Oxygen monitoring and hypoxic air fire prevention

oxygen monitoring and hypoxic air fire prevention

Why oxygen monitoring is critical for operator safety & fire prevention

Hypoxic air fire prevention systems, also known as oxygen reduction systems, are widely used in a range of sectors, most notably in data centers, archival storage vaults, and closed-area storage and warehousing. They work by reducing the concentration of oxygen in the ambient atmosphere to a level below which it is possible for fire to break out – this is based on the principle that it’s better proactively to manage the risk of fire rather than the traditional use of gas flooding or water mist suppression systems, which only react once a fire has started.

There are two conventional models to describe fire risk: the older ‘fire triangle’ and the more recent ‘fire tetrahedron.’ These models share three common elements, all of which need to be present for fire to occur: a combustible material, heat to raise the temperature of the material to its ignition threshold, and oxygen to sustain combustion. The fire tetrahedron adds a fourth element: an exothermic chemical chain reaction in the material. Removing any one of the first three elements will prevent fire occurring, while removing the fourth element will prevent a fire being sustained.

Oxygen reduction

Hypoxic air fire prevention systems essentially remove or reduce one of these elements, oxygen, and replace it with an inert gas.

Typically, hypoxic air fire prevention systems use a nitrogen generation unit, where pressurized atmospheric air is forced through hollow fiber membrane cartridges. These use the effect of partial pressure between the inner and outer surfaces of each hollow fiber to allow the primary components of air – oxygen and nitrogen – to be separated into two gas streams. An alternative system uses pressure swing adsorption vessels where the adsorption properties of activated media such as carbon are exploited to isolate nitrogen from the other gasses in a high-pressure air stream.

The nitrogen-rich gas stream is then fed to the area that requires protection, with a network of oxygen sensors being used to monitor ambient gas concentrations. In this way, it is possible to reduce and maintain the concentration of oxygen in the protected area to below the previously determined ignition threshold.

Oxygen thresholds

The air that we breathe contains 21% oxygen and 78% nitrogen. The human body can, within certain limits, function normally at both reduced and elevated concentrations of oxygen. In the USA and UK, for example, the minimum safe level of oxygen when working in confined spaces is set by the respective health and safety bodies (OHSA and HSE) at 19.5%, with an upper limit of 23.5%.

It is possible to work outside these limits, subject to strict precautions being exercised. Depending on the environment, these include the use of appropriate PPE and, most importantly, suitable devices to monitor oxygen concentrations in real-time.

From the perspective of fire prevention, the safest approach is to reduce oxygen concentrations to the lowest possible levels. In environments where human operators need to have access, however, this must be balanced against the health or safety of human operators who may need to enter the area to carry out, for example, essential maintenance or equipment replacement. Consequently, an oxygen concentration level of between 15% and 16% is generally considered as the best possible compromise.

Oxygen monitoring systems

Modern oxygen monitoring systems use advanced sensor technology to measure oxygen levels accurately in real time. Oxygen monitoring systems usually consist of a control panel, one or more sensors, and visual and audible alarms and are often networked with fire and building management devices. When the sensors detect a change in oxygen levels that exceed the predefined limits, alarms will automatically be triggered to alert personnel and, in some cases, activate other safety equipment such as ventilation or fire suppression systems.

Although there are several types of oxygen sensors, the two most effective are based on zirconia or electrochemical technologies. These are used in our latest generation of Gasenz Oxygen Monitors.

  • Electrochemical sensors are widely used in oxygen monitoring systems due to their sensitivity, reliability, and affordability. They work by measuring the current generated by a chemical reaction that takes place between oxygen molecules in the atmosphere and a specialized electrolyte in the sensor. The current is directly proportional to the concentration of oxygen, providing a fast, simple, and accurate reading.
  • Zirconia-based oxygen sensors utilize a solid-state electrochemical cell constructed from zirconium dioxide (ZrO2) that is stabilized with yttrium oxide (Y2O3). These sensors work by measuring the difference in oxygen concentration between the sample gas and a reference gas, typically ambient air. Zirconia-based sensors are known for their stability, high accuracy, and fast response times. Learn more about our zirconia sensors.

  • Gasenz Oxygen Monitors

    Our Gasenz Oxygen Monitors have been designed for a wide range of critical oxygen monitoring applications, especially where hypoxic air fire prevention systems are being deployed in line with standards such as ISO20388 and UL67377. Gasenz is a compact, rugged, and easy-to-use instrument with a measuring range from 0-25% for ensuring the safety of both key assets and operating personnel. With the option of either electrochemical or zirconia sensors, Gasenz devices can be powered by an AC or 24V DC source, have configurable alarms to alert users to decreasing and increasing oxygen levels, and incorporate light towers and sound beacons.

    For more information, please visit: Ambient Oxygen Analyzer - Ntron Gasenz (processsensing.com).

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