L.B. White Pilot Light Ignition Installation And Service Manual

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L.B. White Pilot Light Ignition Installation And Service Manual Manual pdf 50 page image

Thermocouple and Power Unit TestsComponentTestingEquipment RRequired:Volt / Ohm Meter with DC ScaleThermocouple Test Kit, Part Number 500-08506The thermocouple is used in conjunction with the pilot safetycontrol valve in supplying pilot gas to the pilot orifice.Here iis HHow iit WWorks:A thermocouple converts heat energy into electrical energy.When the tip of the thermocouple is properly positioned inthe pilot flame, the temperature difference between the hotand cold junction generates a small amount of electricalenergy in the form of direct current (DC) which is fed into thespring-loaded power unit within the control valve. Thecurrent holds the power unit open to allow gas to be fed tothe pilot orifice.As long as the pilot gas stays lit and the pilot flame isdirected to the thermocouple, the thermocouple willcontinue to create the small amount of electrical energynecessary to keep the power unit open. The followingillustration shows the electrical circuit of the thermocouplewith its relationship to the pilot safety control valve.If the pilot flame is extinguished, or is too small to reliablyheat the thermocouple, the DC output being generateddrops below the minimum level required to hold the powerunit open. The spring within the power unit pushes down thesafety valve plunger, thereby discontinuing the flow of gas tothe pilot and main burner.If pilot outage problems are occurring, there are severalsteps that you must complete before assuming thethermocouple, or the pilot safety control valve are defective.These are as follows:■ Check inlet pressures to the control valve to make surethe pressure is within parameters specified on theheater’s dataplate.■ Clean the pilot assembly components, or if necessary,replace the pilot orifice if it is plugged.■ Insure that dirt is not located between thethermocouple’s power unit contact and power unitwithin the safety control valve.■ Insure that the thermocouple is properly positionedwithin the pilot burner bracket, that the pilot flameenvelopes the thermocouple’s tip and thethermocouple’s contact is snugged at the gas controlvalve. (Tighten finger tight plus 1/4 turn.)■ The pilot shield must be tight against the burnercasting.■ The pilot gasket (if applicable) is properly installedbetween pilot shield and casting.■ If these items are satisfactory, then proceed to checkboth the DC output strength of the thermocouple anddrop-out strength of the electromagnet within thepower unit of the pilot safety control valve.August 19996.3-11PILOTBURNERTHERMOCOUPLESAFETY VVALVEPLUNGERTHERMOCOUPLELEADHHOOTT JJUUNNCCTTIIOONNPPOOWWEERR UUNNIITTCCOONNNNEECCTTOORRCCOONNTTAACCTTTTHHEERRMMOOCCOOUUPPLLEECCOONNNNEECCTTOORR NNUUTTCCUURRRREENNTT CCOONNDDUUCCTTOORRSSIINNSSUULLAATTEEDD WWIIRREECCOOPPPPEERR TTUUBBIINNGGCCOOLLDDJJUUNNCCTTIIOONN OOPPTTIIOONNAALLAATTTTAACCHHMMEENNTTNNUUTTMMOOUUNNTTIINNGGCCLLIIPPPPOOWWEERRUUNNIITTCCOOIILL