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Forge Explosion!


rwolfe

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Look up "Producer Gas" and "Water Gas"  both of which predate even WWI considerably as they were the gas used in gaslights during the 19th century.

From Wiki:

Producer gas, mixture of flammable gases(principally carbon monoxide and hydrogen) and nonflammable gases (mainly nitrogen and carbon dioxide) made by the partial combustion of carbonaceous substances, usually coal, in an atmosphere of air and steam.

Water gas is a synthesis gas, containing carbon monoxide and hydrogen. It is a useful product but requires careful handling due to its flammability and the risk of carbon monoxide poisoning. The gas is made by passing steam over a red-hot carbon fuel such as coke: H2O + C → H2 + CO

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You can also "crack" coal for various weights of oils and gasses. Methane is common in coal mines, complex ventilation systems are used to keep them reasonably safe. 

It is or was pretty common to see smoke generators  bolted, strapped etc. to the back of vehicles and ducted to the engine to produce fuel gas so they can get around. 

It's ancient tech not magic or even a mystery. Heck popcorn's a magical mystery if you don't know how it works.

Frosty The Lucky.

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  • 2 years later...

My Side Blast Forge just has an Explosion in the Air Supply Flex Pipe and Blower. I had a hot fire burning and added green coal around the fire. I was getting a lot of smoke from the green coal and the Supper Sucker Hood was sucking 99% of the heavy smoke up the chimney. I turned the Blower OFF (big mistake!!!!)

When I turned the Blower on BOOM!!

A Side Blast Forge doesn't have a Ash Dump. Running the Blower with the Guillotine closed allows some leaking air to pass into the fire.

P1040809.thumb.JPG.26e939337947dac9feb5702af6700ea9.JPG 

P1040807.thumb.JPG.6297724eb91b59e17b3434eb4c7bf5a3.JPG

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23 minutes ago, hdvoyager319 said:

I did not get burned. Scarred me more than you could believe

As much as I LOVE jumping on typos I'll give you a break. You meant to say scared as in BOOO! Not scarred as in "The Phantom of the Opera," scars. 

Glad all you got was a good scare, been there felt that. Normally I'd suggest mounting the blower higher than the forge so the heavier gasses won't settle down into the air supply ducting and: blower, bellows, etc. However I'm not familiar with side blast forges so don't know if hot smoke and gasses would flow upwards to the blower like a chimney.

Frosty The Lucky.

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The explosion was like a 22 pistol going off behind me. The blower had only been off for about  2 min. which gave the gas time to fill the air tube and blower.

The smoke and hot gases rise to go up the chimney, no problem here, even with green coal giving off volumes of smoke.. The rising  smoke is combustable and will burn if a flame is present. This isn't the problem on the top[ side of the fire. it's the bottom side there coal gas is the problem.

Green coal added to a hot fire is another matter. In the coking process, coal gas is released. ( In the past, the coal gas that was formed in coking plants was piped to home owners and businesses to be burned for gas lights.) Since the coal gas is heaver than air, it can sink down or if enough gas is formed it can be pushed up, even to bellows mounted above the forge.

So far the only preventive procedure is leave the blower on in the side blast forge. In a bottom draft forge, leave the ash dump door open.

I believe that in the side blast forge the tuyere has a small air hole around 1" or a wee larger. In front of the tuyere is a pile of burning coal/coke which presents considerable resistance to the force of a minor blast. The blower side has the air tube and open blower which presents very little resistance to the blast, thus the blast should exit thru the blower opening as it did in my incident. .

As you can see, I have done a lot of research on the problem, cause and prevention. 

 

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Interesting!

So whats the precaution against this?


Thus far I've been turning off the blower to my (coke) forge, doing a quick tidy up while I make sure nothing is on fire, and then leaving the shop within a couple of minutes.

Should I be doing something better to shut down the forge which is safer?

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I like to make sure things are pretty cooled down before I leave the smithy. Blower off, ash dump opened, pull off the top layer of cooler coke chunks on to my forge table. Do some cleanup. Then I go back and pull more cooled coke from the firepot onto the table. By then I can open a clear path for air from the bottom and things cool down from there. It is still warm in there but definitely out. Finish cleanup, tool dressing and storage and leave. The routine doesn't take long, and has been the routine for a while now. If I an taking a lunch break, I open the ash dump and pile some green coal on top of the hot coke. The open ash dump allows a thin trickle of smoke to sneak out and keeps things alive till I come back and keeps any volatiles out of the airstream. That is what works for me.

