4BE3.5 U3 C2 Obj 3 Easy Medium Hard

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EASY What is the main limitation of using thermocouples for flame failure detection?

Slow response time

What is the maximum boiler input size for which thermocouples can be used for flame failure detection?

120 kW

What is the purpose of the copper outer sheath in a thermocouple?

To act as a conductor

What is the typical voltage output of a thermocouple when exposed to a flame?

20-30 millivolts Signup and view all the answers

What is the purpose of the pilotstat in a thermocouple circuit?

To energize the main SSOV Signup and view all the answers

What is the minimum voltage required to keep the SSOV open?

7 millivolts Signup and view all the answers

What is the main component that makes up a thermocouple?

Two dissimilar wires Signup and view all the answers

What is the function of the cold junction in a thermocouple circuit?

To provide a reference point Signup and view all the answers

In a basic flame rod system, what happens when the flame is present?

The rod, burner, and flame close the flame relay circuit. Signup and view all the answers

What can cause grounding between the flame rod and the burner?

Soot bridging, cracked flame rod insulator, or warped flame rod. Signup and view all the answers

What is the advantage of using flame rods in burners over 120 kW input?

Fast flame failure response time. Signup and view all the answers

What type of photocell is shown in Figure 15?

Infrared scanner. Signup and view all the answers

What happens when a flame failure occurs in an infrared scanner system?

The flame relay opens. Signup and view all the answers

What is the purpose of the threaded ring in a photoelectric cell?

To secure the scanner on the sight tube. Signup and view all the answers

What is the maximum temperature allowed for a lead sulfide scanner?

50°C. Signup and view all the answers

Why are flame rods not commonly used with oil-fired burners?

The rod can become coated with carbon deposits, causing soot bridging. Signup and view all the answers

What is the purpose of the scanner sight tube in a firetube boiler burner assembly?

To observe the pilot and the main burner Signup and view all the answers

What type of rays does the ultraviolet (UV) scanner respond to?

Ultraviolet rays Signup and view all the answers

What is the function of the burner management system (BMS) in automatically fired boilers?

To sequence the startup and shutdown of the firing equipment. Signup and view all the answers

What is the purpose of the flame relay in a photoelectric safeguard system?

To control the power supply to the main SSOV. Signup and view all the answers

Why is it important to ensure the UV scanner cannot see the igniter spark?

To prevent false flame signals Signup and view all the answers

What is the function of the rectifying photocell in a burner management system?

To control the main safety shut-off valve Signup and view all the answers

Why did the programming controller open the main safety shut-off valves in the case study?

Because the scanner detected a spark signal Signup and view all the answers

What is the advantage of mounting the rectifying photocell inside the combustion air supply tube?

It keeps the cell relatively cool and clean Signup and view all the answers

What is the purpose of the pilot burner in a firetube boiler burner assembly?

To keep the burner circuit energized Signup and view all the answers

What is the typical operating temperature range for a UV scanner?

Below 100°C Signup and view all the answers

What is a combination gas valve?

A valve body that combines a gas pressure regulator, safety shut-off valve, and manual gas valve Signup and view all the answers

What is the purpose of an external pilotstat?

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

What is required to relight the pilot?

Closing the main manual shutoff valve and depressing the pilotstat manual reset button Signup and view all the answers

What is the function of a thermocouple?

To detect the pilot and main flames Signup and view all the answers

What is a thermopile?

Multiple thermocouples in series to develop enough voltage to operate an SSOV Signup and view all the answers

What is the advantage of a millivolt system?

It can function even during a power failure Signup and view all the answers

What is the function of a flame rod?

To detect the pilot and main flames Signup and view all the answers

What is the principle of a flame rod?

That flames can conduct electricity Signup and view all the answers

Why does the AC get rectified to pulsating DC in a flame rod circuit?

Because the flame rod and burner have very different surface areas Signup and view all the answers

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

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

MEDIUM What is the reason why the UV scanner should not be able to see the igniter spark?

To prevent the scanner from mistaking the spark for a pilot flame Signup and view all the answers

What is the purpose of the sight tube in a firetube boiler burner assembly?

To allow the scanner to observe the pilot and main burner Signup and view all the answers

What is the function of the rectifying photocell in a burner management system?

To detect the presence of a flame Signup and view all the answers

What is the advantage of mounting the rectifying photocell inside the combustion air supply tube?

It is kept relatively cool and clean Signup and view all the answers

What was the root cause of the problem in the case study?

The new sight glass was too shiny and reflected the igniter spark Signup and view all the answers

What is the principle of operation of the UV scanner?

It responds to ultraviolet rays emitted by the flame Signup and view all the answers

What is the function of the pilot burner in a firetube boiler burner assembly?

