Boiler Flame Failure Detectors: Construction and Operation

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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?

Reference junction Signup and view all the answers

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

Acts as one of the two conductors Signup and view all the answers

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

To minimize the risk of unburned fuel entering the furnace Signup and view all the answers

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 Signup and view all the answers

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

They offer fast flame failure response time Signup and view all the answers

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

It reacts to infrared, ultraviolet, or visible light emitted by fire 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?

It decreases Signup and view all the answers

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

To minimize interference from igniter sparks Signup and view all the answers

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 Signup and view all the answers

In what instances are flame rods commonly used?

In burners over 120 kW input Signup and view all the answers

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. Signup and view all the answers

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. Signup and view all the answers

What generates a small voltage in a thermocouple?

The exposure to a flame Signup and view all the answers

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

The external low voltage power supply Signup and view all the answers

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

Safety Shut-Off Valve (SSOV) Signup and view all the answers

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

To interrupt the main flame only Signup and view all the answers

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

Independence from a transformer for power supply Signup and view all the answers

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

Multiple thermocouples in series Signup and view all the answers

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 Signup and view all the answers

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 Signup and view all the answers

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

To prevent the burner from operating under specific conditions Signup and view all the answers

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

Sufficient voltage Signup and view all the answers

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

It ensures continuous operation during a power failure Signup and view all the answers

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

The process is called rectification Signup and view all the answers