Coalgun- Puff backs & Explosions

rychw wrote:

Bob wrote:Have you tried operating with the high temperature limit set at 170 or 180 and using a 10 differential to see if it would help? From your description it seems as if the level of the fire is still a bit low to ensure that the gases are always exposed to a source of ignition and my theory is that the sensed temperature is affected by the boiler water temperature. Also--do you have the hole in the flapper door fully open?

Bob, I have tried to operate at a higher temperature without eliminating puff backs. I hesitate lowering my ash dump temperature below 110 degrees but that may be my next move. I changed the differential to 5 from 10 that it was at for 5 years. The theory is that there will be less new coal added for only 5 degree differential preventing puff backs. The whole in my flapper is open all tha way.

McGiever wrote:My belief is, if one has "puffbacks" they must get the fire(hot coals) raised higher in their firepot.

What the digital display reads on the controller may not match the owners manual or even what other AHS owners settings are...keep adjusting lower to get the fire higher and the problem will be gone.

"rychw", You may have developed significant inaccuracies in your digital ashing controller...you just need to do as suggested and keep going lower...forget the read-out, do it by appearance of level in firepot...then you will be par w/ all the others as far as puffbacks.

"rychw" -- I note you have the Axeman Anderson and the temperature may not be correctly calibrated.
I recall reading that there is a specific procedure for calibration and perhaps someone can provide the procedure or a link to the procedure.

The key, in my opinion, is keeping the fire sufficiently high so that the generated gases are always exposed to a source of ignition.

Bob wrote:"rychw" -- I note you have the Axeman Anderson and the temperature may not be correctly calibrated.
I recall reading that there is a specific procedure for calibration and perhaps someone can provide the procedure or a link to the procedure.

The key, in my opinion, is keeping the fire sufficiently high so that the generated gases are always exposed to a source of ignition.

The shaking adds coal but there is also more coal add simple due to the weight and breakdown of the bed below.
I found that halfing the hysterisis only causes the ashing to occur more frequently. The time it takes for it to return back above hysterisis is not linear or proportional to the amount of ashing. Also by ashing only half as much it keeps the fire bed low.

The only good solution so far is to lower the sv. Rob did make a good point very early in this thread that with lower SV the fire will rise and you will need to make sure it's not going up too close to the neck of the hooper.

I have not tried the opposite. jacking the sv higher say 140 and increasing the hyst to say 15.

the s260 has no problem maintaing a fire even at sv 95F as long as you keep an eye on boiler temp and keep it above 135F should be ok in lowering op temp. Dont know if the same holds for s130.

I have never seen the thermocouple setup in an AHS, however, I am familiar w/ thermocouples in other applications. There is likely a resistance value for that "type" that could be used as a benchmark test reading that could be a clue to the integrity of the thermocouple junction itself.


Is it mounted in such a way that numerous puffbacks could cause misalignment or otherwise render it inaccurate?

Is it sheathed in some stainless steel cladding and mounted by way of some pipe threads and locking nuts to a bracket to hold firmly in the desired position/location?

Hope it's on right

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IMAG0117 by ValterBorges, on Flickr

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IMAG0121 by ValterBorges, on Flickr

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IMAG0120 by ValterBorges, on Flickr

Thanks Valter for posting pics. It helps to get better representation of how it is orientated in the AHS.

Note to all:
As far as accuracy goes, these thermocouples work on a millivolt level, so any connection in the entire circuit that is less than ideal will cause inaccurate readings at the measuring instrument.
The AHS has 3 connections...2 wire connections at instrument end and 1 at the "junction" at the opposite end near the grate.

McGiever wrote:any connection in the entire circuit that is less than ideal

Can you give some examples of less than ideal?

ValterBorges wrote:

McGiever wrote:any connection in the entire circuit that is less than ideal

Can you give some examples of less than ideal?

Loose terminal screw in instrument and junction end connection could become loose, cracked or separated.
As well, the lead wire could become grounded to boiler frame at some point.

BTW...there all many different cable insulation options available for many different applications.

Working at the millivolt level it takes very little to degrade the accuracy of the signal since the milivolts spans a very large temperature range as it is. The coal boiler only uses a very narrow portion of this overall range...like around a 30-35*F portion.

Type J (Fe/Cu-Ni) thermocouples are widely used in industry due to their high thermopower and low cost. This type of thermocouple has an operating temperature range from 0 oC to +760 oC.

Any good replacement options out there within reasonable price?

How long do you thing these last in general?

Any simple way to test accuracy?

McGiever wrote:Thanks Valter for posting pics. It helps to get better representation of how it is orientated in the AHS.

Note to all:
As far as accuracy goes, these thermocouples work on a millivolt level, so any connection in the entire circuit that is less than ideal will cause inaccurate readings at the measuring instrument.
The AHS has 3 connections...2 wire connections at instrument end and 1 at the "junction" at the opposite end near the grate.

