by Marc Ward
Clay Times July 2004
Last issue I wrote about kiln controllers and the things they can do. Along the way, pilots were often mentioned and I promised to talk more about them this issue. Here goes.
There are two types of pilot functions: Standing pilots and intermittent pilots. Standing pilots are always on; they’re just standing there. Older gas furnaces in houses had them. Old water heaters had them. Most gas kilns have them. They involve pushing a reset button on a safety valve. Pushing the button down causes gas to flow to the pilot where it is ignited. Once ignited it usually energizes a thermocouple that sends a signal back to the safety valve telling it that there is a flame present. Take your finger off the button and gas can now go to the main burner. As long as the pilot is lit (standing), the thermocouple stays energized and the valve stays open. It’s pretty simple stuff. It’s pretty effective. There are other ways to monitor a standing pilot such as UV scanners, and flame rectification, but the main idea is that the pilot light is always on. Using these more technical (and expensive forms) of monitoring a pilot are rare with a standing pilot. The reason that UV and flame rods (rectification) are not often used with standing pilots is their speed of response. UV and flame rods usually have a response time of about 1 second while thermocouples have a response time of 30-45 seconds. The controls that are connected to these faster monitoring devices can read and react faster. There is not a need for the manual aspect (holding a button down) that is usually required with a standing pilot.
So, intermittent pilots are faster. And, because of this, they use electronics to activate them. Intermittent pilots always involve two components: a spark ignition component and a sensor. A signal says to the burner system, “we need heat”. An electric solenoid valve opens and sends gas to the pilot and at the same time a spark starts trying to light the gas coming out of the pilot. The instant the pilot ignites, the sensor tell the intermittent pilot control, “Houston, we have ignition”. Instantly another electric solenoid valve opens to the main burner and we’re off to the races. Like modern gas heating systems in houses, if a temperature is reached, everything is turned off (including the pilot) until heat is again needed. Then, the cycle starts again; open pilot valve, start spark, see pilot flame, open main gas valve, main burner ignites from the pilot. If the intermittent pilot control can’t instantly detect the pilot flame, everything comes to a screeching halt. Now, in the kiln world, you may have an intermittent pilot and think it is a standing pilot because it is always on. Sounds sorta’ confusing huh? It’s not really. Most pottery kilns just demand heat constantly so the pilot and main burner stay on until you turn them off or a controller says that you’ve reached your destination and shuts things down. Most commercial kilns that have some sort of “start” button are using an intermittent pilot. If it lights itself, it’s intermittent whether it stays on all the time or cycles. One of the other main differences is the use of electricity. A standing pilot system does not necessarily require electricity. An intermittent system always has an electrical component. It has to have one. The spark, sensor, and pilot control all need some juice. A thermocouple-standing pilot system doesn’t need but a whisper of electricity. The heat on the thermocouple generates all the electric current needed (a very, very small one).
The two differentiations that I’ve talked about apply to the monitoring of pilot flames, not the actual pilots themselves. Almost any kind of pilot can be set up as a standing pilot or an intermittent pilot. What do I mean as any kind of pilot? I mean the physical structure of the pilot. There are little target pilots that hit a small shell-shaped piece of metal and throw the flame onto the thermocouple. There are pilots that have fins that provide extra grounding area. There are Venturi burners with air shutters that act as pilots. Any of these pilots can be set up as either standing or intermittent systems. It’s all about how they are controlled. It’s all about how they are watched over.