Howard Tomlin gives us an overview of the role of watermist installations in the protection of computer data centres.
Data centres are being built all over the globe to cope with the expanding demand for IT based communications and E-commerce. The most common types of data centres are wholly owned data hubs for large end user businesses and banks, and purpose built units which are rented out to companies who do not need a dedicated facility of their own.
Until recently, data centres have been protected by gaseous suppression systems or sprinkler systems. With the arrival of watermist firefighting systems, however, insurers and end users are seeing that there are benefits in using this technology for these applications.
Watermist works by reducing the heat energy of the fire by discharging water droplets of 10-100 microns into the flame plume. These small droplets of water evaporate very quickly (using up the heat of the fire) and turn to gas, which expands in all directions pushing oxygen away from the fire – this fights two sides of the fire triangle. Once the main fire is suppressed, the watermist system wets the surface to prevent the fire re-growing. All this happens with the minimum amount of water required, as established by third party witnessed real fire tests.
Watermist does not require the room to be completely sealed as it does not require a 10 minute retention time in order to be effective. Gaseous suppression systems, on the other hand, rely on this and so need a room to be sealed, integrity tested and re-tested every time a new penetration is made into the room for cables etc. In fact with watermist, it's actually beneficial for the room to be a little leaky!
Quick response
For data centres, the nozzles used have individual quick response heat sensitive thermal release elements, so only those nozzles in the immediate vicinity of a fire will open to discharge watermist – thus avoiding widespread wetting of equipment not affected by the fire.
The central equipment for Tyco's Microdrop watermist system is very small, typically using a 1m x 2m x 1.5 m high pump skid (a prefabricated frame containing the pump, motor and control panel) connected to a 4m3-6m3 water storage tank. When compared with both sprinkler and gaseous systems, this results in a significant footprint reduction for the central equipment, which means that more floor space can be used for installing computer equipment. This has value added benefits to the owners of rented out data centres as it means more income, and for the end user owned data centres it allows more space for expansion without incurring additional costs.
To ensure they are only ever activated in a real fire scenario, the systems within the data centres are usually controlled by an air primed ‘pre-action' device linked to a ‘double knock' coincidence smoke detection. Microdrop systems are fitted with thermally activated nozzles containing frangible heat sensitive bulbs, the air pilot monitors the pipe for leakage but does not control the activation of the whole system. This ensures that water only ever enters the pipes after smoke is detected in two zones of detection at the same time. Water cannot be discharged through any nozzle in the room until the frangible element closest to the hottest part of the fire is activated by heat. This ensures that only the very minimum of nozzles are activated to control and suppress the fire.
Data centre Microdrop water mist systems are designed using NFPA 750, the FIA Code of Practice for Industrial and Commercial Watermist Systems, and the Tyco Microdrop Design Manual. The assumed maximum area of operation (AMAO) for this type of fire hazard is 72m2. This requires the system to be hydraulically designed to be able to supply enough water for the most favourable 72m2 area as well as enough pressure for the least favourable 72m2 of the data centre. This is undertaken using computer aided design with pressure drop calculations from Firecad or similar.
Within data centres there are a number of fire risks:
- Data rooms contain rows of computer equipment racks, corridors often contain the air conditioning equipment (amongst other services), and technical rooms which are normally distributed around the data centre contain equipment such as switchgear. All of these typically present a Class A deep seated fire risk. These are treated as being equivalent to Ordinary Hazard Group 1( BS EN 12845) fire risks. The system objective is to suppress and control the fire with the minimum discharge time of the system being 30 minutes (NFPA 750). AMAO = 72m2
- Generator sets provide power to the site in the event of an outage from the main grid. Due to the hydrocarbon fuel requirement of these units a Class B Hydrocarbon fire is presented. These are treated as hydrocarbon fuel risks and so are protected using local application or total flood watermist systems. The system objective is to extinguish the fire and cool the surrounding machinery, and the minimum discharge time of the system is 10 minutes (NFPA 750). The AMAO is the complete area or protection of local application (with thought applied to the possibility of fire spread to other areas/equipment)
Generally, all of these areas can be protected from a pump skid with distribution pipe work serving the different risk areas individually controlled by diverter valves. In areas where the generation equipment is very remote or where the client prefers, standalone, cylinder based water supplies can be used to provide for these relatively short discharge systems such as used on the diesel generator sets.
Howard Tomlin is watermist business development manager for Tyco Fire and Integrated Solutions.
