Early and reliable fire detection – a challenging task in the industrial environment
Different fire characteristics can be used in order to detect fires. The best known method consists in detecting smoke by means of smoke detectors according to the optical principle of light scattering or light attenuation. This is usually the method of choice because it is simple and proven, as long as particular ambient conditions do not render the use of optical smoke detectors impossible. In case of dust, moisture or other kinds of vapours or negative effects, smoke detectors cause false alarms and, in addition, they have a significantly reduced useful life. In brief – they cannot be reasonably utilised.
In these cases, alternative methods should be used for detection. In doing so, the following aspects are to be considered:
- Kind of disturbance variables and fire characteristics
(smoke, gases, infrared heat radiation (10µm), infrared light (1-4 µm), heat (hot gases))
- Kind of transmission path of the fire characteristics
(diffusion by airflow or by direct optical path)
- Kind of task
(fire detection and alarm in a building, plant monitoring, process monitoring, detection of plant damages prior to a possible fire)
For that purpose, GTE offers different detection technologies:
- Fire gas detection by means of semiconductor gas sensors
- Linear gas detection by means of laser spectroscopy
- Hot spot and heat detection by means of infrared detector arrays
- Other special technologies are specifically developed for individual applications and customers.
Fire gas detection
During smouldering, different gases are released in addition to smoke (aerosols). Besides CO2 and H2O, this also includes the characteristic low temperature carbonisation gases CO (carbon monoxide), H2 (hydrogen) and a large mixture of various hydrocarbons. Depending on the material, HCl, H2S, HCN, NOX and other components could be generated as well.
The composition of the fire gases is indicative of the type of fire and the material. Furthermore, in this way it can be determined whether there is a fire or not or if it is about a “friendly fire” situation.
In order to make use of this possibility, we offer a detector which reacts, by means of three different gas sensors, to the gases H2, CO and HC (hydrocarbons) and qualitatively to NOX.
The detection sensitivity is of only a few ppm.
Semiconductor gas sensors are used whose useful life is significantly increased in relation to many other sensor principles (e.g. amperometric, so-called electrochemical sensors).
These sensors are based on a semiconductor layer which is applied to a heated carrier substrate. The sensor signal is obtained from the electrical conductance of this layer. Depending on the target gas, the optimal working temperature can be up to 450°C.
The detector and its sensors are integrated into a die-cast aluminium casing; the gases reach the sensor elements by diffusion through a sintered metal filter. In this way, a sturdy and dust-tight design is obtained.
A type-tested casing version is available for dust explosion areas. Although the gas sensors do not react on dusts by its very nature, an additional immunity against dusts is achieved in this way1.
The following table gives a simplified overview of the types of fire and their gas emissions:
|Smouldering of coal||+||++||+|
|Smouldering of wood||+||+||++|
|Flaming fire of PU||+||++||+||++|
|Cold oxidation of coal||(+)||+|
This detector type was originally developed in cooperation with RWE Energie AG in order to be used for the early detection of coal smouldering in coal transport paths within power stations.
For this reason, the standard sensitivity setting is adapted to the power station’s requirements.
The detectors can be parameterised. In this way, they can be optimally adapted to many other application requirements.
The parameterisation is carried out via a service software. Via this software, it is possible to collect, illustrate, analyse and adjust the detector data correspondingly by means of a PC via the Service Bus (“M-Bus”).
Infrared detection technology
In many cases it is not possible to ensure that smoke or smoke gases at the location of the fire reliably reach the detector. An open window or door or a powerful ventilation system often suffice to prevent smoke detection. In this case, an optical detection technology is offered as an alternative: in case of full view of the objects to be monitored, a heated, overheated or burning area can be directly detected by means of a thermal image and this at a detection speed of up to tenths of a second or, in brief, “immediately”. In this way, it is also possible to monitor moving objects, e.g. hot spots on a conveyor belt.
The detector is based on a thermopile array with a number of pixels (local resolution) of 4 x 4 up to 64 x 62, depending on the detector type. The surface of the area to be monitored is projected to the array by means of infrared optics. Subsequently, a thermal image of the monitoring area which is more or less coarsely rasterised is generated. Alerting is now possible by means of appropriate evaluation of the temperature measuring values. In order to do so, each measuring value or a group of measuring values is compared with absolute temperature limit values or a difference analysis is carried out, e.g. between neighbouring fields or other, freely parameterisable fields.
Depending on the application, it is therefore possible to monitor:
- Overheating units, motors, rollers, gears etc.
- Hot spots in warehouse stock or on a conveyor belt
- Auto-ignition processes in stored biomass
(1) Only dusts releasing gases on their part are to be taken into account.