Clean Agent Enclosure Design
Clean agent fire suppression systems are used in enclosures where a sprinkler system could cause damage to sensitive contents, such as rooms containing computer servers, paper files or historical artefacts.
Upon fire detection, the compressed clean agent, which can be a halocarbon or an inert gas, is released into the enclosure causing a peak pressure of around five to 25 pounds per square foot (240 – 1200 Pa) to occur for a fraction of a second. The magnitude is primarily dependent of the total enclosure leakage area.
Once the enclosure is totally flooded, the agent will begin to leak out at a rate that primarily depends on leakage area in the lower part of the enclosure. The distribution of the remaining agent will either be constant throughout the enclosure (due to continual mixing) or will establish an interface with air above and agent below that descends over time as agent leaks from the enclosure. Until 1988, enclosures protected by clean agents used full discharge tests to determine the hold time. Since then, door fans have been used to measure the leakage area, which is entered into formulae found in Annex C of NFPA 2001 to predict the hold time.
Peak Pressure during Discharge
It is common practice for peak pressure calculations to be done for inert agents, but not for halocarbon agents. It will come as a surprise to designers that halocarbon gas discharges can produce as much peak pressure as inert agents, which is why NFPA now requires an evaluation of the maximum peak pressure be performed.
The magnitude of the peak pressure depends primarily upon the ratio between the leakage area of the enclosure and the volume of the room (LVR). In a typical halocarbon agent discharge, as shown in Figure 1, the peak pressure increases with enclosure tightness. This tightness also determines the hold times as shown in the legend. Although a peak pressure evaluation is required by NFPA 2001 Edition 2008 and later, the standard does not describe how it is to be calculated. Retrotec’s software, for example, which can be used for hold time evaluations, provides the peak pressure calculation in its newest integrity software.
A five-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:
- Previously available inert agent formulae from equipment manufacturers typically under-predict peak pressure by a factor of two to four times.
- Under equivalent hold time conditions, halocarbon agents can produce as much peak pressure as inert agents.
- Peak negative pressures from halocarbons are strongly influenced by humidity and may be greater than positive peak pressure requiring Pressure Relief Vents (PRV) that operate in both directions.
Sufficient data was gathered to accurately predict the peak pressure for all agents. Figure 2 shows the new curve (in white) developed for inert agent peak pressure versus leakage to volume ratio (LVR). Previously-existing formulae (dashed lines) all under-predict the peak pressure expected at a given LVR over the typical peak pressure values from 250 Pa to 500 Pa. Figure 3 shows the results of testing of peak pressures versus LVR for all tested inert agents in the research.
Second Leakage Area Must Now be Measured
NFPA 2001 now requires a “specified enclosure pressure limit” that will, in turn, dictate the minimum allowable leakage area for the enclosure. This leakage area can be provided by accidental enclosure leakage, PRV or both.
The enclosure integrity procedure in Annex C of NFPA 2001-2012, has a new test method that provides leakage values for pressure relief and hold time. This measurement is now necessary to fulfil the new requirement in Section 184.108.40.206(10) that states: “an estimate of the maximum positive pressure and the maximum negative pressure” during the clean agent discharge must be made. Section 5.3.7 states: “If the developed pressures present a threat to the structural strength of the enclosure, venting shall be provided to prevent excessive pressures”.
The designer can perform calculations using the new peak pressure equations that have come out of the research project to determine whether or not a pressure relief vent (PRV) is likely to be needed and alter the design using the approaches presented in this article. It is no longer sufficient to simply specify a PRV of a certain size; its leakage rate must also be measured after installation to confirm the vent both opens at the correct pressure and has a large enough free vent area to outdoors to prevent the peak pressure from exceeding the specified limit.
It is a common industry misconception that when a PRV is installed the problems are solved, but unless it is tested as installed there is no guarantee the PRV will work. Common and widespread problems are PRVs installed backwards for halocarbons where the maximum pressure is negative and blocked venting paths. Testing the PRV with a door fan will uncover these problems.
Optimising Peak Pressure and Hold Time Performance
Clean agent discharges can produce damaging enclosure pressures that increase as total enclosure leakage area decreases. Simply providing a lot of enclosure leakage area to solve the peak pressure problem creates another problem, because hold times decrease as the leakage area increases.
One solution is to add a PRV that will provide increased leakage to reduce the peak enclosure pressure; the enclosure can then be made tight to provide the specified hold time. An analysis must be performed during the design stage where parameters such as total flooding height, agent type, discharge time, protected height can be manipulated to maximise protection and avoid having to make expensive last minutes changes such as adding agent or moving suspended ceilings or adding PRV.
Ironically, many inert agent protected enclosures have PRVs installed where they are not needed, while other enclosures (protected by both inert and halocarbon agents) need PRVs but they are not installed. This situation can be resolved by using the new enclosure integrity evaluation procedure along with the new peak pressure formulae so the installer will be given sufficient time to acquire the correct PRV for the job.
