Basics of testing flame detectors: Part 1

Editor's note: This is the first of a two-part series on how to test and troubleshoot various types of industrial flame detectors. The second part will appear in the October issue. The flame detector is one the most important safety devices within the combustion chamber of industrial heating equipment such as steam boilers, drying kilns, or industrial ovens.

By George Allen September 10, 2004

The flame detector is one the most important safety devices within the combustion chamber of industrial heating equipment such as steam boilers, drying kilns, or industrial ovens. The flame detector detects if the pilot light or main flame is actually lit. When properly installed and serviced, flame detectors are designed to prevent boiler explosions caused by the ignition of fuel accumulated within the burner chamber during a flame failure.

A flame failure occurs when the flame within the combustion chamber of the boiler, oven, or kiln fails due to equipment malfunction or faulty operation. All combustion-type, plant heating equipment, whether it is a furnace or a boiler will monitor the flame signal with some type of flame detector.

This article presents the principles and applications for flame detectors and the processes to test these devices as they apply to industrial plant heating equipment, specifically gas or oil-fired units. To properly test and maintain flame detectors, the technician or engineer must have a solid comprehension of the normal operation of the heating equipment, an understanding of electrical principles, and experience with modern digital multimeters (DMMs).

Basic operation of flame detectors and burner controls

Modern heating equipment manufacturers typically install a solid-state flame detector at the factory. It is mounted to sense flame within the combustion chamber and wired into a burner management control system located within a control panel on the heating equipment. The control system might be a modern microprocessor controller or an older electromechanical controller. The burner control system maintains a specific sequence of operation to safely start up and shut down the equipment in the proper order. This sequence includes:

  • Prepurge
  • Ignition
  • Pilot flame verification
  • Main burner ignition and flame verification
  • Run time monitoring
  • Burner shut down
  • Post purge

During pilot flame verification, the flame detector sends a signal to the burner controller unit. If a flame is established within the burner chamber during an acceptable time frame (typically 1-2 min), it sends a continuous signal to the controller, allowing the combustion process to move to the next sequence, which typically is ignition of the main burner. However, if the flame signal is not adequate or is not present at any time during the combustion cycle, the burner controller locks out the control function, which cannot be restarted until it is manually reset. Typically, burner control lockout is the flame safeguard function, which closes all fuel lines (pilot and main burner supply) feeding the burner.

Flame detector applications

Flame detectors are available in many sizes and shapes for different applications. The boiler, furnace, oven, or kiln has a flame detector installed at the factory. The type of detector used depends on the type of flame, the fuel used, the size of the burner, and the ease or difficulty in viewing or sensing the flame. Temperature to which the flame detector is exposed is also a major consideration. Difficult applications can include infrared burners, dual fuel and coal-fired burners, hot refractory, and ignition interference conditions.

Types of flame detectors and sensors

The types of flame detectors used with combustion heating equipment are:

Cad cell

Cad cell (cadmium sulfide) is a photoconductive flame detector used with oil primary controls. The detector, which is a plug-in, light-sensitive cell, is installed inside the air tube of the burner where the cell can view the flame and is wired to the terminals of the oil primary control (Fig. 1). The photocell is a ceramic disk coated with cadmium sulfide and overlaid with a conductive grid. Electrodes attached to the ceramic disk transmit an electrical signal to the primary control. In darkness, cadmium sulfide has a very high electrical resistance. In visible light, its resistance is very low, which allows current to pass. The entire cell is hermetically sealed to prevent cell deterioration.


Infrared (IR) flame detector is a lead-sulfide photocell that is sensitive to the IR radiation emitted by the combustion of fuels such as natural gas, oil, and coal. Since more than 90% of the total flame radiation is IR, these detectors receive ample radiation and can detect weak flames as well as flames of higher intensity. The lead-sulfide cell used in the detector cannot distinguish between the IR radiation emitted by hot refractory and the IR radiation from a flame.

Therefore, the IR detection system includes an amplifier that responds only to the flickering characteristic of flame radiation and rejects the steady radiation characteristic of hot refractory. Unfortunately, smoke or fuel mist within the combustion chamber can intermittently reflect, bend, or block the hot refractory radiation, thus making it fluctuate. This fluctuating action can simulate the flickering radiation from a flame, and IR radiation may be present even after the refractory has visibly stopped glowing. Therefore, be very careful when applying an IR detection system to be sure it responds only to flame.

The electrical resistance of lead-sulfide decreases when exposed to IR radiation. If a voltage is applied across the lead-sulfide photocell, current flows when the cell is exposed to IR radiation.

Visible light

Visible light flame detector senses the visible light emitted by fuel oil combustion flames. It is used with flame safeguard controls to provide fuel oil flame supervision in commercial and industrial burners. Under optimum conditions, the flame detector can detect most oil combustion flames at a distance of 6 ft. The critical factors in determining the flame-detector distance separation are the optimized flame signal (current or voltage) and the flame detector temperature. Other factors may be influential and are installation specific. The detector must actually see the flame. Locate the detector as close to the flame as physical arrangement and temperature restrictions permit.

Visible light detectors are starting to be used as a replacement for the cad cell flame detectors. However, the temperature of the visible light photocell must not exceed 165 F and the faceplate temperature must not exceed 200 F.


Ultraviolet (UV) light detectors sense UV radiation emitted from all flames. These flame detectors are solid-state electronic devices for sensing the UV radiation emitted during combustion of most carbon-

based fuels such as natural gas, LP gas, and oil.

The combustion flames of most carbon-based fuels emit sufficient UV radiation to enable the UV flame detector to prove the presence of a flame in a combustion chamber. The detector is mounted outside the combustion chamber. When a flame is present, the detector senses the UV radiation emitted and produces a signal that is sent to the amplifier in the flame safe-guard control.

The amplified signal actuates the flame relay, allowing proper operation of the system. If the flame extinguishes, the relay drops out, consequently shutting off fuel supply to pilot and burner. Since it is necessary for the UV sensing tube to actually see the flame, locate the detector as close to the flame as physical arrangement, temperature, and other restrictions permit.

Flame Rod

Flame rod detectors are used with flame safeguard controls on industrial or commercial gas burners or oil burners with gas pilots. The flame rod and flame safeguard system use a very small amount of current. This sensing current is actually conducted by and through the flame, which acts as a rectifier. The flame rod sensor and the burner, which is at ground potential, are the sensing electrodes to which ac is applied (Fig. 2).

Electricity flows to ground in only one direction, resulting in a dc signal, which can be monitored and measured, and is applied to the input of the flame safeguard control. The flame safeguard shuts off the fuel supply if the flame is extinguished.

Author Information
George Allen is the Marketing Manager for Meterman Test Tools. He has worked in the test and measurement industry for eight years. George holds degrees in chemistry, law, and business. He can be reached by phone at 206-550-3627 or by e-mail at .