Clearing the mist on fire safety

Fire in healthcare facilities poses various challenging situations. Fires will usually involve furnishings giving off quantities of heat and smoke, which is the primary cause of deaths from fire. Fire detection is a standard requirement but, in healthcare facilities in particular, is of limited value. The patients/residents often have limited ability to respond to any alarm, as they are either incapacitated physically, through injury or infirmity, or incapacitated mentally, through age or dementia.
The problem is compounded due to the limited capacity of staff to be able to evacuate all those unable to do so themselves. Staffing levels are rarely high enough to carry out such arduous labour. For full evacuation, staff would have to rely on the arrival of the Fire & Rescue Service, however, the time for a fire crew to arrive is variable – as current and projected cutbacks in Fire & Rescue Service resources bite.
One of the most effective ways to safeguard these vulnerable members of society is to install an automatic fixed fire protection system which would react to incipient fire and suppress it at source. One such system is a watermist fire protection system.

Watermist, as the name implies, utilises water broken up into very small droplets. Although the concept was discovered over 100 years ago it has only been in the past 20 years that the knowledge and technology has existed to exploit its fire-fighting benefits.
Although a common commodity, water has unique properties with regard to fire fighting. Firstly it absorbs a relatively large amount of heat to raise its temperature. Secondly, once it reaches 100°c it converts to steam, and in doing so absorbs a very large amount of heat. Thirdly, in converting to steam it expands 1620 times its volume. These characteristics can only be exploited if and when the water is delivered in the form of small droplets with sufficient momentum to penetrate the fire plume.

For example, an automatic sprinkler produces droplets around 1mm diameter with a litre of water having surface area of 2m2. In contrast a watermist nozzle delivering droplets of around 100 microns produces a surface area of 200m2 from one litre of water. This greatly increased surface area of the water droplets means a similarly increased ability to absorb heat and thereby cool the surroundings, significantly improving survivability of those threatened by fire. This increased water droplet surface area also enables the droplets to capture soot particles in the killer smoke, thereby improving visibility.
The small water droplets have relatively small mass and thus remain airborne longer to enable them to continue to absorb heat and, at the same time, are able to flow with the thermal air currents, so water is drawn into the seat of the fire even when it may be shielded from the direct discharge from the watermist nozzles.
As a result, watermist extracts heat, cools combustion gases and blocks the radiant heat transfer, thereby preventing the spread of fire. By exploiting the unique properties of water through delivery as small droplets, fire suppression (and fire extinguishment where flammable liquids are involved) can be achieved with significantly reduced quantities of water compared with other water-based fire suppression systems. The benefit of this comes in reduced water damage and thus reduced time and effort to restore premises for re-occupation when there has been a fire. It also follows that the watermist system pipework is smaller and less obtrusive and the size and space required for water supplies is similarly reduced.

Features and benefits
For healthcare facilities where ordinary combustibles are present, the watermist systems utilise automatic nozzles fitted with quick response frangible elements. Each nozzle will respond independently to the heat from a fire in its incipient stages, thus watermist is only discharged onto the seat of the fire. The flow of water activates the water supplies and signals a ‘fire’ alarm.
The nozzles are fitted into small bore pipework which is hydraulically designed to ensure the reduced water flows are delivered at the required flow and pressure to any fire area. The pipework is connected, via a control valve, to dedicated water supplies usually comprising one or more small pumps and a water storage tank.
The benefits of a watermist fire suppression system are: it provides effective fire suppression; discharges small quantities of water; makes the conditions in the vicinity of the fire survivable for longer through cooling of the fire and the surroundings; soot particles from the smoke are entrained thereby improving visibility; and its safe for people and the environment, with minimal contamination and no harmful residues.

One of the key issues for protecting existing premises is the practicality of installing watermist, or any other fire protection system, including the feasibility of doing so without disruption or displacement of residents – this is particularly critical for facilities where the elderly and/or infirm may reside. Today most of the world’s cruise liners are protected by automatic watermist systems in their accommodation, restaurants and shopping areas. Mist is chosen because of its ease of installation and was originally adopted because of its ease of retrofit. During the 2011-12 retrofit of watermist systems in the Katriina Hospital in Vantaa Finland, eight patient wards, plus daytime operating theatres, geriatric outpatient facility and a neighbourhood health centre were protected without affecting the 24/7 operations of the hospital.
Mist systems are recognised by major insurance companies, for example Factory Mutual has dedicated a specific section of its standards to the engineering of watermist systems as well as setting out a representative fire test protocols to establish the design basis for mist system designs. Other internationally recognised bodies such as the Loss Prevention Council (LPC) UK and VdS (Germany) have established test and approval protocols.

In 2008 the 15000m2 Pikonlinna Hospital in Kangasala, Finland, was refurbished and fitted with automatic watermist fire protection covering four wings each six stories high providing senior care and a nursing home.

Whilst the marine industry was first with IMO design and test protocols, NFPA then created NFPA750 and Europe has a technical specification TS14972. BSI UK has published the DD8489 series – guidance documents for commercial and industrial fixed watermist firefighting systems. These are now in their final stages of being updated and converted to full British Standards. Once published, these will increase the recognition and acceptance of fixed watermist fire protection systems for protection of life and property.
All these standards and approval bodies have a common basis for the design of watermist systems – the need for representative fire tests carried out by independent laboratories. Each watermist manufacturer thus needs to have both its nozzles and the proposed design layout to be performance verified by fire tests before these can be translated into a validated design manual for that specific application.
Any prospective buyer and specifier is entitled to see these test reports to substantiate the viability of any watermist protection proposal which covers the healthcare facilities to be protected. A typical case in point are the Isala Clinics in Zwolle, Holland, comprising 104,000m2 of offices, research, treatment, nursing wards and laboratories over four buildings fully fire protected with automatic watermist systems fulfilling Dutch authority regulations.

Valuable additions
Finding companies with the necessary experience and expertise in engineering watermist fire fighting systems is easy as they will be members of major fire industry trade associations, such as the Fire Industry Association (FIA) and the British Automatic Fire Sprinkler Association (BAFSA). These associations require their membership to hold BS ISO 9000 quality assurance schemes in place – with independent third party auditing of their design and installation procedures.
Watermist fire protection systems, successfully tested for protection of the various areas of healthcare facilities, would operate automatically in the event of fire with one or more nozzles in the immediate vicinity of a fire operating by means of a heat sensitive quick response bulb in each nozzle. Whilst these bulbs are similar to those in automatic sprinklers, the water quantities delivered and the fire suppression mechanisms are different from those of sprinklers.

Watermist systems are not sprinkler systems, but they do provide a valuable addition to fire and safety professionals service the healthcare sector.
Fire risk assessments for healthcare facilities are likely to identify that staffing levels, especially at night, may be too low to carry out evacuations of all those at risk in the event of fire.

One of the most effective means of ensuring the safety of both staff and patients, as well as attending fire service personnel, is to fit, or retrofit, a fixed watermist fire fighting system.

Further Information