anchor brick failure - could cause be water 5

[RCMRon]

We have an indurating furnace operating at 2300 deg F as part of making iron ore pellets. the roof is a castabel refractroy 9Vesuvius) and is held in suspension by anchor bricks. We experienced a roof failure where all of the achnors broke at hte interface with the refractory. 14 inch achnor bolts and 9 inches of cast refractory. all that was left was the 5 inches of the bolt that was suspended from free moving steel achors on the beams.
Can anyone shed some light into what would have caused such a failure (Machine had just been cooled down)and does water impregnation affect the tensile strength of the anchor brick to the point of breaking? there were signs of excesive water being directed to the area of the roo colapse. it was a hole in the building roof and all of the water was aimed at the location of failure.

Any help is much appreciated

 

[CTW]

You'd probably get more responses if you posted this in the iron and steel forum.

http://www.eng-tips.com/threadminder.cfm?pid=684

 

[Ron]

Yes, water intrusion can affect the strength of brick, both compressive and tensile. Many older brick were not consistently fired through their cross-sections,resulting in a hard "shell", but a softer core. When the hard shell is compromised by repeated wetting/drying, the core ends up being the primary strength of the brick and has much lower strength than the full brick.

Have a petrographic examination done on the brick to check for this.

 

[boo1]

thoughts:
thermal change from the water
steam created
corrosion

 

[dik]

As Ron noted... over a period of time, with frequent wetting and drying the hard outer skin (fireskin) can exfoliate leaving the softer core. This softer core can deteriorate very quickly. Are there signs of brick 'facing' around the area of deterioration?

Dik

 

[corus]

In a furnace the change in temperature can produce high stresses within the refractory itself to the point that the refractory fails either in compression or in tension. Generally refractory is in compression as it expands and is restrained. More so at the hot face, where the higher compressive stresses can cause the refractory to fail. When it is cooled then the differential thermal expansion can cause tensile stresses. In tension refractory has little or no strength and either fails or joints between the bricks open up.

corus

 

[RCMRon]

I would like to thank everyone for the great feedback. This information if shedding a lot of light on this failure. the brick is only 2 1/2 years old.
I am not sure what is meant by "signs of brick facing" so I attached some pictures of the brick. Also, as Corus mentionned, thre were vertical cracks in the section of roof that seemd to fall first and those crack were darkened indicating tht the crack was not new. What is a petrographic examination?

Do people use specific procedures to enter a cold furnace that has a refractory roof? Are any special inspections performed?

 

[RCMRon]

Here is a picture of the repairs showing the placement of the anchor bricks that hold up the castable refractory roof. This may help.

again, I appreciate eall of the help i have been getting on this topic

 

[dik]

Brick Facing... shards of the brick surface. If only 2-1/2 years old, you might look elsewhere for the problem. Saturated brick exposed to heat can cause the surface to fail. Possible 'bad' batch of refractory brick, or something else.

Dik

 

[RCMRon]

Dik, thanks for the info. Did you look at the picture or the brick? Does it help?

Bad Bricks, is thee anything we as a consumer can do to ensure the quality of the brick?

Are there quality parameters, tests or SOPs that can be obtained from the manufacturer or done independently. Could we doa field verification on site (not sure what to look for) to guard agains this or idenitfy bad bricks before they get installed.

Ron

 

[RCMRon]

The anchor bricks in question, PR-85 from snow shoe refractories are an 855 alumina brick with a specified tensile strength of 2500 psi @ 68degF (20 deg C). What happens to the tensile strength at 2200 degF. Does it go down?

Is this the right brick for the application?

 

[corus]

I don't usually deal in tensile strength of bricks as they're supposed to be in compression. Compressive tests of refractory bricks show a marked decrease in strength at higher temperatures. I suspect it would be the same in tension.

Isn't the problem that the anchor bolts failed rather than the refractory? I would suspect that if they failed then the refractory would just fall off. With the high temperatures the strength of the bolts would decrease such that the hotter end would fail, leaving the colder and stiffer, part remaining.

corus

 

[RCMRon]

Corus, thanks for the information, in one of my postings, about 5 or 6 up from this one, i attahed a picture of how our refractroy roof is suspended. It is suspended by anchor bricks. the bricks themselves are held to beam with steel anchors. It is the bricks that broke at the contact point with the refrqactory.

I tried to attached another picture on this post but am having technical difficulties. You can see pictures of the broken brick and the installion of the hanging brick anchors in a few posts above.

Is this the right application. I don't know why we use bricks. Steel anchors would seem to make more sense. What would you suggest?

 

[CTW]

Yes steel anchors would make more sense. I've never seen a refractory brick in tension, as well as using brick to hold sprayed refractory in place. My experience is that steel anchors are always used when spraying refractory.

What is Vesuvius's opinion of the failure?

Since your last post contradicts your first post regarding the method of failure, can you clarify what was seen at the moment someone was able to look in the furnace?

"We experienced a roof failure where all of the achnors broke at hte interface with the refractory. 14 inch achnor bolts and 9 inches of cast refractory. all that was left was the 5 inches of the bolt that was suspended from free moving steel achors on the beams."

The "bolts" in the above quote are referring to the bricks, correct?

Were all of the bricks broken? Are the bricks 14" long and 5" of brick was left hanging after the failure?

 

[RCMRon]

Yes, I should not have used the word bolt because we have anchor bricks and not anchor bolts.

Yes, all of the anchorbricks were broken. They snapped of at the interface with the roof and 5" of the brick was left hanging.

Vesuvius are strugling with this and want to blame everything on water.

I am definitely no expet on the subject but heating and coling bricks in tension just does not seem like a good idea. Is there a good resource or website that i could get ideas on good steel anchors?

 

[Ron]

RCMRon....Product data shows the PR-45 has a 1000 psi min. tensile strength.

From your photo, the failures are localized. Do they correspond to the roof leak area?

One reason to use brick in this application is to mitigate thermal movement. If everything were "locked" down with steel anchors, you would experience localized failures of the refractory attachment and possibly have thermal fatigue issues with the steel.

Also, exposed to 2300F, the steel properties would change tremendously...both physically and metallurgically. The result would be significantly lower strength in the steel.

A petrographic examination is a microscopic examination of the brick to look for manufacturing anomalies and physical defects, and to characterize why those occurred.

 

[RCMRon]

Thank you Ron.

It is a PR 85 that we use.

Yes the failure is localized to the area under the roof leak.

the bricks are held in place on the beams in a manner in which they can move due to thermal expansion / contraction.

Even though the brick has a 2500 lb tensile strength at 68 deg F, whan happen at 2200 and 2400 deg F to the tensile strength?

the furnace also suffers some heat excursions, and temperature profiles for heating and cooling are note all that tight. there are also indications of flame impingement and roof damage on the inside indicating direct flame damage.

again, thanks for the help on this matter, the feedback is very usefull!