Published: 10th Oct 2005
New developments in the field of gas sensing technology allow for subsequent advances in the provision of better gas safety systems.
In this article, Leigh Greenham, the new Administrator of CoGDEM (Council of Gas Detection and Environmental Monitoring) brings us up to date with gas detection issues, by highlighting some recent industrial safety incidents.
Graphic and memorable television coverage of the Piper Alpha disaster in 1988 has allowed a whole generation of safety engineers to have a visual reminder of the worst-case scenario of explosions, particularly those related to oil and gas exploration, refining and storage. More recent footage of the Texas City explosion in March 2005 shows that the potency of such explosions has not diminished in the 17 years separating the two events.
Flammable gas detection apparatus may not have been able to prevent either of these two incidents, but certainly played a part in averting a potentially explosive incident on another North Sea platform. Shell's Bravo platform's Emergency Shut-Down system was automatically activated in September 2003 when sensors detected a massive gas leak in a compartment within one of the rig's legs. The portable gas detectors carried by two maintenance workers who were in the vicinity of the leak also went into alarm. The ESD immediately isolated and contained the gas within the confined space where the leak had occurred, and allowed the gas to be safely exhausted, without endangering the integrity of the platform. Unfortunately, although the rescue team was ready within minutes, they were unable to rescue the maintenance workers until the gas concentration had been reduced to safe levels, by which time the two men had already died from suffocation.
Flammable gas sensors and developments
Conventional flammable gas detection apparatus uses catalytic bead sensors. Commonly known as pellistors, these consist of a matched pair of elements, one of which is an active catalytic detector and the other a non-active compensating element. Each element contains a coil of very fine platinum wire embedded in a bead of alumina. In the case of the detecting element, a catalytic coating is applied.
Flammable gas contacting the catalytic surface of the detecting element is oxidised, causing a rise in the temperature of the bead. This rising temperature increases the resistance of the platinum coil. There is no such change in the compensating element. The output signal of the detector is based on the imbalance between the two resistances.
Pellistor sensors are able to give accurate readings under adverse environmental conditions as any change in ambient temperature, humidity or pressure will impact equally on both elements. However, pellistors can be poisoned or inhibited by silicones, sulphides, chlorine, lead and halogenated hydrocarbons. The detectors therefore require regular cleaning and calibration, increasing the costs of maintenance. Pellistor sensors also require the presence of oxygen in order to operate.
To overcome these shortcomings, it is becoming the norm to use infrared gas detectors. Gases that contain more than one type of atom absorb infrared radiation. Hydrocarbon gases such as methane, propane and butane are gases of this type. An infrared gas detector consists of an infrared source and an infrared detector. When flammable gas passes between the source and detector, the gas absorbs infrared radiation and a lower intensity is registered at the detector. Specific gases are detected by measuring the amount of absorbed infrared radiation at specific wavelengths, the difference being related to the concentration of gas present.
Infrared detectors are immune to poisoning effects and operate in inert atmospheres. They are therefore suitable for use in confined spaces where oxygen depletion might limit the effectiveness of a pellistor detector. Infrared detectors have a fail-safe design. If the detector becomes obscured or fails, no infrared radiation is recorded and alarm signals are activated. Infrared detectors are available in either a fixed-point format, in which gas diffuses into the detector, or open-path format, in which the source and detector are separated by distances of tens or even hundreds of metres. In this way, a line-of-sight beam is formed and a gas cloud passing through the beam will be detected.
A very recent and exciting development of fixed-point infrared gas detectors has been to miniaturise them and package them in identical housings to those which have been traditionally employed for pellistors. By also including electronic circuitry within the same housing, it has been possible to simulate the electrical characteristics of the pellistor, allowing the device to be used as a plug-in replacement. This combination gives the advantages of the failsafe nature of infrared techniques without the need to change the electronics of the pellistor-based instrument or system.
Gas leaks from high pressure gas storage or distribution sources often create an ultrasonic sound emission, so it is also possible to spot the characteristics of a leak using ultrasound detectors. In an outdoor industrial plant, this can offer the advantage of remote leak detection without the need for a gas cloud to reach a conventional gas sensor. New ranges of gas detectors that employ this technique are now available.
Reports of fatalities from the agricultural sector have highlighted the lack of understanding within some industries as to the power and veracity of toxic gas releases. The presence of noxious substances in this industry is often accompanied by strong odours, although the human nose can become insensitive to some gases once the concentration becomes high enough to paralyse the nasal membrane. An example of this occurred in the UK in August 2004, after two workers entered a confined space within a meat processing works.
The air quality around the plant, where the remains of dead animals were routinely processed, was frequently poor, leading to complaints from nearby residents. On a day when offal from cattle was being processed, a blockage occurred within the main processing machinery which could not be cleared from the outside, so a worker entered the confined space to investigate. Reports state that although he may have been carrying a portable oxygen depletion monitor, he had nothing to warn him of the presence of huge concentrations of hydrogen sulphide and ammonia that had been released from the processed offal and trapped within the machine. When the worker collapsed, a colleague entered to try and rescue him, but was also exposed to the gas and collapsed nearby. When the Fire and Rescue Service arrived, they realised the likely cause of the double collapse and deployed the multi-gas detector that was carried on their vehicle. Even at the entrance to the machine, this detector went into its highest state of alarm and the reading went off the scale. Rescuers with Breathing Apparatus were eventually able to retrieve the casualties, one of whom died the following day from the acute toxic effects of the gaseous atmosphere. It surprised many people that despite the "cloud" of toxic gas, the level of oxygen had not been depleted to a point where a "low oxygen" alarm would have been raised.
