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NVQ Level 2/3 Gas Installation and Maintenance.1
Appliance GasAppliance Gas
Safety DevicesSafety Devicesand Gas Controlsand Gas Controls
NVQ Level 2/3 Gas Installationand Maintenance.
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IntroductionIntroduction1. Basic Requirement for controls
2. Reason why we have controls3. The Control Rigs
4. Look at the 14 controls What they do
How they work
Faults
Maintenance
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OverviewOverviewGas safety device and controlscan be divided into 5 groups.
Pressure regulation
Gas ControlFlame protection controlThermostatsMultifunction control
Over the years many specialised components and controlsystems have been devised, but whatever the degree ofsophistication achieved modern appliances mustincorporate the following features:
Safety
Device
CH4
IgnitionControls
Flame
NG
21mb
+/- 2mb
Gas
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4
Basic RequirementsBasic Requirements A safe and effective method or procedure for the initial
lighting of the appliance A flame failure safeguard to prevent a dangerous situation
arising from the re-introduction of the fuel to the mainburner after flame failure has occurred
Safety devices at the appliance as a precaution againstexcessive system pressures and over-heating
Control of the maximum heat input to the appliance so as toprevent bad combustion or overheating under normal use
The provision of manually operated controls adjacent to theappliance for the purpose of turning off the gas supply to themain or pilot burner
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5
Reasons why we have controlsReasons why we have controls
Safety Comfort
Economy
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The Control RigsThe Control RigsWhat they look like ..
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List of 14 ControlsList of 14 Controls1. Gas Tap/Cock/Valve
2. Safety Cut-off Valve
3. Bi-Metal Strip4. Thermo-electric
5. Liquid Expansion
6. Oxygen Depletion Device (ODS)
7. Flame Rectification8. Solenoid valve
9. Mechanical Thermostat
10. Electric Thermostat
11. Relay Valves12. Governor ( Regulator )
13. Multifunctional Valve
14. Zero Governor
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No.1No.1 Gas Valves, Cocks and TapsGas Valves, Cocks and Taps
SEE PAGESEE PAGE 1212
These devices are the simplest forms of manualcontrol of any gas flow, but in the gas industry
each item is normally used with slightly differentapplications of the gas flow, although there areexceptions to the rule.
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Gas ValveGas Valve
Full Bore Lever Ball
Valve.
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Gas CockGas CockThis manual control is
normally located onInstallation pipework
and also as the emergency
(meter) control. The flow ofGas is controlled by a slotted
Tapered plug which is rotated
Through 90 to turnON/OFF.
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Gas TapGas TapA Gas tap operates in a similar
manner to the tapered plug gascock, but is fitted as an integral
part of a gas appliance.
Gas Taps can be found oncookers for example.
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Faults and RemediesFaults and RemediesGas Valves, Cocks and Taps can dry out and leak.
Appliance isolating cocks are seldom turned on and
off, so they become stiff to turn in the course of
time.
Use a High temperature Graphite grease
(Molykote)to lubricate or replace faulty unit.
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True or FalseTrue or False
The MCV should have a
handle that falls to the openposition ?
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True or FalseTrue or False
The gas tap is the simplestThe gas tap is the simplestform of manual control toform of manual control to
turn a gas supply on or off?turn a gas supply on or off?
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A cooker hob gas tap might leak gasA cooker hob gas tap might leak gas
when it is turned to the off positionwhen it is turned to the off positionbecause?because?
A:A: It gets too coldIt gets too cold
B:B: ItIts designed to leaks designed to leak
C:C: The heat from the cookerThe heat from the cooker overover
time dries out the grease.time dries out the grease.
D:D: All of the aboveAll of the above
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No.2No.2
Cooker Safety CutCooker Safety Cut--off Valvesoff Valves
Cooker hotplates are frequently supplied with drop-down lids.
These lids may be constructed of glass or metal. The British
Standards specify that any drop-down lid constructed of glass
must have a device that prevents any gas from flowing to the
burners on the hotplate when the lid is in the closed position.
Many manufactures provide a cut-off device even when the lid is
constructed of metal.
The majority of devices are operated by a lever which closes a
gas valve ( fitted at the rear of the appliance ) beneath the hob
as the lid is closed, thus shutting of the gas supply to any burner
whose tap/s are still in the on position.
SEE PAGESEE PAGE 1313
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Cooker Safety CutCooker Safety Cut--off Valvesoff Valves
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On the RigOn the Rig
Lid closed Gas to hotplate
is therefore off.
Lid Open
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Faults and RemediesFaults and RemediesIt is possible that the valve can become stuck in the open
or closed position over time, due to a build up of grease
and grime.
