Confined Gas Hazards
Evolving safety in the mining industry
The first known requirement to test for gas started in the coal mining industry two centuries ago. A by-product of coal is methane. Methane is explosive between 5% and 15% in the general body of a mine atmosphere. It is also lighter than air and would gather at roof level or coal workings that were developed to the rise.
In the early days, mine ventilation was not as efficient as it is in today’s modern mines. This allowed methane to collect and accumulate in explosive amounts. The miners would arrive at work using naked flames as the source of light and the inevitable would happen.
The recorded figures for a 10 year period between 1870 and 1880 were 2,700 miners (men and boys) losing their lives to explosions alone. The numbers not recorded prior to this period would have, if they were available, made very poor reading.
The miners discovered that if an individual was brave enough (or stupid enough) to go around the workings with a naked flame prior to the miners entering the working, this individual could seek out the methane and ignite it, making it safe for the other miners.
The fireman would cover himself from head to foot in old wet rags to protect himself as much as possible. He would then enter the workings and search out accumulations of methane. He would ignite these with his flame, which was a candle on a stick. While this was not a particularly good idea, it was effective up to a point. It reduced the number of miners’ lives put at risk and, if the accumulation of methane was low, the fireman survived. Unfortunately, in most cases the fireman had no way of knowing how much methane he was igniting other than to estimate how much could accumulate at the roof or void. If his assessment was wrong and he set off a large volume of methane and an explosion occurred he often paid with his life.
As mining developed, another gas problem came to the fore – that of carbon monoxide. Carbon monoxide is produced by incomplete combustion. It is therefore produced when fires are either starting or as the fire is almost burned out. It is also produced when a fire cannot access enough oxygen to burn efficiently. Coal is liable to spontaneously combust (self-heat) when exposed to air (oxygen), which is a very inefficient form of burning. As carbon monoxide is colourless, tasteless and odourless, it was extremely difficult for miners to detect its presence.
There are two effects of carbon monoxide. One can be the miner being overcome by a large volume of it, which would cause the miner, almost instantly, to become unconscious followed by death. The other, which is the most dangerous, is the carbon monoxide being present in small volumes. This problem is worsened by the fact that blood has a far greater affinity to carbon monoxide than it does for oxygen. Humans need oxygen to live. Oxygen is transported round the body by blood. If blood is given the choice of transporting the vital oxygen or the poisonous carbon monoxide it will choose the carbon monoxide. The mixture of oxygen and blood is not very stable, which means the blood will collect and release the oxygen with relative ease. On the other hand, blood and carbon monoxide is extremely stable, forming carboxyhaemoglobin, which it then retains, meaning the blood cannot transport oxygen. This gradual build-up of carbon monoxide in the blood affects the unaware miner who is eventually overcome.
Note: a worker suffering from oxygen deficiency would appear as very pale with cyanosis (blueness) of the lips and other areas of the body where tissues near the surface of the skin have a low oxygen content. A worker suffering from carbon monoxide poisoning will appear a ‘healthy pink’ colour.
One of the first methods used to detect small concentrations of carbon monoxide in the air was to take straw (not oxygen rich) coloured blood and expose it to the atmosphere. If it turned pink or red that meant carbon monoxide was present in the atmosphere.
Another method used to detect carbon monoxide was to introduce a warm-blooded animal into the atmosphere. This was generally, but not always, a canary. When the canary was exposed to carbon monoxide it reacted before the level of carbon monoxide became dangerous to humans. The canary became ‘excited’ and agitated. It would show this by flying and jumping around its cage. In extreme cases the canary would fall off its perch, dead. When the miners or the rescue workers noted this, they would retreat to a place of safety. While doing so, the canary was revived by closing the door of their Haldane Safety Cage, opening an oxygen supply and filling the cage with oxygen.
The canary and the Haldane Safety Cage were still ‘legally’ being used at Mines Rescue Fresh Air Bases up until the 1980s.
The obvious problem with the aforementioned methods of gas detection was that they were not accurate and put the lives of people and animals at risk.
Flame safety lamps
These were invented and developed in the early 1800s to provide a safe means of illumination in potentially explosive atmospheres.
