Room Integrity for Gaseous Fire Suppression Systems
Gaseous fire suppression systems are effective options for protecting valuable assets by extinguishing fires without damaging equipment in the space. In many applications however the best designed system might not be fully effective if the integrity of the room with the protected equipment itself is not adequately considered, provided, or maintained. Here we’ll review how gaseous suppression agents work, why the integrity of the room matters, how that is tested, and finally controls that can be set in place to ensure the integrity of the room is maintained.
How do gaseous suppression agents work?
Gaseous fire suppression agents work fundamentally the way any fire suppression media works; by removing one of the components of what was traditionally referred to as the fire triangle and now more appropriately, the fire tetrahedron.
Unlike water, which primarily works by removing heat, most gaseous suppression systems suppress fire primarily by reducing the available oxygen for combustion with some secondarily inhibiting the chemical chain reaction required for combustion. A minority of the agents do have a primary mechanism of heat absorption.
Protection can be provided in a “total flooding” or “local application” approach. Local application design focuses discharge on a specific process or piece of equipment while the total flooding approach fills an entire space, typically a room defined by 4 walls, a floor, and a ceiling. Here we will be focusing on total flooding applications.
In order to reduce the available oxygen below the threshold which combustion can be supported, the gaseous agent must be introduced into the space and mix with the air in that space at a concentration that is specific to the particular gas chosen. Design concentrations can vary based on the agent as well as the fuel class being protected. Specifics of this can be found in NFPA 2001, Standard on Clean Agent Fire Extinguishing Systems, for clean agents and inert gases while NFPA 12 Standard on Carbon Dioxide Extinguishing Systems, can be referenced for carbon dioxide.
One of the most important concepts to understand within this discussion is because the properties of these extinguishing agents do not significantly reduce heat, the concentrations must be held for a minimum period of time. This minimum period of time is typically no less than 10 minutes or a sufficient amount of time for trained personnel to respond. If the concentration of the gaseous agent disperses from the space then oxygen levels could once again increase to levels high enough to support combustion and with the heat and fuel still present, the fire could reignite.
This is why room integrity becomes important. The room must be air-tight enough to maintain concentrations for the minimum hold times.
Room Integrity – Pressure
While not the focus of this discussion it should be noted that the initial discharge of gaseous suppression systems can cause significant pressure changes- both negative and positive- and the structural integrity of the enclosure should be evaluated to determine whether venting is required.
Room Integrity – Leakage
All rooms will have leakage when positively pressurized. Openings such as doors, windows, as well as those for HVAC ductwork or electrical cabling are a necessity but will also never be sealed perfectly. It is not the intent to make sure that they are either. Rather, the intent is to ensure that minimum design concentrations can be maintained within the space for the minimum amount of time. As a general rule, all openings, notably doors and ventilation fans and/or openings, must be secured prior to discharge in conjunction with the detection and alarm system.
In the case of clean agents this is specified by NFPA 2001 as a minimum concentration of 85 percent of the adjusted minimum design concentration held at the highest height of protected content within the enclosure for a period of 10 minutes or for a time period sufficient to allow for response by trained personnel. The design of the system plus the air-tightness of the room must combine to meet this requirement.
Testing for room integrity
The ability of a compartment to maintain adequate agent concentration is a function of the leakage of the compartment. Annex C of NFPA 2001 describes a complete procedure for evaluating agent hold time as a function of compartment leakage measured by the door fan pressurization method. This method is called a room integrity fan test or, more commonly, a door fan test. This procedure specified in Annex C of NFPA 2001 determines the leakage of the entire enclosure envelope. It is determined by measuring the enclosure leakage under both positive and negative pressures and averaging the absolute values of the readings. While it seems rather complex, most of the readings from the test can be input into computer programs that will perform the calculations and provide the results. This method evaluates a worst-case leakage for the enclosure and actual performance can be expected to show much less leakage than the results of the door fan test.
Maintaining room integrity
Even if the system was designed correctly and initial integrity tests show that sufficient concentrations can be maintained, modifications to the space can have the potential to drastically change this condition. In order to ensure the integrity of the room is maintained, annual inspection of the enclosure is required, or the enclosure must be monitored by a documented administrative program for changes in barrier integrity or enclosure dimensions. Additionally, any penetrations made through the enclosure need to be sealed immediately. If there is any question as to the impacts any changes might have on the ability for design concentration to be held, integrity testing should be performed again.
Gaseous fire suppression systems are significant investments in protecting valuable equipment and assets. Part of ensuring their effectiveness goes beyond the system itself and must also focus on the room so that any fire can be suppressed and limit the chances for reignition.