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, according to the safety body.
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, leading to a false conclusion that the work area was free of flammable vapour.
The HSE observes that this incident has highlighted the importance of selecting the correct systems for gas detection and verifying the effectiveness of the detection system.
It reports that during laboratory testing of the gas detector and at a concentration of 50%LEL (lower explosion limit) of the substance involved, the sample tube extended the time to achieve a non-zero reading by more than one minute. This extension was considerably longer than the time taken to test at any particular location with a recommended sample tube fitted.At 50%LEL of the substance involved, the time to achieve 90% of the final reading was over 15 minutes. The same gas detector and sample tube had a response time of less than five seconds to the calibration gas (methane).
The phenomenon of adsorption of some substances on sample tubes is known, says HSE. It is mentioned in each of the references to this safety alert and has been studied in previous HSE research (eg Research Report RR635).
Review of manuals for gas detectors from a range of manufacturers has identified that most manuals include little or no information on the importance of selecting a sample system of a suitable material.
While the incident to which this safety alert refers involved a highly flammable substance and measurement of LEL, similar issues may apply to some other gases, particularly reactive gases such as H2S and NOx.
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.
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, three seconds per metre. The combination of gas detector and sample tube should be considered unsuitable where this time is exceeded.
The complete text of the safety alert is accessible via the link below.