gas safety devices controls 14 2006 vle (rigs)

8/11/2019 Gas Safety Devices Controls 14 2006 VLE (RIGS)

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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


4/110NVQ Level 2/3 Gas Installation and Maintenance.

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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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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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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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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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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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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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 multifunction

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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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.



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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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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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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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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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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).



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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

gas safety devices controls 14 2006 vle (rigs)

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