John Baymore on sat 21 sep 96
--------------------------------------------------------
From: Ken Moore
Subject: gas kiln cuts out
Can anyone speculate on what would cause a gas kiln, with a safety
thermocouple, to shut down at about 2000 degrees on a calm, cool day? I
thought I ran out of gas,....
-----------------------------------------------------------
Julie,
A number of possibilities come to mind here, although more information on your
kiln, flame safety, and combustion system would be helpful. I assume from your
post and your probable rural location that you are firing propane and probably
using a BASO valve (brand name) with thermocouple probe. Also that a single
thermocouple is the shutoff for the whole kiln. (Only one burner on this kiln?)
Also that you have more than one gas storage tank.
First of all, as a flame sensing thermocouple gets older, it's output deminishes
a bit. So it requires a solid flame to maintain enough electricity generation
to keep the BASO valve locked in the open position. So exact positioning,
while less critical when new, becomes more important. If a slight breeze from
an unusual direction developed, it might be enough to deflect the flame away
from the probe.
If the thermocouple is monitoring the main burner flame (which is generally a
bad idea) instead of a solid well oxygenated pilot flame, this situation woud be
particularly possible if the kiln were in a reducing state where the main burner
flame was under less draft directing it into the burner port (dampered down)
and was of a "softer" fluffy quality rather than the "hard" more oxygenated
flame.
Another thought is that some mounts for thermocouples on pilot burners place
them into the "cooler" section of the flame near it's base inner cone. This is
good design because of the fact that it subjects the thermocouple armor (the
jacket of metal over it) to less heat and extends its life. But if this
positioning is just "off" it can make the thermocouple prone to false shut off's
with the slightest breeze from the "wrong" direction. Also if the primarty air
on the pilot is shut down (should not be done), the "soft" pilot flame can miss
the thermocouple.
Another potential problem is the use on some burner systems of little pilots
(with the thermocouple monitoring them) that resemble those on gas stoves!
These little "pinpoints" of unoxygenated yellow flame are particularly prone to
flame-out when under good draft conditions (such as those found on a cool,
still, early autumn day ). They also don't like breezes, dust and dirt, bad
positioning, slight gas pressure variations, and bad phases of the moon .
If this is what you have, replace it with a good venturi-type pilot.
It is also possible that a speck of rust, dirt, or scale got into the pilot
orifice from the pipe supply side (while under pressure) and plugged it up.
This would cause the pilot to flame-out and shut down the kiln. When the
pressure was off, it is possible that the scale dropped off the inside of the
orifice and it is no longer exhibiting the symptoms. Unlikely, but possible.
Usually, when a pilot orifice gets plugged, it stays plugged until cleaned out.
The thermocouples do fail once in a while. Usually they get "cranky" about
positioning first and then they just "die". You simply replace the unit with a
new one. Once it dies, it will not come back to life, so if it locks the BASO
valve on since the unusual shutdown, that is not the culprit.
BASO valves also can finally "die". They are pretty long-lifed units in
general. I know ones that are 25 years old still in service and working fine.
They don't come back to life either, so if it can be made to lock on now, then
that is probably also not the problem. If it is dead, simply replace it.
BASO valves are known to be prone to shutting down when they are subject to a
sharp shock (as in blow, not electricity). A brick falling on the pipe near the
unit can cause it to shut off. Anything that might have fallen onto the
pipeline right near the kiln during your mystery firing?
Another real possibility (and the one that I suspect) is that you "froze up" the
tanks, even though they were half full of liquid propane. If this is the first
time they were down to only half full (ish) and the outside air was this cool,
this is a distinct and very probable possibility.
You used the plural, so I assume that you have two or more tanks yoked together.
If these are the vertical so-called "100 lb" cylinders (or smaller), they have
little surface area to allow evaporation. They were designed for low peak draw
units like gas stoves. They store a very small amount of liquid propane to
provide thermal mass. To adequately service even a very small kiln, you'd need
a bunch of the 100 lb. cylinders yoked together. Unless your kiln is very
small, your tanks are probably the source of your "shut down" problem.
To burn the liquid propane you have, you need to convert it from a liquid to a
gas. Most potter's burners are designed to burn gas, not liquid. In most
systems this is accomplished by simple evaporation of the gas from the surface
of the liquid. (Special "evaporators" are available.) The amount of gas that
can be evaporated is directly related to the exposed surface area of the liquid.
