Industrial Gas Detection

Two recent and successful prosecutions brought by the UK Health and Safety Executive (HSE) act as a stark reminder of the importance, among other systems and procedures, of having appropriate and functioning gas detection equipment to help protect employees from serious injury or even death.

On 24 July 2009 multi-national group Bodycote H.I.P. Ltd of Macclesfield Cheshire was fined £533,000 after pleading guilty to breaching section 2(1) of the Health and Safety at Work etc Act following the deaths of two of its workers at its manufacturing plant in Hereford.

In June 2004 the company’s Works Manager and Maintenance Engineer were found collapsed on the stairs leading to a concrete-lined pit into which argon gas had leaked from a large pressure vessel. The court heard that the oxygen alarm system, which would have warned of the oxygen-depleted atmosphere, was inhibited, with the audible alarm not able to sound, and that the ventilation system was not running.

On 25 June 2009 Brechin rendering plant Sacone Environmental Ltd, was fined £12,000 after pleading guilty to breaching Section 3(1) of the Health and Safety at Work etc. Act 1974 after two workers were overcome by hydrogen sulphide in July 2007 as they delivered animal waste from the neighbouring abattoir to a waste pit in preparation for incineration. The pair collapsed and lay unconscious in the waste intake area of the plant before being rescued and taken to hospital. The court heard that the waste intake area had no gas monitors or alarms installed and how its enclosed nature meant there was little exchange of air.

The HSE inspectors involved in investigating the incidents said that both were foreseeable and preventable - a warning to other companies about potential risks to their employees, particularly those working in confined spaces. Both inspectors stressed the importance of gas monitoring equipment.

What gas detectors can and can’t do

Gas detectors do not prevent leaks occurring or indicate what action should be taken, so they are no substitute for safe working practices and regular maintenance. They can, however, provide an early warning of a problem and help ensure the safety of employees, protecting them from the effects of explosion and fire, or short-term and long-term ill-health arising from flammable, toxic and asphyxiant gases. They are predominantly used to trigger alarms if a specified concentration of gas is exceeded but they can also measure workers’ exposure to gases.

gas-detection2.jpg Health and Safety International magazine

What and where you are monitoring

The importance of the use of gas detectors may be more obvious for some types of work and workplaces than others. For instance, in sampling from bore holes on landfill sites and in testing the operation of boilers and flues or in those industrial processes that use flammable and/or toxic gases such as in the solvent extraction process or combustion of gas and oil, their benefits are clear.

One of the most critical uses of a gas detector is, however, for working in confined spaces, as illustrated by the above case studies. On average, gas-related work in confined spaces kills around 10 people every year in the UK. Gas/vapour may be produced by work inside the confined space from, for example, welding, flame cutting, spray painting, use of adhesives or solvents or from combustion processes. It may, however, build up more unexpectedly from sources that may not be readily obvious e.g. from biological or chemical action in sewers and manholes, or anthropogenically, e.g. carbon dioxide may accumulate in pub cellars following leaks from compressed gas cylinders. The key point is that all employers need to ensure that they carry out a thorough risk assessment of the processes and adopt a safe system of work which either eliminates the risk or minimises it through effective controls or use of personal protective equipment such as appropriate gas detection.

Portable and fixed monitoring

Portable and fixed monitoring provide different types of information on gas concentration levels and are used for different purposes.

Portable detectors

These detectors are principally used to protect a worker from flammable, asphyxiant (ie oxygen deficiency) and toxic hazards, which can occur either individually or together. Portable detectors perform a vital role testing an atmosphere in a confined space before entry and monitoring during entry; they are also used for tracing leaks. They are carried typically by a mobile workforce. The detectors can either be capable of being worn on the lapel, on a belt/harness or handheld. They are inexpensive pieces of kit, especially the single gas, disposable monitors, which have a lifetime of a couple of years. Two types of alarm are possible: instantaneous and time-weighted average, depending on the hazard.

