Boiler Flame Failure Detectors: Construction and Operation
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Questions and Answers

What is the main limitation of using thermocouples as flame safeguards on boilers?

  • Slow response time to flame failure (correct)
  • High cost of installation
  • Limited heat resistance
  • Incompatibility with gas burners

    • Which of the following types of burners are thermocouple flame safeguards limited to?

  • Large gas burners with intermittent pilots
  • Electric burners with electric ignition
  • Small gas burners with continuous pilots (correct)
  • Oil burners with intermittent pilots

    • What are the two dissimilar wires in a thermocouple welded together to form?

  • Hot junction
  • Measuring junction (correct)
  • Reference junction
  • Cold junction

    • Which part of a thermocouple circuit is connected to the opposite ends of the two dissimilar wires?

    <p>Reference junction</p> Signup and view all the answers

    What is the function of the copper outer sheath in a thermocouple designed for flame monitoring?

    <p>Acts as one of the two conductors</p> Signup and view all the answers

    Why are thermocouples found mainly on small gas burners with continuous pilots?

    <p>To minimize the risk of unburned fuel entering the furnace</p> Signup and view all the answers

    What is the function of the flame relay in the burner management system?

    <p>Amplifying the signal from the flame unit and providing power to keep the safety shut-off valve open</p> Signup and view all the answers

    Why are flame rods used in burners over 120 kW input?

    <p>They offer fast flame failure response time</p> Signup and view all the answers

    How does a photoelectric cell (flame scanner) react to the flame?

    <p>It reacts to infrared, ultraviolet, or visible light emitted by fire</p> Signup and view all the answers

    What happens to the resistance of a lead-sulfide cell in an infrared scanner when it receives more infrared light?

    <p>It decreases</p> Signup and view all the answers

    Why are ultraviolet (UV) scanners mounted as close as possible to the flame?

    <p>To minimize interference from igniter sparks</p> Signup and view all the answers

    What is the purpose of a rectifying photocell (cad cell) in a burner management system?

    <p>Emitting electrons that are picked up by the anode, creating a signal that closes the contact points of a flame relay</p> Signup and view all the answers

    In what instances are flame rods commonly used?

    <p>In burners over 120 kW input</p> Signup and view all the answers

    What is the difference between a rectifying photocell and a lead-sulfide cell scanner?

    <p>The rectifying photocell is mounted inside combustion air supply tube while a lead-sulfide cell scanner is mounted on a sighting tube.</p> Signup and view all the answers

    How does a lead-sulfide cell scanner respond to flame pulsations?

    <p>It causes a fluctuating voltage (called the flame signal), which is amplified by an electronic amplifier to hold the flame relay in closed position.</p> Signup and view all the answers

    What generates a small voltage in a thermocouple?

    <p>The exposure to a flame</p> Signup and view all the answers

    What energizes the main Safety Shut-Off Valve (SSOV) in the presence of a pilot flame?

    <p>The external low voltage power supply</p> Signup and view all the answers

    What type of valve is referred to as a combination gas valve?

    <p>Safety Shut-Off Valve (SSOV)</p> Signup and view all the answers

    What is the purpose of an external pilotstat that provides 100% shutoff?

    <p>To interrupt the main flame only</p> Signup and view all the answers

    What is one advantage of a millivolt system for small residential boilers?

    <p>Independence from a transformer for power supply</p> Signup and view all the answers

    What is necessary for an SSOV to operate in a millivolt system?

    <p>Multiple thermocouples in series</p> Signup and view all the answers

    What is the principle behind a flame rod's operation as a flame detection device?

    <p>Rectification of alternating current by the burner and flame rod</p> Signup and view all the answers

    What happens if the flame rod is accidentally grounded to the burner?

    <p>The burner monitoring circuit accepts the AC signal and shuts off the SSOVs</p> Signup and view all the answers

    What is the purpose of a limit switch in a boiler's SSOV circuit?

    <p>To prevent the burner from operating under specific conditions</p> Signup and view all the answers

    What does a thermopile develop that allows it to operate an SSOV independently from an external power source?

    <p>Sufficient voltage</p> Signup and view all the answers

    How does a millivolt system contribute to small residential boilers during a power failure?

    <p>It ensures continuous operation during a power failure</p> Signup and view all the answers

    What happens when AC is converted to DC in a flame rod system?

    <p>The process is called rectification</p> Signup and view all the answers

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