The Stack Archive

Gas vs. High Pressure Water Mist in data centre fire suppression – what are the realities?

Wed 7 Dec 2016

fireworks-ltd-sponsoredBarry Elliot reviews the current options for fire suppression in the data centre sector in Europe, and examines the logistics and science that should inform the choice…

Since it is so counter-intuitive to address the possibility of electrical fires with water, it may be difficult to understand why water-based systems are growing in the data centre fire suppression market in the UK and mainland Europe. What do these apparently brave souls know that we don’t?

First, we need to consider the problem in itself – the possibility of component failure causing an electrical fire in a high-production rack, in a manned data centre operation where the consideration of staff safety is closely followed in importance by the business-driven need to bounce back quickly from a fire-induced outage.

The current range of options for data centre fire suppression takes in four approaches: permanently reducing the oxygen content of the room to below 14% by injecting nitrogen (known as the hypoxic method); injecting an inert gas into the room when a fire starts, in order to reduce the oxygen level to below 14%; injecting a halogenated gas into the room when a fire starts, which both reduces the oxygen content and interferes with the combustion process; and spraying a fine high pressure water mist onto the burning area, which both cools and reduces the oxygen level locally.

The zero-tolerance gas scenario

Gas release, undoubtedly effective though it is, has several consequences. Firstly, it is a highly committed solution whose reset costs are considerable both in time and money, since all the gas is released upon every incident, usually with 100% coverage of a room or plant, in service of a localised fire.

All of this presumes as well that the build was either made with gas-based suppression in mind, or has since been retrofitted, at some expense, to provide the semi-pressurised environment that makes gas-based suppression an effective solution.

By contrast, high pressure water mist-based fire suppression utilises a network of secure stainless steel piping and nozzles which only release at the point of the incident, leaving the remainder of the data centre operations and equipment unaffected, requiring no pressure seals or extended evacuation of personnel, and involving minimal reset times with no associated high cost. Compared to conventional gas suppression, when using high pressure water mist, the room itself does not have to be pressure-proofed to the same level, e.g. by the addition of overpressure plates penetrating the external walls.

As a data centre fire suppressant, gas comes with a lot of political and regulatory history, as well as an uncertain future around the world – and especially in Europe.

Gas-based systems also require that the ventilation system to the room is shut down to keep the gas in and to prevent oxygen being fed to the fire. Ventilation control is not necessary to the same extent for a water mist fire suppression system and indeed the ventilation can remain running which will also assist in smoke removal.

How High Pressure Water Mist reduces the risk of false activation

One of the concerns of data centre operators is the prospect of water-laden piping systems in their computer rooms leaking and dripping onto equipment. This is impossible: there is no water contained in the pipes under normal circumstances. Only when the room enters a fire alarm condition will water be released to the overhead piping system. This is known as a ‘pre-action’ system which also helps to reduce false alarms and unnecessary discharge of the suppressant. Furthermore only nozzles that are effected by the heat from a fire will discharge water.

Chemical insecurity

As a data centre fire suppressant, gas comes with a lot of political and regulatory history, as well as an uncertain future around the world – and especially in Europe.

One family of gases known as chlorofluorocarbons (CFCs) – and their brominated cousins known as halons – were very popular as refrigerants and fire suppression gases until 1984 when scientists started to question their effect on the ozone layer – ultimately concluding that the halogens contained within CFCs and halons were depleting the atmospheric layer essential in protecting us from excessive harmful ultra violet radiation from the sun.

The 1989 Montreal protocol banned this family of gases, and, as of January 1st 1994, the United States has banned the production and import of halons under the Clean Air Act. The EU ban on the use of halons in fire extinguishers came into force in October 2000, and was implemented in the UK in 2003 (although some ‘critical’ applications are still allowed).

So what systems are ‘approved’ for fire suppression?

The market splits into two here: one type is known as inert gas and it works solely by reducing the oxygen content of the room down to about 14% by injecting large amounts of a mix of nitrogen and argon. This is seen as a very ‘green’ solution, as the nitrogen and argon were extracted from the atmosphere in the first place.

The downside is that you need a lot of it. It is bulky and quite expensive and, like all gas suppression techniques, your room needs to be effectively sealed and be able to survive the large pressure event that a gas release will initiate.

The other method works by a combination of chemical and oxygen-reduction means. Gases here are known as hydrofluorocarbons, HFC, and another group called Fluorinated Ketones.

The most widely recognised list of approved fire suppression gases is the NFPA 2001:2015 Standard on Clean Agent Fire Extinguishing Systems. Here we will see a list of inert gases (IG series), HFCs and FKs (Fluoroketones).

There isn’t exactly an equivalent European list, but we do have the ‘F-Gas’ Directive, that only allows gases with a zero ozone depletion potential. The F-Gas Directive lists HFCs and PFCs (Perfluorocarbons) that are allowed to be used in the EU. This includes familiar brand names such as HFC227ea (FM200). But there is no list that specifically authorises inert gases or fluorinated ketones.

Unlike sprinkler and gas systems, high pressure water mist systems requires full scale fire testing to demonstrate the efficacy. Currently there is only one internationally recognised standard which is published by FM Global 5560 (2016) Appendix M and N. An advantage of this test protocol is it takes into account forced ventilation that reflects the real life conditions within a data centre.

Gas and global warming

Having hopefully won the battle regarding ozone depletion potential, the Global Warming Potential (GWP) of a substance is now coming under increased worldwide scrutiny. GWP is the ability of a substance to trap the sun’s heat within the atmosphere and not allow it to radiate back into space – the ‘greenhouse effect’. GWP is normally compared to the global warming potential of carbon dioxide which is rated as ‘1’.

This is where HFCs now have a problem. If we look at a typical (and very popular) HFC fire suppression gas such as HFC227ea, it has a GWP rating of 3220!

Politicians are now considering the GWP potential of substances and this led to the meeting of world leaders on the 14th October in Kigali or, to give it its full title, The Ozone Secretariat for the Vienna Convention for the Protection of the Ozone Layer and for the Montreal Protocol on Substances that Deplete the Ozone Layer.

The outcome of this is the intention to reduce HFCs dramatically in the environment. The main target is the air conditioning industry – but the fire suppression industry will be caught up in the slipstream.

According to the BBC, richer economies like the European Union, the US and others will start to limit their use of HFCs within a few years and make a cut of at least 10% from 2019. Some developing countries, including nations in Latin America and island states, will freeze their use of HFCs from 2024. Other developing countries, specifically India, Pakistan, Iran, Iraq and the Gulf states will not freeze their use until 2028. China, the world’s largest producer of HFCs, will not actually start to cut its production or use until 2029. India, will start even later, making its first 10% cut in use in 2032.

In Europe however we can expect to see changes over the next few years as HFC227ea will be viewed as a very ‘un-green’ option.

This leaves us with inert gases, Fluoroketones, which have a GWP of 1, just like CO2 – and water mist fire suppression.

High pressure water mist – purity over complexity

High pressure water mist works by forcing clean water out of holes in a specially designed and tested nozzle at high pressure, forming a mist of very fine droplets. The evaporation of water significantly cools the fire combustion zone and displaces the oxygen locally around the fire.

To sum up the advantages of high pressure water mist: it’s an approved and certified solution, a very ‘green’ solution with no issues of ODP or GWP; the room does not have to be sealed in the same way that a gas suppression system requires; there is no overpressure impact on the structure of the room; ventilation does not need to be turned off; there is no safety impact upon people in the data centre; and a localised response to a fire can be given without resorting to total room flooding.

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