Generally speaking, the more surface area, the more gas can be evaporated. This
is why a propane tank is never completly "full" when it is full.
This evaporation requires heat to "drive" it. The heat comes fron the
surrounding air and that energy stored in the thermal mass of the liquid in the
tanks. As the liquid evaporates, the remaining liquid cools as the stored heat
is used to turn liquid to gas (just like warter evaporating). As the liguid
cools, the amount of gas that it will "give off" as evaporated gas per square
inch of surface layer DECREASES.
At some point, it is possible that the rate at which it is able to evaporate
becomes less than the rate at which it is being consumed. At this point the
pressure of the gas in the line begins to drop as the tank pressure falls below
that at which the main regulator is set. If gas consumption continues, the
pressure eventually will drop to a point where the burners almost (or do) go
out. The thermocouple detects this loss of flame or extreme decrease of flame
as a fault, and shuts the kiln off.
This pressure drop can be appear to be quite sudden, but the events leading up
to it have taken quite a while to set up the situation. Let's say that the
pressure in the open area of the tank is 90-100 lbs./sq. in under "normal"
ambient air temperature and no draw conditions. Lets also say that the main
pressure regulator to supply the kiln is set to 2 lbs. / sq. in. for the rating
of the burners. Until the pressure in the main tank drops to about 2 lbs. (the
main regulator setting), you will not see any real visable change in the flame
on the burners. The main regulator is doing it's job. You are merrily firing
away, not realizing that the main tank pressure is dropping steadilly.
But by the time the main tank pressure approaches the requlator setting, the
main store of gas is almost completly cooled off and cannot evaporate much gas
at all! That last 2 lbs. of pressure drop (change) represents a very samll
percentage of the total affect of the cooling on the liquid storage. The last
few pounds of drop can happen very quickly.
So if you happened to check the kiln just before the tank pressure hit the
regulator setting, and then left to come back in an hour to re-check it, you
could have missed the sudden decrease in pressure and the obvious visable
changes happening to the flame over the next half hour. You came back in an
hour, and the kiln was just off. The flame safety system actually may have done
it's job perfectly in this case.
Once the evaporation of the gas stops, the tanks begin to warm up (from heat in
the ambient air), and they will again be able to evaporate gas. This is not
instantaneous............ it takes a while for the liquid to warm up. The
warmer the surrounding air, the faster the recovery.
The sizing of liquid propane storage is based on the peak BTU draw of the kiln
and the minimum ambient air temperature that the unit will be operated in. A
kiln wth a 500 ga. storage tank may fire fine in the summer, but "freeze up" the
tank in the winter. It may require a 1000 gal. tank to evaporate enough gas
during a firing so that the kiln can be operated in the winter.
Have you found your tanks getting cold when firing? Possibly even getting a
frost coating on the outside? This is a clear sign of excessive cooling, and
not enough storage capacity. Slight condensation on the outside is possible.
Lots of it (or actual frost!!!) is indicative of a sizing problem.
Make a habit of monitoring the main pressure guage on your tanks as you fire as
well as the pressure settings on your burner manifold. If you don't have one,
have it installed. It can tell you if the sizing of your storage is adequate
for what you are attempting to do. If it drops significantly, it/they is/are
too small. You may have to add another tank or two to your "array". Better
yet, get a single tank that is sized correctly.
I design and build kilns professionally. There are very few kilns I have put in
over the last 25 years that have less than a 500 gal. propane storage tank.
I am sure that there are many other possibiltiies that others here on CLAYART
will suggest. Hopefully one of these ideas will solve your "mystery". Good
luck.
...............................john
John Baymore
River Bend Pottery
22 Riverbend Way
Wilton, NH 03086
76506.3102@Compuserve.com
Ken Moore on sun 22 sep 96
John, Thank you very much for the advice. I believe from what you described
that the tanks (two 100 gal) cooled down and the pressure dropped. I went to
the tanks after the kiln cut off and they were covered with condensation and
very cool, although there was no frost. I started the firing at 3pm which is
later than I've ever started one. Also, the two previous firings I had done
with this kiln were in the summer and the temperature was probably in the
80's...Anyway, I think this is it...
Thanks again. Julie
BTW, the kiln is an AIM updraft three burners about 20 years old that I
bought from a neighbor who had it for 15 years and never fired it.
