John Baymore on wed 23 sep 98
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Fred,
=3Csnip=3E
OK now the solenoid shuts off the gas including the pilot. The radiated
heat from the burner port is keeping the thermocouple nice and hot.
=3Cclip=3E
I mentioned this in my first post on this subject........ to Tom, I think.
Couple of days ago.
It is important (for the reason you just described) to locate the
thermocouple and pilot in such a way to minimize this =22phantom=22 heating =
of
the thermocouple by radiant heat from the kiln interior or hot
refractories. If the main burner nozzle is located correctly well away
from the face of the exterior kiln wall to allow suffcient cooling
secondary air to flow around it, if the port for the burner is correctly
sized, and the pilot is located to the bottom (for propane) or the top (for
natural) off axis of the main burner, the BASO should drop out relatively
quickly.
The slow response time is one of the drawbacks to the inexpensive BASO type
flame safety systems. You trade response time and reliability for price.
To get the =22most=22 out of these systems that they are capable of, you =
really
have to fine tune the exact installation. A bit of fiddling with the
placement of the sensor makes a huge difference.
This slow response and tendency to show =22false proof=22 of flame because =
of
hot refractories or radient heat is one of the prime reasons that in many
situations, simple BASO type valves are not considered adequate for flame
safety systems.
=3Csnip=3E
The gas comes back on and since there is no pilot the kiln either does not
light and there is gas all over the place because the BASO valve is open
due to the radiated heat on the thermocouple. Or if the kiln is hot
enough I think the burner will light itself and not too much will happen if
it light immediately but if it doesn't light a bad situation is occurring.
=3Cclip=3E
If the firebox refractories in the gas /air flow path are at least at dull
red heat, the flame will reignite, (and quickly too), assuming the mixture
is reasonably well areated (but not over lean). Below red heat it won't.
If the kiln is below red heat, the hot refractories and radiant heat should
not be a significant factor in falsing the thermocouple into thinking there
is a flame present, so the BASO should drop out pretty fast. At even low
red heat, it should have little falsing effect either. The real falsing
problem usually occurs at elevated firebox temperatures. At those
temperatures the flame will readily reignite, so the problem is somewhat
self-limiting.
Also if the kiln is below red heat, the output setting of the main burners
would be pretty low, so the total aerated mixture spewing forth in the time
it takes the properly sited BASO to drop out would no be all that great.
Don't know if that would really qualify as =22...gas all over the place=22.
Remember, the tracer added to odorless propane is very powerful. The human
nose can detect very low concentrations....... it usually smells worse than
it is. Which is QUITE deliberate....because a gas leak is nothing to fool
with in general.
That is not to belittle the importance of trying to minimize any unburned
gas / air mixture sort of getting loose.
The key thing you want to protect from is the kiln gas supply shutting down
while at a pretty high output rate, staying off until the firebox area
refractories are below red heat, and then coming back on and filling the
kiln with an explosive, aerated mixture that eventually finds a small area
of the chamber that is still at ignition temperature. Or possibly backs
out of the kiln and finds another ignition source ....like the pilot on a
home gas heater, the minute spark caused when a light switch is thrown, or
something else.
=3Cclip=3E
Any way I am going to put a normally open relay in series with the power
line and wire it so that I have to manually push a button to close it.
There is a way to hook it up with a second set of contacts on the relay so
it seals itself in as long as the power is on but drops out when the power
is off and won't close again until someone pushes the button after the
power goes back on. With the relay open --no juice to the solenoid or the
blowers.
=3Cclip=3E
This would be a good setup, but might be unnecessary if you get the
thermocouples located so that you get reasonable response time from the
BASO's you have. And it adds one more thing that could =22fail=22 (the =
relay)
and be a PIA for firing.
It is a good idea to have the blowers interlocked something like you
describe. That way if the system goes down because of a flame out on a
burner, the blower is not blasting uncombined combustion air into the
chamber. Also if the system goes down because of electrical interuption
and than the power is restored before you can get to it to relight the
burners ........at around .... oh say..... maybe 1063F (a-b quartz
inversion). Might make the difference between a saved load and a disaster.
=3Csnip=3E
This relay will also serve another purpose in my setup because I have a
Yokogawa thermocouple meter that will open a contact causing the relay to
open when the temperature reaches a set maximum and thus can be used as a
kiln sitter for the gas kiln. Not that I intend to use it to end the firing
but it is a safety device if for some reason I am not there at the set
point. With this setup the solenoid valve in front of the BASO valves
should be
safe. What do you experts think?
=3Cclip=3E
If you want to invest in a little =22cleaner=22 system, there are small
thermocouple activated N.O. switches made by various sources including
Johnson Controls that can be wired into the system in series to monitor the
pilot burner flames, and this curcuit can be used to control the function
of the 110 VAC solenoid valve directly. Eliminate the BASO valves
altogether from the system. Then your high limit, flame proofs, and AC
power (and hence air supply) sensors all drive the same electronically
operated valve. One key feature is that the switches have a little faster
response times than a typical BASO valve. The single 110VAC solenoid then
becomes the main safety valve for the whole system.
You can get all manner of relatively inexpensive sensors that can be wired
into this type of system.... like air flow sensors that go in the blower
line, high/low gas pressure sensors that make sure the gas line is at the
correct level, or even wire in a simple interlock that won't let you start
the burners unless you have remembered to open the damper on the chimney.
It usually is a series circuit...... lose any one point in the sensor array
and the system shuts down. If you are handy with logic diagrams and simple
electrical wiring, it is not all that hard to do, and add in some slicker
functionality too.
You can switch the entire system to a 12 VAC system with a transformer at
the 110VAC source, and make the wiring of all control functions low
voltage. You utilize a 12 VAC solenoid valve instead of the 110 VAC one as
the main control unit on the gas supply.
I have used these thermocouple switch units on multi nozzle burners that
utilize a single gas/air mixing unit and blower. Otherwise you'd have to
stack a series of BASO's one after another on the single gas line feeding
the single mixer.... one for each nozzle. Ugly, in-elegant solution prone
to too many mechanical moving parts being avaliable to fail at critical
times. Or go to more expensive flame rod or UV detection systems.
It is possible to also wire in a remote alarm with this type of system
(battery powered) that will sound if one or more flame monitors drops out
(triple pole double throw relay). Let you know you have a problem
happening.
Well....... hope these thoughts are of help. And I stongly second the
opinion expressed that the flame safety system is NO SUBSTITUTE for being
around when a kiln is firing. Other than MAYBE overnight pre-heat on pilot
sized burners (with flame safety in place), a firing kiln should have a
live operator around at all times. Particularly a kiln that is of the
typical type utilized by craft potters......... usually far less than state
of the art in =22combustioneering=22 and flame safety systems.
The less fuel firing experience the kiln opeator has... the better the
flame safety system really ought to be..... and for those of us who have
lots of fuel firing experience, it is important to remember the old
=22familiarity breeds contempt=22 saying.
BEst,
.............................john
PS: BTW..... Tom,........forgot to mention it last time........ North
American and Pyronics both have nice tiny, stable true venturi pilots.
They call them =22blast=22 pilots. Check them out if you need to change the
pilots on your installation.
John Baymore
River Bend Pottery
22 Riverbend Way
Wilton, NH 03086 USA
603-654-2752
JBaymore=40Compuserve.com
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