Safety and health in mines research advisory board
Annual Review 2004
Advanced Fire Detector for Coal Mines
In UK underground coalmines, fire alarms detect fewer fires than mine personnel, particularly during the early combustion stages of smouldering and smoke emission. However, as collieries become more automated, there are fewer personnel to detect fires. A recent survey by UK Coal showed that current fire detectors, based on semiconductor sensors which detect gaseous products of combustion, are under-utilised. This is because they are (a) susceptible to interferents, resulting in false alarms and instrumental drift; and (b) not user-friendly. They are also virtually obsolete. Carbon monoxide sensors, however, are better for detecting spontaneous combustion and more developed fires. More recently, a withdrawal of men from Kellingley Colliery occurred where equipment did not detect a significant conveyor failure, which produced large amounts of smoke. This joint project, involving HSE, the Health and Safety Laboratory (HSL), UK Coal Ltd and TES Bretby Ltd, was therefore initiated to investigate the performance of proposed solutions to the sensor shortage, and explore the potential of an 'intelligent' fire detection system, i.e. an array of sensors having the sensitivity of the existing system but not susceptible to typical interferents, e.g. diesel exhaust, methane, coal dust.
A wide range of sensor types were initially employed: potential replacement semiconductor sensors, ionisation smoke detectors (as used in domestic alarms), photoelectric smoke detectors (as used in domestic alarms), advanced two-colour photoelectric smoke detectors (as used in commercial aviation applications), a current coalmine fire detector ('FIDESCO') for products of combustion and carbon monoxide, nitric oxide and nitrogen dioxide electrochemical sensors, and a smoke ionisation alarm (produced for coal mines). Fire tests were performed in the full-scale, surface training gallery at UK Coal's Welbeck Colliery. The sensors were exposed to smouldering conveyor belt, coal, wood, oil and grease; a flaming diesel pool fire; and diesel exhaust fume from a free-steered vehicle. Tests were also carried out in the laboratory to investigate interferences and sensitivity for certain sensors.
The first series of tests at Welbeck, coupled with collaboration with Kidde plc, who loaned the smoke detectors and provided further experimental support and technical advice, and the knowledge that the radioactive ionisation detectors would generate safety problems and be obsolete in the near future, led to a refinement of suitable sensors. Sensors chosen for further tests were a two-colour photometric smoke detector (blue and infrared) that
can distinguish smoke from coal dust but cannot distinguish smoke from diesel exhaust; nitrogen dioxide or nitric oxide sensors to distinguish smouldering fires from diesel exhaust; and carbon monoxide sensors for general body monitoring.
Further tests at Welbeck and at a belt testing laboratory, which generated high concentrations of fume, highlighted the requirement to improve the sensitivity of the photometric smoke detection method in order to detect fires in mine roadways at extended intervals. The proposed solutions were to test a commercial, high sensitivity laser-based smoke detector (but higher cost than the two-colour photometric detector) or to modify the two-colour photometric detector, which will require further development (by Kidde). The first option is now being pursued. If successful, it is hoped that commercialisation of the multisensor system (high sensitivity smoke, oxides of nitrogen and carbon monoxide sensors) will ensue.
A paper, Development of a fire detector for underground coal mines, Walsh et all, describing this work was accepted for the Safety in Mines Research Institutes conference at Brisbane in October 2005, as were two others on work describing in previous SHMRAB reviews: Review of experimental comparisons ofropean coal mining explosives and the recent history of permitted explosives in the UK, Pickering et al, and Introduction of new personal dust sampling methodology into the UK coal mining industry, Gilmour et al.
Explosion Protection of UK Coalmine Methane Drainage Systems
This HSL study was undertaken at the request of HSE's mines inspectorate to compare the existing measures in UK coalmines to prevent the transmission of flame through methane drainage systems with National Standards and Guidance. It was prompted by the increased use of methane flaring and utilisation for heat and power generation as a result of the UK's ratification of the Kyoto Protocol, which requires a cut of 5% of the UK's greenhouse gas emissions below the 1990 level.
Visits were made to a number of collieries to collect details of the different methane drainage systems at each. An overview best practice and guidance available in UK and International Standards for explosion protection was prepared.
The key devices in colliery applications are the safety devices employed in methane drainage systems to prevent explosion propagation from the surface back down into the mine. The different types of passive device (deflagration and detonation flame arrestors) and active device (slam shut valves and extinguishing systems) were described. The relative merits of both types of device in terms of operational requirements, maintenance and reliability were compared.
The study concluded that the use of passive deflagration and detonation flame arresters as part of a mine's methane drainage explosion prevention control measures (which are based on established explosion prevention hierarchy), is probably still the best available method of ensuring that flames are not transmitted along the methane drainage pipe work. Further work is being considered.
Reaction to Fire of GRP Methane Drainage Pipes
HSE's mines inspectorate commissioned HSL to compare the performances in a fire situation of steel pipework and glass reinforced plastic (GRP) pipework used in a methane drainage system following a small fire in a booster fan at a mine where GRP pipework was installed in the same roadway. The parameters of particular interest were:
- Resistance to ignition
- Contribution to the fire load if involved in a fire from another source.
- Integrity of containment of methane when pipes are involved in fires.
The GRP pipes in use underground are manufactured in various sizes using a phenolic resin with glass fibres. In order to impart a degree of resistance to the hazard from accumulation of static charge, the material is impregnated with an electrically conductive material. The manufacturers, Flexadux, were understood to have subjected their products to various flammability tests, which yielded favourable results.
