Blisters in firetube boilers

[deckie]

What causes a blister in the furnance tube of a firetube boiler. I know it is due to plate lamination but what causes the blister after 20 years of service with no problems ?

 

[metengr]

The best source for this is the NBIC, Part 2; I have included an excerpt below

3.4.7 BULGES AND BLISTERS
a) A bulge may be caused by overheating of the entire thickness of the metal, thereby lowering the strength
of the metal which is then deformed by the pressure. Bulges may also be caused by creep or temperature
gradients.
b) A blister may be caused by a defect in the metal, such as a lamination, where the side exposed to the fire
overheats but the other side retains its strength due to cooling effect of water or other medium. Blisters
may also be caused by a hydrogen environment (See NBIC Part 2, 3.4.5).


So, you may have blisters that formed from an event and if the event is not repeated, they could be dormant in service. However, this requires careful evaluation.

 

[chicopee]

How do you know that it is a blister and not a bulge?

 

[chicopee]

If it is a blister, there may have been some inclusion in the steel coupled with scale on the waterside, you have an overheated area.

 

[blacksmith37]

After 20 yr of service, I would say Hydrogen ; corrosion on waterside or reducing flame on fireside. If it is hydrogen , high temp H attack is a possibility.

 

[chicopee]

It is all speculation at the moment; perhaps pictures would help.

 

[deckie]

Thanks to all for your help. Picture attached.

 

[chicopee]

Looks like there may have been slag inclusion in the steel. From the picture, the fireside of the tube soften to the point that the two blisters have drooped downward similar to dew drops on a surface from the effect of gravity. Was this condition newly discovered as a result of an annual internal inspection or was this a preexisting condition that may have worsen? and what does the waterside of the tubes look like another word any scale buildup? The only way to know the cause is to cut out a segment of the affected tube and have it examined. I would be of the opinion to replace the entire tube before putting the boiler back in operation.

 

[GenB]

The is ready to blow up. You need a new furnace.

 

[macmet]

What was the expected lifecycle of the boiler? Depending on the situation you're in, it may be better to just replace it.