Module 11 Gas Safety Devices and Controls Flashcards
Which module covers gas safety devices and controls?
11
Gas Safety Devices and Controls fall all fit into the following objectives
- Safety
- Comfort
- Economy
Name three safety features
- Appliance ignition
- Flame failure safeguard
- Protect from excessive pressure and/or over heating
Gas Safety Devices and Controls fall all fit into the following objectives
- Safety
- Comfort
- Economy
Name one economy feature
Max heat input to prevent bad combustion or overheating
What is the correct term for a Flame Failure Device (FFD)?
Flame Supervision Device (FSD)
Which one is not a thermal FSD?
a. Bi-metal strip
b. Liquid Expansion
c. Flame rectification
d. Thermo-electric
c Flame rectification
Bi-metallic strip – explain how works?
Two metals with dissimilar expansion rates bonded together.
When heated different rates cause it to bend.
This bend opens / closes the valve
If a bi-metallic strip fails what is the risk?
Valve does not shut – known as a fail to danger
Liquid expansion devices – commonly found in modern …
Cookers and water heaters
When an oven is turned on the pilot light of bypass flame heats the liquid expansion valve and what happens?
Liquid vaporizes, expands lots down the capillary tube, bellows on expands, valve opens on main gas = full flame
If a liquid expansion valve fails is it:
a. Fail safe
b. Fail to danger
a. Fail safe
Vale shuts
What controls the temperature of an oven?
b. Liquid expansion valve
c. Thermostat
c. Thermostat
LEV confirms the presence of the flame. It is either on or off!
How do you test a Liquid Expansion Valve is working
- Turn oven to max – bypass flame, ~30 seconds later full flame
- When oven at correct temp. flame drops to low
- Turn off oven, wait 30 seconds, relight.
- Flame on low – passed
Often a thermocouple energizes an ….
which holds open a valve and lets the gas flow
Electro-magnetic unit
What metals are typically used in thermocouples?
Chrome-nickel alloy and copper
When a thermocouple is heated an electromagnetic force (EMF) is produced which energises the magnet. What voltage do these two metals produce?
15 to 30mV
As a guide how long should it take for a thermocouple to become energized?
~45 seconds
How do you test that a thermocouple is working?
1) Running at op temp
2) Turn of gas either by service valve or ECV
3) Start stop watch – listen for valve to close
4) Tightness / let by test either at pressure test point on appliance or the meter
FSD – maximum time allowed for a DFE to shut down:
a. 60 seconds
b. 90 seconds
c. 120 seconds
d. 180 seconds
c) 120 seconds
FSD – maximum time for non DFE fires to shut down:
a. 60 seconds
b. 90 seconds
c. 120 seconds
d) 180 seconds
d) 180 seconds
FSD – maximum time for circulators, cooker, hotplates and grills, refrigerators and storage water heaters below 8kW:
a. 60 seconds
b. 90 seconds
c. 120 seconds
d. 180 seconds
b. 90 seconds
FSD for all other appliances:
a. 60 seconds
b. 90 seconds
c. 120 seconds
d. 180 seconds
a. 60 seconds
What type of FSD is shown in the picture
Flame rectification
Does flame recitation
a) fail safe
b) Fail to danger
a) fail safe
Carbon (soot) is a great conductor. If there is a build up between the electrode and the burner will a flame rectification unit:
a) fail to danger
b) Fail to safe
a) fail to safe
Why?
Allows AC to follow around the circuit. No DC = shuts down
Flame rectification is commonly found in:
a) o/f boilers and LFE fires
b) Gas wall heaters
c) Fanned flue appliances and pre-mix burners
c) Fanned flue appliances and pre-mix burners
Flame rectification circuits can be tested by
a) turning off the electrical supply to the appliance
b) Turning off the gas supply
c) Disconnecting the igniter
b) turning off the gas supply
In a flame rectification circuit what type of current is used?
a) AC
b) AC & DC
c) DC
b) AC & DC
Flame rectification, how does the AC get converted to DC?
a) a rectifier in the igniter
b) a rectifier post the burner
c) the gas
c) the gas
Which type of flame conducts electricity the best?
a) luminous
b) blue flame
b) blue flame
Which is the best conductor of electricity?
a) carbon
b) Copper
c) blue gas flame
b) copper
With part of a pre-aerated flame conducts electricity the best?
a) inner cone
b) outer cone
c) outer mantle
b) outer cone - the blue bit!
To optimise a flame rectification circuit the positive plate (the igniter) and the negative plate (the burner) must be
a) the same size
b) igniter x 4 > burner
c) burner x 4 > igniter
d) igniter x 10 > burner
e) burner x 10 > igniter
e) burner x 10 > igniter
By having one plate a lot larger than the other the current goes one way or is rectified!
Another explanation:
Normally an AC current is BIG one way and BIG the other. DC BIG one and SMALL the other. Smaller igniter, big burner, electrons don’t want to return, becomes DC
If a flame rectification circuit keeps locking out how could you test it?
Check amps, see attached
!! Figure 7, page 12!!
