SF6 tracer gas application in Mine Ventilation System
Good ventilation at workplace is very necessary to provide comfort environment to workers in underground mine. This will not only improve the working efficiency of the workers but also improve the productivity of the mine,the most important is to keep safe working in underground. The extent of an underground mine is so large. Therefore, it is necessary to optimize the ventilation network path and to control the leakage so that the maximum quantity of air could reach working place. The estimation of lost air in the path due to various reasons can be done by application of tracer gas in mine ventilation system. One of the state-of-the-art technologies for studies of mine ventilation systems is tracer gas.
Sulfur hexafluoride (SF6) is widely used as a tracer gas because of its detectability at low concentrations. This attribute of SF6 allows the quantification of both small-scale flows, such as leakage, and large-scale flows, such as atmospheric currents. SF6's high detection sensitivity also facilitates greater usage efficiency and lower operating cost for tracer deployments by reducing quantity requirements. The detectability of SF6 is produced by its high molecular electronegativity. This property provides a high potential for negative ion formation through electron capture thus naturally translating to selective detection using Electron Capture Detector (ECD).
Many mines in different area has successfully used sulfur hexafluoride (SF6) as a tracer gas to measure mine air flows under conditions where conventional methods have failed. SF6 was employed experimentally to measure accurately recirculation of return into intake air caused by leakage through an old stoped area, to check for potential leakage from an adjacent mine, to trace 10,000 cfm of "lost" air from an intake airway, and to measure transit air time through uranium mines. It proved useful as a means of accurately measuring airflow volumes in airways of large cross section and very low flow velocity and in determining the recirculation resulting from underground cooling plants.
The ECD,short for electron capture detector,mentioned in the above description,is also a kind of ionization detector. It is a selective detector with high sensitivity. It only has signal to electronegative substances, such as halogen, sulfur, phosphorus and nitrogen. The stronger the electronegativity of substances, that is, the greater the electron absorption coefficient, the higher the sensitivity of the detector, but there is no signal for neutral (non electronegative) substances, such as alkanes. ECD has super high accuracy, sensitivity and stability.
At present, the main representative products of high-precision SF6 gas detector in the market which based on the principle of ECD is as follow,Portable Ultra-high-sensitivity SF6 Trace Gas Leak detector - Q200 of D-Industrial, Plus Gas Chromatograph – GC2010 of Shimadzu and Gas Chromatograph - 7890A GC of Agilent.
Portable Ultra-high-sensitivity SF6 Trace Gas Leak detector - Q200 of D-Industrial, is a portable instrument, supplied in a carrying case(43 x 40 x 23cm) holding the power supply and electronics,a small cylinder of compressed argon gas, and the main display panel. The detector is mounted in a hand-held probe, attached to the main unit by a 10 ft (3.5 meter)umbilical connection. A display panel is mounted in the hand probe enabling the operator to monitor measurements even when working in difficult locations. The instrument is extremely easy to use, and provides highly accurate leak rate measurements.The response is almost instantaneous and the detector recovers quickly, even after exposure to high concentrations of trace gas.Where products are manufactured or maintained to a high leak tight specification the QUALICHEK 200 is an invaluable aid in ensuring quality.The concentration sensitivity can reach to 0.01ppm,response time can be 1 second(85% of reading).