big problem in the boiler tubes..please help.

[chemical321]

I work in a power plant (4unit, each unit 150MW, total: 600MW).
We have a big problem in the boiler of the units. The supporting tubes wich hang the supperheater and reheater tubes; fail after about 5 or 6 years in each unit( I am not sure about the number of years).
Please see the attached file that show one of the failed supporting tubes and cross section of this tube.
(it seams that it is possible only 1 file to attach because of this 2 links are in the lower section of the text)


Some properties of the natural circulation boiler:
Fuel : gas (and some times furnace oil, specially in winter)

P=140 bar
T= 340 oC

Saturated water go to this train:
Boiler drum---Supporting tubes ---supperheater tubes--- turbine

Supporting tubes:
T in = 340 oC
T out= 450 oC
Material: 13Cr-Mo44
Thickness: 4mm
ID=20 mm

The failed tube poses direct hot flow of flu gases and convection and radiation of combustion. This failed tube is lower section of supporting tubes.

There are possible some reasons for this problem:
1- Hot corrosion because of vanadium and sodium oxides on the tube.
2- Unfit material selection for this section of boiler.
3- Incorrect Design of these tubes.
4- Creep failure because of deposit in tubes.

We don’t have any problem in supperheater tubes and reheater tubes.
Material of lower section of supperheater tubes (section3) that first meet the flow of gases in boiler: X20 Cr Mo- V121. water in these tubes have the most temperature.

Quantity of deposit in supper heater tubes is much in contrast with supporting tubes deposit. And I think this problem may not be from reason number 4.

in the cross section of the failed tube we can see some parallel lines... what are reason of these lines?

Please help me about this problem and suggest me to how analyze this tube to find correct reason.
What's your idea about this problem.

pictur1: failed tube

http://files.engineering.com/getfile.aspx?folder=2db3c4b4-a888-40a0-95c6-718465a6f74a&file=tube.JPG

picture 2:cross section of failed tube

http://files.engineering.com/getfil...48c4-a609-c76ed87e789b&file=cross_section.JPG

 

[ak1965]

chemical321

1. pics are not very clear. following may be analyzed.

2. External corrosion can be caused due to Flue gases. To establish this, the flue gas composition & sample of tube deposit need to be analyzed. Sulphur and other metal traces present in the gas may cause ash formation and accelerate corrosion. Please see picture below of a typical Deposit Failure initiation.

3.Micro Structural examination of Failed tube specimen should be carried out for indicating specific reason of failure. Please see a typical micrograph showing failures due to various reasons.

 

[JEB66]

You need to have the tubes exmained for failure mechanisum. The furnace manufacture can do this if you cannot do these inspections in house.

My 1st guess is overheat. Heavy oxide scale on the ID with a thick edge failure. I would rune a boroscope up and down the tube to inspect for pluggage.

If the combustion is very poor, you may have flame inpingment. Inspect burners.

 

[MJCronin]

What is the name of the company that designed your boiler ?

 

[metengr]

I work as a metallurgist for our Power Plants. My advice is to send the failed tube to a metallurgical lab for proper failure analysis. There could be several possibilities and to simply name them makes no sense. You need to determine the cause of failure and decide corrective actions after this is completed.

 

[davefitz]

first of all, if the tube is steam cooled , then you have to ensure that there is steam flow whenever the boiler firing rate is over 10% MCR ( or whenever the gas temperature near the tube is over 1000 F) This may require the use of a startup steam bypass valve whenver the main load is not yet available. Also, it means you will need to lower the burner heat input to less than 10% MCR whenever the steam flow has stopped, witin a few seconds of the flow stoppage. If your system is not setup to meet this requirement, then it might not be the boiler mrf's fault.

Next, if the support tubes have a nondrainable low point that colects condensate during outages , then you cannot raise boiler firing rate above 10% MCR until those tubes have "boiled out"- which may take over an hour during a cold startup. Boilout can be confirmed if you monitor maybe 20 tubes at their outlet header stubs, and when the tube to tube temperature unbalance drops below 75 F then you can assume there is steam flow in all tubes.

Finally , the required tube wall thickness of the suppport tubes is based on the allowable stress at the worst tubes max metal temperature, and the axial stress due to gravity load the tube is supporting must be added to the tube axial stress that is caused by the steam pressure in the tube. You must use the allowable stress that is calculated to include creep effects- just using a fraction of yield stress is an incorrect calc.

 

[chemical321]

designer of boiler: Waagner-Biro company
material of tubes is appropriate for this application?

 

[stanweld]

I would also recommend that you determine the depth of the apparent seam like discontinuities on the pipe ID surface.

 

[metengr]

These might be axial-oriented fissures in the oxide from creep deformation. Seen this before, which is why this failure must be analyzed by a lab.

 

[chemical321]

the thickness of tube have varied ....and the part of the tube that pose the direct flow of flue gases have a thin thickness...

 

[chemical321]

and the ID of tube have exceeded in contrast with before installation..upper section of cross section of tube have about 3.7mm thickness and lower section have about 2.5 mm.

 

[boilerone]

1. Corrosion - explains the rusts on the outer surface - bad fuel used, avoid. Might also cause creep happening at much lower temperature.
2. Tube material - explains the material appearing brittle. 13Cr-Mo44 looks correct but check,
a. actual material used? b. heat treatment?
3. Frequent startups and shutdowns - explains the parallel lines (fatigue) on the inside.

A good lab test should tell you the almost exact reason(s), mechanically and chemically.

Boilerone

 

[chicopee]

From your original post ..."The supporting tubes wich hang the supperheater and reheater tubes"... is that really the case?

 

[Guest102023]

Follow metengr's advice and have a proper metallurgical F/A performed. Otherwise we are all just speculating.

 

[Metaljon]

Yes you really need to a full metallurgical assessment to determine cause. Are the tubes seam welded as the main crack appear to be following a linear feature. The fracture looks too thick to be creep Looking at your conditions, I think corrosion fatigue could be a candidate for failure.