by Marc Ward
Clay Times Sept. 1998
The biggest problem with gas kiln safety systems is that they work. They shut down the burner... unfortunately this may happen at the thirteenth hour of the firing with one cone to go.
There are two basic types of safety systems used on gas fired kilns; thermocouple systems and flame rectification systems. The thermocouple system is the most common, mainly because it is far less expensive than flame rectification.
A thermocouple, when continuously heated by a pilot flame, produces a small millivolt charge that is relayed back to the safety valve. While you hold down a reset button that opens the main safety valve, this small millivolt charge causes a tiny electromagnet to pull a lever into place and hold the valve open. If the charge from the thermocouple gets weak or stops, the electromagnet releases the lever holding the valve open. The burner suddenly stops and that's when the cussing begins or you look toward the heavens and thank your lucky stars you had a safety system. In the first sentence of this paragraph, I underlined the word "continuously". Drafts or kiln back pressure can cause the pilot flame to dance around on the thermocouple and degrade the signal back to the safety valve. This is what happens when you have those sudden shutdowns. Or you can have too much flame on the thermocouple causing all of the thermocouple to get hot, not just the tip. This also causes signal degradation. The valve that the thermocouple is hooked up to is called a baso valve. This has become the common name for the valve just as Kleenex is a term for all types of tissue. In both cases, these are brand names that have come to represent a whole class of valves. BASO is a registered brand name of Johnson Controls. If you have a low pressure safety valve that has quit working, it's almost always the thermocouple that needs replacing. If you have a high pressure valve, it could be the thermocouple or the valve itself.
Flame rectification works on a different principal, most commonly an ultraviolet scanner. The scanner has a chemical compound in it that gets "excited" when exposed to UV (ultraviolet) radiation. This excitement produces a current that is amplified by a relay and used as a switch to open a solenoid valve. Can't picture that? Don't feel bad.... flame rectification has quite a few bells and whistles associated with it, any of which can cause a system malfunction. If you are in a situation where you need approval to build a kiln and hear the words "flame rectification", be prepared to spend thousands of dollars just on the safety system and, unless you're real handy, you'll probably need to have the system professionally installed. Since there are so many different components in a flame rectification system, troubleshooting can be a real hassle. The advantage of this kind of system is that shutdown times in the event of a flameout are rapid. Depending on the relay (the thing that reads all the information and passes it on to the appropriate valves), shut down can be anywhere from less than a second to three seconds. This rapid shutdown is why some local codes call for flame rectification when the BTU total of the burner reaches a certain output. BASO valves, on the other hand, are pretty dumb, slow beasts. Shutdown can take anywhere from 30 seconds to 45 seconds.
To give you an idea of relative costs between the two systems, I charge $169.50 to install a BASO system with pilot, gas lines, pilot valves, and various fittings on a Venturi burner. You can buy the parts and do it yourself for less plus, it doesn't need electricity to operate. To install flame rectification on the same burner, you'll start out at a thousand bucks for the basics and head upwards depending on code requirements. You'll need electricity and a real good understanding of wiring schematics.
So, if the inspector mentions flame rectification, you may at least want to change the subject towards BASO valves. At worst, it may be cheaper to move! Both systems have their pluses and minuses but, for most potters, BASO systems are more that adequate. Generally, the only folks that use flame rectification are the ones that couldn't get out of it for code reasons.