Gas detection solutions – hazardous area protection

12 November 2014

Some gases have a strong odour at low concentrations – as is the case with sulphur compounds (mercaptanes, for example) – and people smelling them can move away. But not every hazard announces itself. Carbon monoxide, for example, is odourless. Toxic or flammable gases and vapours can build up, reaching hazardous concentrations or there may be insufficient oxygen in the air which can result in a life-threatening situation. Here, Kevin Honner of Dräger l

A detection system must respond reliably to a variety of gases, identify them, and measure their concentration in the ambient air. Any such system needs to be meticulously planned and carefully considered before being implemented.

With any potential gaseous hazards you need to consider not just the physical properties of the gas but the particular local conditions that they are under.

Assessing risks

Wherever toxic or flammable chemicals are being manufactured, processed, stored or shipped, there will always be a risk of an accident or substance release. Even the smallest release of substances can cause harm to people, damage the environment or even destroy property. Essentially, any gas is potentially dangerous, whether liquefied, compressed or in a normal state.

There are three categories of risk which must be detected reliably in order to protect human life, assets and the environment.
• Ex – risk of explosion by flammable gases
• Ox – oxygen; risk of asphyxiation by oxygen displacement or risk of increase of flammability by oxygen enrichment
• Tox – risk of poisoning by toxic gases.

Without auxiliary tools, people are often unable to recognise these dangers early enough to initiate appropriate counter measures. In a chemical plant, for example, where there are a myriad of hazards presenting themselves at any one time, managing risk and having in place substantial gas detection systems plays a major role in the mitigation or prevention of a major accident or incident.

It is the responsibility of any company operating in a hazardous area to carry out the appropriate risk assessment depending on the probability of the occurrence of toxic or explosive incidents. Certain areas present a higher risk probability yet if such locations are not easily identifiable, it is important to distribute gas detection sensors across the entire hazardous area to help to manage the risk.

It follows that the necessary steps should be put in place and the most appropriate gas detection methods employed, to ensure workers are alerted to harmful gases if the situation arises and human lives, as well as industrial facilities, are protected.

Sniffing out gases

When it comes to gas-related accidents, most eventualities do not ‘just happen’. Each of these accidents is, more often than not, a culmination of a series of individually minor events that combine and add up to cause a more serious problem.

The detection of even the smallest gas leak, for example, can avert a chain reaction that would otherwise take on a life of its own and even threaten lives. These leaks can arise, for example, when cold, liquefied and other pressurised gases are released into the ambient air and mix with it; their concentration decreases and the gas dispersion depends more on the current temperature conditions and ambient air convection than on the density of pure gas.

Usually invisible to the naked eye, often odourless and impossible to hear, many of these substances are undetectable to the human senses – but leaks of these gases and vapours can pose a very dangerous threat to both people and places. Depending on their substance, quantity and location, gas leaks can cause everything from immediate ill health to plant shutdowns and potentially explosive situations.

Of course, there are some highly odorous substances – such as hydrogen sulphide recognised by its ‘rotten egg’ characteristics - which give off a strong smell, even in small, harmless quantities. But over time, we can become used to smells and we become desensitised, failing to detect the substance even when it is present in higher concentrations. Lethal high dose concentrations of hydrogen sulphide are not detectable by our sense of smell and it is vital that assumptions are not made about safe areas which may otherwise prove fatal.

Critical points where the release of gases could occur include all types of storage vessels, including valves and pipes, as well as in machinery and pumps. Dynamic loading as a result of temperature and pressure variations, corrosion and material fatigue are important factors that may lead to leakage, and joints and seals, together with filling and tapping points, also need special attention. There is also the issue of technical leakage to consider, whereby material is deliberately released through valves, diaphragms and overflows to prevent malfunction.

The most notable incidents are those in which dangerous substances are released. In this form of release, gases form a cloud around the leakage point, and the edge of the cloud becomes a turbulent boundary zone.

