WardBurner@aol.com on mon 23 sep 96
Hey folks,
Been outa' town and just read Julie's post concerning kiln shut down....
Julie wrote:
<<<<<thermocouple, to shut down at about 2000 degrees on a calm, cool day? I
thought I ran out of gas, but the company came to fill them and said they're
still half full.
Julie Moore
Dirty Bird Pottery
Manassas, VA>>>>>
Below is a past article I wrote on thermocouples that appeared in "Clay
Times". It may be of some help....
Marc Ward
Ward Burner Systems
PO Box 333
Dandridge, TN 37725
USA
423.397.2914 voice
423.397.1253 fax
wardburner@aol.com
Q: The thermocouples on my burners are continually burning out. Is there
something I can do to keep this from happening?
A: Thermocouples are designed to be low maintenance items. I just replaced
one on my kiln that was installed ten years ago. A thermocouple works by
sending a signal to a safety valve telling that valve that "hey there's a
pilot flame burning on me so everything is fine". This signal is a very small
voltage current that is produced by the pilot flame and the "difference"
between two metals that are welded together. The flame heats up the bulb at
the end of the thermocouple while the back end of the bulb stays relatively
cool. This "tension" between the front and back of the bulb produces the
current. The current is anywhere from 10mV (10 millivolts or 1/100 of a volt)
to 35mV (35 millivolts or 3.5/100 of a volt). If the back end of the bulb
gets too hot in relation to the end on which the pilot flame is burning, the
signal "drops out". In other words, the safety valve is no longer getting the
message from the thermocouple and the safety valve shuts the burner down. If
the back end of your thermocouple bulb gets constantly overheated, it will
fail. With excessive back pressure coming from the burner ports,
thermocouples can get burned up quickly.
There are many types of thermocouples, most of which are interchangeable.
K16, K15, K19, 17D, 50, ect. are some of the types used on burners. Also,
these types come in an industrial or residential grade. The residential type
can be picked up at the hardware store and used in a pinch, but you're better
off using industrial quality thermocouples such as Johnson Control's "Husky".
The industrial type is a tiny bit more resistant to burning out if the rear
of the bulb is overheated. The biggest advantage with the industrial
thermocouple is it's signal strength or millivolt output. These types of
thermocouples have an output range of 25-35mV. Again, whatever type
thermocouple you are using, if the back end or "cold junction" gets too hot,
you'll have a shutdown.
Another cause of thermocouple problems is an unstable pilot flame. If the
flame is "dancing around" on the thermocouple, the difference between front
and back is lessened causing the signal to degrade and producing a shutdown.
Burners should be protected from wind and positioned properly so that
secondary air entering the kiln does not cause the flame to dance. A burner
port should be large enough to have the pilot within it's perimeter with room
around the pilot. This doesn't mean the burner head and pilot are inside the
burner port. All open port systems should have the burner and pilot set back
from the outside edge of the kiln anywhere from 1/2" to 1". If your burners
are too close or inside the port, the life of the burner will be shortened,
the pilot flame will be unstable, and your combustion will be less than
ideal. If moving your burner back doesn't solve the problem, then you're
getting too much back pressure from the port. You need to open the damper to
decrease the kiln pressure. If that doesn't stop the back pressure, you may
be forced to enlarge your flue and/or chimney.
So, you don't want to rebuild your kiln and you still want to sock the
reduction to it. What now? You can wrap a small amount of ceramic fiber
around the back end of the bulb to keep it cooler. Another thing that is
commonly overlooked is wire kinks. The wire coming out of the back of the
bulb needs to be straight for at least 1/2". Where the wire enters the safety
valve also needs to be straight for the same distance. In between these two
points, the wire can be bent and twisted as long as it isn't severely
crimped.
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