Clean valve
Replace valve
If appropriate, it should be included in a Landlords Gas Safety
Record inspection.
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True or FalseTrue or False
The cooker lid must be in theThe cooker lid must be in theopen position in order toopen position in order to
correctly carry out a gascorrectly carry out a gastightness test.tightness test.
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No.3No.3
BiBi--Metal StripMetal StripThis device works on the principle of two dissimilar metals
attached to each other, with different expansion coefficients.When heat is applied to the strip ( via the pilot flame ), the
different expansion rates cause the strip to bend and open the
main gas valve.
See pageSee page 44
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BiBi--metal strip flame supervision devicemetal strip flame supervision device
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OperationOperation
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On the RigOn the Rig
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MovieMovie
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Faults and RemediesFaults and Remedies If the metal strip becomes damaged in someway.
Examples bent out of shape or due to corrosion, breaksapart. Then the device would fail. This device could stayclosed but is more likely to open the gas way ( Fail toDanger ).
The valve spindle becomes bent keeping the gas valveopen or closed.
These Devi ces cannot be r epai r ed and w i l l need
r eplaci ng when t hey f a i l .
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True or FalseTrue or False
A BiA Bi--metal strip flamemetal strip flamesupervision device needs asupervision device needs a
pilot flame to be present topilot flame to be present toopen the gas valve?open the gas valve?
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True or FalseTrue or False
A BiA Bi--metal strip flamemetal strip flamesupervision device if found onsupervision device if found on
an appliance in working orderan appliance in working orderwould be classified as ID?would be classified as ID?
SS 66
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No.4No.4
Thermoelectric ValveThermoelectric Valve Heat is used via a pilot to heat two different
metals. They, in turn generate a tinyelectrical current which passes down a wireto a magnet in the main thermoelectric
valve. The magnet therefore becomes anelectro magnet keeping open the main gasvalve which is being pressed against the
magnet. Whilst heat is applied the valvewill remain open. When heat is removed thevalve will shut.
See pageSee page 66
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Thermoelectric FSDThermoelectric FSD
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Push and HoldPush and Hold
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ThermocoupleThermocouple
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IdentificationIdentification
OperationOperation
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OperationOperationWhen the joint is heated by a pilot flame, the voltage energises themagnet thus holding the armature to it in a spring-operated gas
valve and allowing gas to flow to the main burner.
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Test Procedure (FSD)Test Procedure (FSD)
1. Ensure that all primary and secondary controls are
set so that the burner will not be turned off during
this procedure.
2. Light the appliance, and allow the burner to reach
its normal working temperature.3. Turn off the appliance shut-off device ( appliance
isolation valve ) and simultaneously start a stop
watch.4. Halt the stop watch when the valve is heard to
close.
Maximum shutMaximum shut down timesdown times
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Maximum shutMaximum shut--down timesdown times
Appliance FSD beingtested
Maximum shut-down time in
secondsDFE gas fire 120
Other gas fires 180
Cookers ( see next page ) 90
Circulators and storage waterheaters less then 8kw input
90
All other appliances 60
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Additional informationAdditional information Note: For cookers, the gas valve should shut down
within 60 seconds if the burner is situated in acompartment (oven) and within 90 seconds in thecase of an open burner, covered burner or griddle.
Always take guidence from manufacturesinstructions.
If the thermocouple did not receiveIf the thermocouple did not receive
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If the thermocouple did not receiveIf the thermocouple did not receive
heat from the pilot flame the gas valveheat from the pilot flame the gas valvewould.would.
A:A: Close the gas valve instantly.Close the gas valve instantly.
B:B: Close the gas valve after aClose the gas valve after a
short period of time.short period of time.
C:C: Open the gas valve instantly.Open the gas valve instantly.D:D: Open the gas valve after aOpen the gas valve after a
short period of time.short period of time.
M i hM i h t d i id ti i
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Maximum shutMaximum shut--down time indown time in
seconds of a DFE is?seconds of a DFE is?
A:A: 60 seconds60 seconds
B:B: 120 seconds120 seconds
C:C: 180 seconds180 secondsD:D: 240 seconds240 seconds
E h h fl i l
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Ensure that the flame is correctly
positioned, generally it should be on thetip, or the top __ mm of the thermocouple.
A:A: 16 mm16 mm
B:B: 3 mm3 mm
C:C: 12 mm12 mm
D:D:
2 mm2 mm
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No.5No.5
Liquid ExpansionLiquid Expansion When liquids are heated sufficiently, they are
turned into vapour, or boil. The vapour has a muchgreater volume than the liquid from which it cameand so causes a rapid increase in pressure whichcan be made to operate the valve in the liquidExpansion flame protection device.
