By Patrick van Hees, Lund University, Division of Fire Safety Engineering, John Barton, Lund University, Division of Fire Safety Engineering, Martin Nilsson, Zurich Insurance plc (Ireland), Sweden Branch, and Brian Meacham, Meacham Associates
In recent years, hypoxic or oxygen reduction systems (ORS) for fire prevention have become increasingly used as part of fire prevention and mitigation strategies in a wide range of facilities, from cold storage to computer facilities to museums. Such systems operate by creating an environment of sufficiently low oxygen concentration to prevent, or significantly inhibit, fire initiation, development and spread. The fundamental operating principle of ORS for fire applications is to displace ambient oxygen in an enclosed environment with one or more nitrogen generators.
As of early 2018, two test standards provided guidance on design, installation and maintenance of ORS for fire: VdS 3527en (2007), Oxygen Reduction Systems Planning and Installation (VdS, 2007), and EN 16750:2017 E, Fixed firefighting systems — Oxygen reduction systems — Design, installation, planning and maintenance (which replaced British Standards Institution PAS 95 [BSI PAS 95], Hypoxic Air Fire Prevention Systems). In addition, efforts have been underway in the International Organization for Standardization (ISO) to develop an international standard for these systems.
While ORS systems are in use, since standard test methods are available, several concerns have been raised about the boundary conditions of use and their efficacy in preventing ignition and controlling the spread of fire. In light of such concerns, the NFPA’s Fire Protection Research Foundation (FPRF) commissioned a literature review on the topic, from which boundary conditions could be clarified, gaps could be highlighted and ongoing research needs could be recommended.
For example, one issue already identified in previous research was that the testing procedures specified by the standards for assessing ignition thresholds for materials in reduced oxygen environments may be insufficient — the strength of the ignition source and the fuel configuration do not reflect environments with ORS installed. Nilsson and van Hees (2014) expressed such concerns regarding ignition thresholds in the methods described in VdS 3527 and BSI PAS 95, saying these include:
- The point of igniter application is not defined (research indicates this affects test outcome).
- No sustained external heat flux is maintained once the initial igniter is removed; under actual real-world conditions, an ignition source may be sustained (such as an electrical ignition source or exposure to a fire).
- Behavior of composite materials is not addressed.
- Behavior of a smoldering fire is not evaluated.
- Generation of products of combustion is not evaluated.
Based on these factors, Nilsson and van Hees concluded that the ignition thresholds obtained using the tests described in VdS 3527 and BSI PAS 95 are only valid for the tested conditions and may not represent real-world conditions.
Independent work by other researchers support this conclusion. For example, research conducted by Xin and Khan (2007a, b) concluded that based on laboratory scale testing, the required oxygen concentration for preventing ignition in “real-world” environments is lower than the threshold oxygen concentrations found in the VdS 3527 or BSI PAS 95 methods. These conclusions have been reconfirmed through subsequent testing on a larger scale (FM, 2018).
Although not expressly assessed in previous experiments, similar concerns exist about the recently developed standard EN 16750:2017 E (CEN, 2017), which also provides exemplar data on minimum oxygen concentration levels for ignition of select materials, using essentially the same test method as described in VdS 3527.
The main research questions for this in-depth review of existing literature were:
- To what degree do existing test standards for ORS for fire establish ignition threshold values (reduced oxygen levels) that are appropriate for the system to be deemed effective as a fire prevention system?
- To what degree do the ignition sources and strengths in existing test methods adequately reflect real-world ignition scenarios?
- To what extent might different performance requirements be proposed, and/or alternative test method(s) to those currently available, that provide a closer representation to real-world conditions?
- What other considerations for design, inspection, maintenance and use are important to assuring effective ORS for fire?
In conducting the review, the Project Technical Panel established several boundary conditions. First, one of the major focus areas of the studies has been, but not limited to, ignition prevention and associated test methods. The findings apply to all ORS for fire, but the review does not address the efficacy of ORS in cases where ignition is not prevented and a fire develops in a reduced oxygen atmosphere. It was determined and recommended that additional research is needed in this area.
Second, safety of people in reduced oxygen atmospheres, without protection (e.g., breathing apparatus), has not been addressed. In this case, there is a wide range of regulatory restrictions in different countries. The medical data do not adequately reflect the specific use, and research by appropriate medical personnel is warranted.
In total, more than 20 findings and recommendations have been made. These include limitations on existing test methods and the need for more data regarding real-world scenarios, better data about required levels of oxygen concentration to inhibit ignition and under what conditions, and more guidance on the inspection, testing and maintenance of ORS for fire. The final literature review report is available on the Fire Protection Research Foundation website (https://www.nfpa.org//-/media/Files/News-and-Research/Fire-statistics-and-reports/Suppression/RFOxygenReductionSystems.pdf). A poster on this work will be presented at the SFPE Europe Conference in Malaga, Spain, in May 2019.
Acknowledgments
The authors gratefully acknowledge the financial support provided by the Fire Protection Research Foundation and the project sponsors, and for the helpful feedback and guidance on the scope and content of the report provided by the Project Technical Panel.
References
BSI. PAS 95:2011 — Hypoxic air fire prevention systems: specification, BSI, London, 2011.
EN 16750:2017 — Fixed firefighting systems — Oxygen reduction systems — Design, installation, planning and maintenance, CEN, 2017.
FM Global — Evaluation of oxygen reduction systems (ORS) in large-scale fire tests, Project ID 0003058222, FM, 2018b.
Nilsson, M. and van Hees, P. Advantages and challenges with using hypoxic air venting as fire protection, Fire and Materials 38(5):559–575, 2014.
Van Hees, P., Barton, J., Nilsson, M., and Meacham, B.J. Review of Oxygen Reduction Systems for Warehouse Storage Applications, Quincy, MA USA: Fire Protection Research Foundation, (https://www.nfpa.org//-/media/Files/News-and-Research/Fire-statistics-and-reports/Suppression/RFOxygenReductionSystems.pdf), 2018.
VdS 3527en:2007, Guidelines for Inerting and Oxygen Reduction Systems, VdS Schadenverhütung, 2007.
Xin, Y., Khan, M. Flammability of combustible materials in reduced oxygen environment, Proceedings Fire and Materials conference, San Francisco, CA USA; Interscience, London, 2007a.
Xin, Y., Khan, M. Flammability of combustible materials in reduced oxygen environment, Fire Safety Journal 42:536–547, Elsevier, 2007b.