Explosion Corner on Fluidbed boilers 1

[Charan5]

Does anybody know what are the NFPA guidelines in specifying safety or explosion corners in fluidbed boilers. I know for sure that we use on recovery boilers.
Especially with USA laws. Any help is appreciated.

 

[rmw]

Any boiler that is built with a rectangular furnace will be stronger (against furnace explosions) at the corners than along the walls. It was one of the first things I was taught as a young pup in the industry and it was taught to me regarding recovery boilers when it was said. I watched the old hands, and if they had to be out on the firing aisles for any reason (other than the guy rod-ding out the ports) you would find them hanging around the corners if they didn't have to be anywhere else. I followed suit.

Since learning that lesson, I have seen my share of furnace walls bulged out after the occurrence. I never saw a recovery boiler furnace explode although there was a link to the explosion at I-P Vicksburg on one of these fora recently. I believe that it was a natural gas explosion rather than a smelt / water explosion as it was during the start up phase after a plant shutdown and it seemed to be too early into the start up sequence to be on liquor yet - but that is just my read. Opinions are like elbows; everyone has a couple and I just gave one of mine.

However, I never knew that it was a specification by NFPA or anyone else. If it turns out to be, I will be the beneficiary of some new learning here. But then again, I learn a lot here.

rmw

 

[boilerone]

I designed both recovery boilers and fluidized beds. I know the exact answer to your question. For the people curious about what explosion corner is I can explain that it is to make one or more of the furnace wall corner connections much weaker than the others so that in the case of explosion happens inside, the corner seal will quickly break and let the pressure out. This is much more effective than the traditional explosion doors which open slowly and typically do not have enough opening areas for releasing the explosion gas.

The explosion corner design came from recovery boilers and the explosion corner guideline is from BLRBAC (Black Liquor Recovery Boiler Advisory Committee) particular to pulp and paper industry because explosion damage coming from water contacting with the blazing char bed can cause severe explosion damages and there is no other good way to deal with it rather than to open up the furnace walls. I have heard that explosion corners and other important safety designs according to BLRBAC guidelines have saved lifes in paper mills. The BLRBAC site is below:

http://www.blrbac.org/

Unfortunately I have not seen NFPA guidelines adopting the explosion corner design. I saw only NFPA asks to design the furnace and connecting ductwork to resist minimum +/-35 inches WG. This is more of for holding the pressure inside rather than releasing it. The common knowledge is that explosion pressures are always much higher than 35 inches so things can still break and damages can happen at unpredicable places. The idea of explosion corner is much better because it makes some thing weaker and you always know where is going to break and just need to be sure not to get close to it then you will be safe. I think this is exact the reason of you asking question but the reality is the other way. Due to lack of guidelines and the ineffectiveness of the traditional explosion door designs, many coal-fired boilers have even gotten rid of the explosion doors so there is almost no good protection against furnace explosion at this moment.

Even though there is no good guideline for CFB and BFB, for European designs they have already made the explosion corners standards to all the CFB/BFB products including those sold to US but there could be always debates among US clients regarding using it or not so some of the US customized designs were changed back to solid corners.

I don't know what's in NFPA's mind but what I can think of that the reasons of not using the weak corners are:
1. If you don't barricade or limit access to the areas around the outside of the weak/explosion corners, it is even more dangerous and becomes safety hazard. Recovery boiler users always know about it and even avoid putting platforms near the corners but this is not the case for the most PC plus CFB and BFB customers.
2. Weak/explosion corners tend to crack open prematurely if you have a lot of furnace pressure variations which cause the fatigue on the weak corner seal material. You start to see sparks coming out of the corners and this becomes maintenence nightmare to certain clients. If otherwise the furnace holds constant negative pressures then there is not such problem.
3. It is a little bit harder to install explosion corners and need special attentions. Bad installations can cause premature failures.

The above are not big problems and can be easily caken care of. Besides, having weak corners can also reduce the cost of materials because do not have to design things very strong now. One more advantage is that, if NFPA adopts it and customers use the design, I am sure the insurance companies will follow up and consider this to be a good safety feature for the boiler and will then lower the customer's payments.

Overall, as my opinion explosion corner is really a good and only working design for safety against explosion and it should be a must-have for all boilers and NFPA should adopt it. In addition, NFPA should also pay attention to what I mentioned above and add terms to avoid access to the surrounding areas and, that's it.

 

[boilerone]

I want to take back what I said about the explosion corner design to be a BLRBAC guideline. This was just my impression and to be caureful I went to their site but found no such content. I even went to TAPPI site and did not see any related information either. I can now only say that this is a commonly accepted recovery boiler design feature but might not be backed up by any organization. My apologies.