It was then realised that the flame safety lamp could also be utilised to test for methane in the mine atmosphere. If methane was present in the mine, either in accumulations to the rise or in the ventilation general body, the flame of the lamp burned higher with a blue tinge. Some lamps were equipped with a metal gauge to measure the height of the flame, thus giving a percentage of methane present. Mining law was developed to ensure a margin of safety was introduced to protect the miners. Remember, methane is explosive between 5% and 15%; above this volume the methane displaces oxygen and prevents ignition.
Methane exposure safety:
- At 1.25% methane in the general body, electricity was switched off to remove a potential ignition source
- At 2% all miners were removed from the mine
It was also realised that miners could place the safety lamp close to the ground to detect oxygen deficiency and other gases, such as carbon dioxide, which are heavier than air and that would be found in low lying areas, also called the dip of the mine. The effect of these gases or oxygen deficiency would be to extinguish the flame of the lamp.
The flame safety lamp continued, as a legal requirement, to be used in the United Kingdom coal mining industry up until 2015.
This, in effect, created a safe means of detecting both methane and oxygen deficiency.
The early days
Intrinsically safe electrical devices to detect methane were invented and developed from the 1940s. These became known as methanometers.
The most reliable and used of these was the Mine Safety Appliance D6 methanometer. This was a handheld instrument that weighed approximately 0.47kg. Due to the size and weight of the instrument, it was suitable for taking spot samples of methane in the atmosphere. The D6 would take methane readings in the 0-5% range, detecting methane before it neared the explosive range.
The instrument worked on the Wheatstone bridge principle. This created an electrical imbalance in a circuit depending on the volume of methane present in the atmosphere. This was then measured and shown on a meter (later models were digital). High reading methanometers in the 0-100% range based on the same principle were also available.
I was once in charge of a rescue team deployed to fight a fire that had broken out in an underground substation. The smoke from the fire was travelling around the mine in the ventilation system. The miners on an operating coal face were travelling through the smoke wearing filtering respiratory protection (self-rescuers), which would have given them just over one hour’s protection against carbon monoxide.
Our remit was to reduce the smoke and carbon monoxide in the mine environment to make it easier for the miners to travel to safety. The team had to crawl for 100 metres along a distorted mine roadway to arrive at the fire area, where they immediately commenced firefighting. On pressing the D6 activation button to take a methane reading I was shocked to note the pointer going off the scale, indicating more than 5% methane. We set up a fire hose to spray water towards the fire area and retreated to an area we knew was fresh air. We tested again with the D6 and had the same effect. We re-tested with a flame safety lamp and it indicated no methane. Using the flame safety lame to test the atmosphere, we returned to the fire area to find that our fixed fire hose had been very effective in reducing the smoke. The men from the face made it to safety thanks, in the main, to the filter self-rescuers protecting them against the carbon monoxide. The fire took a further two days to extinguish completely.
Upon investigation, the counterbalance on the end of the D6 methanometer had fallen off, presumably after we had left our fresh air base where it had been checked, and probably while we were crawling in the roadway.
The lesson learnt? Protect your gas monitors; they can make life so much easier and less stressful.
Gas detector tubes were developed in the 1930s. These glass detector tubes have a scale on the side of them. They are filled with a chemical reagent that changes colour when exposed to the gas that is being detected. The sample of air is drawn through the tube by a hand pump.
The tubes are initially sealed at both ends. When a gas reading is required, the tips at each end are broken off and the tube pushed into the hand pump. The direction of insertion is clearly marked on the tube. When the sample is being taken the hand pump is operated for a predetermined number of pumps – anything from one to 10 – to draw the appropriate volume of air through the detector tube.
As the air being sampled is drawn through the tube it causes a reaction with the reagent, which cause it to change colour. The proportion of the reagent that changes colour is checked against the scale and then you have your gas reading.
There are thousands of chemical detector tubes available for numerous gases.
The disadvantage detector tubes have is with accuracy, cross contamination between gases and, in some cases, the time it takes to have a reading available, as 10 pumps takes time!
Due to the huge range of detector tubes available they are still in successful use in many industries today.
Modern detection devices utilise sensors to detect a gas (or vapour). It is of the upmost importance that you ensure that you select the device with the correct sensors and required capability: recording, memory, downloading, peak reading, alarm setting, and transmitting.
The choice of device is firstly fixed or portable.
