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UK HSE issues safety alert for personnel specifying and selecting devices for measuring concentrations of flammable and toxic gases

16 September 2020

The UK HSE issued a safety alert on September 10 to highlight the risk of misleading gas detection readings associated with the use of sampling tubes with pumped gas detectors. 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.

The HSE says that 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.

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. 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.

This incident has highlighted the importance of selecting the correct systems for gas detection and verifying the effectiveness of the detection system. The purpose of this safety alert is to highlight the risk of adsorption if an unsuitable sample tube is used.

Action required

The HSE has issued four actions to be undertaken:

-  Operating instructions for most gas detectors recommend a function check (often referred to as a 'bump test') before each day’s use.  This is additional to the requirement for periodic calibration.  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.  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).  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.

Click here to read the Safety Alert in full.

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