Earlier this year, a member of the SAFTENG FaceBook group posted a question about this type of system being used in a warehouse within the USA. Many of us were caught off guard as it seemed very risky to drop the Oxygen level to 13% in an occupied space, even with all the alarms and such in place. Many of us are familiar with Fixed Systems like the old Halon systems in computer rooms in our administrative buildings. But those spaces are not normally occupied and will have pre-discharge alarms so anyone inside knows the system is preparing to discharge and they have ample time to exit the space. This same concept is now being utilized (OUTSIDE of the USA) in occupied spaces such as warehouses, museums/libraries, etc. For me, doing it in “public spaces” is even more insane, as we have a long history of how humans respond to alarms and safety devices!
But with these systems already in place, FM Global has come to our rescue again. FMDS0413 – Oxygen Reduction Systems provides us with a nice description of these systems, how they function as a fire protection system, and the pros and cons of using such as system. And they just updated their Engineering Data Sheet with:
- Provided other options to maintain the needed nitrogen concentration for 12 hours within a room/building/equipment enclosure due to loss of power.
- Provided additional options for achieving N+1 nitrogen generator designs.
- Provided additional information on where to provide O2 sensors and how many sensors are needed for much larger building volumes.
- Provided additional information on O2 sensor maintenance and replacement schedules.
Here are the significant parts of their standard.
System Description
Commercial oxygen reduction systems are designed to maintain a reduced oxygen concentration in a building/room/equipment enclosure to limit propagation of a fire. The system could be designed to prevent ignition, but it would not be practical to maintain the oxygen concentration within a large volume. Some systems are designed to keep the building at some reduced oxygen level and, upon detection of smoke, discharge more nitrogen into the building to reduce the oxygen concentration (like a clean agent system).
These systems have been used to eliminate the need for automatic sprinkler protection. In Europe, since code allows these systems, companies have started installing them in warehouses. There are a couple of reported installations in North America. However, codes in the USA and Canada do not currently recognize the systems, and employee safety laws limit the minimum oxygen concentration in continuously or occasionally occupied spaces.
Occupancies for which oxygen reduction systems might be considered include the following:
- High-value occupancies [museums, galleries, libraries, archives]
- High-bay warehouses
- Cold storage (freezers)
- Data centers
The main components of an oxygen reduction system are as follows (also see Figure 3.1):
- Nitrogen generation system
- Electronic control devices and alarms
- Oxygen sensors
Nitrogen is produced as required and stored temporarily, if necessary. This may involve central nitrogen supply facilities. The storage quantities may be either liquid or gaseous. The release of nitrogen into the building/room/equipment enclosure is strictly gaseous. An oxygen reduction system monitors the oxygen concentration in the building/room/equipment enclosure using suitable oxygen measurement technology. The limitations of the oxygen sensors need to be considered when determining the oxygen concentration design point. During normal operation, the nitrogen is added to the enclosure automatically when the control system reaches the set point oxygen limit.
This system is recognized by various codes and standards worldwide, although almost all are based on VdS 3527, Oxygen Reduction Systems: Planning and Installation. The systems are designed to maintain an oxygen concentration in the protected space from 17% down to 13%, depending on the type of material being stored.
Determining Oxygen Concentration Limits
FM Global has completed two studies aimed at defining the oxygen limits for various solid fuels. The first test series used the Fire Propagation Apparatus (small scale). The second test series used a two-tier rack located inside an enclosure that was arranged to permit full control of the oxygen concentration during the test.
The research identified oxygen concentration targets that could limit flame propagation in a warehouse. The oxygen concentrations ranged from 11.1% to 13% for the common commodities investigated. These values are not affected by ignition source duration. Fire testing intended to determine the oxygen limit for a volume that will impact a fire must consider many issues that can influence the measured value. These include the following:
- The goal of the design. Is the goal to prevent any ignition, allow ignition but prevent spread, prevent ignition for a limited duration, or prevent ignition from a certain size ignition source? Oxygen concentrations that would prevent ignition (i.e., inerting concentrations) are impractical for large volumes.
- The occupancy of the building/room/equipment enclosure. The configuration of the combustibles in the volume are used to determine what oxygen level is needed. Parallel surfaces, enclosed volumes (i.e., boxes or totes filled in a standard air atmosphere), storage heights, commodity types, commodity moisture content, etc., can all affect the oxygen design concentration. Both issues are extremely difficult if not impossible to address with small-scale testing. The goal is to use the minimum oxygen level necessary to prevent significant fire spread, and not require levels that prevent all combustion. The lower the oxygen level goes, the higher the cost to maintain it. It is important to know how the oxygen concentration design value was generated. The currently accepted small-scale test for setting oxygen concentrations can show plywood won’t burn in a standard air atmosphere.
Using limiting oxygen values developed for inerting may be lower than what is needed in a large building because that testing is aimed at preventing any ignition, not just preventing spread. FM Global designed a larger scale fire test that can determine the needed LOCFP for rack storage arrays. This test method allows replication of storage geometries, testing of real commodities, and measurement of oxygen concentrations that prevent significant fire spread that are independent of ignition source duration.
