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SIL assessments for safety related systems - to be presented at HazardEx 2012

Author : Steve Sylvester, Associate Director, Environmental – AECOM

20 December 2011

In New South Wales (NSW), Australia, regulators (the Department of Mines or DoM) has recently stipulated that SIL assessments need to be conducted for safety related systems at the mines within the NSW area.

In one mine in the north west of NSW (Narrabri), the mine had nearly completed the construction of a stockpile recover tunnel, which was vented and contained a number of electrical and electronic components (e.g. conveyors, motors, apron feeders, lights, etc.). The owner of the mine had provided ventilation in the tunnel to minimise the potential for gas or dust accumulation and therefore he did not consider the area to require hazardous area classification. However, the DoM insisted on a review of the area and indicated that in the event of fan failure the area could become hazardous and the dust and methane could accumulate and exceed flammable concentrations. 

The owner had installed a considerable amount of electrical equipment, which would result in a reasonably expensive re-installation and project delay (the project delay was the main cost, could be a couple of hundred thousand dollars per day). The solution was to integrate the HAC with a SIL assessment using gas detection, power isolation and fan flow failure device. Hence, in the event of a gas detection (methane release from the coal) or fan failure (potential for dust accumulation) there could be an ignition and explosion. Under these circumstances all power is cut to the tunnel, eliminating potential ignition sources.

Whilst this would appear to be a fairly simple application of SIL/HAC, the paper will introduce some discussion in relation to a paper published by Paul Gruhn (of the International Society of Automation or ISA). Paul raises concerns regarding using gas or fire detection in SIL applications, particularly in relation to the position of such devices and the potential for these to fail to detect the incident, rendering the overall security of the system in jeopardy. I would like to introduce some further discussions in relation to the use of gas & fire detection systems. SIL analysis and HAC within the paper, using the Narrabri Coal Mine as an example where such approaches can work. 

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