Portable gas detectors should be inspected for damage before every use and checked in-line with the manufacturer’s recommendations. It is also important to perform a ‘bump test’ before use which entails checking that the detector responds to a known concentration of test gas, having first obtained a zero reading is obtained in a clean atmosphere and that the detector responds to gas. The battery charge level should be sufficient for the job.

The ability to datalog is a useful feature. Historical data can be analysed to understand trends in exposure and events, leading to improvement in health and safety performance; examination of detector ‘events’, e.g. bump test results, can also increase the efficiency and effectiveness of detector use.

gas-detection3.jpg Health and Safety International magazine

Fixed detectors

A fixed (static) detector is permanently installed to provide continuous monitoring of plant and equipment. It is particularly useful when there is a possibility of a leak into an enclosed or partially enclosed space where flammable and/or toxic gases could accumulate. The gas detector should be set to alarm at a level low enough to ensure the health and safety of workers, but high enough to prevent false alarms which are most likely to be caused by changes in sensor output due to, for example,the gradual, natural degradation of sensors and drift in electronic components over time, extreme environmental conditions, very high concentrations of the target gases, exposure of some types of sensor to poisons and corrosive gases. If a specified gas concentration is exceeded, the detector system should trigger an alarm, audible and/or visible and distinguishable from the fire alarm as it is likely that different actions will be required. In determining the required alarm levels for fixed gas detection systems, it is important to take into account:

• Any industry standards and recommendations;

• Where appropriate, the lower explosion limit of the gas or vapour

• Whether the area is occupied

• The toxicity of the gas or vapour

• The size of the potential leak and the time to reach a hazardous situation

• The alarm response time of the detector

• The time required to respond to an alarm

• The actions to be taken following the alarm

A suitable safety margin should be incorporated to account for ventilation dead spots, where vapours could accumulate, and the variability of natural ventilation. One option is to set more than one alarm level. The lower alarm(s) act as a warning of a potential problem requiring investigation. The higher alarm triggers, either automatically or manually, an emergency response such as activating a control, e.g. ventilation, shutdown, evacuation.

Fixed detectors should be included in the plant maintenance schedule as the performance of most detectors deteriorates overtime, especially in dusty, corrosive or damp environments.

Additional important considerations

Gas detectors can’t do the job on their own

A safe system of work is the primary means of minimising risk from gasrelated hazards. The use of gas detection is secondary but nevertheless important as a tool for safe and healthy working practice. Portable gas detectors should only be used by those staff who have been trained in their use. Training is usually provided by the gas detector supplier. A typical training schedule could include:

• Ensuring that the detector is suitable for the purpose and is correctly configured

• Checking the instrument is functioning and reading correctly

• Using the instrument properly to obtain true readings

• Interpreting the results

Workers should also be trained in:

• The hazards and properties of dangerous substances on site

• The need for personal protective equipment (selection, correct fitting and use of respiratory equipment require specialist expertise and training)

• The action to take if the portable gas detector alarms

• Emergency procedures

Is the atmosphere potentially explosive?

It is essential that instruments for the detection of flammable gases and those for oxygen and toxic gases, which are used where flammable substances may be present, are certified as being safe for use in potentially explosive atmospheres (i.e. ATEX certification in the EU).

Am I up to date on workplace exposure limits?

Exposure limits of toxic substances are set and disseminated by the appropriate regulatory authority. New substances can be added, existing ones deleted, and exposure limits can change, invariably to lower values. It is therefore important to keep up-to-date. Revised limits should be considered in the context of the workplace risk assessment and how this may influence adoption of appropriate procedures to ensure adequate control of exposure. Detectors may need to be upgraded to be capable of meeting reduced limits, assuming the technology is available at a reasonable cost.

gas-detection4.jpg Health and Safety International magazine

Confined spaces: what do I need to know?

