Portable Gas Detection
Published: 10th Jan 2008
Minimising workers risk to atmospheric hazards
There are a large number of occupations that require the entry to a confined space. A definition of a confined space varies in the legislation from one country to another, but the differences in effect are slight.
In essence it is a space that is large enough to enter and perform work but has limited or restricted means for entry and exit and is not designed for continuous human occupancy. Anyone entering such a space by virtue of its enclosed nature is at increased risk of being overcome by toxic fumes, vapour and oxygen deficiency, drowning, extreme temperature or explosion.
Some areas that fall within the definition of a confined space may only be dangerous occasionally, for example during certain maintenance activities or when containing hazardous cargo. The subject of this article is to outline atmospheric hazards with the options available to minimise worker risk.
Gas hazards within a confined space
A hazard within a confined space is of special occurrence, this means that it is a combination of the place of work being confined and the presence of substances and conditions that when taken together increase the risk of injury to the worker. There are a number of common gases that need to be considered in most confined space applications, as well as additional hazards unique to a specific area of work. The nature of the risk depends on the gas that is present, in general we divide gas hazards into 3 main categories; flammable (or combustible), toxic and asphyxiant. The common gases monitored by portable gas detectors are flammable, carbon monoxide, hydrogen sulphide and oxygen depletion / enrichment.
Monitoring for flammable gases
Flammable gases present an obvious risk of explosion, typical examples are methane, hydrogen, propane and pentane although there are many more. Flammable gas is monitored in the Lower Explosive Limit (LEL) range, this is the minimum concentration needed for ignition. Typically LEL alarms are set at 10% or 20% LEL giving the worker an early warning of rising flammable gas levels. It is important that the instrument has been calibrated with or near to the flammable gas being monitored. For example, methane calibration when danger from leaks from natural gas pipelines. If an instrument is being used across a site or location with varying flammable gas risk a flammable gas selector feature is valuable. This feature allows the user to change the calibration factor meaning the instrument is suitable for monitoring a different flammable risk other than what it has been calibrated for.
Detection of toxic gases
Toxic gases are directly harmful to health, mainly by damaging tissue or causing a metabolic reaction in the human body. Usually the hazard is present at much lower concentrations from flammable gases. Due to this the concentration is measured in parts per million (ppm). The effects of exposure to toxic gas can be instantaneous or long-term where prolonged exposure in sufficient concentrations is damaging to health. The way these gases are measured and monitored, therefore, is more complex than flammable gases. Values over time must be monitored and often records of exposure maintained. The exposure over time value is known as the Time Weighted Average (TWA). Two time periods are used, these are Short Term Exposure Limit (STEL) which is averaged over a 15 minute period, and the Long Term Exposure Limit (LTEL) which is averaged over an 8 hour period.
The levels are based on current scientific knowledge and reviewed by bodies such as the HSE in the UK. This monitoring is unique for the individual using the instrument. This means that if an instrument is being used by more than 1 user then there should be a feature allowing for this to be recorded. This will keep the monitoring accurate for each different user. The uploading of recorded data to a PC or laptop should be available and simple.
4-gas variety monitoring instruments
In most confined spaces the presence of hydrogen sulphide and carbon monoxide cannot be ruled out. It is safer to be cautious and include for the monitoring of these gases on a portable instrument. This is the reason why most portable gas instruments sold are of the 4-gas variety, H2S, CO, O2 and flammable. Hydrogen Sulphide is produced by rotting organic material and also from crude oil and oil products. Carbon Monoxide is a by-product of incomplete combustion it is commonly created in heating systems and motor vehicles. Obviously there are many more toxic risks such as ammonia, chlorine, nitrogen dioxide and sulphur dioxide. These are site and industry specific but it is important to consider the ability to monitor uncommon gases when selecting instrument type. Questions such as ‘Can I add a different sensor to the instrument at a later date?’ and ‘Would this be a simple process?’ should be asked. Similarly, if a sensor is no longer required it should be easy to remove the sensor thus reducing service costs. A flexible instrument could be crucial in saving downtime and money if a new monitoring requirement crops up.
When the toxic risk is from a reactive gas such as ammonia, chlorine or nitrogen dioxide it is important for worker safety to realise that it is more difficult to get an adequate sample to the sensor. These gases are easily absorbed into sample line and internal components of the instrument. When the detection of a reactive gas is required, a significant feature to look for in an instrument is assisted diffusion. This uses an integrated pump on a lower sample level to help draw the ‘sticky’ gas to the sensor. This increases response time and lessens the risk of higher levels of gas being present without the user being aware.
Detection of toxic gases
Often the hazard is not just the flammability or toxicity of the gas but its ability to displace oxygen in the atmosphere. Gas purging, where a gas such as nitrogen is used to prevent the possibility of an explosive atmosphere by removing hydrocarbons and also oxygen, will by its nature create an oxygen deficiency. Biological processes such as occurring within sewers and fermentation in silos or a chemical action such as rust formation can also lead to oxygen deficiencies.
