Why do rooms with a gas suppression system blow up or still catch fire?

Enclosure integrity testing for gas suppressions (as per NFPA2001 and ISO14520)

The system discharge and hold time

Clean agent fire suppression systems are used in enclosures where a sprinkler system

would cause damage to sensitive contents such as 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. Once the enclosure is totally flooded, the agent will begin to leak

out at a rate that primarily depends upon the leakage area in the lower part of the enclosure, although higher leakages play a significant role as well. 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 the agent below that descends over time as the agent leaks out as shown in Figure 2. The gas suppression system is designed to protect critical systems within the enclosure for a certain time once the agent is released, which is called the ‘hold time’. This will generally be 10 minutes or in some cases 20 minutes, or any other time frame dependent on the exact situation and used standard. To keep the agent inside the enclosure, the enclosure will have to be airtight enough to prevent the agent from leaking out too fast, the less leakage, the more hold time an enclosure will have. The actually allowed leakage to keep the agent inside the enclosure differs for every enclosure, as this depends on a lot of different factors such as used agent, agent concentration, equipment height, etc. To make sure a room is protected it will have to be tested on the enclosure integrity by a capable technician, with the use of a calibrated blower door system, as well as up-to-date software that is capable of calculating the expected hold time as per the latest applicable standards.

Without an enclosure integrity test, it will be merely a guess whether or not the system will actually suppress a fire in case of disaster, or whether the agent will just leak out directly and allow the fire to spread again

Graphical representation of the NFPA 2001 Edition 2015 Annex C

Why does peak pressure matter?

Once the agent is released within an enclosure, the pressure in the enclosure will go up, and a peak pressure will be reached (before the start of the required hold time). For inert agents (such as nitrogen and argon) a positive peak pressure is reached due to the large quantity of agent that is released from a highly pressurized cylinder. For halocarbons (such as HFC-227) both a negative and positive peak pressure will form due to the cooling effect caused by a phase change of the agent (negative pressure), as well as the nitrogen (pressurizing agent) blowout at the end of the discharge (positive pressure).

Pressure Vent are used to release excessive pressure (by Pressurevent.com).

The peak pressures, both positive and negative, can be high enough to severely damage the enclosure. Damages can vary from cracks in structures to completely blown-out walls, doorframes, and ceilings. Once the damage is done, the agent will leak out of the enclosure and the fire will not be suppressed as intended.

Preventing high peak pressures can be done in multiple ways. The main part will always be to measure the actually provided vent area with a calibrated blower door system, after which the software is capable of calculating the expected peak pressure. In case the developed peak pressure is higher than the pressure the room would be able to handle, action will be required. The most common solution for high peak pressures is to install a pressure relief vent.

Pressure relief vents are designed to release excessive pressure while closing once the pressure drops to make sure the agent will remain within the enclosure.

Examples of damages due to peak pressures

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). As standards currently require PRVs to be tested at pressures up to 125Pa, a vent that opens at any higher pressure only will not provide any vent area during an integrity test as per the standards. Furthermore, the standards require the vent to be tested on a free-flowing vent area based on the leakage they actually provide at 125Pa, which will generally be different than the often used sizes of the vent itself (due to restrictions in air flow, etc.).

Solution

As many enclosures are currently not tested on the enclosure integrity, there is a high potential for fire to spread as the suppression system may not be working. Furthermore, due to the possible high peak pressures, untested rooms can be considered as ticking time bombs, waiting to blow up once the gas suppression system is triggered.

An enclosure integrity test that is done correctly will provide data on both the hold time as well as the predicted peak pressure. Only when both hold times and peak pressures are within the required thresholds, a room can be considered to be passed for the integrity test. It is common to re-test enclosures on the enclosure integrity annually. If any changes would be made to the enclosure that may affect the air leakage (like changing ducts, adding cables, etc.), it will also be required to re-do the enclosure integrity test.

Enclosure integrity tests are generally done as per the latest version of NFPA:2001 in the Middle East region. To do a proper test as per the applicable standards there are many requirements that a tester, or witness, should be aware of. It is highly recommended to start with extensive training on the subject before witnessing a test, or before actually performing an enclosure integrity test.

For more information on enclosure integrity testing, testing equipment, training, and calibration, give us a call at +971 (0) 455 17140 or email us at info@buildingdoctor.ae.