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Control RigControl Rig
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The device is used for a number of appliances including room heaters,water heaters and cooker ovens. Its advantage is that any damage
resulting in loss of liquid causes the valve to fail to a safe condition.
Some older devices contained mercury.
Control Rigg
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Faults and RemediesFaults and Remedies Apart from failure of the sealed system containing the
liquid, which entails changing the device, the leversystem and valve may require cleaning and lubricating
from time to time.
The main source of complaints results from incorrectposition of the probe ( Phial ), relative to the pilot
flame.
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No.6No.6
Oxygen Depletion Device (ODD)Oxygen Depletion Device (ODD)
Atmospheric Sensing Devices ( ASD ) Vitiation Sensing Devices ( VSD )
Oxygen Depletion Sensor ( ODS )
Also Known as
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Vitiation ?Vitiation ?
Means to impair the quality of the combustion air.
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Answer:Answer:AN OXYGEN DEPLETION SYSTEM (O.D.S) IS A SAFETYDEVICE, WHICH ACTIVATES THE APPLIANCES FLAME SUPERVISIONDEVICE IN ANY SITUATION RESULTING IN THE PRESENCE OF
EXCESSIVE LEVELS OF CO (and the deadly carbon monoxide),WHICH REDUCE THE LEVELS OF OXYGEN CAUSING THE PILOTFLAME TO BECOME UNSTABLE AND LIFT OFF THE THERMOCOUPLETIP. THE APPLIANCE WILL THEN BE AUTOMATICALLY TURNED OFF
LONG BEFORE THE SITUATION MIGHT BECOME DANGEROUS FORTHE OCCUPANTS OF THE ROOM, IN WHICH THE APPLIANCE ISINSTALLED.
Question:Question: WHAT IS AN OXYPILOT OR AN O.D.S.?WHAT IS AN OXYPILOT OR AN O.D.S.?
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Types of appliance with ODSTypes of appliance with ODSMostly installed on domestic gas appliances where
there is a danger of products of combustion from theappliance contaminating the air in the room in which
the appliance is fitted. Generally, these are flueless
space heaters and water heating boilers. This follows
a ruling from the European Gas Directive that from 1st
January 1996 all appliances connected to a flue for the
dispersal of products of combustion must be so
constructed that in abnormal draught conditions thereis no release of products of combustion in a dangerous
quantity into the room concerned.
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OperationOperationThe device relies on a precision pilot holding open a
thermo-electric valve via the thermocouple. Thepositioning of the pilot flame in relation to the
thermocouple is critical. For this reason if the
thermocouple or pilot become faulty they must be
exchanged as a unit.
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How it worksHow it works Air supply satisfactory:precision pilot heats
thermocouple tip
As the air becomes
contaminated the
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oxygen level falls, the
precision pilot liftsaway, cooling the
thermocouple. If this
situation persists the
thermo-electric valvewill shut off the gas.
As the air becomes
further contaminated the
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further contaminated the
precision pilot hunts for
air and completely lifts
away from thermocouple
tip. The thermo-electric
valve will shut off the gas.
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ODS on the RigODS on the Rig
Faults and RemediesFaults and Remedies
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Faults and RemediesFaults and Remedies Because of the critical nature of this device it is essential
that the correct servicing procedure is used,manufacturers instructions must be followed, and all
safety checks must be carried out if any work is carried outon the appliance or there appears to be a problem. Themain checks to be applied are:
1. The ODS is complete and not damaged2. The unit is securely mounted in its recommended position
3. The aeration port is clean and free from obstruction
4. The flame picture is not distorted and the sensor flame
and cross lighting flame are correctly positioned
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Yes or NoYes or No
Vitiation means there is tooVitiation means there is toomuch combustion air.much combustion air.
1:1: YesYes
2:2: NoNo
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Flame RectificationFlame Rectification
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Appliances that use FRAppliances that use FRThese devices have the
advantages of instantresponse and immediate
shut-down in the event of
flame failure. They arecommonly used in
conjunction with full-
sequence automatic
control of gas burners.Commonly fitted to
Boilers.
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Faults and RemediesFaults and RemediesAny build-up of dust or condensation cannot affect the
rectification and cannot simulate a flame.
These devices have high reliability, however proper
electrical polarity and grounding are critical to this type
of flame sensing system.
A FR device can be measured for correct operation
using a meter capable of measuring in A ( micro-Amps )
If the device is faulty then replace the unit.