Selection of Specified Enclosure Pressure Limit
Formulae have been used for over a decade to predict peak pressures and to size PRVs for thousands of enclosures without damaging those enclosures. Since the research project showed that the actual peak pressures exceeded those predicted by the previously used formulae by at least 100 percent, and many of those enclosures were discharge tested with inert agents, it is safe to say that a wide range of enclosures handled 10 PSF (500 Pa) of peak pressure. One can therefore assume that a double-sided wall, securely fastened top and bottom, will easily handle 10 PSF (500 Pa). This can also be tested using a high pressure door fan.
While thicker walls can take more pressure as shown in Table 1, false ceilings can only take about 1 PSF(50Pa), requiring sufficient area of venting to protect them from damage.
Selection of an Appropriate Hold Time
NFPA 2001 requires a hold time of ten minutes or a time period to allow for response by trained personnel.
However, ten minutes may not always be the appropriate hold time. The designer must consider what the response time for trained personnel is to determine if longer hold times are necessary. Shorter hold times might be appropriate for small enclosures that are always occupied. Reducing the hold time could solve one of the most costly and pernicious problems that installers face, where getting these enclosures tight enough to pass the ten-minute requirement becomes very difficult.
One of the most common mistakes made by designers is to overlook the fact that the agent will be mixed during the hold time. Mixing may often be the only way to pass enclosures where protection is required at high elevations in the enclosure. Knowing this in advance will allow for using 30 percent more agent than is usually needed to facilitate the continual mixing hold time extension.
Enclosure Design Approaches
The following design strategies have the potential to do one or more of the following:
- Reduce installation costs.
- Reduce the risk of damage created by discharge pressures.
- Ease maintenance.
- Improve fire protection.
- Reduce the risk of smoke damage.
These strategies are meant to be considered during the design phase. The installed performance of the PRVs must be checked during installation to determine that they open at the correct pressure, in the correct direction and that the free vent area of the entire vent path falls within the specification. A very different leakage test, with PRVs closed, is performed to check adequate retention time.
- Seal the walls to the upper slab. Extending walls to the upper slab and sealing them is the only defence from fire and smoke entering from outside the enclosure. Paragraph C-1.2.1 (2) in NFPA 2001 states “…enclosures absent of any containing barriers above the false ceiling, are not within the scope of Annex C,” meaning the enclosure will be difficult to test and verify.
- Flood the entire enclosure with agent. The higher the initially flooded height, the leakier the enclosure can be, producing less peak pressure but yielding longer hold times. Typically, the small savings generated by flooding only to the bottom of a false ceiling are offset by the increased air sealing costs needed for adequate hold time, and may also require PRVs more often. If a false ceiling is needed, nozzles should be specified above and below the false ceiling.
- Use an automatic door closing system. Doors often get wedged or propped open when the enclosure is in use. This practice impairs the clean agent system. A better solution is an automatic door release mechanism that will close the doors whenever the first alarm sounds. A mechanism should be specified that will close the door when it is de-energized so it is failsafe.
- If a false ceiling is specified, lower leaks should be sealed first until the specified hold time is reached and then leaks above the false ceiling should be sealed until the peak pressure limit is reached. The air leakage determination will require measuring upper and lower leaks separately, as described in Section C.2.7.2 of NFPA 2001 and shown in Figure 6.
- Increase the initial concentration of agent a further 15% over design concentration if continual mixing will occur, to ensure a long enough hold time. If air handlers continue to run during the hold time, then continual mixing is certain, but even equipment cooling fans or thermal effects can be sufficient to cause continual mixing. Increasing the margin between the initial and final concentration in the continual mixing case has the same effect as making the room taller in the descending interface case. For non-mixing cases, the agent is allowed to drain out until it hits the protected equipment, which is typically at 60 percent to 75 percent of the enclosure height, allowing 40 percent to 25 percent of the agent to run out before the equipment is no longer protected. If additional agent were not added, only 15 percent of the agent would have to be lost before the equipment loses its protection, since the standard requires that the final concentration at the end of the hold time at the top of the protected equipment be not less than 85 percent of the design concentration. NFPA 2001 uses an integration formula that increases the hold time prediction, but it is still important to add this additional agent, otherwise the enclosure will fail the hold time after only 15 percent of the total weight of agent is lost.
If no mixing will occur, the height of the protected equipment should be kept to a minimum. If the equipment height exceeds 75 percent of enclosure height, continual mixing may be the only way to achieve a reasonable retention time.
Pressure Relief Vents
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 is vented.
- Vents should open at pressures no lower than 2 PSF (100 Pa) so they do not open unintentionally under normal HVAC pressures and no higher than 3 PSF (150 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 outdoors has not been accidently restricted.
Peak Pressure Evaluation
PRVs must be tested at a reference pressure of 2.6 PSF (125 Pa) in a temporary pressure box constructed around the damper or inside the tested enclosure. A large door fan flow will be required to test these vents in their open position. Dual acting PRVs open in both directions, so they must be tested in both directions.
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