New watermist system unveiled
Published: 01 July, 2005
The new PDX Firemist technology is most certainly in the ‘exciting innovation’ category and one could argue that such advances in the field of fire suppression, brought by Pursuit Dynamics plc, will finally establish water-mist as the obvious and natural replacement for the now ‘outlawed’ Halon systems.
The new PDX Firemist technology is most certainly in the ‘exciting innovation’ category and one could argue that such advances in the field of fire suppression, brought by Pursuit Dynamics plc, will finally establish water-mist as the obvious and natural replacement for the now ‘outlawed’ Halon systems.
As a fire protection option, watermist systems have been around for some time. However, it is only recently that the technical team at Pursuit Dynamics plc was able to realise the full potential of a process that creates water droplets that are so tiny, that they behave and conform just like a gas.
The fundamental mechanisms of watermist, in relation to fire suppression, are only just becoming clear to scientists who know full well that the concept works! However, it has taken extensive testing in zero-gravity conditions to fully understand the physical processes involved.
Because of this it has been difficult to establish accepted standards for watermist systems and to date, before they are accepted for specific risk protection, they must undertake pain staking and expensive evaluation tests in large fire halls to assess each system’s suitability and effectiveness.
The NFPA 750, IMO and European version in draft present typical standards, although the soon to be released FM 5560 will provide the most comprehensive single source of watermist test requirements in the world for land-based applications. This standard aims to provide a ‘one-stop-shop’ covering system and components together.
PDX suppression performance
What makes Pursuit’s PDX system stand out from conventional watermist systems? Quite simply the answer lies in its ability to create smaller droplets in gas-cloud formation and project this fine mist, or cloud, to distances never achieved before by conventional watermist systems that function in low or high-pressure mode.
The ability to discharge vast quantities of this finely divided PDX Firemist into large volume spaces and establish a rapid gas-like fill rate (40m3/s) using a rotating single nozzle ensures rapid fire knock-down by: cooling; radiant heat attenuation; and oxygen dilution.
This means that the fire is rapidly suppressed due to a reduction in the oxygen content of the compartment, down to a level that will not support combustion. The compartment is instantly cooled and the massive reduction from the fire’s radiated heat flux caused by the PDX Firemist ensures that surrounding contents, surfaces and materials within the compartment are not damaged further.
The PDX Firemist system has suppressed fires in large volume compartments faster than any other system has ever achieved in total flooding situations.
However, the fire suppressive qualities of the PDX Firemist system are even further advanced when used in a direct streaming mode. The nozzle itself consists of a small cylindrical unit which inlets steam, as the driving source through an annular channel.
This channel serves to create a supersonic high shear flow of entrained water, where steam condensate and high shearing forces lead to a finely-dispersed mist of micron-sized water droplets. Despite the use of steam as a driver, temperature at the nozzle outlet is measured at 52ºC falling to ambient within a couple of metres.
The supersonic flow measured within the nozzle itself demonstrates velocities in excess of 1,500 m/s (3,000mph), which give rise to extremely low pressure effects within the annular chamber. These effects may be used for further entrainment of additives (or additional water) through several ports inside the unit, which again atomises to a very fine mist cloud due to the rapid Mach 3 velocity changes.
In effect, the ability of PDX Firemist to discharge so rapidly and at such high velocities, sees very large class ‘B’ pool or pressurised spray fires snuffed out within micro-seconds!
What risks can be protected?
Obviously large volume spaces such as warehouses, aircraft hangars, road and rail transport tunnels and marine engine spaces are ideal situations where PDX Firemist could offer fire protection capabilities. The ability to flood large volume spaces with a watermist that presents true gas-like qualities has obvious advantages.
The rapid deployment of the mist and the projection capabilities of the PDX nozzles suggest that risks and occupancies that were not able to achieve effective protection from conventional watermist systems in the past may now look again at this new innovative technology.
The ever-increasing trend towards new global power generation options see the combined cycle gas turbine (CCGT) power plant as having a real future over the next 30 - 100 years. Steam Turbine Power Generators are seen as an ideal ‘risk’ where the PDX system could provide a cheap and economical form of fire protection.
Recent fires at the Vindhyachal STPS and TCL, Mithapur plants, as well as at Kelanitissa, Sri Lanka, have demonstrated how CCGT generators are at risk from fire.
When disaster strikes
On March 22nd, 2004, at Kelanitissa, Sri Lanka, a major fire broke out in a 163 MW auto-diesel fired combined cycle gas turbine (CCGT) power plant completely destroying a steam turbine. As a result, the supply of 65 MW of power to the National Grid was suspended. Such consequential losses to a country are difficult to quantify but a simple and economical PDX Firemist installation would have prevented this situation.
The increasing global trend in the provision of further road & rail transport tunnels, coupled with a spate of multi-fatal fires over the past five years, sees a pressing need for some form of reliable fixed fire protection to quench fires, reduce smoke production and enable people to escape safely. There have been several fairly successful trials using conventional watermist systems in such tunnels over the past years but a problem in application has arisen through the natural conflict caused between flow-rate and droplet size.
In tunnel fires a high flow-rate of water is needed but the higher the flow-rate becomes, the less the watermist generally retains its gas-like qualities. However, the PDX Firemist system is able to deliver higher flow-rates whilst still maintaining an effective mist cloud.
The PDX flow-rates are variable but for example, flow-rates of 0.48 l/s deployed with high momentum transfer velocity achieve Dv50 and Dv90 of a droplet range never seen before from a watermist system.
This means that at Dv50 (for example) the PDX Firemist presents 1,446m2 of water surface area (droplets) to the super-heated environment of a fire compartment compared to just 120m2 available from conventional systems. This massive amount of cooling power is unleashed at supersonic velocity from the PDX nozzles with projection distances up to 40m.
In conclusion
When I first saw the PDX Firemist deployed I was astonished to see its ability to perform like a true cloud! It was as if we had reached up into the sky and pulled one of Nature’s wonders down for a closer look. The tiny microscopic water droplets suspend themselves within the steam condensate and I was convinced we were on the verge of controlling the weather!
The concept of PDX Firemist is most certainly innovative and as a fire suppression system is offers so much potential. Its true gas-like qualities and its ability to project a fine cloud of condensate and finely divided water droplets into large volume spaces and with such great speed presents us with a most viable and environmentally alternative to Halon gases and chemically based replacements.
Clearly, this fire suppression system offers many benefits for applications such as tunnels, hangars, warehouses, processing plants, petrochemical and oil platforms, installations and power plants. The list is endless.
