Ventilation controlled fires in tunnels
This Health and Safety Laboratory (HSL) project is jointly funded by the European Coal and Steel Community (ECSC) and the Health and Safety Executive (HSE) mines and railways inspectorates. HSL’s part is an experimental investigation of the conditions for and characteristics of ventilation controlled tunnel fires - fires in which the heat output is related to the rate of supply of air to the fire rather than the availability of fuel. This has particular importance for:
Progress has been made during 2001 towards finalising the experimental design and programme to be carried out during the spring and summer of 2002. These experiments will be conducted using 600 mm ducting and comprise two seats of fire. The first will comprise a gas burner located near the duct inlet merely to provide a vitiated atmosphere further downstream in the duct. This atmosphere, characterised particularly in terms of oxygen, CO and CO2 concentrations, will then be used to ventilate a series of test fires using a variety of materials including conveyer belting PMMA (Perspex - a common fire test material), a flammable liquid pool and a wooden crib. Downstream the duct will be instrumented with thermocouples, smoke opacimeters, load cells and gas concentration sensors. Some initial experiments to examine the variation of flame morphology with ventilation rate will be followed by tests to measure the response of smoke generation, CO/CO2 ratio, heat release rate and rate of flame spread with ventilation rate and the extent of upstream vitiation.
Prediction of Fire Effects in mines
The University of Nottingham (UoN) are also now involved as partners in the ECSC underground fires project, having taken over the contract from IMC Technical Services (IMC-TS). UoN are to conduct a series of Computational Fluid Dynamics (CFD) simulations to replicate a series of fire gallery tests performed in 2000 by IMC at the former Swadlincote test facility and reported in last year’s review. In spite of the delays caused by the contract reallocation, the UoN team is on schedule to complete the work before the end of 2002.
Fire Safety Testing of Conveyor Belts
The large scale test gallery facility at Swadlincote has been used for many years for the assessment of the fire propagation properties of conveyor belting intended for use underground in the UK and is referred to in BS 3289:1990. Its imminent closure prompted HSE to sponsor Cerberus (Mining Acceptance Services) Ltd to carry out a project with the following objectives:
The main contributors to the project were Fenner Ltd, who are the only manufactures of this type of conveyor belting in the UK, and Cerberus (Mining Acceptance Services) Ltd, who continued to test conveyor belting following the privatisation of British Coal.
The project satisfactorily achieved all three objectives. A new test method has been provided together with drawings of the apparatus needed and proposed acceptance levels.
The results have been published as Contract Research report 407/2002, which is accessible on the HSE Website and in hard copy from HSE Books. Subject to further discussion with the DMCIAC, the outcomes of the research might form the basis of amendments to BS 3289, which is a standard referred to in the Model Rules that accompany The Coal Mines (Owner’s Operating Rules) Regulations 1993.
The aim of this HSL project is to produce a risk assessment tool to improve fire safety systems and awareness in underground mines. This is to be achieved by reviewing incident history for fires in both coal and non-coal mines, the regulatory framework, and the inventory of combustible materials used underground, along with the range of standard fire tests applied. This information is to be input into a risk assessment framework, identified during the project, to provide a means of quantifying the risks present by particular groups of materials and benefits of applying conditions to their use and various possible means of mitigation.
The assessment system has now been developed and passed to HSE’s Mines Inspectorate, the project sponsors, for comment. Broadly it is based on the use of a flow chart and a number of check sheets used to assign a simple ranking to areas of possible concern. Those showing a high ranking, either due to past involvement in incidents, the presence of fuels and ignition sources together, or known susceptibility to standards of housekeeping or maintenance are treated in further detail. The output of the system is a documented record of all aspects of the areas considered, well considered fire suppression/detection systems, and input into the mines fire and evacuation strategy through a knowledge of the nature of possible fires and the number of personnel at risk.
Two applications of the methodology will be undertaken, viz: the assessment of fires on conveyor systems and vehicles used underground. The system might also be piloted in an audit of a mine’s fire safety strategy.
Idler bearings used in coal mine belt-conveyers have long been recognised as a potential fire source. They are designed to meet a L10 failure rating: under full axial and radial load conditions, 10% of a sample of these bearings are expected to fail within 106 revolutions. There are many of these bearings in use underground and inevitably failures occur which go undetected for long periods and in the past have led to lubricant and coal dust accumulation fires. What is not so well understood is whether bearing failures might present an ignition hazard in relation to flammable gas and coaldust-air atmospheres by some mechanism other than that which results in fire. The European Explosive Atmosphere Directive (ATEX) covers such non-electrical ignition sources and European Standard PrEN13463-1 will require the hazard to be assessed. This project was set up to get a better understanding of what the hazard involves and the extent of the assessment which might be appropriate.
HSL is working in conjunction with Continental Conveyers Ltd (CC), one of the main suppliers of conveyer systems. The project is progressing on two separate phases in parallel. These are:
The basic design of the apparatus, developed in conjunction with CC, was completed by the year end and will use driving wheels to turn the roller avoiding drive-belts, which CC found to have limitations in similar testing applications.
CFD modelling of gas flows in coal mine goafs
This three-year HSL project began in April 1999 and is part-funded by the ECSC programme. The main objectives are to use computational fluid dynamics (CFD) modelling to improve understanding and control of methane emissions from the goaf and longwall faces. In addition CFD will be used to improve understanding and control of spontaneous heating in the goaf by optimisation of inert gas injection systems.
The control of methane emissions from goafs and at longwall faces has been studied using two differing CFD approaches:
The first examined the flow in a longwall face, back return and goaf. Three dimensional simulations have been undertaken to examine flow behaviour and the build up of methane concentration at the return end of the face. The effect of sealing efficiency on methane concentration at the face return has been demonstrated.
The second set of simulations demonstrates an approach to the simulation of methane drainage. CFD and geotechnical modelling have been used together in these simulations. The geotechnical modelling allows rock permeabilities to be calculated that are based on strata specific data and include influences of mining on permeability, for example, fracture formation. More recently, further simulations have been undertaken, improving the representation of the strata and the methane drainage process in the CFD model.
Mines Rescue Arrangements – Future Options
IMC-TS completed a study for the HSE which examined (a) the present structure for mines rescue within the UK, (b) the structures of other, overseas, mines rescue organisations and (c) the structures of other search and rescue organisations.
During the study, discussions were held with all the principal mining organisations within the UK who are associated with the UK Mines Rescue Scheme. These included the MRSL functions at Mansfield, Dinas, Houghton-le-Spring and Crossgates, UK Coal, Scottish Coal, Tower, Betws Anthracite and the Coal Authority. Overseas mines rescue organisations examined included those in France at HBL, Germany, Spain, the USA, Canada, Ukraine, Australia and the RSA. Outside mining, discussions also took place with the DTLR search and rescue functions within the Maritime and Coastguard Agency and the UK Fire and Rescue Service.
The study demonstrated that, throughout the world, there exists a wide range of models for providing emergency services to mines. However, despite each one purporting to provide adequate arrangements for an effective response in the event of an incident, none provided the level of cover demanded in the UK. Relatively few organisations guaranteed a response within one hour and many only guaranteed attendance of a first response team, not the first and second response teams.
The study concluded that the existing UK mines rescue arrangements provide one of the most flexible and low cost means of ensuring effective mines rescue cover to both large and small coal mine operators. The findings of the investigation were presented to the DMCIAC at their meeting in January. The final report should be published in 2002.