Watermist
Published: 01 October, 2005
Virtually every industrial or commercial building contains rooms classified as machinery spaces. These may contain machinery as diverse as generators, pumps, hydraulic power packs, incinerators or transformers, writes Jukka Nieminen.
The fire risk in such spaces normally involves the accidental release of flammable liquids onto nearby hot surfaces or live electric circuits. Typically, this could be a pressurised fuel or lubrication oilpipe or hose, which ruptures and spills hot oil onto a hot exhaust manifold. Because an oil fire can be so intense, a fixed fire protection system is usually necessary to reduce danger to people, equipment and buildings.
What should the system do?
The extent of the risk depends on a number of factors such as the amount of flammable liquids present, the potentially hot surfaces, and the size of the space. Additional factors include whether the space is manned or unmanned, and whether the space is near other key rooms or spaces.
The fixed fire protection system in a machinery space must therefore be able to quickly extinguish a fire and cool surrounding areas so that re-ignition cannot occur, with minimal damage to equipment in the space. The system should also not be harmful or a danger to people or the environment, whether set off in a fire or through an accidental discharge.
A diesel-engine-driven generator is a typical example of machinery to be protected. The biggest hazard is caused by the failure of fuel system or lubrication system. In case of transformers, the transformer oil also poses a hazard. In switchgear, the hazard is posed by an overload, which may set insulation material on fire.
High-pressure watermist systems
Well-engineered, high-pressure watermist systems are increasingly seen as a better fire protection solution for many machinery spaces as they provide fast extinguishing, cooling to prevent re-ignition and radiant heat absorption to prevent fire spreading and further damage.
A gas-tight enclosure is not required - indeed, fire tests are carried out with door openings. Watermist systems are effective and safe for people, the environment and the protected equipment.
Marioff has developed a range of HI-FOG watermist fire protection solutions for a wide variety of applications. For protection against machinery space fires a range of HI-FOG solutions can be offered depending on the size of space, protection time requirement and availability of electric power.
The HI-FOG systems usually have a ‘total flooding’ design. They are effective against spray and pool fires as defined in the relevant fire test performance criteria such as the new Factory Mutual Class 5560 Water Mist Standard.
HI-FOG systems can be released immediately after the detection of the fire, as plain potable water and nitrogen or compressed air is used. Thus, there is no need to evacuate personnel. No potentially toxic by-products are to be expected. HI-FOG features effective yet gentle cooling of the environment.
To put the fire out quickly is not enough, however. If the contact between the burning material and the ignition source exists, the re-ignition risk is significant. Hence, a prolonged continuous protection with cooling of the space has to be provided. A short fire protection agent discharge, or non-continuous discharge, is of little value. Agent that leaks out of the space may fail to give adequate protection.
System advantages
It should be noted that applying fine watermist on live electric equipment has been studied and found safe; testing has shown that arcs or leakage currents are not developed due to the presence of watermist.
Clean-up and reinstatement time will be minimal compared with traditional fire protection systems. The system can be put back to operation by just replenishing water and nitrogen or compressed air cylinders. The HI-FOG systems can be tested at any time - without any significant cost.
For small enclosed spaces Marioff will supply pre-engineered self-contained pressure cylinder systems which are simple and need no electric power. For larger spaces nitrogen or compressed air powered pump systems can be supplied which also need no electric power. For the largest spaces, or where a HI-FOG sprinkler system is installed, an electrically-driven pump system is installed.
Typical components
HI-FOG systems for protecting machinery spaces consist of a power unit, nozzles, interconnecting tubing and necessary valve & control units. Systems are tailored to the customer’s specific requirements, in particular with regard to any interface with detection systems.
The simplest system, suitable for small spaces and basic local protection, is the MAU (Machinery Accumulator Unit), which uses stored nitrogen cylinders and non-pressurised water cylinders to protect volumes up to 260 m3 (9,175 ft3). Nitrogen, when released, drives the water from the water cylinders to the HI-FOG spray heads. The typical protection time is 10 or 20 minutes.
The GPU (Gas-driven Pump Unit) is approved for protecting spaces up to 500 m3 (17,645 ft3). It uses nitrogen or compressed air cylinders to drive a special pump, feeding water to the distribution network. Water for 30 min protection is normally stored in a 1.5 m3 dedicated tank at the unit. A GPU can be used to protect several hazards when using zone valves. Appropriately listed GPU local application systems are also available.
Both MAU and GPU are pre-engineered, self-contained units, solving problems with power and agent supply. As the systems are used in conditions ranging from offshore platform in the arctic to sandstorm prone hot desert, they are often built in protective or heated skids.
Where is it being used?
Very large machinery spaces may require engineered HI-FOG solutions. Then, SPU (Sprinkler Pump Unit), which incorporates a number of electrically driven pumps, is used. Should electric power be unavailable the SPUD direct diesel driven pump units are specified instead.
HI-FOG systems bear type approvals from FM Approvals, and VdS Loss Prevention, and several Marine classification societies. Marioff has supplied HI-FOG systems to protect machinery spaces around the world.
Examples include large centralised systems protecting transformer and machinery rooms in a number of hydroelectric power plants in Norway and Sweden.
A variety of HI-FOG systems are protecting all the machinery spaces on one of the world’s largest offshore oil production vessels operating in the Bonga field offshore West Africa. Self-contained systems are protecting modular generator sets worldwide for a major original equipment manufacturer. And Local Application systems are providing vital protection for high-risk areas in UK gas compressor stations and Finnish paper mills.
Watermist is the first choice fire protection technology for protecting all kinds of machinery spaces. The technology combines optimum suppression performance and cooling which prevents re-ignition and minimises damage to equipment.
Watermist systems do not require gas-tight enclosures and are inherently safe for personnel and the environment. HI-FOG is the leading watermist technology with more approved systems and a larger reference list than any other water mist manufacturer today.