The HSL investigation has begun with a review of the literature available relating to the fire performance of glass fibres impregnated with phenolic resin. The results of this suggest that the material is resistant to fire and maintains its integrity at relatively high temperatures, when compared to other types of resin impregnated glass fibre products.
HSL are to perform some fire tests using GRP pipework of similar type to that used underground aimed at evaluating the strength of such pipework to resist rupture under explosion pressures such as may be generated by the ignition of a methane-air gas mixture in a concentration within the flammable range. The integrity of GRP pipes will also be evaluated when these materials are subjected to heat from an external fire.
Scoping Study for Ignition Prevention on Underground Conveyor Idler Rollers
As part of a wider program to investigate the problem of bearing failure in relation to ATEX compliance, HSE's mines inspectorate also commissioned HSL to review possible techniques that could be used for early failure detection. Following meetings with the mines inspectorate and UK Coal, a review of different techniques has been carried out and Explosion Control Section at HSL has identified possible techniques for detecting bearing failure, but also other potential failures.
The methods identified for detecting a failing bearing fall into two categories: sensing the heat generated or monitoring the changes in vibration (noise). Issues such as the suitability of such techniques for use underground, and how they would be implemented (subdivides into continuous monitoring, sampled or manual monitoring) were discussed. Many of the methods identified are commercially available, but require some adaptation to the mining environment. The large number of bearings involved has practical implications, and makes cost and time considerations of paramount importance.
The sensing methods identified are:
- Thermometry techniques. These include infrared, optical fibre, thermocouples and thermistors.
- Temperature transition identification techniques. These include heat sensitive paints, low melting point alloys, bimetallic strips and thermomagnetism.
- Thermal imaging - already implemented in a prototype study but probably needs to be backed up by a further technique.
- Noise and vibration monitoring. These include accelerometers detectors, ultrasound microphones and associated digital signal processing techniques.
The next stage will be to identify a strategy based around a couple of these techniques (such as thermal imaging plus temperature sensitive paints) which may then lead to testing underground and at HSL.
Tests to Compare Failure of Idler Roller Bearings
An earlier HSL project in conjunction with Continental Conveyer Ltd looked at the failure of bearings in idler rollers with a view to establishing the risk of ignition of explosive mine atmosphere. This work was initiated because of the increasing number of lubricant fires that occurred with such equipment, and the need to identify and prevent all sources of ignition as required by the ATEX Directives. While clearly lubricant fires directly cause a source of ignition, they occur at much lower temperatures than for an explosive mine atmosphere based on the relative auto ignition temperatures.
The earlier work had developed a test apparatus for 25 mm diameter bearing rollers and showed that failure could lead to temperatures across large surface areas which were in excess of auto ignition temperature of explosive mine gas and dust atmospheres. However, the work also identified a potential shortcoming in the use of nylon caged roller bearings in such rollers, as this type of roller may fail to produce an incendive situation more quickly than other types of bearings. The mechanism for this to occur is that after a temperature rise of 100 to 150oC in the bearing, the cage softens leading to rapid loss of the ball bearings. This results in steel rubbing against steel, and can lead to temperatures greater than 600oC occurring quite rapidly.
This project, again with Continental Conveyer Ltd, will use the test apparatus at Buxton to study the failure of metal caged bearings and investigate their speed of failure, the temperatures generated and so begin to understand if these bearings pose a reduced risk to those with plastic cages.
UPTUN - Upgrading Methods for Fire Safety in Existing Tunnels
Mines Rescue Service Ltd (MRSL) completed its contribution to this project under theropean Commission (EC) Fifth Framework programme with research on two tasks:
Task 1: State of the Art Review, targeting six specific areas:
- Safety management and emergency preparedness issues.
- Rescue under conditions of high physiological stress, in particular extreme conditions of heat.
- Irrespirable atmosphere physiological effects, oxygen costs of escape and life support options.
- Movement through smoke in terms of orientation and wayfinding.
- Systems of refuge, concentrating on maintenance of a life-supporting atmosphere and control of the refuge thermal environment.
- Tunnel communications technology.
Task 2: Development of an Evacuation Support System
The major work involved the investigation of a purpose-designed active audible/visible beacon system to aid guidance and evacuation through smoke; the research considered visible, audible and tactile directional cues. To assist in direction finding through heavy smoke, sound localisation techniques were considered.
Each beacon is independently powered by an internal battery, which is inductively charged from a line carrying a high frequency current that couples contactlessly through each unit. The single charging line is also used to send and receive commands from individual units or groups of beacons. This provides a real-time facility to monitor environmental conditions and call alerts at each beacon, together with the potential capacity to update direction information, responding to the development of a fire. Each unit is fitted with a precision temperature sensor and a dual range CO sensor, providing an ability to detect fires and subsequently monitor fire situations throughout the tunnel or structure.
The use of a high fire withstand, ceramic clad wire is proposed for the charging line, which could also provide tactile cues. In addition, the system could provide a platform to incorporate acoustic instruction and guidance information.
The strategy has been to reduce beacon cost and installation complexity, to allow beacons to be relatively closely spaced, and to provide a near continuous sequence of guidance cues, even where tunnel refuges or intermediate exits are relatively widely spaced. With further commercial development, this proof-of-concept system is considered to have application potential across the generality ofrope's road, railway and metro tunnels.