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Safety and health in mines research advisory board

Annual Review 1998


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 1998.

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 1998 on research within SMRAB's remit.

FIRE AND EXPLOSION

Ignition Control in Rapid Drivages

International Mining Consultants Ltd (IMC) completed a study, supported by the European Coal and Steel Community (ECSC), into the more effective prevention of gas ignitions in rapidly advancing headings driven by continuous miners.

The project investigated the application of remotely located water spray nozzles for improving control of frictional ignitions. An optimised system, comprising 14 solid cone spray nozzles located on the machine jib covering a cutting width of 3.3m, was developed. Operated at a pressure of 22 bar and a flow of 84 l/min, this system was capable of reducing the mean ignition probability for the area across the top of the cutter drum from 40% to a predicted 8%. Furthermore, it was shown to give at least the same ventilation of the cutting zone as the current system of 8 hollow cone sprays which it was intended to replace. However, the penalty for this reduction in ignition probability was a significant increase in water consumption by a factor of more than four. The increased water requirement is likely to cause problems at sites which have water-susceptible mudstone floors.

During the course of the project, machine and equipment manufacturers had renewed efforts to develop wet cutting head systems for continuous miners, intending to overcome previous poor reliability problems. Joy Mining Machinery developed a wet head for their range of continuous miners and this has undergone field trials in the USA. Hydra Tools offered a phased water supply to nozzles with the potential to provide greater ignition source quenching over the full cutting zone and efficient dust suppression for similar rates of water usage. A further collaborative project to study such wet head systems was started late in the year. Surface trials were undertaken on a wet head system manufactured by Hydra Tools, fitted to a Joy 12CM5 machine loaned by RJB Mining. This phased system, where 56 of the 84 picks on the 3.1 m cutting head are wet at any one time, was shown to be capable of ventilating the cutting zone. However, to provide equivalent ventilation to the current standard system of remote hollow cone sprays required the 1 mm orifice pick sprays to be operated at a minimum of 5.5 bar, 1.3 l/min/spray, with the head rotating. Such levels of water application are also sufficient to influence ignition probability, reducing this to approximately 8%. Required total water usage for such a system is of the order of 80 to 90 l/min.

Of the two systems described, each requiring similar levels of water consumption, the wet head system appears to have the greater potential. However, water consumption is believed to be too high for routine use at most sites in the UK. More practical options, involving wet cutting on selective areas of the cutting head are currently being considered.

Ignition Control on Longwall Faces

IMC also Investigated the effect of pick body material hardness on both wear rate and frictional ignition probability with picks in an unprepared condition; i.e. de-tipped and without the tip recess.

Long reach, point attack picks with three separate hardness ratings were examined; 300, 400 and 500 Vickers (30, 40 and 48 Rockwell C). In each case the contact area increased rapidly during the first 100 pick strikes. The high initial wear rate was attributed to the truncated cone shape of the pick body and the initial wearing of the sharp edge. As anticipated, the softest pick (300V) had the highest wear rate. In comparison, both the 400V and 500V picks developed a wear flat of approximately half that obtained with the 300V pick over the same number of pick strikes.

Experiments to investigate the influence of hardness on frictional ignition probability showed a general trend for ignition frequency to increase as the contact area of the pick increased. The experiments conducted with the three prepared picks yielded mean frictional ignition probabilities of 81.3%, 86.7 % and 76.5 % for the 300, 400 and 500V hardness ratings respectively. The results indicated that for a given contact area, variations in pick body harness of between 314 and 554 Vickers have no significant effect on frictional ignition probability

Although pick body hardness was shown to have little effect on ignition frequency, hardness did have a significant effect on the contact area. Given that the maximum ignition probabilities were obtained with the largest contact areas, the results suggested that a 300V pick losing a tip whilst cutting underground would reach the maximum ignition frequency in approximately half the time of a pick with a hardness value of 400 to 500V in the same condition. The results confirmed that the standard hardness rating of 400V, used currently in the UK, is possibly the best compromise between wear resistance and strength.

