UV Photocells & Flame Sensors (QRA, QRB, RAR): How Burner Flame Detection Works
- Flame sensors act as safety relays, signaling fuel shutdowns within seconds if combustion is lost.
- QRB photocells detect visible yellow light, making them optimal for light diesel oil (LDO) installations.
- QRA series UV sensors detect ultraviolet rays, designed specifically for gas burners and dual-fuel burners.
- Weekly cleaning of sensor glass prevents soot accumulation that blocks light and causes false lockouts.
Flame sensors detect the presence of combustion. Photocells (like QRB1/QRB3) detect visible yellow light in oil burners. UV sensors (like QRA2/QRA53) detect ultraviolet radiation in gas or blue-flame burners. Ionization rods detect flame conductivity. If the sensor fails, the controller shuts off fuel flow immediately.
In the world of industrial burners, safety is paramount. If fuel is sprayed into a hot combustion chamber but does not ignite immediately, it will vaporize and can cause a severe explosion the moment a spark occurs.
To prevent this, burner controllers rely on Flame Sensors. These sensors continuously monitor whether a flame exists during the run cycle. If the flame goes out, the sensor signals the controller, which shuts off the fuel solenoid valves within 1 to 5 seconds.
Here is a guide to the three main types of flame detection technologies used in industrial plants.
1. Photoelectric Cells (Visible Light Sensors) - Common Models: Siemens QRB1, QRB3, Riello photocells, Danfoss LDS. - How they work: These sensors contain a light-sensitive resistor (LDR). When exposed to the bright yellow light of a burning oil flame, their resistance drops to a few hundred ohms. In the dark, their resistance rises to millions of ohms. - Application: Strictly for yellow-flame oil burners (Diesel/LDO). - Limitation: They cannot distinguish between a real burner flame and glowing red-hot combustion chamber bricks. Therefore, the controller checks for ambient light before starting (extraneous light test).
2. Ultra-Violet (UV) Sensors - Common Models: Siemens QRA2, QRA10, QRA53, Honeywell C7027A (Minipeeper). - How they work: A gas flame or clean-burning blue oil flame does not emit much visible light, but it emits strong Ultraviolet (UV) radiation. UV sensors contain a gas-filled tube that becomes conductive when exposed to UV light, sending an electrical signal to the burner controller. - Application: Gas burners, dual-fuel burners, and blue-flame oil burners. - Maintenance: The quartz glass lens of a UV sensor must be wiped clean weekly. Dust or oil film on the lens will block UV rays, leading to false safety lockouts.
3. Ionization Flame Rods - How they work: Combustion produces ions, which make the flame conductive to electricity. By placing a metal rod directly in the flame and applying a small AC voltage, a tiny DC current (called rectification current, typically 2 to 20 microamps) flows from the rod through the flame to the grounded burner body. - Application: Clean-burning natural gas/LPG burners. - Key advantage: Simple, low-cost, and instant response. - Limitation: The rod must sit directly in the hot flame zone, causing it to oxidize and degrade over time. It requires regular wire-brush cleaning and alignment.
Troubleshooting a 'False Lockout' If your burner runs for 5 seconds and then cuts out, the sequence controller is likely not receiving the flame signal: 1. Clean the lens: Remove the QRB or QRA cell and clean the glass face with a dry cloth. 2. Check alignment: Ensure the sensor is pushed fully into its bracket and has a clear line of sight through the diffuser plate to the flame. 3. Verify wiring: Ensure shield wires are properly grounded to prevent electrical interference from high-tension ignition cables.
Burner Spare Part Comparison Table
| Parameter | Siemens QRB1 | Siemens QRA2 | Ionization Rod |
|---|---|---|---|
| Detection Type | Visible Light (Yellow) | Ultraviolet (UV) Radiation | Flame Conductivity / Rectification |
| Application | Diesel / Oil Burners | Gas / Oil / Dual-Fuel | Atmospheric Gas Burners |
| Mounting | Plug-in slide tube | Flange clamp housing | Direct combustion zone contact |
| Service Life | Long (Optical sensor) | Moderate (UV tube wears) | Short (Oxidizes in flame) |
"We run a textile dyeing unit in Surat. Sintering and fabric heating require absolute flame stability. Our stenter burner suffered false lockouts due to dirty photocell lenses. The engineers at Gajanand Engineering & Services set up a preventive maintenance cycle and replaced the worn QRB1 sensor. The line now runs without interruption."— Mahesh Desai, Managing Director, Shree Textile Processors, Surat
Frequently Asked Questions
Can I use a Siemens QRB1 cell on a gas burner?
No. A gas flame does not emit enough visible yellow light to trigger a QRB1 photocell. Gas burners require a UV sensor (like QRA2) or an ionization rod.
Why does my UV sensor trigger a lockout before the burner starts?
This indicates 'extraneous light' error. The cell might be detecting ambient light leaking into the burner, or the UV tube has degraded and is shorting out internally, giving a false flame signal.
Where can I buy authentic Siemens QRB1 or QRA2 flame sensors in India?
You can buy genuine visible light photocells and UV flame sensors from Gajanand Engineering & Services based in Ahmedabad, Gujarat.
How often should the lens of burner flame sensors be cleaned?
Gajanand Engineering & Services recommends cleaning the glass window of flame sensors weekly to prevent false soot-induced safety lockouts.
What is the difference between photocells and UV flame sensors?
Photocells detect yellow light in oil burners, whereas UV flame sensors detect ultraviolet radiation in gas burners; Gajanand Engineering & Services supplies both varieties.
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Sukhdev founded Gajanand Engineering & Services in 2012 and has over 12 years of hands-on technical expertise calibrating, servicing, and supplying industrial combustion systems and original boiler spares across Gujarat and India.
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