Yanche wrote:There have been a few times when I've found my barometric damper partially detached from its pivot point. My AHS S-130 in a detached shop building from my house so I've never been a witness to what happened. Obviously, some rapid ignition of coal gases occurred. I've pondered the conditions that would cause this. I liken the conditions needed like those when a hand fired stove is stoked with fresh new coal. A hot spot of glowing coal ignites the gases given off by the newly added coal. One needs to be an owner of a "coal gun" boiler and familiar with the boiler operation to understand how this set of conditions could occur. It seems to me the trigger event would be the aquastat reaching the high limit trip point. The combustion blower would have been running and coasting to a stop. So there would be good hot coals to serve as a ignition point. Now if new fresh coal enters the firebox at just this instant and the coal have some "fines" with it you could have ignition of the new coal gases. I have seen when fresh coal is added all of a sudden. As if the coal from the gravity feed hopper got stuck and then just let go. This jerk might release the coal "fines". I addition when the combustion blower stops the oval flapper door is making a transition from the closed position to the open draft position. Transient conditions are always the most difficult to understand and I suspect this is one of them. I have made sure all my stove pipe, stove pipe tees and elbows and barometer dampers are all securely fastened. Perhaps our barometric damper is is not adequate for coal use. I do know there are differences for coal and oil vs. gas boilers. I'll do a bit more research.
I noticed in the 1953 US Dept of the Interior, Bureau of Mines study of the Axeman-Anderson Anthratube (a unit similar to the AHS
coal gun), it stated that when burning #1 buckwheat coal in the unit, the air over fire must be increased to burn off CO. This is done by drilling a 7/16 hole in the sight tube door of the combustion chamber. This is necessary because the unit's air flow was designed only for burning pea coal. (Buckwheat was to be an alternative in "emergencies") It states: "without this air, the carbon monoxide in the products of combustion would frequently during start-up periods, build up to some 13 percent or enough to create an explosive mixture. The mixture would ignite and cause a 'bump' or small explosion. "
I believe Yanche is absolutely correct in that the volatiles, under certain circumstances, and in certain combinations, which are hard to detect, will cause that combustion bump.
I believe the fines might approximate the buckwheat coal described in the study. Their greater surface area for the release of volatiles and the reduced combustion rate from restricted air flow might put the CO/air mixture into the explosive range.
Most likely an auger feed would reduce the amount of fines picked up versus the pail or shovel. Perhaps drilling holes in the pail would allow the fines to shake out and prevent them from going into the hopper. In your situations, do not drill the hole in the sight door unless you are burning buckwheat and only with mfr confirmation--please!
Just a suggestion from a rookie theoretician still waiting for his boiler to be completed at A-A!
(Yanche-pm me with your address and I'll send you a copy of this study FYI.)