To provide a stable flame for the main burner Signup and view all the answers

What happened when the programming controller opened the main safety shut-off valves in the case study?

The boiler was shut down due to a safety fault Signup and view all the answers

What is the primary direction of electron flow in a flame rod system?

From the burner to the flame rod Signup and view all the answers

What is the primary reason why thermocouples are limited to small gas burners?

They have a slow flame failure response time. Signup and view all the answers

What is the purpose of the insulated inner wire in a thermocouple?

To prevent the inner wire from contacting the copper sheath. Signup and view all the answers

What is the purpose of the relay in a basic flame rod system?

To amplify the signal from the flame unit Signup and view all the answers

What happens when the hot junction of a thermocouple is exposed to a flame?

The voltage output of the thermocouple increases. Signup and view all the answers

What can cause soot bridging between the flame rod and the burner?

All of the above Signup and view all the answers

What is the function of the relay in a thermocouple circuit?

To energize the main safety shut-off valve. Signup and view all the answers

What type of photocell responds to infrared, ultraviolet, or visible light emitted by fire?

Photocell Signup and view all the answers

What is the main component of a thermocouple that is exposed to the flame?

The hot junction. Signup and view all the answers

What is the effect of a flame failure on the resistance of the infrared scanner cell?

The resistance increases Signup and view all the answers

What is the purpose of the sighting tube in a photoelectric cell installation?

To aim the scanner at the flame Signup and view all the answers

Why is it important to ensure the thermocouple voltage drops below a certain level before the safety shut-off valve closes?

To prevent unburned fuel from entering the furnace. Signup and view all the answers

What is the purpose of the pilot burner assembly in a firetube boiler?

To provide a continuous pilot flame. Signup and view all the answers

Why are flame rods not commonly used with oil-fired burners?

Because they can become coated with carbon deposits Signup and view all the answers

What is the advantage of using thermocouples for flame failure detection in small gas burners?

They are simple to install and operate. Signup and view all the answers

What is the function of the burner management system (BMS) in automatically fired boilers?

To sequence the startup and shutdown of the firing equipment Signup and view all the answers

What is the effect of grounding between the flame rod and the burner on the burner monitoring circuit?

The circuit does not accept the AC signal and shuts off the SSOV Signup and view all the answers

What is the advantage of using flame rods in burners over 120 kW input?

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

What is the purpose of a combination gas valve?

To combine the functions of a gas pressure regulator, safety shut-off valve, and manual gas valve Signup and view all the answers

What happens when a pilot flame fails in a system with an external pilotstat that provides 100% shutoff?

The main SSOV is closed and the gas supply is interrupted Signup and view all the answers

What is the purpose of a thermopile?

To generate a high voltage to operate an SSOV Signup and view all the answers

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

To prevent the burner from operating if a problem is detected Signup and view all the answers

What is the principle of a flame rod?

The flame conducts electricity between two electrodes Signup and view all the answers

Why does the AC get rectified to pulsating DC in a flame rod circuit?

Because the flame rod has very different surface areas from the burner Signup and view all the answers

What is the advantage of a millivolt system?

It operates without external power during a power failure Signup and view all the answers

What is required to relight the pilot?

Closing the main manual shutoff valve, opening the pilot gas valve, providing a source of ignition, and depressing the pilotstat manual reset button Signup and view all the answers

What is the function of a thermocouple?

To convert heat energy into electrical energy Signup and view all the answers

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

To interrupt the gas supply to the pilot and main burners Signup and view all the answers

What is the reason why thermocouples have a slow flame failure response time?

The thermocouple has to cool down sufficiently to close a safety shut-off valve Signup and view all the answers

What is the purpose of the insulated inner wire in a thermocouple?

It prevents the inner wire from contacting the copper outer sheath Signup and view all the answers

What happens when the hot junction of a thermocouple is exposed to a flame?

It generates a small voltage capable of causing a small current to flow Signup and view all the answers

Why are thermocouples limited to small gas burners?

They have a slow flame failure response time Signup and view all the answers

What is the function of the pilotstat in a thermocouple circuit?

To energize the SSOV when the thermocouple voltage is high enough Signup and view all the answers

What is the purpose of the external low voltage power supply in a thermocouple circuit?

To power the pilotstat Signup and view all the answers

What happens when the thermocouple voltage drops below about 7 millivolts?

The SSOV closes Signup and view all the answers

What is the purpose of the copper outer sheath in a thermocouple?

It acts as one of the two conductors in the thermocouple circuit Signup and view all the answers

What is the primary reason why the UV scanner in the case study detected a flame signal of 3.5 volts?

The igniter spark reflected off the new sight glass Signup and view all the answers

What is the purpose of the curved metal cathode in a rectifying photocell?

