Managing Gas on Open Cut Coal MinesMines safety bulletin no. 186 | 11 August 2020 | Version 1
During 2020, there has been 6 Directives and 5 Substandard Condition Practices (SCPs) issued to open cut coal mines involving flammable and toxic gases in open cut coal mines where the onsite systems, knowledge and/or practices have not met a standard required to ensure an acceptable level of risk. Flammable and toxic gases have been emitted from a range of situations, including spontaneous combustion events, blast holes, post blast areas, blast fumes and old underground workings. Coal mine workers at open cut sites are often not aware that flammable and toxic gases may be present, and pose a significant risk during normal mining activities at open cut operations.
Specific areas of concern include general awareness of flammable and toxic gas risks, knowledge and competencies for managing gas, the use and maintenance of gas monitoring systems, and the practical application of Trigger Action Response Plans (TARPs).
Why it is important?
Coal mine workers at open cut mines should be aware of the toxic and flammable risks associated with gases encountered on site. This should include how to effectively monitor, assess and manage potential risks. Some types of gases can displace oxygen while others are flammable and/or have acute toxic effects. The potential risk from each type is determined by the concentration of gas in air. Therefore, the detection and measurement of gas in air provides a critical role in the mine’s safety and health management system for gas management. Coal mine workers who conduct gas monitoring and assessment of risks need to be aware of the physical properties as well as the hazardous properties of the specific gas types likely to be present at the mine site. There are 3 primary hazard categories of gaseous atmospheres likely to be present in the open cut mine site environment:
- Flammable/explosive atmosphere
- Toxic gases
- Oxygen deficiency
In March 2007, the inspectorate released Mine Safety Bulletin 61 – Flammable and toxic gases in open cut coal mines which provided general information regarding the characteristics and potential health effects resulting from overexposure to some of the more common gases in surface coal mining. The safety bulletin included a summary of the flammability limits in air, time weighted average exposure limits (TWA) and short term exposure limits (STEL) relevant to each type of gas. It should be noted that Safe Work Australia (SWA) has recently recommended, and released for comment, a reduction in the workplace exposure standards for a number of gases. This includes carbon dioxide, hydrogen sulphide and nitrogen dioxide. It is important to take this into consideration when reviewing the mine safety bulletin, and developing procedures as part of your mine safety and health management system.
What are your regulatory requirements?
The Queensland Coal Mining Safety and Health Regulation (Section 142 Flammable or toxic gas), prescribes that:
- A surface mine’s safety and health management system must provide for protecting persons from risks from flammable or toxic gas at the mine.
- Identifying and monitoring parts of the mine where the gas might accumulate and create a hazard;
- Fixing flammable gas concentration limits for the work environment;
- Testing for flammable or toxic gas in the work environment, before, during and after carrying out work;
- Recording information about flammable or toxic gas detected in the work environment;
- Controlling flammable or toxic gas in the work environment;
- Establishing emergency procedures for flammable or toxic gas in the work environment.
- The system must include a standard operating procedure that provides for the following—
The integrity and effectiveness of the gas monitoring equipment should be managed in accordance with AS/NZS 2290 which sets out requirements for the inspection, maintenance and performance verification of gas detection and monitoring equipment that forms part of the coal mines safety and health management system. (AS/NZS 2290.3 Electrical equipment for coal mines – Introduction, inspection and maintenance; Part 3: gas detecting and monitoring equipment).
In addition to AS/NZS 2290.3 there are other AS/NZS that should be considered when selecting gas monitoring equipment for use at open cut coal mines. These include the following.
- AS/NZS 3007:2013 Electrical equipment in mines and quarries – Surface installations and associated processing plant. This standard provides requirements for:
- safety critical electrical systems including gas monitoring; and
- application of gas monitoring in reclaim and transfer tunnels in coal mines
- AS/NZS 4641:2018 Electrical equipment for detection of oxygen and other gases and vapours at toxic levels – General requirements and test methods. This standard provides guidance relevant to manufacturers and suppliers on performance specifications for gas monitoring equipment.
Monitoring gas on site
Portable electronic gas detectors are the most common type of gas monitoring system used in open cut coal mining operations. There are other methods of gas detection available, including colorimetric (‘stain’) detection tubes and ‘bag’ sampling for subsequent laboratory analysis. These later methods are typically more labour intensive and less popular than the electronic detectors although may provide a more suitable alternative in some situations.
Portable gas detectors (PGD) are handheld devices capable of measuring and recording gas concentrations upon entry into a work area. Modern PGDs have onboard digital processing and memory capacity, and contain a number of sensors capable of detecting and measuring a select range of gases. These typically include oxygen, flammable gas, carbon monoxide and additional toxic gases. The PGD may have an onboard sampling pump to support active gas detection or otherwise it will rely on passive diffusion sampling. The type of PGD and sensor combinations used must be selected based on an initial hazard identification and risk assessment process whereby the potential types of gas release events and the potential composition of gases have been determined for the mine site. For example, where flammable gases are being sampled, it is recommended to select an instrument with suitable intrinsic safety protection (Ex ia) in order to prevent ignition.
Gas monitoring should be conducted by authorised coal mine workers (preferably open cut examiners) who have completed formal training and assessment by a registered training organisation, for:
- (i) the atmospheric monitoring of coal mine gases (RIIENV301D; UCMDSA301A or equivalent) and including
- the inspection and maintenance of gas monitoring equipment in accordance with AS/NZS 2290.3.
