27 Questions
What is the main limitation of using thermocouples as flame safeguards on boilers?
Slow response time to flame failure
Which of the following types of burners are thermocouple flame safeguards limited to?
Small gas burners with continuous pilots
What are the two dissimilar wires in a thermocouple welded together to form?
Measuring junction
Which part of a thermocouple circuit is connected to the opposite ends of the two dissimilar wires?
Reference junction
What is the function of the copper outer sheath in a thermocouple designed for flame monitoring?
Acts as one of the two conductors
Why are thermocouples found mainly on small gas burners with continuous pilots?
To minimize the risk of unburned fuel entering the furnace
What is the function of the flame relay in the burner management system?
Amplifying the signal from the flame unit and providing power to keep the safety shut-off valve open
Why are flame rods used in burners over 120 kW input?
They offer fast flame failure response time
How does a photoelectric cell (flame scanner) react to the flame?
It reacts to infrared, ultraviolet, or visible light emitted by fire
What happens to the resistance of a lead-sulfide cell in an infrared scanner when it receives more infrared light?
It decreases
Why are ultraviolet (UV) scanners mounted as close as possible to the flame?
To minimize interference from igniter sparks
What is the purpose of a rectifying photocell (cad cell) in a burner management system?
Emitting electrons that are picked up by the anode, creating a signal that closes the contact points of a flame relay
In what instances are flame rods commonly used?
In burners over 120 kW input
What is the difference between a rectifying photocell and a lead-sulfide cell scanner?
The rectifying photocell is mounted inside combustion air supply tube while a lead-sulfide cell scanner is mounted on a sighting tube.
How does a lead-sulfide cell scanner respond to flame pulsations?
It causes a fluctuating voltage (called the flame signal), which is amplified by an electronic amplifier to hold the flame relay in closed position.
What generates a small voltage in a thermocouple?
The exposure to a flame
What energizes the main Safety Shut-Off Valve (SSOV) in the presence of a pilot flame?
The external low voltage power supply
What type of valve is referred to as a combination gas valve?
Safety Shut-Off Valve (SSOV)
What is the purpose of an external pilotstat that provides 100% shutoff?
To interrupt the main flame only
What is one advantage of a millivolt system for small residential boilers?
Independence from a transformer for power supply
What is necessary for an SSOV to operate in a millivolt system?
Multiple thermocouples in series
What is the principle behind a flame rod's operation as a flame detection device?
Rectification of alternating current by the burner and flame rod
What happens if the flame rod is accidentally grounded to the burner?
The burner monitoring circuit accepts the AC signal and shuts off the SSOVs
What is the purpose of a limit switch in a boiler's SSOV circuit?
To prevent the burner from operating under specific conditions
What does a thermopile develop that allows it to operate an SSOV independently from an external power source?
Sufficient voltage
How does a millivolt system contribute to small residential boilers during a power failure?
It ensures continuous operation during a power failure
What happens when AC is converted to DC in a flame rod system?
The process is called rectification
This quiz covers the construction and operation of flame detectors in the context of boiler flame failure detection. It discusses various methods such as thermocouples, flame rods, thermopiles, and photoelectric cells commonly used in this application.
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