Introduction

A flame sensor, also known as a flame detector, is a sensor designed to detect and respond to the presence of a flame or fire. It serves as a critical safety and control component across many industrial and residential applications that use a flame, allowing system activation or emergency shutdown if unsafe conditions are sensed.

This article provides a comprehensive overview of flame sensor working principles, types, applications, selection factors, installation considerations, and usage in modern safety control systems.

Flame Sensor Working Principles

Flame sensors operate on different physical principles to detect the light, heat or combustion products from a flame and provide a corresponding electrical signal. Here are the main detection methods used:

Ultraviolet (UV) Flame Detection

Flames emit light in the ultraviolet spectrum which is invisible. A UV flame detector senses this UV radiation:

• Sensor contains a UV sensitive photocell to convert UV into electric current
• UV is focused onto the photocell using a lens and filter
• Output current indicates strength of UV, signaling a flame
• False triggers avoided by ignoring visible/IR

Provides fast response and high sensitivity but can be affected by arc welding and sunlight.

Infrared (IR) Flame Detection

Flames emit wideband infrared radiation. An IR flame sensor measures IR:

• Contains an IR sensitive photodiode or pyroelectric cell
• Optical bandpass filter passes IR radiation to the sensor
• Variable electric signal indicates the IR intensity
• Fast reacting and low cost but may false trigger on hot objects

Ultraviolet/Infrared (UV/IR) Dual Sensing

Combines UV and IR sensors:

• Independent UV and IR sensing channels
• Outputs combined through logic circuits
• Triggers only when both UV and IR detected
• Avoid false triggers from individual sources
• More expensive but robust detection

Flame Ionization Detection

Uses two electrodes to detect ions generated by a hydrocarbon flame:

• Voltage applied across two metal electrodes in the flame path
• Flame combustion produces ions causing current flow
• Higher current indicates greater ionization from larger flame
• Very fast response to flame initiation

Prone to contamination and saturation effects.

Thermopile Infrared Sensing

Thermopile sensor measures infrared heat emitted by the flame:

• Multiple thermocouple junctions convert IR into voltage
• Changes in flame heat alter the output voltage
• No spectral filters needed, detects all IR wavelengths
• Withstands high temperatures, resistant to corrosion

Slow response time and lower range. Mainly used for fire detection.

Flame Rectification

Uses flame’s ability to conduct alternating current to detect combustion:

• Sensor consists of two isolated probes in flame
• AC voltage applied; current flows due to flame conduction
• Higher current indicates larger flame
• Quick responding and simple

Effective only for open flames and certain contaminants can affect readings.

Flame Sensor Types

Flame sensors are made in different forms suited for specific flame monitoring applications:

Infrared (IR) Point Flame Detectors

• Contain IR sensor aimed at a specific point in field of view
• Used to monitor flames in furnaces, boilers, gas trains
• Low cost, compact size but narrow monitoring area

Ultraviolet (UV) Point Flame Detectors

• Monitor a point flame source in UV spectrum
• High sensitivity with fast response time
• Applied in aircraft engine exhaust, industrial furnaces

UV/IR Point Flame Detectors

• Combination IR and UV sensors
• Look at a specific flame point
• Avoid false triggering from environment
• More expensive than IR or UV only

Infrared Line-of-Sight Flame Monitors

• IR sensor with linear field of view
• Detects flame anywhere along a line path
• Used to monitor large fuel storage tanks
• Cover wide areas from a single location

Flame Imaging Flame Detectors

• Contain multi-element IR sensors for flame spatial imaging
• Analyze flame image patterns to verify true flames
• Advanced false alarm immunity
• High cost limits major installations

Flame Scanners

• IR sensor scanned across coverage area using mirrors
• Sequentially monitor large angular regions
• No blind spots with complete coverage
• Complex mechanical construction

Applications of Flame Sensors

Flame sensors serve as a critical protection and control component in the following applications:

Industrial Furnaces and Boilers

• Monitor main burner flames
• Shut off fuel on flame failure to prevent unsafe accumulation

Gas Turbines in Power Plants

• Monitor combustion zone
• Improve efficiency by optimizing fuel-air ratio

Aircraft Engines

• Monitor jet engine exhaust flames
• Feedback for engine performance control

Fuel Storage Areas

• Detect leaks and fires in refineries, tank farms
• Emergency isolation of unsafe tanks

