What rings your bell?
Published: 01 January, 2006
There is now a wide choice of detectors available to the fire engineer - and detector selection can now be focussed on to the particular products of combustion and site specific conditions.
There is now a wide choice of detectors available to the fire engineer - and detector selection can now be focussed on to the particular products of combustion and site specific conditions.
Even if the best and most reliable detectors are chosen, they are of no use whatsoever if they are not located correctly as part of an overall monitored system and appropriate routine testing and inspection is carried out to ensure ongoing effectiveness. Selecting your detectors based on the likely fire types and hazard configurations is only the first step. It’s vital that an ongoing assurance programme is in place and that 'responders' whether operators shutting down processes or Emergency Responders carrying out fire attack are aware of their roles and responsibilities in the event of a fire detection signal. Let’s consider the technology involved in fire & flame detection:
Fire & flame detection
Fire detection can be broadly categorised as either smoke, heat or flame detection. The type of detector selected will need to take into account the expected nature of combustion ‘products’ given off by any fire. These may include smoke, heat, visible flame and incipient fire gases such as carbon-monoxide (CO). Thus, one of the first steps in selecting the correct type of detector is to review the fire types that can occur at a facility and determine the associated fire ‘products’. Recent developments in combination smoke/heat/gas sensors and video-linked detectors have also added to the choice of detector available.
Flame detectors may be used where flames are the prime indicator of fire. Such detectors may be optimised to sense infrared (IR), ultraviolet (UV) or a combination of both (UV/IR) portions of the flame emission spectrum. Flame detectors are ‘cone of vision’ devices and must be ‘looking’ at the expected source of flame in order to be effective. They are able to distinguish between flames and other sources of radiant energy on the basis of the wavelength of the received radiation.
* Infrared (IR) Flame Detectors - most infrared (IR) sensors are broadband detectors which respond to a wide range of wavelengths and require optical filters to narrow the response to wavelengths of interest. Discrimination between hot body radiation and fires is usually achieved by monitoring the characteristic flame flicker frequency. A disadvantage of most IR detectors is that fires not containing carbon, e.g. hydrogen fires, will not be detected.
The latest generation of IR devices are able to detect three separate wavelengths present in the IR spectrum of a burning fuel. This can enable the detector to ‘see’ previously unseen fuel types and may represent a cost-effective alternative to the provision of separate IR and UV devices or other combined units.
* Ultraviolet (UV) Flame Detectors - UV flame detectors are sensitive to most fires, including hydrocarbon, metals, sulphur, hydrogen and ammonia. However, the presence of large quantities of smoke, contaminants and UV-absorbing gases or vapours in the detection area may attenuate radiant energy, decreasing detection efficiency.
* Combined UV/IR Flame Detectors - Combined UV/IR flame detectors consist of UV and IR sensors within one unit. The two sensors operate independently, and an alarm is usually generated only when both sensors detect a fire. Current UV/IR sensor technology provides a very sensitive and stable detector, although in some cases the limitations are often a combination of both UV and IR detectors.
* Combination Detectors - To reduce the incidence of false alarms, a number of manufacturers now offer ‘combined’ or ‘intelligent’ conventional smoke and heat detectors meeting the requirements of EN-54, the European family of standards for automatic fire detection.
A typical device will be calibrated to sense whether expected fire phenomena such as smoke, heat and incipient fire gases such as CO are present in sufficient quantities or the right proportions. In effect, the units are making an ‘intelligent’ decision over whether a true fire condition exists.
Gas detection
One of the great advances in the field of gas detection in recent years is the introduction of infrared absorption-based detectors to replace ‘pellistor’ catalytic types.
Specifically, improvements in infrared (IR) technology have resulted in both point and ‘open-path’ types of detector, the latter also known as ‘line-of-sight’ devices.
This technology allows a response alarm to a wide range of gases, and is capable of offsetting most of the problems associated with maintenance, drift and calibration of catalytic devices. For both point and open-path devices, IR detectors utilise the fact that gases absorb infrared energy at certain wavelengths.
The point IR device is effectively a tube containing both IR transmitter and receiver. In this case the read-out is proportional to the amount of IR radiation absorbed by the gas. The principles which govern the operation of the open-path gas detector can be likened to those of optical beam smoke detector.
Smoke detection
Smoke detectors are widely used to detect smouldering or flaming fires capable of generating quantities of smoke. Such detectors may be of the ‘point’ or ‘volumetric’ types. Point detectors themselves may be of either the ‘ionisation’ or optical, i.e. ‘photoelectric’, types.
Incipient fire detection
Incipient smoke detection, or ‘aspirating’ detection, has been developed as an effective way of providing protection in ‘clean room’ environments such as computer suites or control rooms containing sensitive electronic equipment. This type of detection relies on sampling air within the protected space via an array of detection pipework. The sampled air is then drawn back to a central or local detection unit for analysis.
Heat detection
Heat detectors respond to an increase in temperature associated with developing fires. Such detectors may be of the point ‘fixed temperature’, ‘rate-of-rise’ or ‘rate-compensated’ types.
‘Linear heat detection’ (LHD), comprising a cable or tube which can detect hot spots at any point along its length, is also useful for specific applications. They tend to be used where smoke detectors cannot be used due to environmental conditions or for fires which do not produce copious quantities of smoke.
z Fixed-temperature Heat Detectors: Fixed-temperature devices operate when the temperature of the operating element reaches a predetermined maximum level. Specific types of fixed-temperature heat detector include the Bimetallic type, the Fusible metal type and the Frangible bulb type.
z Rate-of-Rise Detectors: Rate-of-rise detectors respond when the rate of temperature rise exceeds a given value, e.g. 5°C per minute, regardless of the surrounding temperature level.
z Linear Heat Detection: Linear heat detection (LHD) is an accepted way of achieving a cost-effective detection strategy for a wide range of applications, such as the protection of open-top floating roof storage tanks, pumps, cableways and service tunnels.
