Coalgun- Puff backs & Explosions

To the uninitiated, it might be a little disconcerting to learn, on the 28th page of a thread about boiler explosions, that some coal boilers are supplied with tombstones. Just sayin'.

Mike

Pacowy wrote:it might be a little disconcerting... that some coal boilers are supplied with tombstones.

LMAO No more than the building that airplanes aim for being called "the terminal".

As I understand the "tombstone" cover, it was an improvement over the previous design. The need is to keep the flapper door from being damaged when a severe "puff back" occurs. The first design was simple a horizontal bar, one that went across the lower part of the flapper door, restricting travel. I had my boiler upgraded to the "tombstone" when I had it back at the factory for other mods. My secondary theory on the benefit of the "tombstone" is it creates additional idle draft. With the flapper door ajar, air heated by the front surface of the boiler rises creating a draft. I fully acknowledge it may be so small it doesn't make a difference at all.

Well... I started this thread eons ago; but, I had another ka-boom on Tuesday night 10/9/12... I offer the following to my fellow AHS drivers out there:

1. Watch your fire "height" very closely. It varies as a function of boiler load; and, load varies as a function of weather---so watch it carefully. I realize lifting the tombstone cover off the sight tube to inspect your fire is a pain in the arse; but, it sure beats writing four digit checks to the oil/propane company...

2. Further---The height of the fire is directly proportional to the Dwyer programmable controller Set Variable (SV) setting.

3. I made the mistake of just "leaving" the controller "SV" set at the "factory recommended" 130 degree set point---this was a mistake; because, for my current load, 130 allowed my S260's fire to get way too low…

4. With the low fire, coal gas volatiles accumulated (obviously) following the boiler's normal run cycle fan shutdown; and even with good draft---KABOOM!!!

5. I've since lowered the "SV" to 115; and, the fire is back up where I can see a nice red/orange glow through the sight tube.

6. Of course the usual coal burning requirements apply: clean exhaust pipe, good draft, coal hopper loaded correctly, etc.

7. I highly doubt coal quality/deep mined/strip mined/red ash/white ash has any bearing WRT this "booming" problem; although, I recently started burning Blaschak again. BTW: This batch of Blaschak looks really nice, clean, shiny, etc.

Comments are invited!

Happy Burning

watkinsdr wrote:Well... I started this thread eons ago; but, I had another ka-boom on Tuesday night 10/9/12... I offer the following to my fellow AHS drivers out there:

1. Watch your fire "height" very closely. It varies as a function of boiler load; and, load varies as a function of weather---so watch it carefully. I realize lifting the tombstone cover off the sight tube to inspect your fire is a pain in the arse; but, it sure beats writing four digit checks to the oil/propane company...

2. Further---The height of the fire is directly proportional to the Dwyer programmable controller Set Variable (SV) setting.

3. I made the mistake of just "leaving" the controller "SV" set at the "factory recommended" 130 degree set point---this was a mistake; because, for my current load, 130 allowed my S260's fire to get way too low…

4. With the low fire, coal gas volatiles accumulated (obviously) following the boiler's normal run cycle fan shutdown; and even with good draft---KABOOM!!!

5. I've since lowered the "SV" to 115; and, the fire is back up where I can see a nice red/orange glow through the sight tube.

6. Of course the usual coal burning requirements apply: clean exhaust pipe, good draft, coal hopper loaded correctly, etc.

7. I highly doubt coal quality/deep mined/strip mined/red ash/white ash has any bearing WRT this "booming" problem; although, I recently started burning Blaschak again. BTW: This batch of Blaschak looks really nice, clean, shiny, etc.

Comments are invited!

Happy Burning

My experience is fully consistent with yours. Since I stopped adhering to the recommended SV and began monitoring/controlling fireheight I no longer have puff backs.

lsayre wrote:I read the 1952/53 published U.S. Bureau of Mines report 4936 on the Axeman Anderson Anthratube 130 boiler (posted on this forum by Yanche), and it states this regarding puff-backs:

The equipment was tested under regular house-service operating
conditions. To use Buckwheat in the Anthratube it was found necessary, owing
to its more compact fuel bed, to drill a 7/l6-inch-diameter hole in balance
plate W (fig. 4), so that more air could be provided over the fire. Without
this air, the CO 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.

Here is an Internet link to the archived 4936 report:
http://digital.library.unt.edu/ark:/67531/metadc38592/m2/1/high_res_d/metadc38592.pdf

My personal experience is that I continued to experience puff backs with the hole in the balance plate fully open and the SV setting as specified in the AHS manual. It was only when I began to adjust SV to achieve a "higher" fire height that I stopped the puff backs.

Forum member "steamup" post a link to a 1945 conference. See:

http://nepacrossroads.com/download/file.php?id=37035

There are many interesting sections. One in particular is on the "Antratube". This is a term coined by Penn State that describes burning anthracite in a narrow diameter pipe. The physics of how coal burns in such a tube is described with considerable suggestions for how it can be used to develop a heating appliance. Read and understand what the report says and you will have a better understanding of the "puffbacks". I would expect the severe "puffbacks" occur when the fire portion of the coal varies greatly from the suggestions in the "Antratube" design characteristics. All the suggestions forum members have made for ashing temperature controller set temperatures is really about adjusting the position of the burning coal in the "Antratube" column. I'd like to post a more complete explanation but I just don't have the time right now.

The discussion of the Anthratube starts on page 58 of the bulletin.

i also just wanted to include that mine stopped with dropping my ashing temp down to 110 with a 8 deg. diff. and therefore raising the fire in the pot...works for me

Same here. Although the SV differential is 5 degrees.

Maybe it's just me, but there is seems to be a lot of variability in the settings required for boom-free operation of the S130 and S260.

In reading the pages of this thread, the key seems to be the fire height, not the settings. As each install and operation characteristics are different the settings will change from machine to machine. Somebody's non-boom settings might not work for you. What appears to work is the flame height no matter what setting you have to use on your individual installation. Booms are not just for the coal guns. They can happen on any coal burning appliance when you do not have enough fire to burn all the gasses off. Take a hand fired cover the bed to the point where you have very little or no flame and see what happens!!

Could I recommend instead of concentrating on who has what settings, concentrate on estimating the fire height where no booms occur and then adjust your settings to carry that consistent fire height?

What was the estimated fire height on your equipment when the booms went away?

blrman07 wrote:Booms are not just for the coal guns. They can happen on any coal burning appliance when you do not have enough fire to burn all the gasses off.

Do people really have this issue with flat/inclined or underfeed stokers? I've operated a bunch of those in an assortment of designs and sizes over a period of many years and never had anything resembling a boom.

Mike