Chapter 9
Gas detection
Published
- Fixed gas detection systems: design principles and when they are required
A fixed gas detection system continuously monitors toxic, flammable or oxygen hazards and triggers alarms and safety actions. When it is required, how to design it from the hazards up, voting, alarm actions, documentation and the IEC 60079-29-2 and IEC 62990-2 guidance.
- Catalytic (pellistor) vs infrared flammable gas detectors
Catalytic sensors burn gas on a heated bead and detect almost every flammable gas, including hydrogen, but need oxygen and can be poisoned. Infrared sensors measure light absorption, cannot be poisoned, but cannot see hydrogen. How each works and how to choose.
- Electrochemical toxic gas sensors: how they work and their limitations
An electrochemical gas sensor is a small fuel cell whose current is proportional to the toxic gas concentration. How the cell works, which gases it measures, and its limits: cross-sensitivity, finite life, and the effects of temperature, humidity and pressure.
- Where to install gas detectors: height, distance and common mistakes
Gas detectors must sit where the gas actually goes: near likely leak sources, high for light gases, low for heavy ones, in the path of air movement and within reach for testing. How density, release, ventilation, coverage and mapping studies decide placement.
- Setting gas detector alarm levels for flammable and toxic gases
Flammable gas alarms are set as a fraction of the LEL, typically 10 to 25 %LEL for the first alarm. Toxic gas alarms start from exposure limits (TWA, STEL), but fixed detectors use instantaneous ppm levels. How to choose levels, latching and actions.
- Gas detector calibration and bump testing
A bump test proves a gas detector responds and alarms; a calibration adjusts its reading to a certified gas. Portable monitors are bump tested before each day's use; fixed detectors at intervals proven by records. Gases, response factors and records explained.
- Hydrogen sulphide (H2S) detection: exposure limits, sensor selection and alarm settings
H2S is toxic at a few ppm, kills within minutes at several hundred ppm and deadens the sense of smell from about 100 ppm. Exposure limits (EU, UK, OSHA, ACGIH, NIOSH), sensor types, alarm settings and detector placement explained.
- Open path gas detectors: line-of-sight detection for large areas
An open path gas detector sends an infrared beam across up to hundreds of metres and measures the gas along the whole line in LEL·m. How it works, what LEL·m means, where beams are used, alignment, limits and how to combine them with point detectors.
- Portable gas detectors and confined space entry
Before entering a confined space, test the air from outside with a bump-tested portable monitor: oxygen first, then flammable gas, then toxic gases, at several levels. Keep monitoring during the work. 4-gas monitors, sampling pumps and limits explained.
- Gas detection and functional safety: SIL for gas detection systems
A SIL applies to a whole safety instrumented function, from gas detector to logic solver to valve, not to a detector alone. How IEC 61508 and IEC 61511 apply to gas detection, voting, proof testing, PFD calculation and what a detector's SIL certificate really means.
- Oxygen deficiency: nitrogen, argon and other inert gas hazards
Nitrogen, argon, helium and carbon dioxide kill by displacing oxygen, with no smell and no warning; one or two breaths of pure inert gas can cause collapse. Physiology, oxygen alarm levels, sources such as liquid nitrogen and purging, room calculations and monitoring.