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Safety and health in mines research advisory board

Annual Review 1999


CONTENTS

INTRODUCTION

The Safety in Mines Research Advisory Board (SMRAB) is appointed by the Health and Safety Commission (HSC) as one of its independent advisory bodies. It is chaired by Her Majesty's Chief Inspector of Mines and has eight members representing employers and employees in the British coal mining industry and eight adviser members with relevant expertise in research or mining. Current members are listed in the appendix along with others who contributed to SMRAB's work during 1999.

As well as advising HSC, SMRAB's remit includes providing information to the deep mined coal industry on research matters relevant to improving health and safety standards in the industry. This review summarises progress made during 1999 on research within SMRAB's remit.

FIRE AND EXPLOSION

Ignition Control on Longwall Faces

International Mining Consultants Ltd (IMC) continued a study, supported by the European Coal and Steel Community (ECSC), Ignition Control in Longwall Faces. Experiments were conducted to assess the influence of pick body material on frictional ignition probability. Five materials were examined including the nickel/chromium/molybdenum high tensile steels types EN24 and EN25, boron steel, stainless steel type 17-4PH and the current standard pick material EN19. The materials were chosen for their ability both to be heat-treated to the required hardness and withstand the induction coil brazing process, used for attaching tungsten carbide tips. To ensure that the experimental parameters remained constant, picks from each material were heat treated to 400 Vickers. The de-tipped picks were prepared by dry cutting until a wear flat developed and consistent ignitions obtained. Subsequent dry cutting experiments were conducted to obtain the mean frictional ignition probability of each material. The results of the experiments yielded mean frictional ignition probabilities ranging between 50% and 87%. The lowest mean ignition frequencies (53% and 56%) were obtained with EN25 and Boron Steel, whilst the highest (87%) was obtained with the current standard pick material EN19. The results suggest that pick bodies manufactured from either EN25 or Boron steel could reduce the likelihood of frictional ignitions.

Work by other researchers has suggested that the presence of coal dust in the air reduces the lower ignition threshold of methane/air to below 5%. These experiments were conducted using a small test chamber, an electrical ignition source and relatively high dust concentrations. As the results could have implications for cutting zone ventilation requirements, it was important to verify the findings in a more practical situation. To enable methane/dust ignitions to be conducted on a larger scale, the Frictional Ignition Rig was equipped with a coal dust disperser and dust monitoring instrumentation. Initial experiments, to determine the effect of the ignition source on the ignitability of methane/air only, indicated a minimum ignition threshold of 5.5% using a electrical spark, and 4.7% using a worn de-tipped pick. The moisture content of the rig atmosphere was found to be a contributing factor, with minimum ignition thresholds falling as humidity reduced. Initial experiments, conducted in the presence of fine, powdered coal dust at concentrations that may typically be expected around a shearer cutting drum, have indicated no reduction in the minimum ignition threshold.

Pressure Balancing for Fighting Fire

Following the Channel Tunnel fire in November 1996, a proposal was made to the Health and Safety Executive (HSE) that if the technique of pressure balancing had been used, the fire might have been extinguished much more rapidly. The technique has, of course, been used successfully for fighting fires in mines since the early 1950s. The area around the fire is sealed off from the rest of the mine and the pressure is balanced either side of individual stoppings or across the whole fire zone, to limit the air supply to the fire and so to extinguish it. The technique requires at least a number of days to successfully extinguish a fire. Application of this technique in the Channel Tunnel would have involved balancing the pressure either side of the fire zone through the controlled use of the Supplementary Ventilation System

The Health and Safety Laboratory (HSL) was commissioned to review the technique of pressure balancing for fighting mine fires and to discuss its possible relevance to fighting fires in transport tunnels, particularly the Channel Tunnel. Although the report concluded that the technique does not appear to offer an improved method of fighting fires in the Channel Tunnel, it was nevertheless decided that the technique should be investigated experimentally. A reduced scale tunnel rig was under construction at Buxton at the year end. This comprises a duct 0.3m in diameter and about 20m long. It will be equipped with suitable ventilation dampers and fans to either end to equalise the pressure at each end of a centrally placed fire zone. The duct will be equipped with thermocouples and pressure tappings and the response of fires to different ventilation regimes will be monitored. The fires will comprise wooden cribs and liquid pools with heat outputs in the region of 100kW.

Human Factors Aspects of the Design of Safe Havens

The aim of this work by HSL was to conduct a human factors evaluation of safe haven designs currently being developed within UK deep mines. All the designs evaluated were of portable enclosures which needed to be erected and connected to vital services (air supply, telecommunications) before use. A number of prototype designs were evaluated and the following key ergonomic and engineering aspects were identified:

Pan-European Comparison of Mining Explosives

As reported in the Annual Review for 1998, a European project has been undertaken to compare the performance of mining explosives against national standards in France, Germany, Spain and the UK. The aim of the initiative is to enable mining explosives from the participating countries to be used across Europe. HSL and the equivalent testing houses in the other participating countries (DMT, INERIS and LOM) have exchanged explosives. A UK Group P4/5 has been examined in Spain and the performance of Spanish Class II and Class III explosives have been examined at HSL Buxton. All experimental work has now been completed.

The Spanish class II explosive, supplied in 26 mm diameter cartridges, had a performance and a degree of safety in flammable atmospheres equivalent to a British type P3 permitted explosive, when tested as received. However, when re-cartridged in the normal UK charge size of 32  mm diameter, the explosive failed the P3 test but met the requirements for a P1 explosive. The Spanish class III explosive was supplied in 32 mm diameter cartridges and all tests were performed using this size of cartridge. Tests showed that the class III was not suitable as a type P4 permitted explosive, but its performance, deflagration resistance and level of safety in flammable atmospheres were equivalent to those of a P5 explosive. The final composite report is in preparation.

New Test Appararus for High Current, Low Voltage Applications

New electrical equipment intended for use in explosive atmospheres has to comply with the safety requirements of European Council Directive 94/9/EC. Low voltage equipment can be certified intrinsically safe if it passes a European standard test method, provided the current is also low. Some low voltage equipment, such as miners' battery-powered cap-lamps, fail this test because the test apparatus overheats with currents greater than two amperes, even if an incendive spark is not produced. A simple, reliable and reproducible ignition test, or a method of assessing ignition capability, is needed if such equipment is to be shown to comply with the Directive.

HSL lead a European project, involving test houses in Germany (PTB) and Spain (LOM), to study existing intrinsic safety test methods and evaluate the consequences of using higher currents; and to develop a test method suitable for high current low voltage circuits and draft a procedure for its use. Following a survey of all known test methods, three sets of prototype apparatus were built at HSL incorporating modifications to a standard International Electrotechnical Commission apparatus. The modifications ensure better speed control by using a low voltage DC motor and allow for testing at up to 20 Amps by using larger diameter contact electrodes and a better brush contact system. Round robin testing has been completed in the three laboratories using the prototype apparatus and good agreement has been achieved. The final report has now been accepted by the European Commission.

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Updated 2012-01-16