Oxygen is the priority measurement in confined spaces since it is impossible to sustain life without it and other hazards such as flammability are less relevant without the basic safety measurement. Oxygen is not flammable on its own, however, when an atmosphere becomes oxygen enriched, the flammability of ordinary materials increases, for example, cotton or grease. This is the reason that a lower and upper alarm level is set on the oxygen range, these typically being 19.5% and 23.5%.
It is worth noting that an instrument designed for confined space work should not be used during inerting operations. It is a common misconception that all gas detectors are the same and can be used for any operation where gas or vapours may exist. This is not true and can be dangerous. An LEL measurement is an indication of the flammability of the atmosphere, this being accurate for the gas the instrument has been calibrated on. Gas is only flammable in the correct gas / oxygen mix. Removing the oxygen, such as during inerting operations, means that the gas is no longer flammable, therefore, no longer having a relevant explosive limit. A confined space monitor instrument will, therefore, give a zero indication of LEL but it is still likely that flammable gas is still present. This is a potential hazard as the introduction of oxygen could then produce a flammable atmosphere. It is essential that an instrument suitable for inerting operations is used, typically monitoring the % Volume level of the hydrocarbon along with oxygen content.
The use of a portable gas detector, with appropriate action taken if dangerous levels of gases are detected, can help prevent worker injury. As the atmospheric conditions within a confined space are unknown, it is important that a check is carried out prior to entry to ascertain current conditions. The most reliable method of doing this is to use sampling line and the integrated electric pump of the instrument to draw a sample from the area to the instrument. This allows the worker to view the conditions from a safe area and determine whether a toxic, explosive or oxygen (too high or low) danger exists. It is important to take samples from the top, middle and bottom of the area to locate and quantify varying layers of gases and vapours.
The density of a gas / vapour is a measure of how ‘heavy’ it is relative to air. A density of greater than 1, the gas will fall. A density of less than 1, the gas will rise. It is worth noting that the density of methane is 0.55, carbon monoxide 0.97 and hydrogen sulphide 1.19. If the instrument alarm is activated indicating a hazard during a pre-entry check, company procedures should then be followed to decide on appropriate action. This could mean the requirement for air extraction, a ventilation system or the need for breathing apparatus.
The importance of using the correct detector
It is important when selecting an appropriate portable gas detector to match the instrument specification with the intended application. Confined spaces exist in various sizes and depths. For example, a tank, vessel, sewer, ship hold or deep trench may require pre-entry checking to depths of 30 - 40 metres. It would be very time consuming if using an instrument with no pump to hand aspirate a sample from this depth; likewise using an instrument with a weak pump. Not only is time being wasted but also an element of danger is introduced. The worker can become confident on the safety of the area without the instrument being exposed to the actual atmosphere of the area.
After confirming that the area is safe to enter it is necessary to continue to monitor whilst work is ongoing. Where the instrument has an integrated pump, it is important that the pump can be switched off when continually monitoring. This combination of pumped operation and, when suitable, diffusion operation increases the instrument battery life. Within the confined space the instrument is now being used to alert the worker to atmospheric changes that could be hazardous. It is possible that the area can become hazardous due to leakages, vapour release or the worker disturbing sediment thus creating a vapour plume.
The instrument should have audible and visual alarms that are clearly audible and visible despite often being used in harsh conditions. A good portable gas detector will mean that any alarm event is almost impossible to miss, having bright LEDs visible through 3600 and ultra-loud audible alarms capable of being heard over any machinery being used.
The alarm set points for the relevant gas or vapour being detected should be set in accordance with national regulatory levels or to company standards. These are set at time of instrument manufacture and should only be able to be changed via a secure password protected menu system. This means that only competent personnel can alter alarms.
As indicated, there are particular features of an instrument for confined space use that allows for easier, more accurate monitoring. As important is the actual instrument construction. The instrument should fit comfortably into the hand, onto the belt and into harnesses. It should be lightweight and not be too bulky. The instrument body should be tough and robust, easily surpassing minimum drop height tests to ensure years of use often in harsh environments. All of these available features mean that a purchaser or user should demand the highest specification available in a confined space monitor. Price alone should not be the determining factor. Consideration should be given to the flexibility and suitability of the instrument to the intended use. Flexibility in that it is capable of switching to different hydrocarbon monitoring without re-calibration or sensor change. It should also be possible to add / remove sensors without downtime or a complicated costly process. The suitability for use is key; can multi-users be accommodated through time weighted average monitoring?; data uploading for review and analysis should be simple; a sufficiently strong pump to draw a sample from the required depth is significant and ultra loud and bright alarms be the norm. Remember, safety is not cheap, it is priceless.
Published: 10th Jan 2008 in Health and Safety International