|Clean Agent Enclosure Design|
Issue 74: Clean Agent Enclosure Design
By Colin Genge
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 artifacts. 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 5 to 25 pounds per square foot (240 - 1200 Pa) to occur for a fraction of a second. The magnitude is dependent of the 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, as shown in Figure 1.
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 20011 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. Inert gas discharge can produce as much Peak pressure as inert agents.
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 shown in Figure 2, 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, 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:2
Sufficient data was gathered to more accurately predict the peak pressure for all agents. Figure 3 shows the new curve (in white) developed for inert agent peak pressure versus leak 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 to 500 Pa. Figure 4 shows the results of testing of peak pressures versus LVR for all tested inert agents in the research.
A second leakage area must now be measured
NFPA 20011 now requires a "specified enclosure pressure limit” which will, in turn, dictate the minimum allowable leakage area for the enclosure. This leakage area can be provided by accidental enclosure leakage or the area of any dampers that will be open during the discharge period.
The enclosure integrity procedure in Annex C has also been changed to allow the measurement of two leakage area values, one used for the calculation of the hold time and another used for evaluating peak pressure during discharge. These values must be measured after the enclosure has been built.
The leakage area measurement is now necessary to fulfill the new requirement in Section 188.8.131.52(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 leakage path to outdoors to prevent the peak pressure from exceeding the specified limit.
Optimizing 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. Another solution is to consider the design parameters that affect peak pressure and hold time so that both requirements are met without using PRVs. Even if this design effort still results in the need for PRVs, optimizing the enclosure will increase the level of fire protection and possibly allow the use of smaller PRVs since more passive protection was built in.
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. Adding PRVs is costly, sometimes impossible and often a source of unwanted risk, since they may fail to open and could damage the enclosure.
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 project2 showed that the actual peak pressures exceeded those predicted by the previously used formulae by at least 100%, and many of those enclosures were discharge tested with inert agents, it is safe to say that a wide range of enclosures handled 0.07 PSI (500 Pa) of peak pressure. This has also been verified with the use of a high output fan to pressurize enclosures where no effects were observed at 0.07 PSI (500 Pa). One can therefore assume that a double sided wall, securely fastened top and bottom, will handle 0.07 PSI (500 Pa). This can also be tested using a high pressure door fan.
While thicker walls can take more pressure3 as shown in Table 1, false ceilings can only take about 0.007 PSI (50 Pa), so they must be protected from pressures higher than that with vented tiles.
Selection of an appropriate hold time
NFPA 2001 requires a hold time of 10 minutes or a time period to allow for response by trained personnel. However, 10 minutes may not always be the appropriate hold time. The designer must consider what the response time for trained personnel to determine if longer hold times are necessary.
Enclosure design approaches
The following design strategies have the potential to do one or more of the following:
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.
If no mixing will occur, the height of the protected equipment should be kept to a minimum. If the equipment height exceeds 75% 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:
Peak pressure evaluation
PRVs that are designed to open at a certain pressure must be tested prior to and/or after installation to verify they open at the prescribed pressure. 0.002 PSI (125 Pa) is the pressure generally used to test PRVs because it is representative of the peak pressures that may be encountered. This pressure can be imposed upon the damper in a test box, or the entire enclosure can be pressurized, or a temporary pressure box can be constructed around the damper. A large flow at a fairly high pressure will be required to test these vents in their open position, so testing them in a test box can be advantageous. Once the position at test pressure of 0.002 PSI (125 Pa) is determined, the vanes must be locked in that position while the damper leakage area is tested.
If installed in a test box where there are no bias pressures, it can be tested in the direction of intended venting. If installed in the enclosure, it should be tested in both directions to compensate for any bias pressures and to achieve a more accurate test due by increasing the amount of data collected. The PRV should be tested in the flow direction that will occur during discharge.
There are dual acting PRVs that will open in both directions, but their free vent area differs with respect to direction, so they must be tested in both directions.
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