Evaluating System Availability
In addition to the work aimed at defining oxygen design limits, a full availability study is needed for these systems. The goal is to define the availability of an oxygen reduction system for comparison to an automatic sprinkler system. Since these systems are active systems (i.e., in operation at all times), the ability to know their availability is easier to determine than an automatic sprinkler system, which requires regular inspections to determine its availability. An evaluation of one system demonstrated that this type of system can have availabilities equivalent to or better than automatic sprinkler systems.
One critical element that was identified was the availability of replacement parts, and personnel qualified to repair the equipment. If a system fault occurs that disables all of the nitrogen generators, the building/room/equipment enclosure needs to be isolated (i.e., all openings closed and operations stopped). A properly designed enclosure will be able to maintain the reduced oxygen concentration for some time, possibly over 24 hours. Repairs need to be completed before the oxygen concentration is elevated above allowable limits.
Potential System Applications
Oxygen reduction systems are not fire protection systems. They are fire prevention systems or fire limiting systems (i.e., they can prevent ignition or limit fire spread, depending on the fuel and oxygen concentration). From a property loss prevention standpoint, it may be beneficial to use this type of system in very high-value occupancies that are susceptible to nonthermal damage, or occupancies where the use of sprinklers is either very challenging and expensive, or for which there is no available design. Long-term energy costs for oxygen reduction systems are high because the system is normally in operation. Automatic sprinklers, on the other hand, present limited long-term costs (i.e., periodic inspection and maintenance). In addition, due to the required oxygen concentrations, these systems should only be used in a space that is not normally occupied. Various codes and regulations define “normally occupied” differently. The simplest way to define this targeted type of enclosure is as a confined space.
Storage Enclosures Inside Protected Buildings
The easiest application for these systems is to protect enclosures inside sprinklered buildings. There are storage units for extremely high-value molecular samples, product samples, rare books/documents, and computer tapes, which would all be lost if a fire grew to the point that it operated an automatic sprinkler. Even the use of a clean agent system creates challenges in these enclosures due to over-pressurization concerns and potential damage due to the decibel level associated with the gas discharge. This type of scenario lends itself to strong prevention efforts to achieve a good level of property protection. These systems can easily be applied to these types of scenarios, and automatic sprinklers could still be provided in these enclosures
as a last line of defense.
Automatic Storage and Retrieval System (ASRS) Warehouses
ASRS warehouses rely on robotic retrieval of stored commodities. These warehouses typically use open-top combustible containers to hold the commodity to ensure consistent robot functioning. These spaces are commonly unoccupied due to the electrical and physical hazards associated with the robots that operate in the aisles between the racks. In many cases, the roof height is in excess of 40 ft (12 m). The use of open top containers makes using ceiling-only sprinkler protection impossible and instead requires numerous levels of in-rack sprinklers. The extreme storage heights also severely challenge ceiling-only sprinkler designs, as well as final extinguishment of the fire. Manual firefighting efforts are unable to extinguish a fire at high elevations within the racks, and the robots make accessing even the lower levels of the racks challenging.
Deep Freezer Buildings
These occupancies are challenging to provide automatic sprinkler protection for because, even with specially designed water control valves, an accidental system trip will require the removal of all of the sprinkler pipe so it can thaw out. Additionally, failure to control the moisture levels in the piping can produce ice plugs that are difficult to identify.
RP 1 (Tissue) Paper Storage Warehouses
RP 1 paper is typically stored in an open array due to handling practices. Full-scale sprinkler testing has demonstrated that open array storage of RP 1 cannot be protected using automatic sprinklers. Data Sheet 8-21, Roll Paper Storage, doesn’t currently have any protection options for open array storage of RP 1. More work is needed to define the required oxygen level for this type of storage.
Rooms and Buildings with High-Voltage Electrical Equipment
One example of this type of occupancy is high voltage direct current (HVDC) converter stations. The valve hall normally has plastics, and older halls can have large quantities of mineral seal oil. Getting automatic sprinklers in this type of occupancy is very challenging due to the severe electrical hazards in the space. Oxygen values for mineral seal oil still need to be developed, but if a value is not available, using the limiting oxygen concentration for mineral seal oil will work.
Challenges in Using This System as Building Protection
Beyond knowing the oxygen concentration to limit fire propagation, there are several other issues to address with the installation of this type of system in a building/room/equipment enclosure that does not have automatic sprinklers.
- How to facilitate building access for repairs or maintenance needs to be determined. Sprinkler system shutdown is not something that is frequently needed unless work is required on the sprinkler system itself. Raising the oxygen levels for regular maintenance should not be a normal procedure. When the oxygen levels are increased above the oxygen design point, the system should be considered impaired.
- Fire service response will need to be clearly defined. Fire service personnel enter buildings to fight and extinguish fires, or at least confirm the lack of a fire. Allowing air into a building with an oxygen reduction system could initiate regrowth of the fire without any fire protection in the volume. Sites with building/room/equipment enclosures that have reduced oxygen environments will need to work with their fire service to develop a plan for responding to a fire that maintains the oxygen design point in the volume.