Brittle CS boiler tubes - hydrotesting 1

[mjpetrag]

We have a boiler that has several tubes that have undergone overheating tube ruptures due to poor water treatment and subsequent tube blockages. We have fixed the feed water quality issue and have just completed an acid wash of the boiler to remove any blockages from the tubes, steam drum, mud drum, and water wall headers. We found 3 other tubes that were visually overheated and confirmed with Brinnell Hardness testing that we will be replacing.

After the acid wash, we hydrostatic tested the boiler to 1.5x MAWP at 70 F water with no leaks.

The only problem I see is if we have tubes that are brittle that we have not picked up on visual inspection could potentially fail in operation. We are going to test every other tube on the water walls for hardness while we repair the 3 overheated tubes to help pick up any potential brittle tubes.

My question is, if the boiler hydrotest passed at 1.5x MAWP at 70 F, but we have brittle tubes that have not been tested or repaired, however we have cleared all restrictions in the tubes and can maintain good circulation to prevent further overheating, is it likely those brittle tubes will fail in operation? Is hydrostatic testing a surefire way to pick up a brittle fracture of a tube before putting the boiler back into service?

-Mike

 

[MJCronin]

Where is it written that "hydrostatic testing a surefire way to pick up a brittle fracture of a tube before putting the boiler back into service" ?

I am a retired Mechanical/Power engineer with 43 years of experience and have read virually everything on boiler repair, boiler design and metalurgy - have never read this (especially the "surefire" part)

... but I still learn something everyday...

The "Boiler Tube Failure" Handbook, may help ...

https://mmengineering.com/boiler-tube-failure/

Please respect us and let us know about your final resolution to this problem..

Best of luck to you !!

MJCronin
Sr. Process Engineer

 

[EdStainless]

Hydro will find existing leaks, and nothing else.
The stresses are way to low for failures to occur, unless they are already partially failed.
How much metal did you remove in your cleaning?

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P.E. Metallurgy, consulting work welcomed

 

[Dhurjati Sen]

The acid wash could be more dangerous unless the acid has been neutralized perfectly. This could lead to pitting corrosion, specially in thr horizontal portion of the water wall tubes in the coming years.

Brittleness could be diagnosed using spot metallography of the suspect tubes.

The cut tube samples need to be sent to a lab to carry out a stress rupture test to determine the remanant life of the tubes and hence, the boiler.

DHURJATI SEN
Kolkata, India

 

[mjpetrag]

So what we ended up doing was condition-based hardness testing of the boiler tubes. We also tested the tubes that had previously failed to confirm with our readings and found the areas to be extremely soft with zero hardness. We found several tubes with hardness ranging from 400-700 HB, however we attribute this to layers of hardened metal at the test sites, which had to be cleaned up more. Soft tubes were the ones of more concern. Most of the tubes were in the range of 70-160 HB. We ended up following API 579 section 11 for a level 1 FFS using the hardness readings and tube dimensions/thickness readings.

As for the acid wash, we performed this process under the guidance of a chemical cleaning contractor who specializes in boiler acid washes, as well as B&W, the boiler OEM, and their chemical washing expert. Everything was neutralized after acid washing and done under a nitrogen blanket.

-Mike