 

[rmw]

Which corner is it? Although it is a remote possibility, if I ever get back on a BL recovery boiler I don't want to hover near that one.

rmw

 

[davefitz]

Never heard of such devices prior to this discussion, but it seems it should be easy to to provide such capability by replacing the solid pins at corner buckstay connections with hollow shear pins.

 

[boilerone]

(to davefitz)
Yes, the explosion corner design is always combined with using shear pin connections at buckstay corner connections.

 

[boilerone]

(to rmw)
The weak corners are normally either one or both of the furnace rear wall corners of the RB. The owners normally have stripe marks painted on the walls also hang warning signs around if you can still get access to these areas. They also always have special trainings and exams for you to pass before allowing you to the RB's buildings. The weak corners and other dangerous places such as dissolving tank areas should normally be mentioned in the trainings. Generally speaking, explosion tendency and chemical hazard make recovery boiler much more dangerous than other types of boilers. Be careful.

 

[dougarthur42]

Good discussion We have both a recovery boiler and 2 CFB boilers. Our Recovery boiler will blow out the floor, as the floor slopes toward the smelt spouts. The area in the basement is roped off in case it happens. I have never known our CFB to have blowout corners. The lower furnace is all 6" thick refractory, corners and all. However we do have a couple of buck stay connection on the upper floors with shear pins that I never knew what they were used for.
This is definately the weak spot.

 

[Charan5]

Thanks for everybody's input. I agree with most of your comments. It is commonly accepted practice to have weak corner design in recovery boilers as compared to fluid bed boilers. I am trying to assist a definite reason, if there is one, to use one on fluid bed boilers and if do use, will there be any complications in plant insurance liabilites.

 

[rmw]

BoilerOne,

How far back does this type of design go? I would estimate that it has been somewhere between 5-7 years since I have been in a paper mill or around a recovery boiler and in the years prior to that some mills had instituted the training programs, videos, gas masks etc as a protocol for entering the mill and/or even specific portions of it, especially the pulpmill. But I, like DaveFitz never knew about this concept until now. As DougA.. mentions, I have seen plenty of areas below the RB around the smelt tanks roped off, but that was always the case because of the danger of their going off if the smelt spout plugged and released all at once. But not even all of them were roped off, just some of them. I've helped rod ports and I have helped install automatic port rodders. I even once (and I was young and really foolish in those days) -pre OSHA of course-
went into an operating recovery boiler ID fan to try to adjust the soot blower nozzle angle to try to get better blade cleaning. My senior partner at the time a man in his '60's (I was in my 30's) was the ringleader of the effort and the only admonition that he gave me was to come out when I felt my ears getting hot.

Back before they did all the odor control, I actually liked the smell associated with them. Boy this thread is bringing back a lot of memories. I had forgotten how much time I did spend around and in RB's over the years.

rmw

 

[boilerone]

I miss the paper mill smell, too. It is not stinky odor thsy the unfamiliar people might think. Especially when you know that the smell is from cooked wood. To me it is good smell and it smells career and life.

I have known that recovery boiler have explosion corners for almost 20 years so the history should be longer than that. I have seen recovery boilers or drawings made by B&W, CE, Ahlstrom/Andritz, Tampella/Kvaerner/Metso and they all have explosion corners. If your boiler does not have it, the boiler must be really, really old. One other reason could be that people have forgotten about it after years or generations of safe operations. Be sure to ask the operators or check the drawings. Look for the furnace cross-section and the buckstay corner connection drawings. If some corners look different than the others, they are normally explosion corners. Look at the detailed enlarged vies and you can quickly figure how the corners work.

Weak corners for fluidized beds are relatively new. Only a few models or suppliers have them and this is also the topic of this post.

 

[rmw]

My grand father lived downwind of a large mill. As a kid, I just knew that times with Grandad were great times and the smell always brought back those memories. I would get right in the middle of the pulp mill and breath in profoundly. Others with me thought me sort of mad. Now the smell is gone and they still do.

Thinking back, only one recovery boiler that I was on to any degree (vent stack scrubber issues mostly) was built within the last 20 years, but I don't remember any particular markings or warnings on any of the corners. I remember being on the port rodder level several times and had a project to do with the furnace camera on that boiler once as well. The rest would have been much older than that. The one that I went into the fan on was in another mill and it was new at the time I did that stunt which would have been mid '70's. I still think of it as a new boiler but that old gal would be over 30 years old now. That mill did build a newer one, but I never had occasion to go up on it so I may have missed the corners.

rmw