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3.2 Explanation of Control System
Safety System
The purpose of the safety system is to shut off the flow of gas if
the pilot flame goes out. It is comprised of the flame itself, the
thermocouple, and the flame failure gas valve.
The pilot flame is lit by holding in the gas control knob, which in
turn temporarily pushes the plunger inside the safety valve
open and allows gas to flow through.
Once the burner is lit, the thermocouple will begin to generate
millivolts (after about 10 to 30 seconds of being heated) and
will energize the electromagnet inside the gas valve.
Once energized the electromagnet holds the plunger inside the
gas valve in the open position. The plunger has to have been
pushed all the way in for the electromagnet to be able to hold it
in place.
If the burner flame goes out, the thermocouple will cool after
about 10 to 30 seconds and stop generating millivolts. The
electromagnet will then de-energize, and the plunger will snap
shut, cutting off the flow of gas.
Detail of each component in the safety system is explained
below.
Thermocouple
The thermocouple is a device that generates electricity when
heat is applied to the tip.
Insulator
Nut
Internal
Wire
Figure 3.2a
The tip of the thermocouple is located in the pilot burner flame,
and the nut at the other end of the thermocouple screws into
the back of the gas valve.
Inside the copper tubing is a wire which is joined at the tip but
insulated from the rest of the tubing.
These two parts (the copper tubing and wire) make up the
'Wiring' for an electrical circuit. When these two dissimilar
metals, wire and tip, are heated an electrical voltage is
produced.
This type of thermocouple generates between 7 and 30
millivolts when heated in the pilot flame.
Electromagnetic Flame Failure Gas Valve
The purpose of the safety valve is to shut off the flow of gas if
the pilot flame goes out.
Inside the body of the gas valve is an electromagnet connected
to a spring loaded plunger. When the electromagnet is
energized, it holds the plunger in, allowing gas to flow through
the valve. When the electromagnet is de-energized, the
plunger snaps to the closed position, stopping the flow of gas.
Millivolts are provided to the electromagnet by the
thermocouple (not shown) which generates millivolts when
heated. The thermocouple screws into a fitting at the back of
the gas valve to make an electric connection. By pressing in
the gas control knob, the plunger can be temporarily held open
while lighting.
Blue Seal Evolution Series - G47 Gas Pasta Cooker
Amendment 6
Conductor
Tip
There are two reasons for this; gas has to flow through the
safety valve to make it possible to light the pilot burner,
secondly the plunger has to be pushed all the way in for the
electromagnet to hold it in. i.e. the electromagnet is strong
enough to hold the plunger in once there, but is not strong
enough to pull it in by itself.
Gas flow
Gas flow
Sometimes a problem with the flame not staying lit after
releasing the button can be attributed to not pushing the
plunger all the way in.
This millivolt circuit is interrupted by a safety cut-out
thermostat. If the cut-out trips it cuts the millivolt supply to the
pilot valve magnet, shutting off the burners. The pasta cooker
is then inoperable until the cut-out has cooled below 100°C.
The purpose of this is to shut off the burners should the tank
run dry.
The Troubleshooting Guide (Section 5) should be used to
identify any incorrect operation. On correct identification of the
operating fault the Troubleshooting Guide will make reference
to the corrective action required, or refer to the Fault Diagnosis
section and / or Service section to assist in correction of the
fault.
11
Operation
Thermocouple
Electromagnet
Plunger
Shaft
Knob
Plunger
Control Knob
Figure 3.2b
© Copyright Moffat Ltd, March 2016
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