Protecting wind turbines against burnout
Published: 01 October, 2005
Semco Maritime’s Pelle Madspjerg told IFJ: “The risk of fire in wind turbines has always been considered very small but now there have been several cases fire protection has become more relevant through increased demands from authorities, insurance companies and owners, “ he comments.
How does fire begin in the nacelle of a typical wind turbine? The causes will typically be a thermal overload in brakes and gearbox, a short circuit or lightning strike. Fuel is provided by the oil and hydraulic fluids present; there is also glass-fibre and other materials which will combust when the temperatures have increased to high enough levels.
“Several different gas based systems exist that all displace oxygen and hereby suppressing the fire. However, a gas-based system has several requirements; the gas concentration must be maintained longer to avoid re-ignition, doors and hatches must not be opened until it is known that the fire has been properly extinguished and the nacelle cooled down below re-ignition temperature,” says Madspjerg.
“Finally, gas provides a potential hazard for occupants of the rooms, hence gas based fire fighting must be limited to small, enclosed spaces such as inside switchboards, transformer rooms and similar areas.”
The wind turbine’s nacelle is a unit which follows the wind direction and is constantly turning, making it unsuitable for locating equipment outside the nacelle. Consequently, the watermist system is built as a compact unit for mounting inside the nacelle.
“The water is stored in a tank, where it is protected from frost and always ready for use. The pump itself is driven by a 24-volt DC motor with battery supply, and the control system always ensures it is fully charged. The system is activated by a detector system which starts the pump. The watermist is then distributed through nozzles in the nacelle; these extinguish the fire, cool the surroundings and prevent re-ignition,” explains Madspjerg.
Watermist systems
A high-pressure watermist system combines the qualities of both gas and water. This means that it extinguishes fires like a gas system by displacing the oxygen from the fire and like a water-based system it also cools the surfaces, avoiding re-ignition.
“Water mist does not cause the same damage as traditional water-based systems,” reports Pelle Madspjerg. “Tests show that generators and other equipment which have been exposed to watermist remain 100% functional even though they were only protected to IP23.”
Watermist is simply water sprayed into a burning room under high pressure. Tiny droplets which stay in the air for a long time either hit the flames directly or are drawn into the fire through the turbulence created by the strong heat generation - and subsequently vaporise.
First and foremost, one litre of water vapour absorbs a large amount of energy through the vaporisation and it produces approximately 1.7m3 of steam. The steam displaces the oxygen in the same way as inert gas does and suppresses the fire. Unlike gas systems, the watermist system continues to produce steam as long as the fire is burning and water is available. Naturally, the water has the same cooling and extinguishing effect as normal water-based systems - i.e. a combination of the merits of gas and water-based extinguishing systems.
Over the years, Sem-Safe has been subjected to many tests demonstrating its fire-extinguishing properties, and the system has been approved for turbine enclosures from Det Norske Veritas - based on requirements from the American standards FMRC from the US insurance company, Factory Mutual.
“Based on these tests we have developed concepts of coverage for various specific areas including coverage of engine rooms in wooden ships, nacelles etc. This means that the dimension designs of the system are based on documented test results from authorised fire laboratories,” concludes Madspjerg.
