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DISTRIBUTED TEMPERATURE SENSING FOR FIRE DETECTION

Sperosens is a leading supplier of total fire solutions to the African mining and industrial industries. Catering for each unique customer’s requirements, these solutions vary from deep mine distributed telemetry systems to carefully designed fire detection and fire suppression systems.

A proper fire protection installation is crucial to prevent loss of lives, damage to valuable assets and down-time in operations. Installations are even more complex when the areas are affected by harsh environmental conditions such as dirt, dust, humidity and corrosive atmospheres in the production, storage and transport of goods.

A proven method for fire detection and temperature measurement is modern fibre optic sensors based on DTS technology (distributed temperature sensing). This method uses a passive fibre optic cable that provides accurate temperature measurements along the length of a conveyer belt, or any other installation, and facilitates the effective monitoring of conveyor belt routes – even very long routes. Fires and overheating can occur during operations and can lead to serious damage. This method can quickly detect and localize the problem within one meter accuracy. The necessary counter measures can then be activated. Thousands of FO-LHD systems (Fibre Optic Linear Heat Detection) have been installed and this technology is now an established solution in fire protection.

Raman OTDR (Optical Time Domain Reflectometry)

Most commercially available FO-LHD (Fibre Optic Linear Heat Detection) systems are based on Raman-DTS, which utilizes the OTDR (Optical Time Domain Reflectometry) – method. Light pulses are coupled into the fibre of the sensing cable. The backscattered light contains information about the temperature of the optical waveguide and thus the surroundings. The backscatter spectrum consists of the so called RAMAN Stokes and Anti- Stokes lines, which are shifted to the lower (Stokes) or higher wavelength (Anti-Stokes) due to an inelastic collision of photons with atoms that form a temperature- dependent electromagnetic oscillator. While the intensity of the Stokes line is nearly independent of the temperature, the Anti- Stokes line shows temperature dependence. The quotient of both intensities constitutes an obvious measure of temperature in the optical waveguide. Measuring the backscattered signals as a function of time together with the information of the speed of the light, one can calculate the distance and thus a temperature profile along the optical fibre (radar principle).

DTS for conveyors belts

Nearly all open fires in mines are caused by an externally supplied ignition. A frequent source of fire is disturbances in active conveyor belt systems. This means most fires develop in the proximity of conveyor belts. Examples of these include stuck or defective rollers, graters, grinding or slipping of the belt and belt misalignment.

FO-LHD has been proven to be an ideal solution for detecting fires in an early stage in the proximity of conveyor belts. A fibre optic based distributed temperature sensing system has several advantages:

• The cable design is robust and resistant against dirt and dust.
• The system is fully certified (FM, UL, VdS and ATEX/IECEX).
• The system can operate over long ranges or distances (up to 10km) while up to 4 measurement channels makes it possible to measure and monitor multiple conveyor belts with one system.
• The system is virtually maintenance-free which implies no down time due to maintenance activities.
• The system is based on a mechanically robust cable design.
• The precise localization of fires and/or hot rollers enables targeted intervention.
• The system facilitates reliable fire detection despite unfavorable environmental conditions.

DTS for smoldering fires
Another frequent source of fire is the self-ignition of coal dust that accumulates below the conveyor belt. The initial smoldering stage of this kind of fire is difficult to detect with conventional technology and requires finely tuned alarm algorithms. In spite of all the security technology that is currently used in conveyor belts, it is still not possible to prevent the ignition of all coal smoldering fires in close proximity to conveyor belts.

Another concern is thermal radiation from a smoldering fire. It spreads out independently of the ambient speed of air currents. For this, a monitoring system is required to detect the thermal radiation continuously and to monitor the area to allow hot spots to be detected at an early stage. Due to the high air currents, smoke or gas detectors are not recommended in these instances.

A good example of an advanced fibre optical temperature sensing system for early detection of smoldering fires at conveyor belts has been installed in the mine Prosper Haniel in Bottrop, Germany. This installation shows that it is possible to install a fibre cable at a conveyer and as part of its permanent operation, under practical conditions. A fibre optical sensing cable roughly 3500m long was mounted on the lower left side of the conveyor belt.

distributed-temperature-sensing-for-fire-detection

Installation of a sensor cable for detecting smoldering fires

A coal dust smoldering fire was inflamed by hot plates. During the test the smoldering fire was monitored by a thermographic camera. The electric heat was turned off after the coal dust had reached a surface temperature of 270 °C and visible smoke with first zones of smoldering fire showed. The smoldering fire progressed until the applicable area was completely covered. The temperature of the smoldering fire rose to between 450 to 500°C. The temperature in all five fibre cable positions, with air flow speeds of 1,2m/s, 3,0m/s and 4,5m/s, were acquired.

Under these test conditions the FO-LHD system successfully monitored the detection of a coal dust smoldering fire with an area of 0,5 x 0,5m (=0,25m²) and a distance of 1,8m (distance between surface of smoldering fire and sensing cable) at an air flow speed of up to 4,5m/s.

DTS for hot roller detection

Rollers on conveyor belts are another source of danger.  The rollers overheat when the bearings suffer enhanced friction after long periods of use.  Conveyor belt systems that transport heavy loads or work at high speeds are more likely to overheat.  These systems are often used in harsh environments where hazardous materials, dirt, dust and vibration are present.  If a fire should occur, danger of personal injury, asset damage and the down-time of the line present a significant financial risk.

Experience gathered from a wide range of conveyor belt system installations, coupled with extensive laboratory and field tests on hot rollers, lead to the conclusion that heat detection for hot rollers can be achieved by a proper installation of a fibre optic sensor cable and configuration of the alarm zones and analyses.

In these applications the placement of the sensor cable is crucial.  A solution was developed to mount the sensor cables with a special clip that ensures a simple and secure installation but also improves heat detection of small hot spots.  Once installed, rollers can still be accessed for maintenance or exchange without affecting the sensor cables (patent pending).

Another method is to install the sensor cable as close as possible along the rollers.  The sensor cable is installed close to the idler roller using a customized mounting frame.   The frame keeps the fibre near to the surface of the idler and ensures that the sensor cables are securely mounted and that the cables are mechanically protected while ensuring that the thermal radiation of a hot roller is concentrated at the sensor cable.  With this method, early detection of overheated rollers is possible at temperatures of 100-150ºC.

distributed-temperature-sensing-for-fire-detection

Mounting frame on an idler station and cable setup

A new distributed sensing technology is on its way.  This may improve the protection of assets in conjunction with FO-LHD .  It utilizes the optical fiber as a distributed microphone.   It still measures the temperature, but in addition it also measures the acoustics along the conveyor belt with the same fibre optic cable.  This adds another level of information to help identify potential sources of danger and to prevent fires – especially conveyor belt fires caused by hot rollers.

Sperosens provides innovation through the design, manufacturing, installation, management and maintenance of fire safety solutions.  We therefore, enable our clients to focus on their core business.

Credit to Henrik Hoff, AP Sensing GmbH for much of the technical information.

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