Safety Thermocouple (or Probe)
The safety thermocouple provides gas shut off in the event of
flame failure. It is important that the probe is rigidly and correctly
fixed to the burner head or pilot and the probe tip only (approx.
5mm) is situated in the flame. This is all that is required to
provide sufficient voltage to stimulate the safety magnet allowing
the gas to flow. The "cold" junction end must be only hand tightened
plus 1/4 turn with a small spanner in the body. Always check for
foreign matter between this junction and the safety valve body.
Do not scratch the metal surface, do not use thread sealant on
this joint and do not over tighten.
Most safety probes have a flattened section for identification,
this is OK but if the probe is severely bent or damaged it should
be replaced. It is not possible to repair these probes. Safety
probes should be replaced every 18 months or so depending on use.
Never subject the burner head or probe to excessive back heat
by having the burner too close to the kiln port. Heat will also
shorten the life of the safety valve.
These valves house the thermomagnet that provides gas shut down
if the safety thermocouple is not heated. Although usually providing
many years of service, over time, heat, dirt and normal use will
necessitate their replacement. However if the gas flame will not
stay lit, when the safety valve button is positively depressed
and slowly released, it may not always be time to throw the valve
out. Assuming the probe has been checked, a common problem is
dirt trapped in the valve causing the button to not be fully depressed.
This should not be disassembled but should be returned to the
burner supplier for service. Keep the burner clean. Dirt or sealant
may force its way under the safety valve seat allowing gas to
flow even in the event of flame out.
Care Of The Gas Orifice
Most atmospheric burners have a gas orifice that is accurately
fixed in the burner throat providing air inspiration. The hole
in the orifice is necessarily small to provide the correct gas
flow and to provide sufficient velocity to ensure there is a suction
available for the correct air inspiration. As this is the smallest
gas passage it is possible for any foreign matter in the system
to be trapped here. The usual sign that there is a blockage in
the burner unit is evidenced by either a yellow flame, short flame
or excessive gas pressure to reach top temperatures previously
effortlessly attained. The burner should generally be returned
for maintenance but if the orifice is removed do not use wire
to clean and remember to reseal to guard against leaks. Any enlarging
of the orifice hole or scratching of the material will cause either
the exact ratio between gas and air to be upset or the gas stream
to fire at an angle into the burner throat. It is imperative that
this orifice hole is drilled absolutely central.