John Baymore on thu 24 sep 98
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I thought I might share a little information that might be useful in
thinking about designing your own more elaborate flame safety system to
meet your specific needs and desires. Many of these functions can be
accomplished far cheaper than you think they can, and don't require very
elaborate electronics.
The main purpose of =22flame safety=22 is to fire and explosion proof the
installation and to protect the kiln unit from overfiring. Obviously, if a
person is present RIGHT THERE all the time any burner is on, and that
person KNOWS WHAT (S)HE IS DOING, you have a pretty sophisticated monitor
on the system. Unfortunately.... most of us can't sit at the gas kiln for
the time it takes to fire it doing nothing else but paying attention to it
=3Cg=3E. And much as I hate to say it, there are many people firing gas =
kilns
that have had little training in the theory or operation of fuel firing
systems. (Blame our schools shrinking technical curriculums for that one
=3Cwg=3E.)
So the flame safety system has to take into account the types of typical
=22failures=22 that could occur and cause problems. It also is intended to
protect the installation (and society) against problems created by an
inexperienced operator, or from operator error. This aspect of the problem
gets a little more sophisticated.
What can go wrong. Well it is actually a long list. Just ask Murphy =
=3Cg=3E.
Some of it includes:
The flame could go out and spew gas into the kiln and/or room, and
eventually ignite causing explosion or fire.
The kiln could overfire and cause a structure fire and/or destroy the load
of ware or the kiln itself.
A component of the combustion system or the flame safety system could fail
and cause the flame to go out (see above).
The gas supply could be temporarily interupted.
There could be a gas leak into the room.
The electric supply could be interupted and cause the mixture of the gas
and air to fall outside acceptable ratios.
The kiln draft flow could become blocked backing combustion products and
heat into the room.
The operator could not have the knowledge to operate the system safely.
The operator could make an error..... such as have a main burner gas valve
fully on when turning the remotely located main gas supply valve on for the
whole installation.... thereby having a lot of gas/air mixture pumped into
the kiln before the mistake is realized. Then if an ignition source is
present (the operator still tries to light the kiln), an explosion could
result.
The operator could forget to open the damper.
The operator could forget the kiln is on and overfire it.
The kiln could achieve a higher temperature in a certain amount of time
than the operator expects, thereby overfiring it.
The operator could become incapacitated and not be ABLE to shut off the
kiln at the appropriate time.
The operator could decide to leave the kiln unattended.
The following listing of functions and logic decisions is typical of some
of the more industrial flame safety systems I have worked on. It is not
showing every detail, and all systems do not have all functions, but it
gives enough to visualize a little bit about how this more elaborate stuff
works.
TURN MAIN POWER SWITCH ON
Is there a ground fault on the system? (The whole installation is on a GF
breaker.) If yes, trip breaker. Else continue.
Is there AC power to the system? If so, light AC power on light. Else
nothing works =3Cg=3E.
Is there low voltage AC to the system? If so, light low voltage power on
light. Else, no light, halt.
Sound alarm horn. Is alarm working? Press manual alarm silence to latch
out alarm, continue. Else, light alarm fault light and halt.
Is the combustion air blower functioning? If so, latch air proof relay and
light indicator light. Else light red indicator light and halt.
Is the gas pressure on the main line within high/low tolerances? If so,
latch gas proof relay and light green indicator light. Else light red
indicator light and halt.
Is the damper on the kiln open? If so, latch damper interlock relay. Else
light red indicator light and halt.
Is the overall room ventilation system on and functioning? If so latch
vent proof relay, and light green panel light. Else light red indicator
light and halt.
Send low voltage AC power to the electrically driven make up air louvers.
Are the automatic makeup air louvers open (for combustion air into the
room)? If so, latch make up air proof relay and light green panel light.
Else, light red indicator light and halt.
Is the kiln below the set point on the thermocouple high limit sensing
system? If so, latch high limit relay and light green indicator light.
Else, light red indicator light, halt, and sound alarm.
Are any main burner SOV actuating switches in the on position? If so,
sound alarm until switches are off, then proceed.