What is a VSD?
a) Visitation Sensing Device
b) Vittles Sensing Device
c) Voiding Sensing Device
d) Vitiation Sensing Device
d) Vitiation Sensing Device
With a VSD the flame lifts off as
a) Carbon dioxide levels fall
b) Oxygen levels fall
c) Nitrogen levels fall
b) Oxygen levels fall
VSD - explain what is happening in the pictures
Alternative names for VSD are
a) atmosphere sensing device (ASD)
b) oxygen depletion sensor (OSD)
c) oxygas pilot (OP)
d) All of the above
d) All of the above
If a VSD keeps ‘going out’ which are possible causes?
a) Damage
b) Incorrectly located
c) Lint in aeration port
d) Abnormal flue or vent conditions
e) All of the above
e) All of the above
Faults on cooker burner controls are often due to what?
Drying out of the lubricant
The correct lubricant for gas taps is:
a) Vaseline
b) Molybdenum disulphide grease
c) Silicone grease
d) Any of the above
b) Molybdenum disulphide grease
Where do you commonly find a safety cut off valve?
1) Cooker lid
2) Oven door when there is a grill inside
What is an SSOV?
Safety Shut Off Valve
A primary thermostat is located in the appliance, where would a secondary thermostat be located?
Remotely, good example is a room thermostat.
Primary and secondary thermostats can be subdivided into:
a) metallic and non-metallic
b) liquid and vapour
c) mechanical and electrical
d) All of the above
c) mechanical and electrical
Name components on this mechanical oil filled thermostat
Outline how you would test an oven thermostat
1) turn oven on to mid point (GM 5)
2) Has FSD energised and burner gone to main flame?
3) Wait ~15 mins for oven to warm up
4) Burner should reduce to low but not go out
An electrical thermostat has
a) gas passing through the unit
b) No gas passing through the unit
b) No gas passing through the unit
To control the gas flow an electrical thermostat uses:
a) bellows
b) diaphragm
c) solenoid
c) solenoid
Solenoid valves open and close due to:
a) an electrical current
b) change in temperature
c) change in gas pressure
a) an electrical current
When the current flows in the coil of a solenoid valve the armature is :
a) drawn into the tube, closing the value
b) pushed out of the tube, opening the valve
c) drawn into the tube, opening the valve
c) drawn into the tube, opening the valve
Solenoids using AC supplies are often noisy due to the reversal if the current causing the armature to vibrate against the pole. This can be solved by:
a) lowering the voltage
b) using a rectifier
c) turning off the supply
b) using a rectifier
When testing a solenoid the electrical supply is isolated and you:
a) listen for the valve to close
b) do a tightness test
c) All of the above
c) All of the above
Listen and measure for pressure loss
A medium and low pressure regulator is used when the supply pressure is:
a) between 75 mbar and 2 bar
b) 21 mbar
c) >2 bar
a) between 75 mbar and 2 bar
What does UPSO mean?
Under pressures shut off
The picture below is a thermoelectric FSD. Name the two key things that identify it
- Push button
- Termocouple
Thermoelectric FSD – outline test method
- Light pilot and establish
- Turn gas off, time until lcoks out
What type of device is shown in the picture?
Bi-metallic strip FSD
How do you test a bi-metallic strip?
- Light pilot and establish
- Turn pilot off, time until burner turns off
What device is shown in the picture
Vapour pressure FSD
How do you test a Vapour pressure FSD?
- Light burner (should light on low flame) after approx. 30sec rising to high flame.
- Turn gas tap off, wait for 90 sec and re-light (should light on low flame)
Where do you find vapour pressure FSDs?
Ovens
How do you test flame rectification devices?
Turn appliance on, make sure flame lights and establishes
Keep the demand on the appliance and turn the gas off. The appliance should go into lockout or keep sparking
What device is shown in the picture and key thing that identifies it:
Mechanical thermostat (vapour expansion)
Dial with numbers indicated
Where are mechanical thermostats (vapour expansion) commonly found?
Ovens
How do you test a mechanical thermostat (vapour expansion?)
- Turn oven on and heat up for 15 min.
- Turn control to lowest setting flame should drop to by-pass rate
What device is shown in the picture and key thing that identifies it:
Mechanical thermostat (rod type) Dial with numbers indicated
Name two places where you would find a mechanical thermostat (rod type)?
- Old cookers
- Old water heaters
What device is shown in the picture and key thing that identifies it:
Electrical Thermostat
How do you test an electrical thermostat?
Heat up appliance, thermostat should turn the solenoid off or modulate the burner pressure down
What is the purpose of a solenoid?
An electrically operated gas valve
How to you test a solenoid
- Turn electricity on valve opens
- Turn electricity off valve closes
What device is shown in the picture?
Multi-function control valve
How do you test a multi function control?
If the FSD, Regulator and solenoid work then the multi-function control is working.
What device is shown in the picture?
Gas tap
How do you test a gas tap?
- Tap locks in the off position
- Tap functions correctly
What device is shown in the picture?
Atmospheric Sensing Device (ASD or VSD)
What is the purpose of an ASD?
To close down the open flued appliance automatically, if the appliance and flue operate incorrectly
What device is shown in the picture?
Atmospheric Sensing Device / over heat thermocouple interrupter
What is the purpose of an Atmospheric Sensing Device / over heat thermocouple interrupter?
To close down the open flued appliance automatically, if the appliance and flue operate incorrectly
What device is shown in the picture?
Zero governor
What is does a zero governor do?
To compensate for the negative pressure caused by the fan, on the gas valve (this could pull more gas into the burner than intended)
How do you test a zero governor?
Attach manometer to burner pressure test point Turn appliance on when the fan comes on the manometer reads negative pressure, quickly going to positive pressure when the burner comes on.
What device is shown in the picture?
Modulating multifunctional control valve
