Underground » Health and Safety
In view of recent mine disasters in the coal mining industry in Australia, there is a need to develop strategies to enable the introduction of Self-Contained Self Rescuers (SCSRs) into underground mines. These strategies must be part of an overall strategy to maximise the likelihood of survival of a person attempting to escape in an emergency involving fire or explosion. In order to fulfil this aim the project team set out to gather field data in order to develop methodology to predict how much oxygen is actually needed by an individual to escape from an underground mine.
The project team considered the effects of personal and environmental factors upon the duration of SCSRs, escape times, distances travelled and average heart rates. The study was an opportunity to assess the capability and comfort of SCSRs. In order to facilitate the introduction of SCSRs, training issues relating to SCSRs had to be identified. This research should assist mines to develop a practical system for escape planning which accounts for relevant issues pertaining to the use of SCSRs.
The SCSR in this project uses a chemical, potassium superoxide (KO2), which reacts with the wearers breath to produce oxygen (O2) and absorb carbon dioxide. A SCSR works in a closed circuit whereby the wearer exhales back through the chemical into a breathing bag for re-use. As oxygen is produced, any excess to an individual's requirement is released to the atmosphere via a pressure relief valve situated in the breathing bag. MSA Portal-Pack SCSRs with a nominal duration of 60 minutes were used in the trials. Approximately 100 litres of oxygen is available to the wearer. If oxygen consumption, or the rate at which the wearer consumes oxygen, is VO2(litres/min), then the duration of a SCSR (oxygen "run out" time) in minutes is as follows:
SCSR Duration (minutes) =
Useable Oxygen (litres) 100
------------------------------- @ ------
Oxygen Consumption VO2
(litres/minute)
Previous studies on oxygen consumption during an exercise described three models which related oxygen consumption to heart rate (one model related oxygen consumption to both heart rate and body weight), thus enabling oxygen "run out" time for a SCSR to be predicted from measured average heart rate. By measuring heart rate during exercise (work) the amount of oxygen consumed by the wearer can be estimated. This physiological model underpins the methodology used in this project.
The major project activities are shown in fig. 1. of the final report and are described in detail in the main body of the final report.
Project Method
The project was conducted in three stages:
Measurement of oxygen consumption while wearing a SCSR.
Duration of SCSRs in underground field trials.
Effects of heat and humidity on duration of SCSRs.
Measurement of Oxygen Consumption
Six volunteers exercised on a laboratory treadmill breathing through SCSRs in a closed circuit, inhaled and exhaled gases were measured. It was apparent that individuals consumed oxygen at different rates. In view of the findings in this stage, the likely fitness levels of the majority of individuals in the Australian Coal Industry and manufacturer's advice it appears reasonable to assume 100 litres O2 will be "useable" by the wearer for the purposes of predicting oxygen "run out" time.
The SCSR is designed to produce oxygen in excess of the individual's needs. This study demonstrated that fitter individuals consumed less oxygen from the SCSR than their less fit colleagues, but lost more oxygen to the atmosphere via the pressure relief valve. Other evidence suggests, that at higher rates of work, when individuals are breathing faster and consuming oxygen quickly the release of oxygen from the potassium superoxide may be less efficient. This situation can occur in less fit individuals, therefore optimal speed of escape is critical to the most efficient use of the SCSR chemical.
Duration of SCSRs
Underground simulated escape trials, using 37 volunteers, were carried out at three NSW coal mines and one Qld coal mine. Each volunteer's heart rate was monitored. The recorded data was later downloaded into a computer program for statistical analysis.
Following the statistical analysis it was apparent that none of the reported prediction models accurately estimated the duration of supply of oxygen available to each subject. Consequently a linear regression analysis was used to develop a "University Of Wollongong" (UOW) model which relates oxygen consumption (VO2) with body weight (W) and average heart rate (HR), as described below:
VO2 = 0.012W + 0.003HR + 0.332
Caution must be observed in the application of any prediction equations for the duration of SCSRs, oxygen needs during escape and for the placement of change over stations. Prediction equations were developed from the results of trials using new SCSRs. Therefore the equations must be regarded purely as a guideline and calculations should allow a "safety margin".
All volunteers completed a subjective assessment of the comfort and operational efficiency of the SCSRs. Most of the subjects reported some discomfort although it did not impede their capability to escape.
Effects of Heat and Humidity
The third stage of the project, using six volunteers, was conducted on a treadmill in a hot and humid chamber to determine the effects of heat and humidity on oxygen "run out" time. Each subject was required to walk on a treadmill at a constant speed wearing full underground apparel on three separate occasions 24 hours apart. The temperature and humidity were increased on Day 3, to simulate heat and humidity conditions in an atmosphere in the vicinity of an underground fire or post explosion situation.
Using the UOW formula, the findings in this stage indicated that the duration of SCSRs would not be significantly affected by extreme heat and humidity. Furthermore, evidence from the scientific literature suggested that the greater portion of the increase in average heart rate in hot and humid environments is more likely to reflect the body's attempts to prevent heat related illness. It was thought that the risk of heat related illness was of a concern during escape in hot and humid conditions. Strategies to counter heat stress must be considered in planning and training for escape.
Conclusions
- This study has shown that oxygen "run out" time can be predicted using body weight and average heart rate. To date the UOW model has the closest predictive value. The UOW model accounts for the degree of physiological strain on the individual and is reflected in the average heart rate.
- A simple table, developed using the UOW model, can be used to estimate duration of SCSRs and be a guide in establishing changeover stations. The UOW model requires a representative sample of mine workers, an established escapeway and minimal technology (bathroom scales and heart monitor).
- Speed of travel, intensity of work (or exercise) and individual characteristics influence oxygen consumption. Heavier individuals would be expected to use more oxygen than persons weighing less. While wearing SCSRs the efficiencies gained by physical fitness are offset by the escape of oxygen via the pressure relief valve.
- Panic is a potentially a major problem, as wearers may hurry and therefore poorly utilise the chemical in SCSRs. The development of escape procedures must include visible markers whereby speed of travel is optimised to achieve maximum use of the available SCSR chemical. It is likely that complying with procedures laid down in training for controlled escape will maximise the likelihood of survival of a person attempting to escape in an emergency involving fire or explosion.
- Further research is urgently necessary to develop a non-destructive method to evaluate the integrity of SCSRs carried by miners during the course of their normal work. Powdering of KO2 granules due to vibration caused by carrying or transporting the SCSRs may have a significant effect on the duration of SCSRs.