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True or FalseTrue or False
Flame rectification works onFlame rectification works onthe principle of voltagethe principle of voltage
displacement ?displacement ?
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A FR device can be measured forcorrect operation using a meter
capable of measuring in _____
A:A: A ( micro-Amps )
B: V ( micro-Volts )
C: mV ( milli-Volts )
D: Either A or B above
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No.8No.8
Solenoid ValveSolenoid ValveA Magnetic effect of an electric current operates the valve.
Attached to the valve is a soft iron rod ( an armature ) free toslide within a brass tube. When the electric current is
switched on the magnetic effect lifts the armature and so opens
the gas valve. A return spring positively keeps the valveclosed when no current is passing.
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How it worksHow it worksMust be
connected to
mains electricity.
Normally can be
found as part ofa multi- function
valve, or in
cooker ovens.
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Solenoid on the RigSolenoid on the Rig
Solenoid integrated as part of
a multi-function gas valve.
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Faults and RemediesFaults and Remedies The solenoid can be prone to hum
The slug seizes up and will not open
1. It is possible to clean the iron slug, and generally
service the valve.2. Replacement of the solenoid valve head or
complete unit.
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True or FalseTrue or False
A solenoid valve is a directA solenoid valve is a directcontrol device?control device?
What type of device is a solenoidWhat type of device is a solenoid
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valve ?valve ?
A:A: Comfort control deviceComfort control device
B:B: Flame failure deviceFlame failure deviceC:C: Ignition deviceIgnition device
D:D: Isolation deviceIsolation device
No.9No.9 Mechanical ThermostatMechanical Thermostat
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These devices are designed to control pre-determinedtemperatures of gas fired equipment by controlling
the flow of gas to the burner.
The methods used to operate thermostats are:
differential expansion of metals liquid expansion
vapour pressure
These methods can be used to control gas valves in
the gas supply to the burner by:
Direct acting or Indirect acting
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How it worksHow it works Rod TypeRod TypeA temperature sensitive device consists of a rod of
nickel-steel alloy inside a tube made of brass. The
two metals are brazed together at one end. Whensubjected to heat the expansion of the tube will draw
the rod into the tube. A valve and spindle is held to
the end of the rod by a spring. On heating the valvecloses and on cooling the valve is forced open.
On the RigOn the Rig
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Liquid Expansion ThermostatsLiquid Expansion Thermostats
Vapour Pressure ThermostatsVapour Pressure Thermostats
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Faults and RemediesFaults and Remedies Phial requires correct positioning. These may
be displaced or distorted by the user when
cleaning the appliance.
Calibration may get out of adjustment and need
resetting.
If any damage is caused to the liquid system a
complete renewal is necessary.
No 10No 10
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No.10No.10
Electrical ThermostatElectrical ThermostatThe electric thermostat is merely an on/off switch
controlled by temperature through the capillary tube.
The capillary tube is a hollow, sealed tube filled with aliquid that expands and contracts with changes in
temperature. This movement of the liquid moves a
bellows, which in turn activates a spring loaded on/offswitch. Electrical thermostats are used in conjunction
with solenoid valves.
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Electric ThermostatElectric Thermostat
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On the RigOn the Rig
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Faults and RemediesFaults and Remedies Phial requires correct positioning. These may
be displaced or distorted by the user when
cleaning the appliance. Calibration may get out of adjustment and need
resetting.
If any damage is caused to the liquid system acomplete renewal is necessary.
Micro switch may fail
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True or FalseTrue or False
A Mechanical Thermostat is anA Mechanical Thermostat is anindirect control device?indirect control device?
No.11No.11
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Relay ValveRelay ValveA relay valve controls gas supply using a remote control device ,
usually a mechanical thermostat. It is a pressure operated valve
controlled by a diaphragm. When the thermostat calls for heat,
gas flows through a weep orifice at a given rate into a weep
tube, this gas is then burnt off at the main burner.
Greater pressure below the diaphragm causes it to lift, allowinggas to flow to the appliance burner. When the thermostat is
closes, the flow through the orifice stops and the pressure above
the diaphragm builds up, stopping the gas flow to the burner.
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Relay Valve on the RigRelay Valve on the Rig
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F lt d R diF lt d R di
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Faults and RemediesFaults and RemediesThe valve depends on the flow of gas through the orifice creating apressure loss above the diaphragm, so it will open, and on the
pressure building up in the weep tube and above the diaphragm so
it will close. Therefore, a blocked weep tube will cause the valveto remain closed. The blockage may be in the tube, thermostat or
at the burner.