Prior to using the gas detector in a potentially hazardous environment, it is important that the user is familiar with the following:
- Calibration requirements
- Instrument limitations, including T50/T90 response times, detector interferences and cross sensitivities
- Audible and visible alarm trigger levels for Oxygen, Flammable and Toxic gases
- Setting-up the instrument (inspection, zero check, bump test & Cal check)
- Operating the instrument to collect valid gas samples, interpreting display readings, responding to latched and unlatched alarms, downloading, recording and reporting data
- Type(s) and characteristics of gases likely to be present
- Sampling strategies suitable for detecting coal mine gases in open cut mine operations.
Maintenance of gas detectors
The maintenance of gas detecting equipment should be undertaken by an authorised person who has been deemed competent to carry out equipment inspections, calibrations and maintenance in accordance with AS/NZS 2290.3.
The following types of inspections/checks should be conducted at specified intervals in accordance with of AS/NZS 2290.3: Table 3.1 Schedule of Inspections and Tests include:
- External Inspection – identifies general deterioration or damage to the instrument which may compromise performance.
- Zero Test – involves applying fresh air or a zero concentration of gas to compensate for detector offset or drift
- Span Test – involves applying span gas to confirm alarm levels trigger and operating range of the detector performs as intended.
- Response Time Test (Section 4.5) – involves applying span gas to confirm the time required for the detector to respond and indicate the gas concentration
- T50 – the time it takes for the sensor to indicate 50% of the gas concentration.
- T90 – the time it takes for the sensor to indicate 90% of the gas concentration.
- Calibration – detector accuracy and linearity is confirmed for multiple span gas concentrations across the normal operating range of the detector. A calibration certificate (preferably NATA) should be issued for each test and a calibration label indicating expiry date secured to the unit.
Where equipment does not meet the acceptance criteria for any inspection, test or calibration, it shall be withdrawn from service until the issue has been rectified.
Trigger Action Response Plan (Gas TARP)
A Trigger Action Response Plan should be established and implemented at the mine to define a systematic approach which detects and responds to incremental changes in gas concentration in such a way which ensures that flammable and toxic gas is controlled to an acceptable level of risk. The Gas TARP should be referenced within the Standard Operating Procedure for gas management at the mine.
The TARP should consider all potential flammable and toxic gas release events identified for the site. The TARP should clearly specify the gas concentration trigger levels, how they are to be measured, how to investigate and what actions need to be undertaken for each incremental change. Gas concentration trigger levels should be:
- Identified through risk assessment
- Set to a level that recognises the time taken to initiate effective response.
For example, flammable gas concentration trigger levels may be increased in increments including ‘Not Detectable’, 5% LEL, 10% LEL and 50% LEL to ensure flammable gas concentrations are controlled to below 100% of the Lower Explosive Limit. Toxic gas concentration trigger levels may be increased in increments including ‘Not Detectable’, 10% OEL, 50% OEL, and 100% OEL to ensure unprotected exposure to toxic gas is controlled below respective OELs.
Gas concentration trigger levels assigned within the TARP should correspond with latched and unlatched audible alarm trigger levels applied to the gas monitoring equipment. This will ensure adequate warning is provided to coal mine workers when the TARP actions should be initiated. Coal mine workers should understand what action to take when a gas detector alarms and how to apply the Trigger Action Response Plan (TARP).
Recording gas monitoring results and maintenance
The results of gas monitoring should be recorded for risk reporting and communication purposes, to inform risk assessment processes, incident investigations and for future reference. It is recommended that as a minimum the results of gas measurements be recorded on the OCE’s statutory inspection report for the shift when sampling was undertaken. The record should include the time when the sample was collected, the type of gas, concentration measured, sample location, and observations of atmospheric/weather conditions and comments. Where multiple samples are collected, the results should be recorded in tabular format and attached to the OCE’s statutory inspection report. The record of gas monitoring results should be communicated in a timely manner and be accessible to all coal mine workers who may be affected and/or who work in the affected area.
Records of gas detector maintenance and inspections should be retained for at least one calendar month (for weekly checks) or 4 times the scheduled test period (i.e. monthly records retained for 4 months from testing date). However, it is recommended that sites retain all calibration, maintenance and repair records for the life of the instrument.
The following recommendations are made in addition to the matters included in this Safety Bulletin.
- Senior Site Executives must ensure that persons instructed to undertake gas monitoring activities at the mine are trained and authorised to operate the gas monitoring equipment. Training courses which include the relevant knowledge and skill competencies required for monitoring coal mine gases in open cut coal mines and maintaining gas detector equipment should be acquired through a registered training organisations. (e.g. Queensland Mines Rescue Service, Simtars and Coal Services).
- Coal mine workers are to be made aware of the potential gases present at the mine, during induction and ongoing refresher training.
- A Trigger Action Response Plan (TARP) should be established and implemented at the mine to define a systematic approach which detects and responds to incremental changes in gas concentration in such a way which ensures that flammable and toxic gas is controlled to an acceptable level of risk. The Gas TARP should be referenced within the Standard Operating Procedure for gas management at the mine.
References and further information
Authorised by Peter Newman - Chief Inspector of Coal Mines
Contact: Fritz Djukic, Senior Inspector of Mines, +61 7 4999 8504 Fritz.Djukic@rshq.qld.gov.au
Issued by Queensland Department of Natural Resources, Mines and Energy