Fire in the mining environment can cause damages to valuable assets, downtime and loss of lives. A proper fire protection installation is made more complex when the area is affected by harsh environmental conditions. Industrial facilities frequently produce dirt, dust, humidity and corrosive atmospheres in the production, storage or transport of goods.
Fire in the mining environment can cause damages to valuable assets, downtime and loss of lives. A proper fire protection installation is made more complex when the area is affected by harsh environmental conditions. Industrial facilities frequently produce dirt, dust, humidity and corrosive atmospheres in the production, storage or transport of goods.
The use of modern fibre optic sensors based on DTS (distributed temperature sensing) technology has established itself as a proven method for fire detection and temperature measurement. A passive fibre optic cable can provide accurate temperature measurement along the length of a conveyor belt, which enables effective monitoring of even very long conveyor routes. Fires and overheating, which can occur during operation, are quickly detected and localised to within one metre accuracy, allowing the safety countermeasures to be quickly activated. Heat detection for rollers requires proper installation of the sensor cable and configuration of the alarm zones. This article is based on conveyor belt tests and case studies, and describes the potential of fibre optic based DTS technology with regard to fire and hot roller detection along conveyor systems.
Thousands of FO-LHD (fibre optic linear heat detection) systems have currently been installed and consequently this technology is now solidly established in fire protection. But a new distributed sensing technology has appeared on the horizon, which may improve the protection of assets in conjunction with FO-LHD. This technology utilises the optical fibre as a distributed microphone, meaning it measures not only the temperature, but also the acoustics along the asset. Adding another level of information helps better identify potential sources of danger and improves the overall goal of fire prevention. Particularly on conveyors, this new technology is gaining interest for hot roller detection, one of the major causes of conveyor fires.
Raman OTDR (optical time domain reflectometry)
Most commercially available FO-LHD systems are based on Raman-DTS, which utilises the OTDR method. Light pulses are coupled into the fibre of the sensing cable and 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 (anti-Stokes) wavelength 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 a measure of temperature in the optical waveguide.
Measuring the backscattered signals as a function of time, together with the speed of light, one can calculate the distance, and thus a temperature profile along the optical fibre.
Using distributed sensing on conveyors
Nearly all open fires in mines are caused by externally supplied ignition. The most frequent sources are disturbances in active conveyor belt systems, and thus many fires develop in the proximity of conveyors. A few examples of these faults include stuck or defective rollers, grinding of the belt, slipping of the belt and misalignment. The initial smouldering stage is difficult to detect with conventional technologies and requires very tuned alarm algorithms. In addition, many conventional smoke/gas detectors do not work properly because of the high air currents.
Fire detection
FO-LHD appears to be a good solution for detecting the early stages of fires in the proximity of conveyor systems. A fibre optic based distributed temperature sensing system has several advantages during normal operation and in the case of a hazardous situation:
• Cable design is robust and resistant against dirt and dust.
• Fully certified systems (FM, UL, VdS and ATEX/IECEX).
• Long range up to 10 km and up to 4 measurement channels monitor multiple conveyors with one system.
• Virtually maintenance-free.
• Mechanically robust cable design.
• Precise localisation of fires/hot rollers enables targeted intervention.
• Reliable fire detection despite unfavourable environmental conditions.
Smouldering fires
In spite of all the security technology currently used in conveyors, it is sometimes not possible to prevent the ignition of coal smouldering fires in close proximity to conveyors. For this reason, a monitoring system is required which is able to detect the thermal radiation from a smouldering fire through permanent monitoring of the area allowing hot spot detection at an early stage.
A good example of an advanced fibre optical temperature sensing system for early detection of smouldering fires at conveyors has been installed in the mine Prosper Haniel in Bottrop, Germany. This installation shows above all that it is possible to install fibre cable at a conveyer and permanent operation under practical conditions. A fibre optical sensing cable about 3500 m long was mounted on the lower side of the conveyor.
Under test conditions, the FO -LHD system successfully monitored the detection of a coal dust smouldering fire with an area of 0,25 square metres at a distance of 1,8 m (distance between surface of smouldering fire and sensing cable) at a weather speed of up to 4,5 m/s.
Hot roller detection
Rollers on conveyor belts can become overheated and a source of danger when bearings suffer enhanced friction due to wear over time. Conveyor 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. Apart from the danger of personal injury and asset damage should a fire occur, the downtime of the line could result in significant financial loss. Extensive laboratory and field tests have been carried out for hot roller testing, in addition to experience gathered from a wide range of conveyor belt system installations.
When a DTS solution is used to detect hot rollers on the conveyor belt, the placement of the sensor cable becomes crucial. A solution has been 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.
DAS (distributed acoustic sensing) is another promising solution for detecting defective rollers in idler stations. Wear out of bearings increases friction and thus an intensified rolling noise, which is detected and localised by the DAS system. However, background noise caused by the operation of the conveyor, as well as from the working environment, is a challenge for a trouble-free utilisation of acoustic sensing. Specialised algorithms and analysing acoustic patterns, frequencies and amplitudes are required to minimise the nuisance alarm rate.
Conclusion
Distributed sensing technologies are widely used to protect transportation systems and the characteristics of FO-LHD are ideal to improve the safe operation of conveyor systems. One system can cover multiple long-distance conveyors, the sensor cable is robust, passive and easy to install and no additional wiring is required. Fires and overheating are detected quickly at an early stage and are localised with accuracy down to 1 metre. Furthermore, it has been shown that FO- LHD is able to detect and localise hot rollers and different methods have been developed and tested. For safe operation of FO-LHD it is recommended to use systems which are certified according to internationally recognised standards and are safe in explosive atmospheres, even when the system is compromised. In addition, distributed acoustic sensing can contribute to the safety of a conveyor by detecting defective rollers before any overheating may cause a fire.
Technology is continually progressing to help address the dangers of underground mining and the complexity of the harsh environments in which the monitoring systems must operate. The latest innovative AP Sensing fibre optic systems offer innovative new solutions to these challenging applications.
For more information contact Marihette Hattingh, Sperosens, +27 (0)12 665 0317, marihette.hattingh@spero.co.za, www.spero.co.za
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