperpowertools.com/brands/airetool/index.htm

After being in the field for more years then I want to admit I have seen all the above problems. 99% of the time it is poor workmanship and/or technique.