Once a gas has matched the temperature of its surroundings, it may be classified on the basis of its density relative to the density of air – as a light, heavy or neutral gas. The gas cloud will either rise in the air (light gas), spread along the ground (heavy gas) or be carried by the wind (neutral gas).

However, some gases often behave differently from these general patterns immediately after they have been released – mainly because the process temperature is very different from its ambient temperature.

As well as differences in gas behaviour, there are also differences between gas detection in confined spaces and in the open air. The conditions for the formation of gas clouds in interior spaces are similar to those in the open air, but gas clouds behave very differently in an enclosed space. In cases like these, the architecture, convective flows and ventilation determines the spread of the gas. Pools of heavy gases are especially dangerous as they can form on the floors of enclosed spaces such as cellars and tanks.

The role of gas detection systems

Gas detection systems are fundamentally products of safety technology, used to protect workers and ensure plant safety. It is important not to misinterpret the role of gas detection systems. They are not simply a handful of detectors spread across an industrial plant; rather they should be considered a bespoke solution developed to meet individual protection objectives.

Gas detection systems can be portable gas measuring instruments or fixed installed gas detection systems. The safety of an area potentially being affected by dangerous gases and vapours to a high degree depends on the reliability of the gas detection system and the quality of the sensors being used. We often find that fixed installed systems are in operation continuously, 24 hours a day, year after year, to ensure that they are alerted under any instance of a random gas release, as well as operating under extreme environmental conditions.

So how do we protect against gases behaving unpredictably?

Many toxic gases are very reactive and under suitable conditions they can change by chemical reactions. Modern gas detection systems utilise a multitude of techniques in their operations: from electrochemical sensors and catalytic bead sensors to infrared technology in point sensors and open path sensors, as well as flame detectors, ultrasonic sensors and gas cameras.

Just as gases can display unusual characteristics, the real innovation in gas detection technology is in the very precise matching of the measuring system to localised circumstances and particular requirements.

With ever-improving sensors, modern detection technology for flammable and hazardous gases enables us to design highly reliable gas detection systems. The foundation for planning any system is well-considered matching of the components, based on a range of different technologies, to specific local conditions.

Explosion protection and plant safety

In almost every individual application, flammable substances are used and gas detection systems must become reliable protection systems. Wherever there are hazardous situations due to the presence of combustible gases and vapours, e.g. in processes involving the use of solvents or in the plastic processing industry, explosion protection measures must be in place as regulated by law, to ensure personnel and plant safety.

These systems are designed to trigger an alarm in sufficient time to inform the operator and perform counter measures for the prevention of a potentially explosive scenario. The safety concept of a gas detection system always follows this protocol; detect dangerous gas, react and avert.

Depending on the application, different measuring principles for the detection of gas and vapours can be used – for example, catalytic bead sensors or point or open-path infrared sensors – the system should be tailored to the individual needs of the operator. When combined with a central controller, it is possible to detect flammable vapours and gases at an early stage, when concentrations are so low that a dangerous condition or risk of explosion can be reliably averted.

Holistic solutions

Safety needs cannot be met by ‘off-the-peg’ solutions and it is important to undertake a detailed analysis of the current situation to ensure that all individual needs in gas detection are met, providing an holistic solution.

To ensure that a gas detection system is able to react quickly and precisely, its components must be carefully matched to the particular circumstances – the physical and chemical properties of the materials, assumption of the most likely leakage points, as well as the location’s constructional, geographical and meteorological conditions.

The prediction of gas dispersion and most likely behaviour based on particular circumstances forms the basis for a gas detection system, whether stationary or mobile, and can detect single-gas point sensors. Fundamentally, in order to ensure a system’s reliability, it must be correctly installed, calibrated, operated and maintained. It follows that within every tailored gas detection system should come a bespoke training and servicing plan – an area which Dräger believes is absolutely imperative in ensuring the safety of human lives.

Make sure you have a robust system in place. It could mean the difference between life and serious injury or even death.

About the author:

Kevin Honner is Materials Development Manager at Dräger. His background is in gas measuring instrumentation, both in industrial gas analysis and in gas detection.