To emit electrons when exposed to visible light Signup and view all the answers

Why is it important to mount the UV scanner close to the flame, but not beyond 100°C?

To prevent the scanner from being damaged by high temperatures Signup and view all the answers

What is the primary function of the burner management system in automatically fired boilers?

To monitor the flame and control the safety shut-off valves Signup and view all the answers

What is the advantage of mounting the rectifying photocell inside the combustion air supply tube?

It keeps the photocell relatively cool and clean Signup and view all the answers

What is the primary reason why the programming controller opened the main safety shut-off valves in the case study?

The UV scanner detected a false flame signal Signup and view all the answers

What is the primary function of the flame relay in a photoelectric safeguard system?

To keep the main safety shut-off valve energized and open Signup and view all the answers

HARD What is the primary reason why the UV scanner should not be able to see the igniter spark?

The igniter spark can cause a false flame signal Signup and view all the answers

What is the primary function of the flame relay in a flame rod system?

To amplify the signal from the flame unit Signup and view all the answers

What is the reason why flame rods are not commonly used with oil-fired burners?

Because they can become coated with carbon deposits, causing soot bridging Signup and view all the answers

What is the primary difference between a flame rod system and a photoelectric cell system?

The type of radiation detected Signup and view all the answers

What is the purpose of the sighting tube in a photoelectric cell system?

To ensure the scanner is aimed at the flame Signup and view all the answers

What happens to the electrical resistance of the lead-sulfide cell in an infrared scanner when it is exposed to infrared radiation?

It decreases Signup and view all the answers

What is the primary function of the burner management system (BMS) in automatically fired boilers?

To sequence the startup and shutdown of the firing equipment Signup and view all the answers

What is the primary reason why the flame rod system is more suitable for high-input boilers?

Because it offers faster flame failure response time Signup and view all the answers

What is the primary function of the threaded ring in a photoelectric cell system?

To secure the scanner to the sighting tube Signup and view all the answers

What happens when the flame relay opens in a flame rod system?

The power supply to the SSOV is cut off Signup and view all the answers

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

The type of radiation detected Signup and view all the answers

What is the primary function of a combination gas valve?

To combine the pilotstat with the manual gas valve Signup and view all the answers

What is the purpose of a thermopile?

To operate an SSOV independently of external power Signup and view all the answers

What is the primary advantage of a millivolt system?

It can operate without external power Signup and view all the answers

What is the purpose of a flame rod?

To conduct electricity between the flame rod and the burner Signup and view all the answers

What is the principle of operation of a flame rod?

Flame rods operate on the principle of electrical conduction Signup and view all the answers

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

To prevent the main SSOV from opening if a fault is detected Signup and view all the answers

What is the reason why the AC gets rectified to pulsating DC in a flame rod circuit?

Because the burner and flame rod have very different surface areas Signup and view all the answers

What is the purpose of an external pilotstat?

To detect the pilot flame and interrupt the main SSOV circuit Signup and view all the answers

What is the function of a thermocouple?

To convert heat energy into electrical energy Signup and view all the answers

What is the purpose of relighting the pilot?

To reset the pilotstat after a flame failure Signup and view all the answers

Study Notes

Flame Failure Detection Methods

Various methods are used to detect flame failure in boilers, including:
Thermocouples
Thermopiles
Flame rods
Photoelectric cells

Thermocouples

A thermocouple consists of two dissimilar wires welded together at one end to form a measuring (or “hot”) junction.
The opposite ends are connected to an electric circuit, called the reference (or “cold”) junction.
When the hot junction is exposed to a flame, its temperature increases above that of the cold junction, generating a small voltage (about 20 to 30 millivolts).
This voltage energizes a sensitive relay, called a pilotstat, which may be built into an SSOV (Safety Shut-Off Valve).
Thermocouples are limited to pilot burners up to 120 kW input due to their slow flame failure response time (up to 90 seconds).

Thermopiles

A thermopile is a flame detector that develops sufficient voltage to operate an SSOV independently from any external power source.
It consists of multiple thermocouples in series, generating around 750 millivolts.
Thermopiles are also limited to burners not exceeding 120 kW input.
They offer the advantage of operating without external power, making them suitable for small residential boilers.

Flame Rods

A flame rod is a flame detection device that operates on the principle that flames can conduct electricity.
It consists of two electrodes (the flame rod and the burner) and the flame, which conducts electricity between them.
The flame rod is a heat-resistant, small diameter metal rod, supported by a ceramic holder that keeps the rod electrically insulated from the burner.
Flame rods are commonly used with gas burners and are suitable for burners over 120 kW input due to their fast flame failure response time.

