subject: Detecting Leaks of Fire Suppression Gases [print this page] Detecting Leaks of Fire Suppression Gases
Advanced gas detection today is based on highly accurate, long life electronic units, with sensors that can cover a broad spectrum of gases or target specific or difficult to detect gases. In essence, that means that monitoring for leakage in automatic fire suppression systems is not especially challenging although it does, naturally, involve careful specification and siting of sensors and design of the overall monitoring system. It should be noted, for example, that the location of the gas storage containers will be the primary but not necessarily the only site for fixed leak detectors as ancillary piping may also contain significant quantities of gas. In addition specific situations may require professional judgement of likely air movements and distance of sensors from possible gas escape sources in order to ensure optimum performance.
Other considerations arise when specifying the monitoring system layout. For example, the characteristics of the gas being monitored (lighter/heavier than air) may be relevant to the placement of leak detection sensors at various heights. Argon is a commonly used gas that is heavier than air and will dictate lower or floor level placement while others may require monitoring of ceiling level pockets. In fire suppression systems that operate using such inert gases and oxygen depletion, it will be the oxygen level that is monitored and the system design and sensor placement will reflect the locations and movement of people within the space. Once again the characteristics of the specific gas/oxygen mixture will be a crucial factor in the height placement of sensors.
Another example is HFC227, which is relatively inexpensive and safe to use in occupied spaces. However, it is much heavier than air and with a 36-year atmospheric life time is especially dangerous environmentally. Leak detectors working with a fire suppression system based on HFC227 will require to be sited at floor level or in underfloor voids. Similar considerations apply to HFC-227ea, which is stored as a liquid and dischargedor will leakas a vapour.
Carbon Dioxide: Because of the immediate threat to human health, CO2 in fire suppression requires the highest level of precautions against leakage as well as untimely discharge. CO2 is a toxic gas with well-defined Occupational Exposure Limits in most countries and with specified safety levels in many specific industries. Exposure at concentrations greater than 10% can cause loss of consciousness in less than 15 minutes. However, in health and safety terms the most widely recognised limits for CO2 are 5,000ppm as an 8-hour time weighted average (TWA) with a short term exposure limit (STEL) of either 15,000ppm or 30,000ppm. The refrigeration industry is the exemplar sector for CO2 usage with ASHRAE standards in the USA and EN378 in Europe. The lower TWA level used in the USA is regarded worldwide as best practice and should be so regarded also in automatic fire suppression systems.
The most widely used technology for real-time monitoring of CO2 is non-dispersive infrared, measuring the gas level as a function of the absorbance of infrared light. Today's miniaturised NDIR CO2 detectors are highly accurate and most are dual detectors that provide both a constant reference and an active signal, the difference between which indicates the CO2 concentration level. In the past, infrared instruments tended to be bulky, expensive and requiring operator expertise for consistent accuracy. Now these NDIR detectors are neat and easily installed, readily incorporated in multi-point monitoring systems (wired and wireless) and not especially expensive in the context of the types of high-value or strategic environment (data centres, clean rooms) where CO2 fire suppression systems are the choice.
Hydrofluorocarbons: now the most common group of gaseous agents used in fire suppression, all HFCs are detectable and concentration levels measurable by electronic gas leak detection and monitoring systems.
Halon: although out of production since 1997 because of its severe ozone depletion effects, Halon was for many years the dominant agent in gaseous fire suppression. For that reason it is still widely present in legacy systems, especially in government and other large facilities which have not been technologically superseded in other respects. Halon is readily detected by specific sensors in leak detector systems.
