AS NZS 60079.29.3:2016 pdf free.Explosive atmospheres Part 29.3: Gas detectors—Guidance on functional safety of fixed gas detection systems.
5.2.6 Poisoning and adverse chemical reaction
Particular sensor technologies suffer from common cause failures; airborne substances can inhibit or poison certain sensor technologies (e.g. catalytic sensors) whereas electrochemical sensors can suffer from adverse chemical reactions.
Sensors using such technologies have a dangerous undetected failure mode (application specific) unless sensor diagnostics detect the loss of sensor sensitivity. If poisoning and/or adverse chemical reactions cannot be excluded for an application, scheduled inspection and proof testing of the sensor (calibration) is required.
If these failure modes cannot be excluded for an application, redundant sensors will not improve the safety integrity as these are common cause failures. The safety integrity will be improved in such cases only by using diverse detection principles.
5.2.7 ppm.hr or %vol.hr lifetime
Particular sensor technologies have a ppm.hr or %vol.hr life time. Sensors having lifetime restrictions may have a dangerous undetected failure mode, therefore scheduled inspection and proof testing of the sensor (calibration) is required unless sensor diagnostics detect the incipient end of life.
5.2.8 Negative gas readings
Unlike process measurement, gas sensors have no negative readings of gas values.
NOTE Signals below zero can be caused by zero drift or adverse cross-sensitivities.
5.2.9 Hazard and risk analysis
Gases and vapours generate numerous hazards. They may be flammable, toxic or both. Oxygen levels can be excessive or deficient. Any hazard and risk analysis should consider all hazards associated with gases and vapours, which include short term and long term effects. Gas dispersion should include modelling with consideration to specific gas/vapour densities. The environmental conditions, including the presence of other gases, should be considered.
5.2.10 Preventative effectiveness or mitigation effectiveness
Some gas detection functions are preventative, while others only mitigate release consequences. Gas detection functions that mitigate release consequences may not be entirely effective even if they activate. Determining the effectiveness of the mitigation function is outside the scope of this International standard but is more a function of plant specific operation and personnel.
5.2.11 Cross sensitivities
Most gas detection sensors suffer from cross sensitivities which may increase or decrease the response to gas. In general, cross sensitivities can generate spurious alarms or prevent alarms from being tripped. Therefore, particular attention should be paid to the gases or vapours which may occur in an application.AS NZS 60079.29.3 pdf download.