Improving the reliability of wind turbine fire suppression

It is estimated that 0.3-0.5 fire incidents occur per 1000 wind farms (onshore and offshore) every year, while a study conducted by SP Safety at the Technical Research Institute of Sweden showed that 10-30% of all loss-of-power-generation-capability incidents in wind power plants are due to fire. Daniel Kopte, expert for safety systems, renewables certification at DNV GL, estimates that worldwide about 120 wind turbines suffer fire damage (not necessarily causing total loss) annually.

Potential ignition sources are mainly inside the nacelle where there is fast moving machinery (generators, gearboxes, etc) that creates heat, and combustible oil and solid material. Lightning strikes can also be a cause of fire, for example, as at Ardrossan in North Ayrshire in December 2011, where a 100 m high turbine caught fire during hurricane-force winds, reportedly due to lightning. The wind turbine was completely burnt out and debris scattered over large distances due to the strong wind.

Fires in wind turbines not only lead to a loss of business continuity and but also, most importantly, are a critical safety issue. Possibly harmful debris can be drifted by the wind in the event of a fire (as in the Ardrossan case) and there is also a significant risk to human lives. When turbines are under erection, commissioning maintenance and repair, escape routes for operators are often long and vertical. Three out of six incidents involve a human presence in the nacelle; hence, a fire becomes a safety concern. In 2013, a crew of four engineers died in Ooltgensplaat, Netherlands, in a wind turbine fire. This devastating loss of life calls for improved review of fire safety to minimise the risk to engineers.

It is estimated that there are currently over 340 000 wind turbines in operation around the world, yet there are thousands of machines without any fire system installed. And even with safety measures in place, a fire can initiate and develop, leading to the possible complete destruction of the turbine.

With projections of much taller towers and more powerful turbines, 13-15 MW to be implemented by the middle of the next decade, the more costly they will be to operators in the event of fire damage.

With some turbines already at heights of almost 200 m, they are effectively outside the range of the fire emergency services; and when it comes to turbines at sea, human intervention in the event of fire is virtually impossible.

Introducing fixed fire extinguishing systems as fire protection is becoming more prevalent because of the increased rate of fire incidents and the rising value and sizes of turbines.

Wind turbines require an active fire protection system, which includes but is not limited to detection (of flames, heat, gas, and smoke), alerting personnel and rescue services, and activating systems for fire suppression or extinguishing. Due to the height and location of wind turbines, classic firefighting methods come up against their limits and therefore fire extinguishing systems are deployed that use gases such as carbon dioxide, inert gases or clean agents such as FM-200® and NovecTM1230, which are especially appropriate for dealing with fires in electrical systems because they extinguish the fire quickly.

However, it is important to note that such fire extinguishing systems require maintenance to ensure they are fully operational and ready in event of a fire. ISO 14520-1:2015(E) assumes that these systems accidentally discharge and leak, while 6.2.4.2 says “means shall be provided to indicate that each container is correctly charged” and 9.2.1.3 states that “the storage container contents shall be checked at least every six months as follows. a) liquefied gases: for halocarbon agents, if a container shows a loss of agent in quantity of more than 5 % or a loss of pressure (adjusted for temperature) of more than 10 %, it shall be refilled or replaced.”

Section 10.5.3.2.2. of the NFPA 850 states that the “maintenance and inspection of total flooding gaseous agent systems and interlocked equipment are critical.” All systems at some time maybe called upon to operate in an emergency situation and reassurance must be obtained that the system can operate to its full potential.

The case for constant monitoring

So there is a clear trend towards including fire suppression in newer units, but with such systems, comes the requirement of maintenance. The assumption in the installation, commissioning and maintenance of gaseous extinguishing systems is that they are highly pressurised and thus risk leaking and discharging, which is why annual inspections are required.

Inspection should include an evaluation as whether the extinguishing system continues to provide adequate protection. This is essential under ISO 14520 where gaseous extinguishing systems have to be designed in relation to the discharging agent hold-time (if the room cannot hold the agent because of leaks the agent will disperse and not extinguish the fire) and discharging agent peak pressure (if the pressure is too high for partition walls or suspended ceilings they will be blown apart or damaged, possibly destroying room integrity). However, routine maintenance is liable to be overlooked or given insufficient attention by the owner of the system.

Given that offshore wind parks require huge investments and are difficult to reach in the event of a fire, emergency automatic fire extinguishing systems are a must for insurance cover. But what if the suppression systems that are installed in the turbines do not release on actuation? Gaseous extinguishing/suppression systems are installed to protect against hazards in critical infrastructure. They deliver the infrastructural resilience that wind turbines require. If it is a known fact that there is a long response time to wind turbine fires, then it is unacceptable that the dynamic suppression systems are left unattended 364 days a year.

There is a call for remote fire alarms in the turbines due to the lack of easily accessible fire servicing stations, so why not also for the gaseous systems? Why have an alarm system to alert fire services if the protection system cannot secure the nacelle? A dynamic system needs monitoring. The reality is that gaseous systems are checked for contents annually because they are pressurised and anything that is dynamic offers risk of loss of contents, but this fails to deal with the probability of discharge or leakage for the 364 days per annum in the interim, between certification checks.

Coupled to this is a complete lack of room Integrity testing after the gaseous system has been installed. As buildings age or their internal use is changed leak sites develop. If the gas cannot be “held” in the room on discharge during a fire event the probability of its suppression diminishes in direct proportion to the size of the leak sites. Room integrity tests are imperative for the determination of both the hold- time and the peak pressure needed for successful fire suppression.

The level of leakage is carefully monitored in order to ensure the correct agent concentration is achieved; room integrity must be ‘tight’ enough to ensure sufficient retention time according to NFPA or ISO, yet remain ‘loose’ enough to prevent enclosure damage at discharge. The presence of undesired and unregulated leak sites reduces room integrity and will hence dramatically impact the hold time and peak pressure, placing room contents and potentially wall structures at risk.

It is accepted that in wind turbines, vibration can loosen connections while dirt, dust, and temperature extremes are known to cause unwarranted discharge. Additionally, openings in the turbine housing can significantly inhibit achieving the designated agent concentration. Devising a solution to overcome these challenges can add significantly to the weight in the turbine.

Ultrasonic solutions

To address some of these issues, Coltraco Ultrasonics has developed Permalevel® Multiplex, a fixed fire suppression monitoring system, designed to provide continuous contents verification. Permalevel® is designed to ensure that fire suppression systems are always fully operational and that no accidental discharge has occurred, which could affect the effectiveness of the overall fire protection system in the event of a fire. With guaranteed systems operations, adaptability for purpose, 24/7 remote access to the systems status, an interruptible power supply and remote real-time monitoring, the Permalevel® offers the capabilities needed for effective wind turbine fire protection.

In addition Coltraco also offers a handheld ultrasonic liquid level indicator, the Portalevel® MAX. This can service a cylinder in 30 seconds (in contrast to 15 minutes by traditional manual weighing) with accuracy of up to 1.5 mm off the true liquid level.

These systems are part of what Coltraco Ultrasonics calls its smart Firetest® solutions, which enable wind turbine owners and operators to improve their fire safety management and achieve a Safesite®.