If all functions above are true, send power to the main gas line SOV's (or
hydromotor valve ....which is an oil hydraulic actuated unit) and therefore
pressurize the burner manifolding with gas.
(Note here.... there will usually be 3 SOV's or hydromotor valves on the
main line. The two on the main flow route open and the one on the vent
line between them closes. See below.)
Are the SOV's (or hydromotor valves) fully open or closed? If so light
green indicator light and proceed, else sound alarm and light appropriate
red indicator light on panel.
IGNITE PILOT BURNER =23X
Is pilot =23X start switch closed?
If so, automaticaly power up pilot burner =23X gas SOV circuit to supply gas
to pilot =23X.
Send current to pilot =23X ignition transformer (runs a spark plug sort of
like on a car).
Check to see if flame is present on pilot burner =23X via UV detector on
pilot =23X.
If so, lock pilot =23X SOV on and send power to main burner =23X SOV (or
hydromotor valve) circuit. Else re-send power to spark transformer for a
preset maximum number of tries. After max tries and no flame proof, cut
power to pilot =23X SOV, trip out entire burner =23X control system, sound
alarm, and light apropriate red pilot indicator lamp on panel.
LIGHT MAIN BURNER =23X
Is main burner =23X start switch closed? If so, send power to main burner =
=23X
SOV (or hydromotor). Else, do nothing.
Is main burner =23X SOV (or hydromotor) fully open? If yes, do nothing.
Else, cut power to pilot =23X SOV, cut power to main burner SOV (or
hydromotor), trip out entire burner =23X control system, sound alarm, and
light apropriate red pilot indicator lamp on panel.
The pattern for each burner is repeated for however many burners there are
on the kiln. After that, the system monitors all the sensors and will drop
out the system, light appropriate panel fault lights, and sound the alarm
if any one of them indicate a problem with what they are monitoring. The
system then requires operator attention to correct the condition before
relighting one or more burners.
(On some really sophisticated systems if the fault is transient, the system
will attempt to relight the burner and bring it back to the condition it
was at before the shutdown.......... really expensive and complicated
stuff.)
Many of the sensors used in this type of system are pretty simple devices,
except for the UV detectors. The logic for a lot of it can be accomplished
with a series of normally open switches (N.O.) in series. Some of the more
elaborate =22If this / then that=22 logic does require serious electronics.
Most of the sensors can be purchased easily and pretty cheaply. The SOV's
are not all that expensive, really. The very small ones for individual
pilots are quite cheap. The simple series wiring for simple sensors should
be run through conduit, routed away from heat as much as possible, and be
HT teflon insulation, but otherwise is not too complicated. You actually
can build a stripped down version of a flame safety system that has some of
the more important functions of the more industrial systems relatively
inexpensively.
Most of he control functions and monitor circuits are performed using low
voltage AC, stepped down by an AC/AC conversion transformer in the main
system panel.....sort of like the one used for many home doorbells. This
minimizes the possibility of sparks and shocks from line level AC, and
allows the use of less expensive wiring conventions.
Air and gas pressure switches with both NO and NC contacts are readily
available. Hi/low gas swiches are also. Paddle type flow switches are
available for insertion in lines where flow needs to be measured. Switches
driven by thermocouples (act like BASO's) are available. Various types of
simple switches are available at places like Radio Shack that can be
converted into sensors to test things like the position of a damper with a
little ingenuity.
An example of how you might incorporate an =22interlock=22 that made sure =
that
the general ventilation fans in your kiln room were on before you could
light the burners is illustrative of this general concept.
To do this you need a main gas line valve installed after the main shutoff
at the supply point, and before the burner mainfolding and controls which
is powered by either line vlotage or low voltage AC. This is the main
device that is the crux of the safety system. Obviously if this valve
fails, your protection is shot.
