Sniffing out gases

05 August 2015

Kevin Honner, gas detection expert at Dräger, explores the options when sourcing gas detection systems.

Our sensory organs are often unable to detect airborne hazards. 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.

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 planned and carefully considered before being implemented.

Assessing risks

Wherever toxic or flammable chemicals are manufactured, processed, stored or shipped, there will be a risk of an accident or substance release. Even the smallest release of substances can cause harm to people, damage the environment or destroy property.

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.

It is the responsibility of any company operating in a hazardous area to carry out the risk assessment depending on the probability of the occurrence of toxic or explosive incidents. 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.

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

The detection of even the smallest gas leak can avert a chain reaction that would otherwise take on a life of its own and threaten lives. Gas leaks to atmosphere from pressurised sources may be at a higher or lower temperature than ambient conditions and the dispersion of the gas may be affected by its temperature until it is diluted in air, as its density on release will be affected by its temperature. 

Highly odorous substances – such as hydrogen sulphide – give off a strong smell, even in small, harmless quantities. Over time we can 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.

Critical points where the release of gases could occur include 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 attention. There is also the issue of technical leakage, whereby material is deliberately released through valves, diaphragms and overflows to prevent malfunction.

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. Architecture, convective flows and ventilation determines the spread of the gas. Pools of heavy gases are dangerous as they can form on the floors of cellars and tanks.

The role of gas detection systems

Gas detection systems can be portable gas measuring instruments or fixed installed gas detection systems. 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.

Wherever there are hazardous situations due to the presence of combustible gases and vapours, eg: 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. 

The safety concept of a gas detection system always follows this protocol; detect dangerous gas, react and avert.

To ensure a gas detection system can react quickly and precisely, its components must be carefully matched to the particular circumstances. It 
must be correctly installed, calibrated, operated and maintained. Every system should come a bespoke training and servicing plan – an area which Dräger believes is imperative in ensuring the safety of human lives.