Testing and monitoring the atmosphere is just one consideration among many when designing a safe system of work. The UK approved code of practice for the Confined Space Regulations identifies the following considerations:

• Risk Assessment - It is necessary for employers and employees to identify all risks which could occur by entering and working within a confined space. Part of the assessment must cover the possibility of atmospheric contamination which may be accounted for by looking at the recent history of the confined space and therefore any potential residues from previously occurring substances. The possibility of gas seepage from adjacent premises or plant and leaching of gases from surrounding land or ground water should be considered

• Preventing the need for entry - If the work can be undertaken without entering the confined space then it should be. This also applies to the pre-entry test of the atmosphere which should be conducted using sampling tubes or probes so that the instrument and its operator remain outside the confined space

• Safe working in confined spaces - For those people who must work in a confined space, this section covers the procedures which must be followed prior to and during entry. It suggests that following the risk assessment, the atmosphere may need to be tested for the presence of hazardous gas of deficiency of oxygen and that regular monitoring may be necessary. Guidance is given on the techniques which can be used to test the atmosphere and suitable gas detectors

• Emergency Procedures - Should an emergency arise while people are working within confined spaces, it is necessary for the relevant rescue personnel to also be equipped so as to minimise the risk to themselves. This may, again, include atmospheric monitoring devices

• Plant and equipment - All equipment provided or used for purposes of securing the health and safety of people either working in a confined space or those rescuing workers from one, should be in good repair and working order. This should include periodic examination and, depending on the equipment, inspection before every use e.g. breathing equipment. Specific mention is made of the need to properly maintain atmospheric monitoring equipment in accordance with manufacturer’s recommendations and that reports of examinations, maintenance and calibration records should be kept

• Training - The ACOP makes clear that employers must train staff in the risks associated in working in confined spaces. Amongst the training recommended is the use of atmospheric testing equipment and the action which needs to be taken depending on the readings achieved

Postscript: legislation, standards and gas detection

Legislation is the primary driver for the use of gas detectors in the workplace. In the EU, member states must enact directives through their own regulations. Although directives may not explicitly mention gas detectors, they may refer to them indirectly through equipment safety control and monitoring. For example, the Chemical Agents Directive 98/24/EC Protection of the health and safety of workers from the risks from chemical agents, (implemented in the UK as the Control of Substances Hazardous to Health Regulations) states that, to prevent or control exposure, (e.g. where required by the safety and health document) monitoring devices measuring gas concentrations at specified places (automatically and continuously), automatic alarms and devices to cut off power automatically from electrical installations and internal combustion engines, must be provided.

Another key piece of legislation in the UK is the Confined Space Regulations which states that so far as is reasonably practicable, no person at work shall enter or carry out any work in or (other than as a result of an emergency) leave a confined space otherwise than in accordance with a safe system of work.

A confined space is defined as a place which is substantially enclosed and there will be a reasonably foreseeable risk of serious injury from hazardous substances or conditions within the space or nearby. Some confined spaces are fairly easy to identify, for example, closed tanks, vessels and sewers. Others are less obvious but may be equally dangerous, for example, open-topped tanks and vats, closed and unventilated or inadequately ventilated rooms and silos, or constructions that become confined spaces during their manufacture.

In recent years the harmonisation of standards throughout Europe has led to the publication of many new standards covering performance and use of gas detecting instruments and systems. For toxic gas detection, the EN 45544 (Parts 1-4) series of standards, generically entitled “Workplace atmospheres - electrical apparatus used for the direct detection and direct concentration measurement of toxic gases and vapours” has been published. New standards for flammable gas detectors (EN 60079-29-1 and 60079-29-2) have emerged, a revised standard for oxygen detectors (EN 50104) is imminent, and work is in progress to produce a EN standard for carbon monoxide and carbon dioxide detectors for indoor air quality, including commercial applications.

วันจันทร์ที่ 26 กรกฎาคม พ.ศ. 2553