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Safety and health in mines research advisory board

Annual Review 2005


CONTENTS

FIRE AND EXPLOSION

Advanced Fire Detector for Coal Mines

In the UK, automatic fire detectors detect fewer underground fires than do mine personnel, particularly during the early stages of smouldering and smoke emission. As collieries become more automated, there are fewer personnel to detect fires. Current fire detectors in use in UK coal mines are under-utilised, are not user-friendly, have performance limitations due to interferences, and are obsolete. 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. Objectives were: to develop a prototype, products of combustion monitoring system for potential use in underground mines based on a sensor array; and, to recommend how the system may be further developed for actual use underground.

The following sensors were found to be unacceptable for use in an advanced underground fire detection system:

The following sensors were found to be the most promising:

The recommendations were that an advanced mine fire detector system should be based on a combination of an optical high sensitivity smoke detector (HSSD) fitted with a cyclone to remove coal dust; and nitric oxide or nitrogen dioxideelectrochemical sensors to distinguish smoke from diesel exhaust. If such a system proves to be too expensive then an alternative could be based upon a combination of blue/infrared optical smoke detector, which distinguish fires and diesel exhaust from coal dust, and a nitric oxide or nitrogen dioxide electrochemical sensor.

Prior to evaluation underground, to investigate its response to coal dust and general pitworthiness, the project partners met to develop the HSSD specifications, testing regime and select a suitable underground testing site.  A stainless steel enclosure was designed and built to contain the HSSD selected for underground testing. The HSSD was characterised and then optimised for underground use. The HSSD samples all airborne particles below 10 μm. In order to prevent coal dust being sampled and causing false alarms a size selective cyclone needed to be fitted upstream of the HSSD to prevent particles larger than about 1 μm from being sampled. The size selective parameters for a cyclone were calculated and a suitable cyclone obtained. However, the cyclone restricted airflow through the HSSD so that it could no longer operate. An in-line booster air pump system was developed to keep the airflow within the acceptable limits of the HSSD and the complete system was tested in the laboratory.

The partners met again at Welbeck Colliery to discuss and identify the optimum test location in the mine; finalise the design of the HSSD, cyclone, pump and enclosures for the specific conditions found at this location; and agree operational protocols. It was decided that the testing underground would be carried out in two configurations: without the cyclone fitted to determine if the HSSD could detect smoke even when it was monitoring coal dust levels; and with the cyclone fitted to distinguish smoke from coal dust if the coal dust levels were too large or variable.  The HSSD sampling pipe work was altered to meet the new requirements and the complete system tested.  Underground testing is to be carried out at Welbeck Colliery during 2006.  If successful, it is hoped that commercialisation of the multisensor system comprising of a HSSD and oxides of nitrogen and carbon monoxide sensors, will ensue. It is expected that development and production of such a system would require collaboration between a mine monitoring equipment supplier and a smoke sensors supplier.

Oxygen Hose Failure

HSL carried out an examination on a failed hose from Rawdon Mines Rescue Station.  The hose had been attached to an oxygen cylinder which was part of a rescue breathing apparatus.  At the time of the incident the cylinder was being recharged with gas at a pressure of approximately 200 bar.  The cylinder had been placed in a bath of water to cool while the charging operation took place and it was under these circumstances that the hose failed.

The items sent for examination comprised a length of 14mm diameter hose, a stainless crimped ferrule that had parted from the hose and a fractured brass connector that had fitted within the free end of the separated ferrule. The construction of the hose comprised an internal lining hose of PTFE, two layers of braided type 304 stainless wire separated by a polymer wrap and an outer polymer sheath.  The fractured wires protruding from the separated ferrule appeared badly pitted and were covered in oxidation products, in particular, the wires of the internal layer of braiding.  Additional evidence of oxidation was found on the remnant coupling still attached to the ferrule and on the PTFE lining of the hose.  

Hardness measurements were carried out on wires from the inner and outer layers of braid both at the failure location and distant from it. A dramatic reduction in hardness was observed in the inner braid wires at the failure location, to a value well below half that found elsewhere in the hose.  Metallographic examination of these wires revealed that this reduction in hardness was accompanied by an alteration in the grain structure from cold drawn to recrystallised, an effect concomitant with exposure to high temperatures.  Heavy oxidation residues on the outer surfaces of the wires and evidence of heat damage to the inner hose supported this evidence.

Overall, the evidence presented by the components indicated that the hose failed as a consequence of the ignition of the PTFE liner and a severe reduction in the tensile properties of the inner braid wires.  This was reflected in the measured hardness levels.  It is very probable that the source of the ignition was a temperature rise at the failed end of the hose consistent with adiabatic compression of residual air in the hose when the recharging apparatus valve was opened.

There was no evidence to suggest that, if the user had no knowledge regarding the conditions giving rise to this form of failure that the hose was inappropriate for its application.

Updated 2009-05-22