Practical requirements for general stone dusting in UK mines

Regulations require the stone dusting of mine roadways to avoid coal dust explosions but these were drawn up many years ago. Mining methods have altered since and it is likely that the particle size distribution of coal dust has changed. Recent research has suggested that the proportion of stone dust should increase as coal particle size decreases. To ensure stonedusting requirements are effective for modern times, HM Inspectorate of Mines (HMIM) commissioned the Health and Safety Laboratory (HSL) to provide the scientific information needed for soundly-based regulations on the practical stonedusting requirements of roadways within UK mines.

Earlier stages of the work, reported in last year's Annual Review, dealt with measurements of particle size distributions and analysis of coal dust deposits taken from mine roadways in six different UK mines. The project has been completed this year with the testing of the explosibility of dusts from two of the mines, Selby and Welbeck, using the standard 20 litre sphere apparatus.

The 20 litre sphere apparatus produces dust explosions by dispersing the dust homogeneously in the reaction vessel and applying a strong 10 kJ ignition source. It thus produces explosions with mine roadway dust mixtures that would not occur in practice. It does, however, clearly demonstrate trends in the explosibility of coal dust / stone dust mixtures and was used for this purpose. Explosibility characteristics (KST values) have been measured.

The results showed that the KST values and maximum explosion pressures of the Selby and Welbeck coal samples progressively decreased with increasing incombustible content. At 72% incombustible content, required by existing legislation for the Selby and Welbeck coals, a weak explosion was generated in the sphere with KST values of 42 bar m/s and 27 bar m/s measured respectively. With 92% incombustible content in the Selby and Welbeck samples the KST values were reduced to 17 bar m/s and 11 bar m/s respectively. The finer Selby coal required a larger quantity of incombustible material to give an equivalent inerting effect on the sample. Approximately 2-3% more incombustible material was required to produce an equivalent KST value in the Selby admixture when compared with the Welbeck coal.

The main recommendation arising from the work is that the current regulations on stonedusting should not be relaxed, as the test results indicate a potential for a weak explosion with the current stone dusting regulations. Whether the stonedusting requirements for UK mines should be increased is a matter for judgement based on experience in mining practice and literature data, as well as the results of this work.

Pan-European Comparison of Mining Explosives

A European initiative to compare the performance of mining explosives against national standards in France, Germany, Spain and the UK began this year. The aim of the project is to enable the reduction of statutory testing barriers to trade in mining explosives between participating countries. Explosives have been exchanged between DMT (Germany), INERIS (France), LOM (Spain) and HSL.

As part of the programme, a UK Group P4/5 explosive is being examined by LOM in Spain and the performance of Spanish Class II and Class III explosives is being examined at HSL Buxton. The HSL test programme included measurements of the strength of the explosive using the UK Ballistic Mortar test apparatus. Early results indicated that the Class II explosive may have performance and safety characteristics similar to those of a British type P3 explosive and the Class III may be similar to a British type P5. The work is continuing.

Mine Fire Detection

The purpose of this further work, recently completed at HSL, was to carry forward progress made on earlier projects on mine fire detection. The objectives were to:

An analysis of mine fire data over the period 1989 - 1996 showed that environmental monitors first discovered 13% of the fire and smoke events which lead to withdrawals. People continue to account for the majority of such discoveries, although the fraction of fires discovered by monitors has increased over the years.

Work on the evaluation of fire detection instrumentation, using the simulated mine roadway at HSL Buxton, showed that products of combustion (POC) sensors have the highest sensitivity for various types of smouldering fire - including belt, coal, wood and oil - compared to carbon monoxide sensors and ionisation smoke detectors. However, POC sensors are susceptible to interferents, resulting in false alarms, and instrumental drift which has limited their use. An instrument with the sensitivity of existing POC sensors but not susceptible to interferents, humidity and drift, would therefore be extremely useful as an advanced warning fire detector. An 'intelligent' monitoring system to address this problem might be based on a sensor array and artificial neural network.

The test methods used by industry to decide on the placement of fire detection equipment were observed, critically reviewed and compared with other available methods. The results of this work have been incorporated into recently revised industry guidance.

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Updated 2010-08-04