Fixed devices are particularly useful when the gas being detected has the potential to migrate or leak into an enclosed space, potentially making it a confined space (with specified risks) and requiring compliance with the 1997 Confined Space Regulations.
A portable detector is normally a handheld device that can be utilised in confined spaces prior to and during an entry.
There are also devices available that, while not portable, are transportable from site to site with the intention of temporarily providing a fixed detection point, either while work is undertaken in an area or a permanent fixed device is being maintained.
Sensor selection depends on:
- The gases to be detected
- The range of the gas concentration
- Point or open path – point measures at the instrument while open path measures the average along the path of a beam
- Other cross contaminants – other gases that could affect readings or damage the sensors
The type of sensors available and suitable for use are:
- Catalytic (pellistor) – point, catalysed reaction, portable and fixed
- Infrared – point and open path, absorption of infrared light, portable and fixed
- Thermal conductivity – point, detection of heat, portable and fixed
- Flame ionisation – point, detects ions in an ionised gas, portable and fixed
- Flame temperature – point, monitors the temperature of a hydrogen flame, fixed
- Semiconductor – point, measures the change in electrical conductivity, portable
- Photo ionisation – point, flame ionisation using an ultraviolet lamp, portable (mainly used to detect minute leaks)
Oxygen detectors can be:
- Electrochemical – electrochemical reaction, fixed and portable
- Paramagnetic – magnetic field, fixed (oxygen atoms are attracted to magnetic fields, more accurate in its calibrated direction than in any other direction)
- Zirconia type – a ceramic that conducts electricity by the movement of oxygen ions
A different type of fixed detection device that is used where the potential for high pressure gas leaks is present is ultrasonic. This device uses an acoustic sensor to detect the sound of a leak.
A further detection method used mainly in mines is the tube bundle. A sample is drawn from many kilometres away to a sample device where a gas reading is obtained. The disadvantage of this system is the time lag between the sample being taken and a reading being obtained. The advantage is there is no risk of the sensor being poisoned, contaminated or damaged.
The following are important considerations when selecting a gas detection device.
Sampling can be by natural diffusion or aspirated (pumped).
Natural diffusion can be slow and aspirated devices can speed up the process, with the disadvantage that they then must be aspirated to get an accurate reading.
Portable devices can be fitted with probes to allow a sample to be drawn to the device.
Some devices will have the ability to memorise and record peaks and troughs in readings. For example, a portable device can be lowered into a confined space and when it is raised to the surface it can indicate the highest flammable reading recorded along with the lowest oxygen reading recorded.
The alarm can be audible and visible. Preferably it should be both and will continue to alarm until an action is taken. The instrument should also alarm under fault and low battery conditions. The device must be capable of alarming at the required trigger points and conditions.
Trigger points can be both location and/or remote and volume of gas, such as:
- Legal or regulatory requirement
- Industry standard
- Lower explosive limit
- Potential leak volume
- Area occupation
- Time for response
- Actions required by alarm
- The toxicity of the gas or vapour
Inspection, maintenance and calibration
A gas detection device is only as good as the inspection, maintenance and calibration regime, and quality controls established to support the device. As explained previously, if legal advice, the manufacturer’s guidelines and good practice are not followed, then the devices will not be effective in providing the protection they were designed, purchased and installed to do.
Knowledge and performance
Those whose lives will depend on gas or vapour detection should have a detailed understanding of the device’s operation and capability.
Even more importantly, they should have extensive knowledge and understanding of the gas or vapour being detected. There should be no lack of understanding of what, where and how the gas or vapour affects humans.
Let me give a very pertinent example of this. The company involved made fuel tanks for the aviation industry. An employee was deployed to clean the finished fuel tank with a cleaning chemical. The worker was supplied with no environmental monitoring or particular personal protection equipment. The worker was so hot inside the fuel tank that he went to unplug the inspection lamp he had been supplied with to provide illumination inside the fuel tank. When he unplugged the light, the spark ignited vapours produced by the cleaning chemical. The worker had extensive burns and is now wheelchair dependent. He has a young family and is unlikely to work again.
There was no risk assessment and no method statement relating to this job.
The worst part of this is that when the risk assessment was carried out and a method statement produced, there was no benefit in using the cleaning chemical – soapy water was just as effective.
In summary: know your detection device. know your gas.