Backfires and whoomphs

[Glenn]

I have been present when forges have backfired. The first was a bellows driven forge that backfired like a 12 ga shotgun going off. The forge had been unattended for many minutes and the air/gasses mix just got to the right mix.
The second was a hand cranked blower. The smith took a little longer than normal at the anvil and when he turned the crank and added fresh air the mix was right. Just a whoomph but blew the connection loose at both the blower and the forge. The backfires will get your attention. It seems like a good topic for conversation, so:

How do you prevent backfires and whoomphs at the forge ?

[Archie Zietman]

what is a backfire?

[irnsrgn]

Its not really a backfire, A backfire would come out the blower.

What is happening is the unburnt gases are going down and into the air supply pipe. I know hot gases go up not down, but stop and consider that the blower is usually higher than the fire or close to it and this makes a chimney effect and actually sucks the unburnt gases (remember how the smoke will burn if you stop the blower) down thru the opening in the firepot. They are too rich to burn till you add air with the blower device and this mixes air with the unburnt gases and you then push them up into the fire. If the right mixture is obtained ( it won't do it all the time), you will get a minor explosion at the bottom of the firepot. What makes an explsion work is it is touched off in a confined space and it expands just like the air full mixture that makes a motor push the piston.

I have seen a ball of flame come out of the firepot and at the same time it launches hot coals in every direction, not a good way to get a wave in your hair or your eyebrows trimmed and some times 1st or 2nd degree burns on the operators face. Not to mention hot fine cinders and coals in your eyes if you don't have safety goggles on.

It won't happen too often, but if you want to stop it , the only way is to install a one way flapper valve in the air pipe close to the entrance at the bottom of the tuyere. Or to mount your air blower down below the firepot which is impratical unless you have an electric blower.

my 2

[Daryl]

When I made my bellows, I installed a one-way valve, just like the one in the old bellows that I took the majority of measurements off of (it was made in 1863).

[Archie Zietman]

The blowers for the forges at Centaur Forge (I visited them recently, for an armour making workshop) were all below the firepit, probabably to prevent backfiring.

[Archie Zietman]

Also, about the gasses, Could you not harness them in some way to help the fire burn? You do that when making charcoal, burning the gas-filled smoke to help cook the charcoal. Works really well.

[HWooldridge]

An older firepot is usually corroded to the point the ash dump leaks enough that the gases can get out or at least minimize the bang plus an electric blower also seems to overcome this tendency. One other thing is that some types of coal are worse than others. I had some Alabama coal that was really great stuff but had a lot of volatiles - it was good for at least one blast per week.

I have not seen it but been told that a bellows can take in enough to actually blow it completely apart.

[Daryl]

The conditions for these 'blowouts' are increased with the amount of unburned (green) coal or coal that has high phosphor and/or sulfur content has been placed on top where there is no air flow up through the tuyre. Once the subject coal begins to approach the temperature where the volatile matter within the coal (sulphur, phosphor, other materials that more readily attach themselves to carbon atoms), it has separated from the parent coal. Since there is insufficient blast/airflow through the tuyre to force this matter up and out, the matter is heavier than air and will now flow down through the pipe to the area with the least heat. From this point, all that is required is a spark OR enough of a temperature rise to spontaneously combust the vapours.
If you remember grade 8 science where the teacher lit a small piece of paper, snuffed it out and let the smoke go up inside a small glass tube. Once the smoke was filled in the tube, the glass was placed over the bunsen burner and one end opened to a spark - poof!
As far as blowing up a bellows, I have personally seen a bellows that used a small weight on the top board - a backfire occurred and we had to retrieve the weight from the other side of the forge. :roll:

[T-Gold]

Archie, re: gases, they're being burnt, don't worry. It's only a concern when you're not pumping, and your air source is higher than your tuyere.

I use a gas forge to prevent backfires and whoomphs. As far as prevention there goes, I built a good burner and mounted it securely, and I am careful how I treat the gas ;)

[ThomasPowers]

Now Weygers showed how he made a collector to feed the smoke back into the inlet of his blower, more for smoke reduction than use of it as a power source.

It was in "The Modern Blacksmith" IIRC and so should be in "The Complete Modern Blacksmith"

Thomas

[Don A]

This is the first thing I preach when I get new helper to pump the bellows. The one we use is pretty heavy. If you get to the point you are needing a gentle blast of air (just a light breeze) it is easy to pull down too lightly on the lever. You can generate a slight breeze using only the bottom chamber with no inflation of the top chamber. When this is the case, as soon as you release the lever, the tuyre pipe acts as a vacuum and draws forge gas into the bellows. As long as there is even a slight inflation to the upper chamber, this will not happen; the bellows exhales as it comes to rest.

When using the bellows, always keep the upper chamber inflated.

[Ed Thomas]

I never have problems in my shop because I use an electric blower lower than the forge, as irnsgrn pointed out. But the most spectacular incidents were in the open on an windy day with a hand crank. Almost every time we'd stop cranking, the gases would back up and WHHOOMPPHH in the blower.

Pretty much any time I demo with a breeze going by, I can count on at least one or two blowbacks. Probably switching the blower to the upwind side of the forge would help.

[Ed Thomas]

Actually, that post probably didn't contribute anything. I just wanted to post at least one good

WHHOOMPPHH



Your blower in action

:mrgreen: :mrgreen: :mrgreen:

[ThomasPowers]

Hmm looks like my backyard...

(actually the trinity site is about an hour's drive away)

Thomas

[primtechsmith]

I have never witnessed the coke and coal come out of the firepot but I have seen the possibility of it occuring. At the forge there are three of us working and usually one person will build the fire and the others will grab some coal to start up to save time and resources. I have noticed that these pyrotechnic bursts of combustables does occur more when using live coal than starting from scratch.

Seems that these gases are building up as the coals are being lit but the fire is not reaching the top of the coal making its own atmosphere around the firepot. Mike routinely while getting is fire up will crouch down close to the fire pot and blow on top of the fire making it light. Its a neat trick if you know what you are doing, and seems to work beautifully to rid yourself of these gases. I know if I tried it I'd have left behind eyebrows my go-tee and some hair! This trick also helps the smoke problem too. the fire helps add convection to help raise the smoke up the chimmney.

well at least thats what I think. I may be wrong...
peyton :D