Steve

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Coal gas is as explosive as propane, and natural gas. Coal gas is a byproduct of the process of cooking green bituminous coal into coke.                                                                                                            

From the responses, most of you are using coke, not soft coal and a Bottom Draft forge with an Ash Gate. .

I wrote to Mark Aspery, designer of the Side Blast Forge that I built. Adding green coal to a burning fire will produce Coal gas. Coal gas is not an issue with coke and bituminous coal that has been coked.

The problem is caused by the exact conditions that I created. a, have a good hot fire going. b, add several shovels full of green coal, green coal that contained 20% fines. Fines are even worse for forming coal gas as they blanket the fire and keep outside air away from the top of the  fire.

Since most of you are using a Bottom Draft Forge, if you add green coal to a hot core fire and turn the blower off and open the ash gate, the prospect of having an explosion is slim. But if you keep the ash gate closed tightly, with the blower off and a load of green coal cooking away, you also may experience skid marks in the jockey shorts.

Many years ago at the Gunmakers Fair held at Dixons at Kempton Pa. the blacksmith from Williamsburg were forging gun barrels. They were using a bottom draft forge that they brought to the show. They would forge a few inches of the barrel and then quit for a half hour or so. When they came back to forge another few inches of gun barrel, they started the blower and they had a real big Boom. The boom cleared a lot of the burning coal/coke out of the Tuyere. 

 

I believe that I may have a solution to the Side Draft Forge potential to make coal gas when green coal is added. As long as the blower is running, the coal gas goes up the chimney. The idea requires the flex air tube to be removed temporarily.. Now, the air intake end of the Tuyere isn't connected to the air tube and blower.  So, the tuyere now has a free opening to suck air, in similar to opening the ash gate on the bottom blast forge. The convection air should exhaust the coal gas with the smoke up the chimney.

If this new idea makes sense to my fellow blacksmiths, let me know by your comments. I can remove the connecting air tube as easy as you can open the ash gate. 

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This picture shows the Blower, the Air flex Tube and in the far right, the Guillotine air volume controller at the entrance to the tuyere. All I need to do is remove the air tube if I leave the forge for a few minutes or for a longer break. The natural draw will keep the forge warm and possibly coke some green coal in the process.

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Thanks for your comment.

Conditions for an Explosion are:: a, a hot fire.  b, green coal is added around the hot fire.  c, the blower is off.  d, the green coal is making coke. e, Coal Gas is flowing back filling the Air Tube and the blower, up to the air intake level, with gas. Result is an Explosion

Coal gas is heavier than air. As a result, the coal gas flows down. For a Side Blast Forge, there is no down as the Air Tube and Blower are level with the Tuyere, thus, back filling the Air Tube and Blower with gas, when the blower is off.

With the blower off, there was rising smoke from the coking process that was flowing up the chimney. But the coal gas apparently is also flowing back out of the Tuyere.

Remember, I am using a Side Blast Forge that does not have an Ash Gate. If you have a Bottom Draft Forge, and the Ash Gate is open, the coal gas will flow down and out thru the Ash Gate, no boom.

I am going to try an experiment. The conditions for an explosion will be set up with one exception-the Air Tube will be removed. Hopefully, I will see smoke/gas coming back out of the Tuyere's air intake.

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  • 5 months later...
On 1/3/2020 at 11:57 AM, hdvoyager319 said:

Conditions for an Explosion are:: a, a hot fire.  b, green coal is added around the hot fire.  c, the blower is off.  d, the green coal is making coke. e, Coal Gas is flowing back filling the Air Tube and the blower, up to the air intake level, with gas. Result is an Explosion

At least in the case you describe, there should also be:

f, blower is turned back on, blowing mixture of air (with oxygen!) and coal gas into the hot fire, igniting the mixture. 

For any fire, you need something flammable and oxygen (or oxidizer) in an appropriate ratio and pressure (pressure just determining how much oxygen is available for reaction), and heat. I found a reference to an autoignition temperature of 750degF for production gas in air. The lowest autoignition temperature I found for a common constituent (based on limited research of what those common constituents are) is 500degF in air for hydrogen. 

I am wondering if coal gas ignition necessarily starts at the forge fire and simply propagates back through the air supply plumbing, or whether the autoignition temperature of any constituent is low enough that ignition could credibly occur in the air supply (or bellows) without an external ignition source.  

I expect that a spark in an electric blower or switch would be one such credible external ignition source. This could substitute for my hypothetical (f) above. 

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That's a well thought out and cogent analysis Chris.

The ignition source you missed are the hot coals that fall through the air grate, making two legs of the fire triangle waiting for a breath of air from the blower.

Not always but most often.

Frosty The Lucky.

