Swaged Tubes in Boilers

[GBurns]

We have an installation that has swaged tubes installed in the lower furnace. Recently we have experienced some failures of these tubes. The boiler rep is pointing towards possible water treatment issues, however, we have had no failures away from the swaged tubes. The tubes were not stress releaved after manufacturing. Any one have any input?

thread794-163695

 

[metengr]

GBurns;
Just to clarify, are you referring to actual swaged tubes where the OD changes in the tube circuit, and not rolled tubes in a lower drum? Also, what is the tube material?

 

[GBurns]

These are 3" to 2 1/2" swaged tubes. They are composite tubing in a chemical recovery boiler (carbon steel with a stainless cladding). These are furnace wall tubes. In the 3" area the tubes are tangent, in the 2 1/2" zone there is a 1/2" membrane between tubes.

 

[metengr]

GBurns;
I would strongly recommend you have a metallurgical failure analysis performed on one of the swaged tubes. There could be several reasons for the tube failure, so without a detailed failure analysis I could only guess at what the problem is.

 

[SMF1964]

Our experience with swaged tube failures has been in the area of strain age embrittlement. You mention that they were not stress-relieved after manufacture; I assume you are saying that there was no stress-relief after the swaging operation? A detailed failure analysis is definitely recommended.

Does the boiler rep have anything to support his claim of boiler water chemistry?

 

[GBurns]

MetEngr,

We are in the process of doing several metallurgical analysis both internally and a third party. I was hoping to find others that have experienced similar failures in case the metallurgical analysis proves inconclusive. It appears that high residual stresses play a key role. Initally we thought that the mechanism was SCC with the high residual stresses and free caustic building up from DNB in pitting on the hot side of the tube. Unfortunately we have now found cracking on the cold side where you would not expect to see caustic concentrating to levels where you would see SCC. Other areas of the boiler are well passivated and the water chemistry appeared to be well controlled.

 

[0707]

If I understood your boiler tubes are cladded inside with stainless steel. 2 ½” swaged tubes are connected to a header or a barrel. Swaged zone connections are liked each other through carbon steel membranes. Is it all right?

If so, in my opinion, swaged zone connections are failing due to differential thermal coephicient between carbon steel and stainless cladding and are also stressed by carbon steel membranes. How does look like your failures? Cracking on swage membranes?

Let us know of your findings.

Good luck

luismarques

 

[metengr]

0707;
The boiler tubes should be cladded on the OD not the ID surface.

 

[GBurns]

The stainless cladding is on the OD. These tubes are cracking from the ID out. Stainless cladding is fairly standard on Chemical recovery boiler furnace tubes.

 

[robsalv]

We have a 30yr old gas fired D type 66000kg/hr package boiler (manufactured by ICAL) which appears to be having bend or swage failures in the convection tubes near the mud drum. By swage I mean that the tubes change diameter and as such we're not able to apply any typical tube NDE techniques to investigate tube wall thickness. I'd be happy to hear of some though - we've tried EC but can't calibrate or centre the probe for the greater diameter, and we've tried IRIS, but the probe is too long and the bend interferes with it.

We're about to embark on cutting out a tube so we can apply metallurgical analysis. Cutting out a tube is a big deal because the package boiler is a positive pressure boiler where the wall tubes are membrane welded together to create a gas side pressure envelope.

The idea of DSAE and/or SCC of some kind has crossed our minds. We're also concerned that some combustion product is accreting on the mud drum surface and becoming acidic when offline and absorbing moisture... the pH of the combustion product we've scraped off is as low as 3. (The steam boiler only ever goes off line for inspections or ancillary equipment problems)

The other concern we have is that we have no idea how many swage/bends may be impacted... short of plugging a whole bunch of tubes to bring back reliability in the short term - some kind of major overhaul is looking likely.

Sooo, I'll be watching this thread closely and looking with interest at any shared experiences. I'll be sure to come back and share ours.


Thanks.

Rob