It is well known that clean agent fire protection systems are a fantastic way to protect rooms containing computer servers, paper files, historical artifacts, and high-value or sensitive electronic equipment. There are many options available between the inert gases, and halocarbons that vary in price, effectiveness, and design options.
Upon fire detection, the compressed clean agent, which can be a halocarbon or an inert gas, is released into the enclosure causing a temporary peak pressure ranging from approximately 5 to 25 pounds per square foot (240 - 1200 Pa). This peak pressure occurs within a fraction of a second and is a critical factor in the design and effectiveness of the fire protection system.
The peak pressure is influenced by several factors, most notably the leakage area of the enclosure. Larger leakage areas can reduce the peak pressure, while smaller, tighter enclosures can experience higher peak pressures. It is crucial to account for these dynamics during the system design to ensure the structural integrity of the protected area and the efficacy of fire suppression.
So, the question is, how do we know if an enclosure is tight enough for hold time but leaky enough to sustain peak pressure?
Historically until 1988, total or a full discharge method of clean agent was used to test the enclosure if it can maintain a certain hold time and peak pressure. However, since then, door fans have been employed to measure the leakage area, which is then used in calculations based on the guidelines in Annex C of the NFPA 2001 to predict the hold time.
The NFPA 2001 now requires a "specified enclosure pressure limit" which determines the minimum allowable leakage area. The enclosure integrity procedure has been updated to measure two leakage areas: one for calculating hold time and another for evaluating peak pressure during discharge. This dual measurement approach helps in ensuring the structural integrity of the enclosure during a clean agent discharge.
The current requirement mandates an estimate of the maximum positive and negative pressures during discharge. If the calculated pressures exceed the structural capacity of the enclosure, pressure relief vents (PRVs) must be installed. The new peak pressure equations derived from recent research assist in determining whether a PRV is necessary.
Why is it so… important?
Clean agent fire suppression systems are essential for protecting sensitive environments, such as computer server rooms, paper archives, and historical artifact storage. These systems are preferred in these areas because traditional sprinkler systems can cause irreparable damage to delicate contents. However, the peak pressure dynamics during agent discharge, if not properly managed, can cause significant structural damage.
Destructive Peak Pressures
The peak pressure's intensity depends on the enclosure's leakage characteristics. If not adequately controlled, this sudden pressure surge can lead to severe structural damage, as evidenced by numerous instances where enclosures have been compromised.
Impact of Excessive Leakage
Excessive leakage can have a similar detrimental effect. If an enclosure has too much leakage, the clean agent will escape too quickly, failing to maintain the required concentration to suppress the fire effectively. This rapid loss of agent means the hazard is no longer protected, leading to increased costs and potential property loss.
In Conclusion
Proper management of peak pressure dynamics is crucial in clean agent fire suppression systems. Both excessive peak pressures and excessive leakage can compromise the integrity and effectiveness of the system, resulting in significant data, financial, and property losses. Ensuring that enclosures are designed and maintained to withstand peak pressures while minimizing leakage is essential for the reliable protection of sensitive environments.
Why do PRV (Pressure relief vents) play an important role?
Firstly, the standards speak for itself!
Peak pressure varies over time depending on the ratio between the leakage area of the enclosure and the volume of the room (LVR). In a typical halocarbon agent discharge, as seen in the graph, the peak pressure increases with enclosure tightness. This determines the hold times as shown in the legend. Although peak pressure is referred to by the NFPA 2001 Standard, the standard does not yet provide guidance on how it is to be calculated.
A 5-year research project was carried out to provide a validated prediction model for peak pressure based on leak-to-volume ratio. This research uncovered many important facts about clean agent discharge pressures and the peak pressure formulae previously used to predict pressure values during enclosure design and testing. In particular, this research found that;
· Available inert agent formulae under-predict peak pressure
· Under certain conditions, halocarbon agents can produce as much peak pressure as inert agents
· Peak pressure from halocarbons is influenced by humidity
Sufficient data was gathered to more accurately predict the peak pressure for all agents. Formulae have been used for over a decade to predict peak pressures and to size PRVs for thousands of enclosures without damaging those enclosures. Naturally, peak pressure varies with the existing condition of the enclosure, e.g., thickness of the walls, integrity of the false ceiling, weakest point in the room like glass door or window, etc.
How to know? Test the enclosure!
Remember, there are other methods to improve the enclosure design:
· Increase Volume
· Increase discharge time to maximum
· Increase pressure limit
· Compare agents
A simpler cost-effective and time-effective option is to install a PRV (Pressure relief vent).
If PRVs must be installed, there are several guidelines to follow to optimize their performance:
· Install vents as high as possible so that the lighter air, not the denser agent, is vented.
· Vents should open at pressures not lower than 0.007 PSI (50 Pa) so they don’t open unintentionally under normal HVAC pressures and no higher than 0.02 PSI (100 Pa) so the pressure is vented early enough to prevent it from becoming excessive.
· Specify the correct direction for venting with the PRV. Inert agent discharges always create positive pressures and must have venting out of the enclosure, but halocarbons may create positive and/or negative pressures creating a need to be vented in either direction or both, depending on the agent and the humidity.
· All PRVs should be inspected annually to confirm they will open according to their specifications and to verify that the vent path to the outdoors has not been accidentally restricted
PRVs that are designed to open at a certain pressure (0.002 PSI (125 Pa)) must be tested prior to and/or after installation to verify they open at the prescribed pressure.
There is a wide variety of pressure relief vents available, although there are only a few vents actually suitable for enclosures with a gas suppression system. The main concerns for pressure relief vents (PRVs) are the correct opening pressure (as standards will generally only allow to use of vent area of PRVs that is provided at up to 125Pa), the fire rating of the product (if required), and the actually provided ‘free-flowing vent area’ or FVA (which has to be measured during an enclosure integrity test).
Optimizing the dynamics of peak pressure in clean agent fire suppression systems is essential for protecting sensitive enclosures. By understanding and applying updated standards, measurement techniques, and design strategies, these systems can effectively safeguard critical assets while maintaining structural integrity and operational efficiency.
Build Tight, Ventilate Right!
Article by: Technical Engineer Rila Khairun
References:
2. NFPA 2001, Standard on Clean Agent Fire Extinguishing Systems, National Fire Protection Association, Quincy, MA, 2012.
4. Retrotec.com