- Automatic operation, self-checking circuits.
- Self-checking ultraviolet flame detectors, Infrared flame detectors with self-checking amplifiers permitted on fire tube boilers.
- Provide one spare scanner and control chassis for each type used.
- Combustion Control System: Automatic control of steam pressure or water temperature, with provision for manual control.
Types of Flame Detectors
Most fire detection technology focuses on detecting heat, smoke (particle matter) or flame (light) – the three major characteristics of fire. All of these characteristics also have benign sources other than fire, such as heat from steam pipes, particle matter from aerosols, and light from the sun.
Other factors such as air temperature and air movement further confound the process of fire detection by masking the characteristic of interest. In addition, smoke and heat from fires can dissipate too rapidly or accumulate too slowly for effective detection. In contrast, because flame detectors are optical devices, they can respond to flames in less than a second.
This optical quality also limits the flame detector as not all fires have a flame. As with any type of detection method, its use must match the environment and the risk within the environment.
There are three types of flame detectors currently available: ‘‘infrared (IR)’’, ‘‘ultraviolet (UV)’’, and a ‘‘combination of UV and IR’’. The spectrum below shows the relationship between these frequencies and visible light.
Infrared Flame Detectors
Infrared detectors have been available for many years; however, it has only been in recent times that technology has allowed for stable, accurate detection to occur. There are two types of infrared detectors: ‘‘single frequency’’ and ‘‘multi-spectrum’’.
Infrared Single Frequency Flame Detectors
The basic principles of operation for a single frequency IR detector are:
The detector is sensitive to a narrow band of radiation around the 4.4 micron range which is a predominant emission band for hydrocarbon fuelled fires. Additionally, the sun’s radiation at this band is absorbed by the earth’s atmosphere, making the IR flame detector solar blind.
Single frequency detectors use a pyroelectric sensor, which responds to changes in IR radiation intensity. In addition, they incorporate a low frequency band pass filter, which limits their response to those frequencies that are characteristic of a flickering fire.
In response to a fire signal from the sensor, electronic circuitry in the detector generates an output signal.
Strengths of the single frequency IR detector are:
- Highly immune to optical contaminants like oil, dirt, and dust
- High speed response under 30 milliseconds for some brands
- Insensitive to solar, welding, lightning, X-rays, sparks, arcs, and corona
Limitations of the single frequency IR detector are:
- Generally not suitable for non-carbon fires
- Some brands will respond to modulated infrared sources.
- Rain, ice, and water vapor on the detector lens will inhibit detection.
Infrared Multi Spectrum Flame Detectors
The basic principle of operation for a multi spectrum IR detector is:
The detector has three sensors, each sensitive to a different frequency of radiation.
The IR radiation emitted by a typical hydrocarbon fire is more intense at the wavelength accepted by one sensor than the other two.
Electronic circuitry in the detector translates the difference in intensity of the three sensors to a ratio that, along with a synchronous flicker, must be present before a fire signal is produced. This allows the detector to reject high intensity flickering black body radiation sources since these sources will not meet the proper ratio criteria.
Strengths of the multi-spectrum IR detector are:
Virtually immune to false alarms
Fire response in the presence of modulated infrared black body radiation with some brands
Long detection range (60 meters to some fires)
Limitations of multi spectrum IR detector are:
- Typical response time is longer when compared to single frequency detectors.
IR detectors are sensitive to most hydrocarbon fires (liquids, gases, and solids). Fires such as burning metals, ammonia, hydrogen and sulphur do not emit significant amounts of IR in the detector’s sensitivity range to activate an alarm.
IR detectors are suitable for applications where hydrocarbon fires are likely to occur and high concentrations of airborne contaminants and/or UV radiation sources may be present. The detector should be used with caution when the presence of hot objects and the potential for ice buildup on the detector are likely.
Ultraviolet Flame Detectors
A UV detector uses a sensor tube that detects radiation emitted in the 1,000 to 3,000 angstrom (one ten-billionth of a meter) range. It is important to note that ultraviolet radiation from the sun that reaches earth starts at 2,800 angstroms. If the detector’s sensor has a wide range, then it will be triggered by the sun’s rays, which means it is only suitable for indoor use.
There are sensors available with a range of 1,800 to 2,500 angstroms. Virtually all fires emit radiation in this band, while the sun’s radiation at this band is absorbed by the earth’s atmosphere.
The result is that the UV flame detector is solar blind. The implication of this feature is that the detector can be used indoors and outdoors. In response to UV radiation from a flame that falls within the narrow band, the sensor generates a series of pulses that are converted by the detector electronics into an alarm output.
Strengths of the UV detector are:
- Responds to hydrocarbon, hydrogen, and metal fires
- High speed response – under 10 milliseconds
- Solar insensitive
Limitations of the UV detector are:
- Will respond to welding at long range
- May respond to lightning, X-rays, sparks, arcs, and corona
- Some gases and vapors will inhibit detection.
- Some UV sensors have a wide detection range resulting in solar false alarms.
UV detectors are sensitive to most fires, including hydrocarbon (liquids, gases, and solids), metals (magnesium), sulphur, hydrogen, hydrazine, and ammonia. The UV detector is the most flexible general purpose optical fire detector available. They are fast, reliable, have few false alarm sources, and respond to virtually any fire.
Ultraviolet / Infrared Flame Detectors
A UV/IR detector consists of an UV and single frequency IR sensor paired to form one unit. The two sensors individually operate the same as previously described, but additional circuitry processes signals from both sensors. This means the combined detector has better false alarm rejection capabilities than the individual UV or IR detectors.
Strengths of the UV/IR detector are:
- Virtually immune to false alarms
- High speed response – under 500 milliseconds
- Solar, welding, lightning, X-rays, sparks, arcs, and corona insensitive
Limitations of UV/IR detector are:
- Not recommended for non-carbon fires
- Some gases and vapors will inhibit detection due to blinding of the UV sensor.
Since the UV/IR detector pairs two sensor types, it will typically only detect fires that emit both UV and flickering IR radiation. UV detectors will respond to virtually all fires including hydrocarbon (liquids, gases, and solids), metals (magnesium), sulfur, hydrogen, hydrazine and ammonia.
IR detectors typically only respond to hydrocarbon fires. Since the IR detector is not sensitive to burning metals, ammonia, hydrogen, and sulfur, the combined unit will not respond to these fires.
The detector is suitable for applications where hydrocarbon fires are likely and other sources of radiation may be present (X-rays, hot surfaces, and arc welding). They maintain constant protection while arc welding takes place. The UV/IR detectors are highly reliable with fast response times and low propensity to false alarms