Chemical Processing Plants

• Monitor flames in crackers and reformers
• Prevent explosive conditions by shutting down fuel

Flare stacks

• Monitor presence and stability of waste gas flares
• Indicates proper combusting of hazardous gases

Vehicle Engines

• Monitor fuel combustion as part of engine management systems
• Provide feedback for optimum fuel efficiency

Fire and Smoke Detection Systems

• Detect fires and trigger suppression systems
• Close fire dampers in HVAC systems on smoke

Burner Management Systems

• Perform pre-ignition safety checks
• Monitor steady flame during operation
• Timely fuel cutoff on flameout

Residential and Commercial Furnaces

• Monitor pilot and main burner flames
• Prevent gas buildup on flame failures

Flame Sensor Selection Factors

Key considerations when selecting a flame sensor:

• Application – The flame type, fuel source, monitoring needs and environment guides selection.
• Detection method – Type that matches flame emission characteristics. IR best for general purpose.
• Sensitivity – Minimum flame intensity required for reliable detection.
• Response time – How fast it detects flame initiation or loss. Under 10 sec ideal.
• Detection range –Distance and angle from which it can monitor flames.
• False alarm resistance – Immunity to normal ambient variations that may falsely trigger.
• Operating temperature – Must withstand temperatures in monitored area.
• Approvals – Certified sensors satisfy legal and insurance requirements.
• Output – Digital, analog, fieldbus output options for interfaces.
• Diagnostics – Internal self-checks for sensor health reporting.
• Mechanical design – Housing, viewing angle, mounting provision etc.
• Cost – Higher detection performance costs more.

Proper flame sensor selection provides reliable detection and avoids nuisance shutdowns.

Flame Sensor Installation Tips

Optimal mounting location is necessary for proper flame sensor operation:

• Install in the combustion chamber for hottest view and quickest response to flame loss.
• For large furnaces, install multiple sensors to monitor primary and secondary combustion zones.
• Must have unobstructed line-of-sight of flame being monitored.
• Mount at an angle to allow any accumulated material to drain off instead of blocking view.
• Keep clean by avoiding areas with soot buildup or material condensation.
• Seal housing for harsh environments but allow ventilation to prevent internal fogging.
• Use fixed rigid conduit for wiring as vibration can break connections.
• Follow manufacturer’s guidelines on proper wiring, grounding and separation from electrical noise sources.

Proper installation position is key for reliable flame strength monitoring and loss detection.

Integrating Flame Sensors in Control Systems

Flame sensors serve as a critical component in burner management and fire protection systems:

Burner Control

• Performs pre-ignition safety checks to allow fuel flow
• Monitors pilot and main flame during firing
• Triggers safety shutdown sequence on flameout

Fire Detection and Suppression

• Detects fires and triggers alarms, suppression systems
• Closes fire dampers in ventilation systems
• Shuts off fuel lines to stop fuel supply to fire

Advanced automation systems integrate array of flame detectors with analytics software for large area monitoring.

Conclusion

Flame sensing is a crucial technique for combustion safety and efficiency in critical processes across industry and infrastructure. Flame sensor selection requires matching detection mechanism with the specific flame and environment characteristics. Proper sensor integration during initial design stages along with correct mounting placement enables reliable flame condition monitoring and emergency response. With robust flame detection technology ensuring safe operating conditions, process industries can reap the benefits of combustion and fuels without the associated hazards.

Frequently Asked Questions about Flame Sensors

Here are some common questions regarding flame sensors and their usage:

Q: How does a flame sensor detect fire?

A: It uses spectral, spatial or conductive properties of flames to sense UV radiation, infrared light, heat or ionized gases and signal a flame condition.

Q: What industries commonly use flame sensors?

A: Power generation, oil and gas, chemicals, aviation, furnace manufacturing, building infrastructure rely on flame sensors.

Q: What sensors offer the highest flame detection performance?

A: Flame detectors combining both ultraviolet and infrared sensing in one housing generally provide the highest detection sensitivity and fastest response with minimal false triggers.

Q: What is the typical detection range of flame detectors?

A: Point flame detectors have a range around 3-6 meters. Line-of-sight models can monitor fires up to 60 meters away. Imaging flame detectors operate up to 45 meters.

Q: How often should flame sensors be maintained?

A: Annual inspection and cleaning is recommended. Harsh environments with soot or salt deposition may require cleaning every 3 months.