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Safety Alert - Failure to detect dangerous gas/vapour due to incorrect specification of sample tube
17 September 2020
Gas detection may be used in support of a risk assessment associated with, for example, hot work or confined space entry. The gas detection system must be suitable for the intended purpose and give an accurate and reliable indication of the presence of the hazardous material.
This safety alert by the Health and Safety Executive聽highlights the risk of misleading gas detection readings associated with the use of sampling tubes with pumped gas detectors. 聽Sampling tubes are sometimes used to extend the reach of the detection device and/or to allow detection at an increased distance from the user.
In a recent incident a gas detector failed to detect the presence of a flammable vapour. Hot work proceeded in the belief that there was no flammable vapour present. The subsequent explosion resulted in a fatal injury.聽
The purpose of this safety alert is to highlight the risk of adsorption if an unsuitable sample tube is used.
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What you need to know
This incident has highlighted the importance of selecting the correct systems for gas detection and verifying the effectiveness of the detection system.
The investigation found that a significant contributor to the failure to detect the flammable vapour was it being adsorbed on the inner surface of the sample tube.聽This meant that no flammable vapour reached the detector before the test was completed and a false conclusion that the work area was free of flammable vapour.
While the incident involved a highly flammable substance and measurement of LEL (lower explosion limit), similar issues may apply to some other gases, particularly reactive gases such as H2S and NOx.
Action to be taken
- Operating instructions for most gas detectors recommend a function check (often referred to as a 'bump test') before each day鈥檚 use. This is additional to the requirement for periodic calibration.聽It is recommended in the case of gas detectors that will be used with a sample tube, at least the first function check ('bump test') for a new intended use and/or a new sampling configuration be conducted using the combination of the gas detector and its sample tube and the substance of interest, where practicable. 聽This is of particular importance if the substance of interest is not the substance used to calibrate.
- For example, the head space in a sample jar containing a liquid sample of the substance of interest at a temperature above its flashpoint would be expected to give an output representing greater than 100%LEL.
- Sample tubes should be as short as possible. The increase in response time should not exceed the response time of the gas detector without a sample tube plus the delay time specified in the gas detector manual or, where no time is specified in the manual, 3 seconds per metre (e.g. BS EN 60079-29-1:2016, section 5.4.15). 聽The combination of gas detector and sample tube should be considered unsuitable where this time is exceeded.
- Particularly for spot testing, users should be aware of the response time of the combination of the gas detector and its sample tube.