If this worries you (or your local gas inspector) the common =22cure=22 is =
some
redundancy in the system. You place two SOV's in the supply line one after
the other with a =22T=22 located between them. These two valves are powered=
by
the same circuit and both will open or close when power is on or off. If
one fails, the other probably won't. Between these two valves on the open
leg of the =22T=22 connector described above is another SOV that is wired =
just
the opposite...... when the two others are open this one is closed, and
vice versa. This valve leads through piping to a remote oudoor location,
and is a vent for the two valve interlock. This system is called =22block
and bleed=22, and is almost always =22code=22 on systems with over 500,000 =
BTU's
of input.
You route your AC power supply (via a ground fault circuit
breaker....another situation monitored) to this valve (and the whole
system) through a main power switch, probably a single pole single throw
(switch only one side of the circuit) toggle type mounted on a large
electrical panel box which will hold all the stuff in the system. As an
added nicety, you can put a little neon indicator lamp on the main panel
next to the switch that will glow when the circuit is energized by using a
double pole single throw switch and wire the second pole to the lamp. When
you turn on the main power to the system then the light come on....telling
you the whole system is =22on=22.
Now if the power this switch controls is routed via a wire directly to the
main gas SOV, when you turn on this switch, the valve immediately opens and
gas flows to the burner manifold. But instead of routing this direct to
the SOV, you can place one or more (almost infinite number) other switches
in the line, that have to be either manually closed or are closed by the
action of a sensor. If any one (or more) of them are NOT closed, the SOV
does not get power, and it doesn't open. Additionally, if a switch was
once closed and the SOV was powered up and that switch is then opened for
some reason, the SOV loses power, and closes.
So to monitor our ventilation system, we merely need to place a switch in
the main gas SOV power supply line that is CLOSED when the system is
operating. One way to =22sense=22 this operating condition is to assume =
that
when the AC power to the ventilation system is =22ON=22, the system is =
working.
So you place a N.O. relay driven by the 110 VAC power supply that controls
the ventilation system, in the role of switching the series circuit of the
main gas SOV. When this relay has 110 VAC power to it (from the
ventilation power circuit), the relay closes, and the N.O. contacts close,
thereby passing the main gas SOV's controlling power through it. If that
is the only sensor in the SOV's curcuit, the SOV then gets its power, and
opens.
Now it might be argued that the ventilation system might have power to it,
but the blower motor could burn out and cause the system to shut down. If
all you monitor is the 110 VAC supply voltage, then the kiln safety system
would be fooled into thinking the ventilation system was working when in
fact, it wasn't. This is TRUE. If you want to protect against this
possibility, you might use a paddle type pressure switch in the ducting of
the vent system in the place of the relay described above. As long as
there is flow in the duct, the N.O. paddle switch is closed, and the main
gas SOV gets power to remain open.
Want to make sure a particular door to the kiln room is fully open (or
closed) before firing? Want to make sure the damper is not closed? Put a
switch on it and wire it in series with the main gas SOV power. Most any
=22digital event=22 (ON vs. OFF) can be simply installed in the series of
switches controlling the main gas SOV circuit.
Remember that in this scenario above, if the main gas SOV closes and
interupts the gas because of one of the sensor switches wired in series
with its power supply opens up, when the sensor switch closes again the
main gas SOV will get power again and IT WILL REOPEN. If you don't want
this to happen, you will need to install a latching relay in the main gas
SOV power circuit that once it is no longer energized, stays that way until
reset by other means. VERY IMPORTANT.
I tend to favor this option in most cases.
Having a BASO valve in the plumbing or the thermocouple driven N.O.
switches in the series control system also gives protection in the case of
flameout and then the powwr being restored to the main gas SOV from some
changing fault condition. Two ways to solve the issue.
Hope this info is useful to someone out there.
Best,
....................john
PS: Any enclosed (interior) gas kiln installation should have a carbon
monoxide detector and a flammable gas detector hard wired into the room
power supply so that they are always on.
PPS: See my other recent posts on this stuff for relevant info too.
PPPS: Someone mentioned in a recent post that natural gas was less
dangerous because it didn't pool in low spots like propane. However it
collects in high enclosed spots like the peaks of roof lines=21 Same type =
of
problem just from a different location.
John Baymore
River Bend Pottery
22 Riverbend Way
Wilton, NH 03086 USA
603-654-2752
JBaymore=40Compuserve.com
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