At 10:11 AM 9/21/96 EDT, you wrote:
>----------------------------Original message----------------------------
>--------------------------------------------------------
>From: Ken Moore
>Subject: gas kiln cuts out
>
>Can anyone speculate on what would cause a gas kiln, with a safety
>thermocouple, to shut down at about 2000 degrees on a calm, cool day? I
>thought I ran out of gas,....
>-----------------------------------------------------------
>
>Julie,
>
>A number of possibilities come to mind here, although more information on your
>kiln, flame safety, and combustion system would be helpful. I assume from your
>post and your probable rural location that you are firing propane and probably
>using a BASO valve (brand name) with thermocouple probe. Also that a single
>thermocouple is the shutoff for the whole kiln. (Only one burner on this
kiln?)
>Also that you have more than one gas storage tank.
>
>First of all, as a flame sensing thermocouple gets older, it's output
deminishes
>a bit. So it requires a solid flame to maintain enough electricity generation
>to keep the BASO valve locked in the open position. So exact positioning,
>while less critical when new, becomes more important. If a slight breeze from
>an unusual direction developed, it might be enough to deflect the flame away
>from the probe.
>
>If the thermocouple is monitoring the main burner flame (which is generally a
>bad idea) instead of a solid well oxygenated pilot flame, this situation
woud be
>particularly possible if the kiln were in a reducing state where the main
burner
>flame was under less draft directing it into the burner port (dampered down)
>and was of a "softer" fluffy quality rather than the "hard" more oxygenated
>flame.
>
>Another thought is that some mounts for thermocouples on pilot burners place
>them into the "cooler" section of the flame near it's base inner cone. This is
>good design because of the fact that it subjects the thermocouple armor (the
>jacket of metal over it) to less heat and extends its life. But if this
>positioning is just "off" it can make the thermocouple prone to false shut
off's
>with the slightest breeze from the "wrong" direction. Also if the primarty air
>on the pilot is shut down (should not be done), the "soft" pilot flame can miss
>the thermocouple.
>
>Another potential problem is the use on some burner systems of little pilots
>(with the thermocouple monitoring them) that resemble those on gas stoves!
>These little "pinpoints" of unoxygenated yellow flame are particularly prone to
>flame-out when under good draft conditions (such as those found on a cool,
>still, early autumn day ). They also don't like breezes, dust and
dirt, bad
>positioning, slight gas pressure variations, and bad phases of the moon .
>If this is what you have, replace it with a good venturi-type pilot.
>
>It is also possible that a speck of rust, dirt, or scale got into the pilot
>orifice from the pipe supply side (while under pressure) and plugged it up.
>This would cause the pilot to flame-out and shut down the kiln. When the
>pressure was off, it is possible that the scale dropped off the inside of the
>orifice and it is no longer exhibiting the symptoms. Unlikely, but possible.
>Usually, when a pilot orifice gets plugged, it stays plugged until cleaned out.
>
>The thermocouples do fail once in a while. Usually they get "cranky" about
>positioning first and then they just "die". You simply replace the unit with a
>new one. Once it dies, it will not come back to life, so if it locks the BASO
>valve on since the unusual shutdown, that is not the culprit.
>
>BASO valves also can finally "die". They are pretty long-lifed units in
>general. I know ones that are 25 years old still in service and working fine.
>They don't come back to life either, so if it can be made to lock on now, then
>that is probably also not the problem. If it is dead, simply replace it.
>
>BASO valves are known to be prone to shutting down when they are subject to a
>sharp shock (as in blow, not electricity). A brick falling on the pipe
near the
>unit can cause it to shut off. Anything that might have fallen onto the
>pipeline right near the kiln during your mystery firing?
>
>Another real possibility (and the one that I suspect) is that you "froze
up" the
>tanks, even though they were half full of liquid propane. If this is the first
>time they were down to only half full (ish) and the outside air was this cool,
>this is a distinct and very probable possibility.
>
>You used the plural, so I assume that you have two or more tanks yoked
together.
>If these are the vertical so-called "100 lb" cylinders (or smaller), they have
>little surface area to allow evaporation. They were designed for low peak draw
>units like gas stoves. They store a very small amount of liquid propane to
>provide thermal mass. To adequately service even a very small kiln, you'd need
>a bunch of the 100 lb. cylinders yoked together. Unless your kiln is very
>small, your tanks are probably the source of your "shut down" problem.
