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 Features
5.2.1 General
Fixed gas detection systems differ from standard instrumented systems in many ways. During the design and engineering phase of any fixed gas detection system it is necessary to understand the features and/or unique demands associated with gas detection. 5.2.2 to 5.2.17 detail those main differences.
NOTE See lEG 60079-29-2 for guidance on selection, installation, use and maintenance of detectors for flammable gases and oxygen. This document gives additional information for many of the subclauses in Clause 8
of IEC 60079’29•2:2007.
5.2.2 Sensor location
Gas detection systems are reliant upon a transport mechanism to move the gas leak/vapour to the sensing element, unlike process safety devices such as pressure and temperature sensors that have direct contact with the process.
Thus the determination of the final sensing point location is not part of this standard. However, it should be realised that the sensing point location can have an overall effect on the operation of any fixed gas detection system, irrespective whether the system is safety related or not.
NOTE 1 See IEC 60079-29-2 for guidance on sensor location. For this document, it is assumed that the gas or
vapour reaches the sensor.
NOTE 2 See IEC 60079-10-1 for consideration of area classification.
NOTE 3 For example, see ISA-TRB4.00.07 for guidance on evaluation of gas system effectiveness.
5.2.3 Sensor filter elements (passive)
Gas sensors may incorporate a passive filter element to protect the sensor gas membrane from airborne dust, dirt or moisture, or may incorporate a metal sintered disk for explosion protection. All types of passive filter elements have a dangerous undetected failure mode (e.g. blockage), therefore require scheduled inspection and proof testing.
The stated safety function per sensing point should consider the intrinsic use of such passive filters, therefore should include within the stated safety function an acceptable response time when a proof test is initiated.
5.2.4 Sensor filter elements (active)
Gas sensors may incorporate an active filter element to condition or change the sensed gas or vapour. These active filters should be specified by the manufacturer and will have a defined life time. All filters of this kind have dangerous undetected failure modes (e.g. blockage, saturation and deactivation) and therefore require scheduled inspection and proof testing.
Gas sensors may incorporate a catalyst filter. Such filters do not have a defined life time. however they will suffer the same effects as passive filters. These filters should be treated as passive filters, unless environmental conditions affect the catalyst material. All filters of this kind have a dangerous undetected failure mode, therefore they require scheduled inspection and proof testing.AS/NZS 60079.29.3 pdf download.