A leak in the sweep tube will prevent the valve from closing as this
simulates the thermostat calling for heat. A blocked orifice will
cause the valve to stay open. It Fails to Danger
Relay Valves have not been used in appliance for almost 30 years
now. Recommendation would be to replace appliance, even if
device appears to be working correctly.
No.12No.12
G ( R l t )G ( R l t )
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Governor ( Regulator )Governor ( Regulator )This device controls the pressure of gas to an appliance/s.
A weighted or sprung-loaded diaphragm moves to keep the gas
pressure to the system / appliance constant.
Essential parts of a meter regulatorEssential parts of a meter regulator
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CutawayCutaway
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O th RigO th Rig
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On the RigOn the Rig
Faults and RemediesFaults and Remedies
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Faults and RemediesFaults and Remedies Diaphragm can leak giving incorrect pressure:-
Replace Diaphragm
Gas valve can stick to seating:-
Clean valve seating
Always remember to check outlet pressure!
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No.13No.13
Multifunction ValveMultifunction Valve
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Multifunction ValveMultifunction Valve A multifunction device can incorporate many if
not all, of the controls necessary to operate a gasappliance in a single compact unit. This willnormally include gas flow control, gas pressureregulation, flame protection, temperature control
and burner ignition. They will normally be acomponent part of an appliance, but may allow forseparate parts of the valve to be changed asnecessary by a gas operative. Gas modulation canalso be a function of this type of valve, which iscommon on the majority of combination boilers.
Typical multifunction gas valveTypical multifunction gas valve
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Typical multifunction gas valveTypical multifunction gas valve
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On the RigOn the Rig
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Faults and RemediesFaults and Remedies
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Faults and RemediesFaults and Remedies When working on the appliance, always ensure
that any flame protection device operates withinthe maximum time period permissible. Any gascontrol tap should have smooth operation.Buzzing may indicate a faulty solenoid. Any gas
carrying joints that are disturbed should be testedwith leak detection fluid.
Refer to manufactures instructions forreplacement part numbers.
No.14No.14 Zero GovernorZero GovernorBasic Principle of Operation
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Functions according to the principle between the force of:
-the adjustable setpoint spring
- the defined counterspring
- the differential pressure at the working diaphragm based on zero atmosphere.- the air impulse pressure
- the force due to the weight of the moving parts
The counter spring acts against the setpoint spring and the weight due to the
force of the moving parts. Depending on the adjustment of the setpoint spring andthe installation position, the force of the counterspring is compensated. When
used as proportionator, adjusting the setpoint spring or biasing for excess
air/excess gas should be performed at low fire. Outlet gas pressure from the FRG
is directly proportionate to the air impulse line pressure. I.e. increasing the air
impulse line pressure increases the outlet gas pressure.In zero governor applications biasing (adjustment of the the setpoint spring leads
to negative or positive regulatory output pressures. Excessive adjustment of the
setpoint spring leads to positive regulator output pressures.
1:1 Ideal Ratio1:1 Ideal Ratio
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Sectional DiagramSectional Diagram
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Sectional Diagram
How it worksHow it works
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How it worksThe fan draws air in through the venturi which is
mounted on the side of the fan housing. This creates a
suction on the outlet of the 1:1 gas valve attached to
the venturi. When the gas valve is energised gas is
drawn out of the gas valve. With a 1:1 gas valve the gas
flow is proportional to the suction generated by the fan.This means that the gas valve can be adjusted so it is
inherently safe, this adjustment is called the offset
adjustment.
No Air Flow = No Gas Flow
Offset AdjustmentOffset Adjustment
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Offset AdjustmentOffset adjustment is achieved as a function of the zero
governor fitted to the gas valve. The offset is easiest
described as CO2 versus versus fan speed. A positive
offset adjustment would give an increasing CO2 level as
the fan speed is reduced and the gas valve would
continue to pass gas with a zero air flow ( Potentiallyhigh CO). A negative offset would give a reducing CO2 as
the fan speed falls and the gas valve would stop passing
gas before zero air flow( Inherently safe).
On the RigOn the Rig
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Faults and RemediesFaults and Remedies
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Faults and Remedies Incorrect adjustment of the Offset
Incorrect adjustment of the Throttle Failed system component
Adjust Offset and/or Throttle, useGas Flue Analyser.
Replace failed component.
Please refer to CITBPlease refer to CITB
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Please refer to CITBPlease refer to CITB
Domestic Natural Gas Safety,Domestic Natural Gas Safety,
Section 4, for further informationSection 4, for further information
regarding control devices.regarding control devices.
ENDEND
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Last Update November 2003 V 1.1