Our kilns have powerful burners, sometimes unattended; very high internal temperatures (up to 2460 F), hotter than basaltic lava; they produce toxic fumes; emit flames during reduction, and create intense ultraviolet and infared light. So what combination of procedures, equipment, and training allow us to fire them safely, to best manage the known risks ? The following safety summary hopes to acquaint you with the collection of protective features that are built into our kilns, firing procedures, and burner fuel controls.
At CAW, all three high-fire kilns are located in our isolated, fireproof kiln room. The room is built of only concrete and cement block; there are no flammable materials within; and it has self-closing fireproof doors. The doors have a small eye-level window to allow view into the room before opening. Kiln exhaust is directed to a dedicated chimney, and the room has two power ventilators to evacuate smoke, fumes, and excess heat as needed. It also has its own fresh air source through a 1.5 ft² vent directly to the building exterior.
The kiln fuel, natural gas, is one of the safer fuels to use since there are no storage tanks, and even slight leaks produce a noticable odor. In addition, natural gas is lighter than air, so unburnt gas floats up to be blown out of the building by the ventilators, it doesn’t fall and pool in low areas where it might be ignited.
Gas is delivered to the burners through a low pressure, iron piping system with multiple shutoffs and failsafe features. Figure 1 shows the fuel schematic for one of CAW’s downdraft burners, all the others are typical. (Click to enlarge.) There are two burners per kiln, and two kilns that follow this plan. Our third high-fire kiln, the Alpine HF-24, has a different burner fuel system, which is discussed separately below.
As the schematic shows, there are five shutoff valves (ie, five levels of safety), in the line to each burner; any one of which can stop the flow of fuel. The first is the main room valve, pictured in Figure 2, which is manually turned on and off with the big yellow handle. When it’s turned to off, every burner of every kiln is shutdown. The valve is located on the wall closest to the clay storage room. The Valve Position Indicator currently shows that gas pressure is OFF.
From there, the piping splits into three branches; one for each kiln. The branches have their own manually operated shutoffs, as indicated in the schematic.
Phoenix’s – shown OFF – is pictured in Figure 3. Branch valves stop gas flow to all the burners of their respective kiln.
As a safety procedure, all the valves to every burner are normally kept in the OFF position. They are only moved to ON when that specific kiln is going to be lit. Our policy is also to check that all the gas valves to every kiln are OFF before opening the room valve. This avoids unused pipes from becoming pressurized.
After the branch valve, the piping splits again into separate links to the two burners of each kiln. Each burner link first encounters an Electrically Activated Valve (shown in the schematic and pictured on the right in Figure 4) that opens only when :
…….1) Electric power is available,
…….2) The kiln Master Switch is turned ON, and
…….3) The associated gray Fuel Switch is turned on.
As safety policy, the Master Switch and all the Fuel and Blower Switches are kept OFF except during a firing. The Master Switch is located on the wall adjacent to the clay mixer, and the Fuel/Blower switches are directly behind each kiln. These are pictured in Figures 5 and 6 below.
Next, each burner has a thermocouple/BASO safety valve system. Going back to Figure 3, it’s shown in the center. BASO valves shut off gas flow to a burner and its pilot if heat is not sensed by a thermocouple mounted in the pilot flame. (Burners are recessed back from the kiln slightly so that the thermocouple senses only pilot heat.) If the flame is interrupted, the probe cools quickly and automatically closes off gas to the entire burner. BASO valve systems depend on no external electricity or electronics to operate; if the heat goes away for any reason or the probe fails, gas flow is quickly shut off. They are very simple, extremely reliable over long periods of time, and similar units are found in many residential gas furnaces.
Last is the Flow Control Valve, which allows a manual adjustment of gas pressure by the operator. The valve is shown in the “off” position on the left side of Figure 3. Pressure increases in proportion to how far the handle is turned from there toward the full-open position at 90〫(ie, handle parallel with the pipe). A pressure gage indicates the change in gas to the burner as the handle is turned.
We normally candle our kilns overnight at low gas pressure and blower settings. Should an electric power failure occur while the kiln is unattended, the blowers shutdown and the Electrically Activated Valves shut off gas to the burners, the pilots then go out and the BASO valves close. If electric power returns later, the solenoid valves reopen and the blowers restart, but the BASO valves stay closed until someone comes to relight the pilots. Thus, the system defaults to a safe condition, with the burners OFF.
