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April 2018 www.coalage.com 41 operating ideas continued nent is the cable itself (usually a tight buffered configuration), which will have a number of (six or more) optical fibers con- tained within the cable itself. How Does a System Find a Leak? There are a two main ways that leakages can be detected through distributed sen- sors. The two methods are called Distrib- uted Temperature Sensing (DTS) and Dis- tributed Acoustic Sensing (DAS), and they respond to changes in temperature and vibration respectively. DAS systems look for the noise gen- erated by the fluid escaping (under pres- sure), which then vibrates the pipe wall that the fiber is connected to. This vibra- tion has been able to detect leaks down to 0.5 liters per second (0.13 gallons per minute) in water pipelines and the abra- sive nature and high density of tailings mean that the noise generation is in fact increased over water. DTS systems rely on a physical prin- ciple called the Thompson-Joule effect, which is attributed to a localized drop in temperature that occurs when any fluid is moved from high to low pressure. An ex- ample of this is aerosol cans getting cold with extended use or a barbeque gas cyl- inder freezing on a hot day. This effect is more prevalent in gases, but does occur in liquids and slurries. This localized drop can be detected and subsequently the presence of a leak can be inferred. Some systems are, based on the Raman principle, using a low cost light emitting di- ode (LED) or a simple fiber-optic data laser in order to transmit the light required. As these systems rely on less stable light sourc- es, they are only able to use the most basic return signal (Raman principle), which lim- its their range significantly and limits them to temperature sensing (DTS) only. Due to the typically aboveground in- stallation of HDPE tailings piles and direct exposure to the elements, these tempera- ture-only systems are not recommended for these applications. When selecting a suitable system for tailings pipelines monitoring, the best choice is a Multivariable FOS (M-FOS). The main advantage of these sensors is the re- sistance to false reporting. Typically, these M-FOS systems are configured in such a way which means that both an appropriate DAS and DTS response should be seen at the same location. In the loosest of terms at a given location for a signal to be confirmed to be a leak, an increase in acoustic signal must be present at the same place as a lo- calized decrease in temperature. This built in "self-checking" method means that not only is the system highly accurate, but also immune to almost all environmental effects (such as rain, snow, wind, etc.) and is all but tamper proof. Due to the optical nature of the sig- nal captured by the device, the systems are immune to all electrical and environ- mental interference, unless the cable is crushed or has severed the system. There are Limitations Like all sensors, there are a number of limitations that distributed fiber-optic sensors have, however, these limitations are based around the difficulty of mea- surement at extreme range (greater 40-km pipe length) and the difficulty of cable ret- ro fit in underground applications. Almost all of the limitations of FOS sys- tems are only encountered when attempt- ing to retrofit to existing infrastructure. Whereas, in new installations, these issues are often handled at the design stage. M-FOS systems also use backscatter components that are incompatible with multi-mode fiber. Due to fiber's require- ment to be on good contact, or "coupled," to the application, existing fiber is also not always ideally mounted for sensing through a distributed sensor. Once buried, it is often not economical to retrofit sens- ing suitable fiber. Some existing fibers, due to their me- chanical design are, although perfect for data transmission, not ideal for sensing due to their physical properties. Fortunately, tailings dam pipelines are typically shorter lengths (10 km or less) and are usually above ground. Meaning that technology and logistical limitations of the technology are rarely encountered. Based in Melbourne, Australia, Mathew Cook is the product manager-fiber optics for Hawk Measurement Systems. He can be reached at mathew.cook@hawk.com.au. Suspended Magnets • Metal Detectors Heavy Duty Feeders Double-Team Dangerous Tramp Metals! Download Catalogs & Product Selection Guides Suspended Magnets • Metal Detectors Heavy Duty Feeders Eriez' Suspended Magnets and Metal Detectors remove tramp metals before they reach your valuable crushers and downstream equipment. 814-835-6000 • www.eriez.com Download Catalogs & Product Selection Guides