Improving management of gas monitoring equipment

Ampcontrol discusses its recommendations surrounding process, procedure and validation of gas monitoring equipment after concerns were raised in Queensland.

A Mines Safety Alert from the Queensland Department of Natural Resources, Mines and Energy in April has reinforced industry discussions regarding the effect of environmental factors on the monitoring performance of methane gas detectors.

Outlining a number of recommendations for site senior executives to consider, one action highlighted within the alert specifically encourages the consideration of the impacts caused by ventilation velocities over the sensors and potential for the filter units on the sensors to become blocked.

Diffusion type gas detectors are used in fixed, machine mounted and handheld applications in underground mines for the real-time detection of flammable and toxic gases. These detectors are reliant on the natural equalisation of gas concentrations inside and outside the detector, driven by the respective partial pressures of gases.

This equalisation can be impeded (to a greater or lesser degree), by the necessary filters and hydrophobic barriers protecting the fragile sensing elements from the typically harsh mine environments. 

This impediment is present in all detectors and typically the gas detector design is verified to comply with the performance requirements of relevant Australian Standards. (AS/NZS 60079.29.1 for flammable gases and AS/NZS 4641 for toxic gases and oxygen).

Where the mine environment is dusty, airborne contaminants are deposited on the gas detector’s external surfaces, including its protective filters and barriers. The gradual accumulation of dust and other foreign matter on the protective filters and barriers can further inhibit the diffusion of gas into a detector, degrading the two key performance criteria of accuracy and speed of response. 

Both are critical for management of gas in mines and confined spaces to ensure that potential ignition sources can be removed by isolating power from mining equipment when methane levels exceed specific values.

Increasing contamination on a catalytic type flammable gas detectors gives rise to two key phenomena, including an increase in the response time of the detector and a reduction in the steady state reading.

Laboratory testing on methane gas detectors was conducted under various environmental conditions and calibration procedures which could be found in underground coal mines. This variation on condition was found to impact the overall performance of the methane gas detector, with results showing that indicated gas levels could be higher or lower than the know applied gas level, depending on the scenario.

Following this testing, the Department of Natural Resources, Mines and Energy has issued a number of recommendations surrounding process, procedure and validation of gas monitoring equipment.

In light of these recommendations, Ampcontrol suggests adopting a regimented calibration process using a well-designed calibration mask. Records of detector readings before and after calibration should be retained and reviewed to identify long-term degradation, and response time testing should be an integral part of the maintenance procedure.

Despite this approach, however, one of the key elements missing from diffusion gas testing and investigation is information regarding what actually constitutes a ‘blocked’ or compromised sensor. Furthermore, the relationships between sensor ‘blockage’ and sensor performance have been limited to qualitative observations.

Ampcontrol is currently pioneering this research, with work well under way on a study of the fluid dynamics of diffusion gas detectors and their associated calibration cups.

Using computationally intensive fluid dynamics software, the operation and characterising features of typical diffusion type sensor heads and calibration cups can be studied in detail, including the identification of factors that enhance and compromise the diffusion process.   Computer simulation studies can be referenced against visualisation techniques using equivalent Reynolds numbers to give a pictorial view of the diffusion process. Using a ‘clean’ (uncontaminated) model as the reference, the effect of contamination of the detector can then be quantitatively analysed, and validated by experiment. 

Such results will inform the development of superior diffusion gas detectors and calibration cups, and provide guidelines as to the maintenance requirements of such detectors.

Industry is encouraged to seek advice and support to comply with these recommendations, with Ampcontrol able to provide technical and operational services such as training and site audits.  

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