Another note regarding accuracy with Thermocouples....

As I recall the lead length and resulting change in resistance also impacts the accuracy of thermocouples. So it would be better to not shorten or lengthen the leads that the thermocouple is mfg'ed with to make it work in your installation if at all possible.

titleist1 wrote:

McGiever wrote:Thanks Valter for posting pics. It helps to get better representation of how it is orientated in the AHS.

Note to all:
As far as accuracy goes, these thermocouples work on a millivolt level, so any connection in the entire circuit that is less than ideal will cause inaccurate readings at the measuring instrument.
The AHS has 3 connections...2 wire connections at instrument end and 1 at the "junction" at the opposite end near the grate.

Another note regarding accuracy with Thermocouples....

As I recall the lead length and resulting change in resistance also impacts the accuracy of thermocouples. So it would be better to not shorten or lengthen the leads that the thermocouple is mfg'ed with to make it work in your installation if at all possible.

Well, not really so much, unless it is an extreme length. Controllers/Instruments have built in compensations for such a constant as fixed lengths...it's the variables ( like poor connections etc.) that make it get out of whack due to inability to compensate for those.

ValterBorges wrote:Any good replacement options out there within reasonable price?

How long do you thing these last in general?

Any simple way to test accuracy?

I think they are as good as any thing else out there in this application, no need to consider an other option.

They do last well if they are built and installed well to start with. That's where the accuracy part comes in.

Testing for our application only needs done by comparing temperature readings to a "good" liquid filled thermometer. If possible, best to compare it in a ice water bath at 32*F...if not possible, then use any temperature bath you desire.

A junction of dissimilar metals will produce an electric potential related to temperature. This is the principle that thermocouples use. There is nothing to calibrate in the thermocouple probe. It's just two pieces of known wire welded together. You can make your own. The difficult part is making an electronic instrument to measure the tiny voltage, especially in a harsh environment. Stray current ground loops are a particular problem.

Read more about how thermocouples work here:

http://en.wikipedia.org/wiki/Thermocouple

There are various methods to sheath the wire junction. Some methods protect it physically from the environment others improve the unwanted noise rejection. Read more about the different types of probes here:

http://www.omega.com/techref/themointro.html

The AHS coal gun uses the thermocouple as a control signal measuring device. There is no need to have it calibrated to absolute accuracy. It makes little difference if the reading the thermocouple electronics display is right or not. What's important is how it controls the height of the fire in the "stack" of ash, burning coal and unburnt coal.

It's far more important to understand how the thing you can set (lower set point, upper set point and hysteresis) influences the "stack". It's the control system that matters.

You can not use ordinary copper wire to lengthen a thermocouple. If you do you have introduced another thermocouple junction, the one between the copper wire and the thermocouple wire. You can shorten thermocouple wires but you must know how to reattach it to the special connector.

Yanche, I totally agree about the fire height/position is the only important factor, and temperature readout is whatever it is...and not all that important.

My previous postings were only to show that how well the thermocouples are installed may have allowed some unwanted variables and would account for some erratic reading.
The differences one might see when comparing settings to the manual or what other owners see in their installations has little meaning. However, a loose connection could be easily found and corrected...and would remove the erratic readings, more stable is more better.

McGiever wrote:My belief is, if one has "puffbacks" they must get the fire(hot coals) raised higher in their firepot.

What the digital display reads on the controller may not match the owners manual or even what other AHS owners settings are...keep adjusting lower to get the fire higher and the problem will be gone.

McGiever wrote:"rychw", You may have developed significant inaccuracies in your digital ashing controller...you just need to do as suggested and keep going lower...forget the read-out, do it by appearance of level in firepot...then you will be par w/ all the others as far as puffbacks.

Reference Fire Height. Sv120, Boiler Temp 160.

Check out this video on YouTube:

http://www.youtube.com/watch?v=HeiuwqQJ ... ata_player

It's been a while since anything added to this post, but a bit of news to add from my AHS buddy. He ended up adding a draft inducer... and still had the occasional boom. Toward the end of last year he made an accidental discovery. He removed the "tombstone" that covers the inspection (view port) door and for one reason or another it got left off. It took some time for him to realize it, but, he has not had one single puff back since it's been left off. He's wondering if the tombstone itself holds some methane? Anyway, if someone else that has the occasional puff back wants to remove their tombstone & see if it makes a difference, it would be an interesting experiment.

I think the only reason for the tombstone is perhaps to keep possible sparks at bay? I don't know, but, certainly don't try this experiment unless you deem it is safe for you do do so. My buddy Charlie's boiler has only concrete all around it, so no chance of fire. ( although I''m not sure that safety is the factory reasoning. I've had some spectacular puff backs & never had embers fly)