Photoelectric Cells

A photoelectric cell is a device that reacts to infrared, ultraviolet, or visible light emitted by fire.
Common types of photocells include:
Infrared scanner
Ultraviolet scanner
Rectifying photocell
A scanner is mounted on the boiler in such a way that it can observe the flame, and its resistance decreases with an increase in the amount of infrared light it receives from the flame.
When a flame failure occurs, the scanner does not sense a flame, and its resistance increases, causing the flame relay to open, which shuts off the fuel to the burner.

Case Study

A case study highlights the importance of proper installation and maintenance of flame safeguard controls to avoid false readings and potential explosions.

Flame Failure Detection Methods

Various methods are used to detect flame failure in boilers, including:
Thermocouples
Thermopiles
Flame rods
Photoelectric cells

Thermocouples

A thermocouple consists of two dissimilar wires welded together at one end to form a measuring (or “hot”) junction.
The opposite ends are connected to an electric circuit, called the reference (or “cold”) junction.
When the hot junction is exposed to a flame, its temperature increases above that of the cold junction, generating a small voltage (about 20 to 30 millivolts).
This voltage energizes a sensitive relay, called a pilotstat, which may be built into an SSOV (Safety Shut-Off Valve).
Thermocouples are limited to pilot burners up to 120 kW input due to their slow flame failure response time (up to 90 seconds).

Thermopiles

A thermopile is a flame detector that develops sufficient voltage to operate an SSOV independently from any external power source.
It consists of multiple thermocouples in series, generating around 750 millivolts.
Thermopiles are also limited to burners not exceeding 120 kW input.
They offer the advantage of operating without external power, making them suitable for small residential boilers.

Flame Rods

A flame rod is a flame detection device that operates on the principle that flames can conduct electricity.
It consists of two electrodes (the flame rod and the burner) and the flame, which conducts electricity between them.
The flame rod is a heat-resistant, small diameter metal rod, supported by a ceramic holder that keeps the rod electrically insulated from the burner.
Flame rods are commonly used with gas burners and are suitable for burners over 120 kW input due to their fast flame failure response time.

Photoelectric Cells

A photoelectric cell is a device that reacts to infrared, ultraviolet, or visible light emitted by fire.
Common types of photocells include:
Infrared scanner
Ultraviolet scanner
Rectifying photocell
A scanner is mounted on the boiler in such a way that it can observe the flame, and its resistance decreases with an increase in the amount of infrared light it receives from the flame.
When a flame failure occurs, the scanner does not sense a flame, and its resistance increases, causing the flame relay to open, which shuts off the fuel to the burner.

Case Study

A case study highlights the importance of proper installation and maintenance of flame safeguard controls to avoid false readings and potential explosions.

Flame Failure Detection Methods

Various methods are used to detect flame failure in boilers, including:
Thermocouples
Thermopiles
Flame rods
Photoelectric cells

Thermocouples

A thermocouple consists of two dissimilar wires welded together at one end to form a measuring (or “hot”) junction.
The opposite ends are connected to an electric circuit, called the reference (or “cold”) junction.
When the hot junction is exposed to a flame, its temperature increases above that of the cold junction, generating a small voltage (about 20 to 30 millivolts).
This voltage energizes a sensitive relay, called a pilotstat, which may be built into an SSOV (Safety Shut-Off Valve).
Thermocouples are limited to pilot burners up to 120 kW input due to their slow flame failure response time (up to 90 seconds).

Thermopiles

A thermopile is a flame detector that develops sufficient voltage to operate an SSOV independently from any external power source.
It consists of multiple thermocouples in series, generating around 750 millivolts.
Thermopiles are also limited to burners not exceeding 120 kW input.
They offer the advantage of operating without external power, making them suitable for small residential boilers.

Flame Rods

A flame rod is a flame detection device that operates on the principle that flames can conduct electricity.
It consists of two electrodes (the flame rod and the burner) and the flame, which conducts electricity between them.
The flame rod is a heat-resistant, small diameter metal rod, supported by a ceramic holder that keeps the rod electrically insulated from the burner.
Flame rods are commonly used with gas burners and are suitable for burners over 120 kW input due to their fast flame failure response time.

Photoelectric Cells

A photoelectric cell is a device that reacts to infrared, ultraviolet, or visible light emitted by fire.
Common types of photocells include:
Infrared scanner
Ultraviolet scanner
Rectifying photocell
A scanner is mounted on the boiler in such a way that it can observe the flame, and its resistance decreases with an increase in the amount of infrared light it receives from the flame.
When a flame failure occurs, the scanner does not sense a flame, and its resistance increases, causing the flame relay to open, which shuts off the fuel to the burner.

Case Study

A case study highlights the importance of proper installation and maintenance of flame safeguard controls to avoid false readings and potential explosions.

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