Flame detector types and applications

Introduction

A flame detector or sensor is an optical fire detection device. It is extremely reliable in harsh environments, as opposed to a smoke detector or heat detector which would be continually triggered in such hostile applications.

Heat, oxygen and fuel are required and form a chemical reaction; to produce fire. When the chemical reaction occurs there is an emission of UV (ultraviolet) and IR (infrared) radiation. Flame detectors are line-of sight devices; which detect the radiation emitted by the flames in the UV, visible, and IR spectrums. They are designed to differentiate between real flames and false alarms.

The following article will explain light spectrums; examine different types of flame detectors and their particular applications.

Light Spectrums

To understand the process by which flame detectors work, it is important to have a basic knowledge of light spectrums.

Figure 1

Light spectrums are essentially wavelengths. The human eye, for example, can see wavelengths of 350-790nm (nanometres). However, there are wavelengths that we cannot see. UV and IR wavelengths are out of our limited view. These however, are the wavelengths that flame detectors detect. The sensor detects the amount of light absorbed within particular wavelengths and produces an alarm if enough light is sensed.

Types of Flame Detectors

UV (Ultraviolet)

Since almost all fires produce UV radiation; this type of detector is a good all-purpose sensor. It responds to radiation within the 185nm-260nm spectral range. It also has a quick response (3-4 milliseconds) and a short line-of-sight (0-50ft). Disadvantages include sensitivity to arc welding, halogen light and sunlight; therefore they are usually located indoors.

IR (Infrared)

This type, responds to fires which emit light in the infrared spectrum (1.1µm and higher). They have a slightly slower response time, under 50 milliseconds. Predominantly used with hydrocarbon fires, they are not susceptible to the same false alarm triggers as the UV type. However, they have better detection indoors; where they are not interrupted by water vapour.

Near IR Array

Infrared radiation is divided into five distinct ranges:

• Near IR (0.75-1.4µM);
• Short wavelength IR (1.4-3µm);
• Mid-wavelength IR (3-8 µm);
• Long wavelength IR(8-15 µm);
• Far IR(15-1,000 µm)

Also known as a visual flame detector, it detects within the near IR range and verifies the alarm by analyzing the pixel array of a charge-coupled (CCD) device.

UV/IR

This combination sensor is the best of both worlds. Since it is a dual-band detector it covers a larger spectral range and has excellent false alarm immunity. It can be used indoors and outdoors. Both sensors must be triggered simultaneously in order to produce an alarm.

IR/IR (Duel IR)

This sensor determines that there is a fire, by comparing the threshold signal in two of the five IR ranges. It operates at a reasonable speed with a range of up to 200ft and can be used both indoors and outdoors.

IR/IR/IR (Triple IR)

This type compares three wavelengths within the IR spectrum. Usually, one sensor looks at a reference range and the other sensors at reference frequencies above and below. They have a line-of-sight and speed comparable to the Duel IR detector.

Visible

Utilizing CCD devices (cameras) and flame detection algorithms, this type of device compares and analyzes the flame and progression of the fire to determine whether a flame is actually present. They are usually found in environments where there is a need to discriminate between process fires and accidental fires. The largest disadvantage is that they cannot detect fires that are invisible to the naked eye. They can also be blinded by smoke or fog.

Applications

To determine which type of flame detector is suitable for a particular application, one must first consider the following:

• What types of hazards is the detector designed to mitigate
• Determine the false alarm features of each type of device
• Note the detection range and response time
• Take heed to the device’s field of view

Once these criteria are determined, one can then choose an appropriate device for the application.

Typical applications for flame detectors are:

• Hydrogen stations
• Combustion monitors for burners
• Oil and gas pipelines
• Offshore platforms
• Automotive manufacturing facilities
• Nuclear facilities
• Aircraft hangers
• Ammunition plants
• Turbine enclosures