Toxic gas sensors and developments
Any gas that can be oxidised or reduced electrochemically can be detected by means of a fuel cell-based electrochemical sensor. Fuel cells are electric batteries that consume gas from outside rather than solid or liquid materials within. Electrochemical sensors are miniaturised fuel cells that react to low concentrations of gas to produce a current that is linearly proportional to the gas concentration.
Other gases may be detected by galvanic electrochemical sensors. In these sensors, electrodes or electrolyte within the fuel cell are used up in the electrochemical reaction. The life span of these sensors is therefore governed by the amount of gas that they absorb.
Solid-state sensors which use heated and semiconductor materials are also available. An example of this is the semiconductor hydrogen sulphide sensor, which is a hybrid device that is most useful for hot environments in which the ambient temperature exceeds 45ºC or where there is a continuous high background of hydrogen sulphide. Under such conditions electrochemical sensors may be unsuitable.
An attribute of many toxic gas sensors is that they can be specifically sensitised for the target gas, and may not respond to other gases that have toxic characteristics. This may be a useful feature, for example, when it is important that the actual concentration of a specific target gas be determined, but could also lead to a false sense of security if the instrument user believes that he is being protected against the presence of any toxic gas. Therefore a requirement exists for a "broad-range" sensor that reacts to a much wider range of toxic gases, including volatile organic compounds (VOCs), many of which cannot be detected by conventional electrochemical or semiconductor sensors.
Photo Ionization Detectors (PIDs) are becoming more commonplace for the detection of a wide range of VOCs such as benzene, and recent developments have seen these sensors become miniaturised so that they can fit into portable multi-gas detectors.
When fitted alongside sensors for flammable gas, oxygen, and conventional toxic gas sensors for the detection of, for example, hydrogen sulphide and carbon monoxide, the PID sensor gives the user of the instrument an added layer of protection. This could be particularly appropriate for members of rescue services who are called upon to enter confined spaces such as drainage tunnels, where unknown hazards may exist, including the potentially toxic fumes from spilt fuel oils. Without PID, the instrument would only respond to the explosive hazard of such spills, despite the fact that fumes from some fuels can be toxic at concentrations well below those at which an explosive hazard could be detected.
As well as the obvious hazards of flammable gas concentrations leading to explosions and toxic gas build-ups leading to poisoning, there is an equal or greater hazard in industry from the depletion of oxygen within confined spaces. Although personnel entering such a confined space would normally be equipped with a portable oxygen depletion monitor (as in the case of the worker at the meat processing plant), it may be sensible to install fixed apparatus in areas where such hazards may occur. Other examples of such locations could be a laboratory or storage area containing liquid nitrogen cryogenic vessels or carbon dioxide tanks. Leakages of either of these two gases could lead to the displacement of normal atmospheric oxygen. With carbon dioxide being heavier than air and liquid nitrogen falling due to its low temperature, it is appropriate to mount oxygen detectors at low levels and it is interesting that the National Blood Service, a major user of cryogenic vessels, have generated their own standards, stating that oxygen depletion detectors should be mounted only 1 metre above floor level.
A similar condition can occur with the bulk storage of argon, which is also heavier than air. A double fatality occurred in June 2004 at a steel-processing factory where argon is used to drive out the normal impurities found in ordinary steel production. Two maintenance workers entered a deep pit under a steel furnace to carry out repairs, unaware that argon gas had leaked into the pit and displaced the oxygen that would normally be present. The possibility of an oxygen depletion hazard was understood by the plant operators so a system of oxygen detectors and remote alarms had been installed, but reports suggest that this had been inoperative at the time of the incident.
Oxygen sensors and developments
Oxygen detectors used in industrial safety applications typically utilise an electrochemical sensor, which contains a lead wool material in contact with electrodes and electrolyte. When oxygen diffuses into this material the electrochemical reaction causes a current to flow. Such sensors are very reliable, but once all of the lead has been consumed the sensor needs to be replaced. Typical sensors have two-year operating periods, but recent developments have seen the operating lifetime extended to three or even five years. In some cases this has been achieved by simply increasing the "reservoir" of lead wool, but it is now possible to buy a conventional "small" sensor which has had its life extended by a reduction in the rate at which the lead is consumed.
Other long-life oxygen sensing technologies are under development, some which eliminate the use of lead altogether. This has the extra benefit of removing a potentially harmful substance from the workplace, as lead is to be banned from use within certain electrical products under a new European Directive that comes into force in 2006. Known as the RoHS Directive (Restriction of Hazardous Substances), gas detection equipment will initially be excluded from having to comply with this Directive, but it is expected that this exclusion will be removed by 2008. ?
Leigh Greenham, CoGDEM
Unit 11, Theobalds Business ParkKnowl Piece, Wilbury wayHitchin, Herts, SG4 0TY UK
Tel: 01462 434322www.cogdem.org.uke-mail firstname.lastname@example.org
Published: 10th Oct 2005 in Health and Safety International