 

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Since the explosion, I have been very careful to disconnect the air pipe at the Guillotine any time the blower is turned off. With the Air Tube off, and green coal is being coked, the heavy gray flammable smoke flows freely back out of the Forge and air intake Guillotine. This is the same looking smoke that rises up out of the fire when making coke from green coal. This smoke burns when hit with a flame.

Any time I leave the fire, the Blower is off and the Air Tube is pulled out of the Guillotine. The Guillotine is closed so there isn't any air feeding the fire, so the fire goes out safely at the end of the day. If I break for lunch, the guillotine is partly opened so the fire stays lite. There never is any green coal on the fire when the fire isn't being used.

Remember, this is a Side Blast Forge so it operates differently than the Bottom Blast Forge. Both forges can explode when the conditions are present to allow the coal gas and smoke to flow backwards into the Air Tube and Blower, and then turn the blower on. The glowing coals in the fire are the ignition source. 

I haven't had any explosions after figuring out the conditions and cause.  

In a Bottom Blast forge, the red hot coals do fall past the Clinker Breaker and accumulate. I would seriously doubt that they will LIGHT an explosion. The fuel for an explosion is coal gas that has backed up below the clinker Breaker and filled the blower and air tube. when the blower is turned, the explosive gas is blown from the bottom into the hot fire--Boom! This can result in the hot fire coals being blown all over the shop and on the blacksmith.

The explosion conditions can be eliminated by opening the Trash Gate under the Clinker Breaker. As heat rises, the air feeding from the bottom thru the clinker breaker will keep the fire burning.  NEVER PUT GREEN COAL OR WET COAL ON THE FIRE UNLESS THE BLOWER IS RUNNING! THE FIRE MUST BE ATTENDED ANY TIME THE GREEN COAL OR WET COAL IS ADDED TO THE FIRE.

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By "guillotine," are you referring to a sliding valve in the air supply?  

I missed this being about a side blast, so yeah, different conditions apply. Pulling the air supply before the guillotine is probably as effective as anything. 

The worst I see in my bottom blast is a little popping when I resume cranking the blower. It's actually good theater at demonstrations, gets everybody's attention safely and gives me something to talk about while the work is heating. 

I have gotten (collected my own fat lazy self) coal that makes more gas and more energetic on first crank pops. I just developed the habit of flipping the ash dump before starting to crank the blower. I keep a galvy steel bucket with a few inches of water in it directly under the ash dump so nothing gets away.

The counter weight on the ash dump is only a little heavier than the gate so if there is much of an explosion most of the force goes down and doesn't blow burning coals out of the fire. The worst one I remember made a fire ball about the size of a party balloon over the fire and scattered ash and coals at my feet. That was when I started putting a bucket under the ash dump. 

Frosty The Lucky.

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15 hours ago, hdvoyager319 said:

Remember, this is a Side Blast Forge so it operates differently than the Bottom Blast Forge.

I knew your forge is and started talking about your case, and then I went and generalized my question without actually elaborating as such.  Frosty's response actually DID answer what I was wondering about, without me actually being clear enough for anyone to know it! :rolleyes:

Wait a second! ::scurries for tin foil hat:: :wacko: Take the thing off for just a little while... Sheesh!

Anyways, I do still think that side blasts are primarily only susceptible to coal gas explosions by blowing the gas into the forge fire (activating the blower/bellows OR some other pressure differential) or an external ignition source (like a spark). That's what got me to wondering how the bellows that I've read about exploding, exploded. Of course, I didn't know whether those forges were side or bottom blast, leading to my question. 

I don't think that it is the accumulation of coals/embers that ignite the gas for a bottom blast in the ignition scenario Frosty mentioned, but single HOT coals that immediately ignite the gas upon falling. 750degF is not unrealistic.

Restarting the air remains the most likely coal gas ignition mechanism for either type. 

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Oh yeah, coals just laying there aren't hot enough to ignite the gasses but a LITTLE puff of fresh air will brighten them right up. I made the dump side of my tuyeres much deeper than the air grate side. It keeps coals and debris farther from fresh air. However I haven't used that forge in a long LONG time so it's the old Buffalo rivet forge I deal with.

I think it's poorly placed bellows that can collect enough coal gasses to maybe blow up. In that case the velocity of the blast is far below the flame front velocity of burning coal gas so it could surely burn back all the way from fire to bellows. Just imagine a great bellows exploding in a closed room with you!:o Even a large shop room would be darned unpleasant. 

On the other hand the flap valve in the bellows should prevent much reverse flow. If it happened anyway I'd sure come up with a more positive valve. 

Frosty The Lucky.

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