(Wed 25th Jun 2008)
Industrial Fire Journal / June 2008:
Much has been written and reported about the effects of fire water run-off from large scale fire incidents. The problem isn't new and there have been several landmark fires over the years to remind us that without careful planning disaster can strike ‚Äì Allied Colloids, Bradford 1992, Sandoz, Basel 1986 and Buncefield, 2005 are seen as the worst case benchmarks for such incidents. Of course, Buncefield didn‚Äôt just force a reappraisal of groundwater contamination, it also highlighted the need to reappraise the risk assessments underpinning the approach to firefighting in storage depots. This article will focus on how foam proportioning itself can become part of the review process so that foam discharge is kept to the minimum whilst maximising firefighting capability and cost effectiveness.
The fire service and process industries alike have always put great store by the use of water as the prime mover in any fixed fire fighting system. If you have pressurised water and you can harness it to drive the means of entraining foam concentrate into both appliance based and fixed systems, it must be good. The problem with inductors and proportioners is that they have never been particularly accurate and lead to a greater than desired pressure loss. With both the spotlight on environmental issues and the cost effectiveness of foam concentrates becoming more focussed month by month, it is beneficial for operators and firefighters alike to find more innovative methods of foam injection.
MSR of Germany developed their FireDos units in answer to the needs of both industry and fire service alike. Each FireDos unit is made up of two main elements: the water-powered motor and the mixer pump. The water supply to the fire hoses passes through the motor and provides its motive power; its rotational speed depends on the volumetric flow rate of the water in litres / gallons per minute. The unit is equally suited to fixed pipe installation as to use with lay-flat hoses.
There is a proportional relationship between volumetric flow rate and the rev. speed of the motor. Changes in the volumetric flow rate have a direct effect on the speed of rotation of the motor drive shaft. The water-powered motor drives the mixing pump. In this way, energy is drawn from the flow of water in the form of pressure and is used as working energy required to drive the mixing pump.
One of the key benefits of the system is its flexibility to deal with different flow rates. Units are available to cover flows from as little as 100 lpm right through o 22,000 lpm. Each unit has a turndown ratio of 10:1 thereby giving a range of options in each size. With the need to mount foam attacks of sufficient volume to ensure control in large tank and process hazards, the capability to provide large amounts of foam solution is the key to success. FireDos provides fireground flexibility because it can be mounted at various points in the supply chain. Typically, a unit can be mounted far enough away from the flammable liquid fire so that the fire doesn't pose an unacceptable threat to firefighters. The run out of hose or fixed pipework is only then limited by the pressure and flow available to mount the foam attack. Equally, it doesn't require specialised discharge devices other than those in common use by both municipal and industrial brigade alike. This overcomes the need to invest in dedicated high volume systems if the risk is within the parameters of such equipment. Large volume firefighting techniques most certainly have their place but it's as well to highlight the middle ground. Smaller inductor / monitor systems may be insuficient but ultra high flows may be equally inappropriate. As always, good risk assessment is the key to identifying the key issues in a hazard specific way.
Whilst oil and its derivatives continue to escalate in price, so does foam concentrate, albeit at a much slower rate. However, both cost and mixing ratios are important considerations in the best value mix. There is general consensus that the less concentrate has to be transported the better it is whilst the quality of foams is key to firefighting capability. For example, 1% concentrate means less product stored on site and subsequently transported to the foreground compared with 3% and 6% foams. There is less margin for mixing error with 1% so it follows that inaccurate mixing of the concentrate can lead to money going, literally, down the drain. Accurate mixing is imperative to minimise both wasteful rich mixtures and to make the best use of modern 1% concentrates. With a directly proportional system such as FireDos, stepless variable adjustment is possible so that injection can range from 0.1% to 1%, 0.3% to 3% and 0.6% to 6% and is not subject to the back pressures that affect inductors. Most 1% foams have a relatively high viscosity but this isn't a problem for a water driven positive displacement piston pump. Wetted parts are either ceramic or PTFE so aggressive foam concentrates don't affect the mechanism.
Foam is increasingly used, either in its traditional role in fighting flammable liquid fires or in class A gel and retardant form to reduce the surface tension of water (so called wetting agents). Again, being able to very accurately proportion these products saves money, especially as they are used in very small concentrations down to 0.1 - 1%.
FireDos units can be appliance mounted in the same way as round the pump proportioners. This offers all the previously described benefits in addition to isolating the potentially corrosive foam solution from pump casings ‚Äì only fresh water is in contact with the appliance pump whilst the FireDos unit and foam are post pump. Again, dosing remains constant despite fluctuations in inlet pressure, outlet pressure or water flow. Moreover, it is possible to build a system which allows both fresh water and foam to be used at the same time. The FireDos units for fire appliances are extremely compact and can be fitted into any convenient space in a fire tender. The mixing pump can be installed on top of the geared setting device, instead of alongside it, this making the FireDos shorter but higher.
Off-shore installations have special requirements. All FireDos units used with salt water are manufactured exclusively from corrosion-resistant materials such as bronze and stainless steel.
Typical applications include Helicopter deck protection on drilling rigs, Freighters carrying flammable liquids, Ships engine rooms,quayside loading and unloading areas, tank farms, and refineries.
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