Forge Explosion!

[rwolfe]

Had a bizarre event happen today. I was working with two Boy Scouts on metalwork (blacksmithing) merit badge and we were going to be away from the forge for about 20-30 minutes so I turned off the blower and banked the fire with a couple of scoops of green coal. About 5 minutes later there was a loud bang and a flash and the 4 inch diameter flexible ducting that connected the blower to the forge flew across the room.  The banked fire did not appear to move; at least we did not notice it. Needless to say, we were quite startled.

I can only assume that when I banked the fire unburned gases were somehow forced into the ducting and then ignited. I’ve never had anything like that happen before – has anyone else?

[rockstar.esq]

Yep.  I have a cast iron fire pot with a cast iron air pipe tied to my hand crank blower.  I was working at the anvil when I heard a loud pop.  It wasn't until I was shutting down that I discovered that the bolts holding my grate to the air pipe through the firepot had their heads ripped off.  My firepot cracked nearly in half.  There was absolutely no water, oil, or any liquid in either the pot or the coal.

It was breezy and I forge outside.  I figure the coal gas backed up through the air pipe and out my blower where a breeze carried it to the lit coal of my forge, igniting the whole thing.

I have an old blacksmithing book that talks about the necessity of a check valve on great bellows to keep coal gas from blowing up your whole shop. 

Sadly, after it happened to me I recalled a local smith telling me to open the waste gate on any unattended (but still lit) forge.  The idea was to let the coal gas fall below the fire where it was less likely to ignite.

 

[Steve Sells]

coal gas does explode, and it collects fast too.  You can read all over the net about mines blowing up from it.

[Smoggy]

Coal gas, also knwon as town gas, carburizing coal in a retort produced coke and coal gas which was collected in gasometers and piped to homes before natural gas took over. Burns a treat apparently!

[Frosty]

This is one reason I really like my exhaust pipe flap cap ash dump. The only thing holding it closed is a little counter weight if coal gas pops it just pips the ash dump open and cleans my tuyere out a treat. It's another reason to keep a tin pail wit a few inches of water under the ash dump.

Frosty The Lucky.

[rockstar.esq]

I've given some thought to a "dead mans' switch" foot pedal that keeps the waste gate closed only when there's foot pressure.  Frosty's idea is less complicated and it's automatic.

[arkie]

When I bank my fire with green coal and leave it for a while, say over lunch, I always open the ash dump below to create an upward draft through the fire.  That way the coal cokes and the gases move upward out of the fire and up the flue.  Sometimes I also open the air supply to also aid in drafting into the fire.

[rwolfe]

Well, I think I'll keep the ash dump open in the future. Seems prudent! 

Thanks for the comments.

 

[Bud in PA]

Wow as someone new to this I never thought about coal gas. I'm glad I joined this forum, it's a treasure trove of information. When I am able to swing a hammer again,  hopefully in August according to the doc, I am going to remove the screw on ash dump and replace it with a flap style. Thanks

[MrDarkNebulah]

Is there an easy way to make a flap style without a welder?

 

[Glenn]

Make the flap, get it to balance, then turn the "handle" 90 degrees. Bolt or rivet this to a piece of flat stock attached to the air pipe.

 

[Frosty]

3 hours ago, MrDarkNebulah said:

Is there an easy way to make a flap style without a welder?

 

I bought a semi truck exhaust pipe flap cap at a truck supply store. They come in a number of sizes clamp on and work a treat. Clamp it upside down on the bottom of the vertical section of the tuyere.

No, leaving the ash dump open isn't a good idea it will divert your blower air. Unless you're talking about doing so if the blower is going to be off very long, then it works pretty well.

Coal gas "explosions" are typically muffled pops you hear through the fire, maybe a little foof of smoke and flame. . . Of course "typically" implies there are exceptions, once in a while you can get a good bang and have coals tossed out of the fire pot.

It's usually nothing, not even a wakeup. However if you were to mount a great bellows below the level of the fire pot you could find yourself picking yourself up off the floor a ways from where you were standing, the shop maybe needing a window and for sure your bellows needing new leathers. This why you see great bellows mounted higher than the forge for the most part.

Frosty The Lucky.

[Daswulf]

Took me a while to get around to doing this. Made a new deeper brake rotor forge pot to try out and since it was apart I figured I'd go ahead and change from the screw cap to a flapper. Haven't had issues with the screw cap other then it getting seized on a few times but this is easier to open and safer.  Not the best angles but I had it setup before I thought to take any pictures.  Originally just had a bolt for the counter weight but it wasn't quite enough so I welded some square stock on. It's ugly but works perfect. Can't feel any air escaping with it shut. 

[Frosty]

Looks good Das.

Frosty The Lucky.

[arkie]

Das, that will work just fine.  Make it easy for yourself.  Here's what my home-made dump looks like.  I can flip it open with my boot toe.

 

 

[Daswulf]

16 minutes ago, arkie said:

Das, that will work just fine.  Make it easy for yourself.  Here's what my home-made dump looks like.  I can flip it open with my boot toe.

Same with mine now. Way better then bending over and unscrewing a cap If it isn't stuck. 

[arkie]

I actually modified mine further.  I drilled a hole near the big knob on bottom, pinned a rod through it and extended the rod up the side of the forge.  Now I just pull up on the rod and it dumps; no more having to do a ballet to dump it with my toe!

[Thats Hot...]

I use this..

[Frosty]

There it is, an exhaust flapper or Rain cap!!

A quick exercise of my Yahoo fu I discovered I should've been calling them "exhaust Rain caps" or Exhaust Flappers" I saw prices from $11.00 to better than $40.00 (if you want rain flapper caps on your Harley's exhaust)

Thank you Mr. Hot. My old coal forge is so buried I can't get a decent pic of the tuyere at all. Your pic is a thing of beauty.

Frosty The Lucky.

[JHCC]

Okay, I'll admit it: in my wilder youth, I used to do this on purpose to scare the neighbors.

[MrDarkNebulah]

While on this subject, I saw someone run his ash dump straight down into a bucket of water, and the water acted as an air seal. What do you guys think of that? 

[Glenn]

May have seen the blueprint on the subject. I placed the bucket under the ash tube and filled the bucket with water to where the air no longer bubbled out of the ash tube, maybe 1 inch into the water above the bottom of the ash tube. This formed an air seal and any ash would drop into the water. No hot ash that way. I will caution you that a bucket full of wet ash needs a fork truck to move it. Do not ask how I know this. (grin)

[MrDarkNebulah]

would it help against the coal gas explosions?

[Frosty]

6 minutes ago, MrDarkNebulah said:

would it help against the coal gas explosions?

As long as it's not too deep in the water, an inch is plenty to seal it but put it in very deep and there's less resistance blowing the coals out of the fire than pushing the water out of the tube.

Frosty The Lucky.

[eseemann]

For what it is worth my dad told me about how the Germans (he grew up in Germany during the war) used coal to produce gasoline. This gas was (from what I have read) a kind of low octane white gas. The Germans to this day gassify coal to make something like LP or LNG. So yup, I can see how this might go boom!