Why caution is needed on oxygen reduction systems
The fire triangle explains that there are three ingredients for most fires – heat, fuel and an oxidising agent. If you want to control a fire, you reduce the fuel, oxygen or heat. Sprinklers, which are the most commonly used fire protection system, address the heat and fuel legs. But now there’s a new kid on the block, which aims to solve the problem differently: oxygen reduction systems (ORS).
It’s not surprising that there is significant interest in this new technology - You need a fire to activate a sprinkler system. While the damage from a fire can be limited using sprinklers, some is inevitable from the fire itself, the smoke or the water used to put it out.
ORS are designed to avoid fire starting or spreading by maintaining a permanently low oxygen concentration in an enclosed protection space. They do this by streaming nitrogen (N2) into these spaces. ORS are sometimes positioned as a more cost-effective way to reduce fire risk, with claims that they can eliminate the need for more costly fire sprinkler installations.
FM Global is seeing an increase in the number of businesses considering ORS as a way of preventing the start or controlling spread of fire. We expect interest in ORS to grow as warehouses get bigger and automated storage retrieval becomes more popular. Combined with increases in robotic technology, rising salaries and land costs, organisations are increasingly building warehouses that are more than 30 metres tall. Sprinkler systems in these environments become elaborate and costly, making alternatives more appealing.
We recently did some testing to advance ORS understanding and reduce risk. Our research centred on the oxygen concentration limits required to prevent fire propagation, testing various ignition source intensities and different fuel sources.
What we’ve discovered should serve as a note of caution for anyone managing fire risk. Our research reveals that the oxygen concentration needed for an ORS to function effectively must be lower than that specified in a commonly referenced standard - the existing German VDS3527.
As a result, we feel that these systems are a potentially viable fire protection method for certain well-sealed, unoccupied spaces but will require significant consideration to replace sprinkler protection.
Research in more detail
To provide more context, I’ll outline how we conducted our lab research into ORS:
- Testing commodities included five standard commodities commonly found in today’s warehouses: Class 3, as well as both cartoned and uncartoned expanded and unexpanded plastics. CUP, CEP, UUP and UEP. Two-tier fuel arrays of standard commodities were set up in rack storage configuration in an enclosure. To represent a large space at uniform concentration, a constant N2/Air mixture flow was supplied into the enclosure at a desired oxygen concentration.
- The target oxygen concentration was varied from nine per cent up to 17 per cent. A premixed propane ignitor with a constant heat release rate (HRR) was used as the ignition source.
- The impact of the test conditions on fire propagation was examined in detail for Class 3 commodities at different oxygen levels. The results showed that oxygen concentration is the dominant parameter controlling fire propagation.
The limiting oxygen concentrations (LOC) that support flame propagation were determined with and without a sustained ignition source using statistical analysis of the large-scale data. The LOC was defined as the oxygen concentration for a five per cent probability of flame spread.
Findings on the appropriate LOC for different commodity and ignition types were as follows:
- Cartoned (Class 3, CUP and CEP) with a sustained ignitor - 11.1%
- Uncartoned (UUP and UEP) with a sustained ignitor - 13.0%
- Cartoned (Class 3, CUP and CEP) with ignitor shut off after ignition - 13.8%
- Uncartoned (UUP and UEP) with ignitor shut off after ignition - 14.7%
Given the criticality of maintaining the LOC and the potential complexity of ORS the design of the specific system should be evaluated to ensure that the fire protection provides the appropriate level of reliability through redundancy, alarming, ready availability of sparing and appropriately trained service personnel.
In temperature control systems, people may not realise that the temperature has spiked until it’s too late. A similar risk exists with ORS. The effectiveness of the system in minimising the spread of fire relies on maintaining the LOC. Failure of electrical power or a system component, or loss of enclosure integrity could cause the oxygen concentration to rise to a level where a fire is possible before the problem can be identified and rectified – leaving the facility unprotected against fire if there is no sprinkler system provided.
There are also personal safety issues to be thoroughly considered. The LOC our research recommends as effective for reducing fire propagation risk will create confined space hazards for any people working in warehouses where ORS are deployed. Any organisation using these systems needs very clear procedures on how a person may enter the space.
Clear cooperation and communication with the local fire department is also critical. Firefighters called out to a fire would need to understand that they’re responding to an event in an area with significantly lower oxygen concentration. There’s also a possibility that by compromising the enclosure integrity fire crews could make matters worse by allowing more oxygen into the building, fuelling the fire.
These words of caution are not to say that ORS are not appropriate for some situations. For those considering how best to reduce fire and fire propagation risk, we recommend thoroughly assessing claims made by providers and consulting with independent partners.
We caution that while ORS offer potential benefits, they must be implemented with caution, all necessary safeguards in place and a back-up plan. At this time, they should be considered an additional fire prevention method, but not a substitute for, traditional fire protection measures. In the future there is the potential for ORS to be implemented as an alternative to traditional fire protection measures.
If you’d like to explore our research in greater technical depth, you can find the full report – Evaluation of Oxygen Reduction System (ORS) in Large-Scale Fire Tests – here.
This article first appeared in Fire Australia.