>
>To burn the liquid propane you have, you need to convert it from a liquid to a
>gas. Most potter's burners are designed to burn gas, not liquid. In most
>systems this is accomplished by simple evaporation of the gas from the surface
>of the liquid. (Special "evaporators" are available.) The amount of gas that
>can be evaporated is directly related to the exposed surface area of the
liquid.
>Generally speaking, the more surface area, the more gas can be evaporated.
This
>is why a propane tank is never completly "full" when it is full.
>
>This evaporation requires heat to "drive" it. The heat comes fron the
>surrounding air and that energy stored in the thermal mass of the liquid in the
>tanks. As the liquid evaporates, the remaining liquid cools as the stored heat
>is used to turn liquid to gas (just like warter evaporating). As the liguid
>cools, the amount of gas that it will "give off" as evaporated gas per square
>inch of surface layer DECREASES.
>
>At some point, it is possible that the rate at which it is able to evaporate
>becomes less than the rate at which it is being consumed. At this point the
>pressure of the gas in the line begins to drop as the tank pressure falls below
>that at which the main regulator is set. If gas consumption continues, the
>pressure eventually will drop to a point where the burners almost (or do) go
>out. The thermocouple detects this loss of flame or extreme decrease of flame
>as a fault, and shuts the kiln off.
>
>This pressure drop can be appear to be quite sudden, but the events leading up
>to it have taken quite a while to set up the situation. Let's say that the
>pressure in the open area of the tank is 90-100 lbs./sq. in under "normal"
>ambient air temperature and no draw conditions. Lets also say that the main
>pressure regulator to supply the kiln is set to 2 lbs. / sq. in. for the rating
>of the burners. Until the pressure in the main tank drops to about 2 lbs. (the
>main regulator setting), you will not see any real visable change in the flame
>on the burners. The main regulator is doing it's job. You are merrily firing
>away, not realizing that the main tank pressure is dropping steadilly.
>
>But by the time the main tank pressure approaches the requlator setting, the
>main store of gas is almost completly cooled off and cannot evaporate much gas
>at all! That last 2 lbs. of pressure drop (change) represents a very samll
>percentage of the total affect of the cooling on the liquid storage. The last
>few pounds of drop can happen very quickly.
>
>So if you happened to check the kiln just before the tank pressure hit the
>regulator setting, and then left to come back in an hour to re-check it, you
>could have missed the sudden decrease in pressure and the obvious visable
>changes happening to the flame over the next half hour. You came back in an
>hour, and the kiln was just off. The flame safety system actually may have
done
>it's job perfectly in this case.
>
>Once the evaporation of the gas stops, the tanks begin to warm up (from heat in
>the ambient air), and they will again be able to evaporate gas. This is not
>instantaneous............ it takes a while for the liquid to warm up. The
>warmer the surrounding air, the faster the recovery.
>
>The sizing of liquid propane storage is based on the peak BTU draw of the kiln
>and the minimum ambient air temperature that the unit will be operated in. A
>kiln wth a 500 ga. storage tank may fire fine in the summer, but "freeze
up" the
>tank in the winter. It may require a 1000 gal. tank to evaporate enough gas
>during a firing so that the kiln can be operated in the winter.
>
>Have you found your tanks getting cold when firing? Possibly even getting a
>frost coating on the outside? This is a clear sign of excessive cooling, and
>not enough storage capacity. Slight condensation on the outside is possible.
>Lots of it (or actual frost!!!) is indicative of a sizing problem.
>
>Make a habit of monitoring the main pressure guage on your tanks as you fire as
>well as the pressure settings on your burner manifold. If you don't have one,
>have it installed. It can tell you if the sizing of your storage is adequate
>for what you are attempting to do. If it drops significantly, it/they is/are
>too small. You may have to add another tank or two to your "array". Better
>yet, get a single tank that is sized correctly.
>
>I design and build kilns professionally. There are very few kilns I have
put in
>over the last 25 years that have less than a 500 gal. propane storage tank.
>
>I am sure that there are many other possibiltiies that others here on CLAYART
>will suggest. Hopefully one of these ideas will solve your "mystery". Good
>luck.
>
>..............................john
>
>John Baymore
>River Bend Pottery
>22 Riverbend Way
>Wilton, NH 03086
>
>76506.3102@Compuserve.com
>
>
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