Hypothetically, if the power interrupt could be so fleeting that somehow the pilots remained lit, then the burners would also remain lit, or just relight; and the firing would safely continue.
On the other hand, if gas were to arrive back at the valve before the thermocouple cooled, (approx 10-20 seconds), then raw gas would blow out of the burners for a few seconds until the thermocouple did continue to cool enough to terminate flow. The blowers would then force the unburnt gas out the flew similar to the way unburnt gas is cleared during reduction conditions.
Another possibility to consider is a blower motor failure. Should that occur, the gas/air ratio to the burner will skew toward reduction, most likely a lazy, even smoky, yellow/orange flame, but the burner will remain lit. This is a safe condition, although somewhat undesirable for the ware, which is typically kept in an oxidation atmosphere until around cone 010.
Alpine Kiln Fuel Supply
The Alpine kiln is discussed separately because it uses an optical flame sensor in lieu of the BASO/thermocouple system. Such systems as the Alpine, known as Flame Rectification, look for an interruption in the flame’s ultraviolet light rather than its heat. Accordingly, a loss of flame or an electric power hiccup trips the electronics and shuts the kiln down within a few seconds. Quick response time is the advantage of flame rectification systems, however their cost and complexity are disadvantages. The schematic below illustrates the fuel supply path, components are shown pictorially in Figure 8.
The Master Electrical Switch on the Control Panel is kept off until a firing is to begin. This de-powers all three Electrically Actuated Valves in the main and pilot lines. Internal springs self-close the valves whenever power is lost. Also, the main burner and pilot Hand Control Valves are turned completely off at the end of each firing, and checked for the “off” condition before the room valve is opened. The Room Valve, recall from Figure 2, is manually turned on and off with the big yellow handle.
Once the Room Valve and the pilot hand-valve are fully turned on, the kiln master switch is turned on. Then both flame sensor safety monitors are reset. From there, logic in the controller directs the sequential lighting of both pilots. The controller powers the electric valves for a short time to allow lighting. Once its flame is sensed from both pilots, the main burner electrically actuated valve is automatically powered and the blowers are energized.
During the firing, loss of either flame sensor signal causes a system trip, wherein all three electrically actuated valves are closed (L & R pilot, and one for the main burners).
At the end of each firing, the main gas valve is closed first, so that gas in the entire piping system is depleted to zero pressure. Then the kiln gas valve is shut off. Following that, the electrical switch are flipped to cut power to the blowers and electric controls. The damper is closed last.
Burns and Eye Damage
Viewing into the spy holes can expose the eye to intense light as well as high flame temperatures. The adjacent picture in Figure 10 shows a spy hole with its plug inserted. Once the plug is pulled, flame shoots straight out about 6 inches from the door. To view into the kiln safely, SP’s stand to the side of the hole, and use tongs or fireplace gloves to pull the plug. Then they briefly view into the kiln with a welder’s helmet. This protects the entire head from contact with high temperature fumes. The helmet glass is not held in the flame for any longer than necessary, basically just a quick look, multiple times if needed.
Figure 11 is a view of the cone pack just before cone 1 drops. Staring into a white hot kiln to see the cones may damage the eyes. Intense ultra violet (UV) and infra red (IR) light is emitted. Ordinary eye glasses or sunglasses may protect from some UV, but are not helpful for IR. A welding helmet used by electric arc welders or glasses specifically designed for kiln work are needed to reduce damaging rays. Eye damage is also reduced by staying as far back as possible from the kiln when looking into it. UV radiation is suspected of increasing the risk of cataracts.
When adjusting the damper gap, it is necessary to grab hold of the damper plate and push or pull to slide it. The plate is very hot, as shown in Figure 12; you can see it is exposed to the flue gas flames for several hours toward the end of the firing. Our procedure is to use woodstove gloves, which protect against extreme heat. Constructed of heavy duty, fire resistant fabric, the gloves allow work and kiln unloading without worrying about burns.
The kiln room is equipped with two power ventilators to control toxic fumes, and a CO and smoke detector to monitor air quality. Any time a kiln is firing, the doors are mandatorily kept closed, and one power ventilator is continuously running. After overnight candling, kiln temperature in the morning is usually around 1500 F, so any organic materials have been burned away before anyone arrives to continue the firing. The ventilator is sized to completely remove fumes associated with the body and glaze reductions; and helps limit the rise in room temperature.