Use of foam for firefighting in tank farms of the oil and petrochemical trade – Part 2

Part One of this text described the everyday incident scenarios at tank farm fires and offered foam as essentially the most appropriate extinguishing agent together with the firefighting equipment mostly used. In Part Two we have a look at foam focus proportioning technologies, cellular extinguishing methods and conclude with lessons realized.
NFPA 11 describes varied kinds of foam concentrate proportioning equipment. In the following, three systems are looked at that are commonest. Tight limits for the proportioning of foam concentrate apply to all of them.
The proportioning fee must not be lower than the permitted values – i.e. 3% for a 3% foam focus or 6% for a 6% foam focus.
The proportioning fee must not exceed 30% above the permitted worth i.e. 3.9% for a 3% foam focus or 7.8% for a 6% foam focus; respectively, the proportioning price is allowed to be an absolute most of 1% above the permitted worth – i.e. 4% for a 3% foam concentrate or 7% for a 6% foam concentrate (the smaller worth have to be used respectively).
To assure appropriate proportioning, the proportioner, together with the proportioning price must be tested a minimal of once a year and its correct functioning must be checked.
Fig. 1 Bladder tank with proportioner.
Bladder tank with proportioner
The bladder tank with a proportioner is a confirmed and cost-effective technology. The bladder tank is a pressurized vessel with a bladder inside which is filled with foam concentrate. The tank is pressurized with water from the fire-extinguishing line and discharges the froth concentrate from the bladder as required. The bladder is linked to a proportioner which operates using the venturi precept. When the fireplace pumps are activated, strain is generated by the pump, causing delivery of froth focus to the proportioner. The extinguishing water flows through the venturi proportioner. The resulting vacuum induces the froth focus into the extinguishing water move.
The benefits of this method are its simple design with out shifting components and its simple operation. No external power is required, and the system is comparatively cheap.
A drawback is that the system is a pressurized vessel subject to corresponding laws corresponding to ASME Boiler & Pressure Vessel Codes. In order to refill foam focus, the system have to be shut down and drained. The rubber bladder is delicate; when broken, water will contaminate the foam focus. At a given proportioning price, the system is suitable only for low variations within the extinguishing water move strain and quantity. Adding or changing individual foam discharge gadgets is possible only to a very restricted extent. The system is also unsuitable for proportioning extremely viscous foam concentrates.
To conduct any mandatory required annual testing, the system have to be activated and premix generated at the venturi proportioner throughout the extinguishing water line. The right proportioning rate must be measured within the premix by laboratory evaluation. The generated premix should then be disposed of, and the consumed foam focus within the bladder tank must be changed.
Fig. 2: Driven proportioning pump with circulate meter.
Driven proportioning pump with circulate meter
The system consists of an atmospheric tank for the foam focus, an electrical or diesel-powered foam focus pump with an electronically controlled valve and a move meter in the extinguishing water circulate line. When the hearth pumps are activated, the froth focus pump drive and digital management system have to be activated. The extinguishing water circulate fee is measured by the circulate meter and the management system adjusts the proper foam concentrate amount by way of the control valve. The foam concentrate is injected into the extinguishing water move by the froth concentrate pump. If there is a change in the move fee, the quantity of injected foam focus is regulated by the management valve.
pressure gauge วัด แรง ดัน น้ำ ’s advantage lies in the exact proportioning of the froth concentrate, impartial of the extinguishing water strain or circulate rate. Foam focus may be topped up during the extinguishing operation. The system is capable of proportioning highly viscous foam concentrates. For the purpose of annual testing, the system have to be activated; nonetheless, the delivered foam focus may be measured through a return line. The proportioning fee is calculated from the extinguishing water/foam concentrate circulate price. No premix is produced; and as the foam concentrate is handed again into the tank, no foam focus needs to be refilled.
Disadvantages are the requirement for an external interruption-free energy supply for the foam concentrate pump and the management system, as nicely as the necessity for a sophisticated management system and the comparatively higher purchasing costs. Furthermore, it must be accepted that a delay occurs between the change of the extinguishing water circulate fee and the newly adjusted foam concentrate amount. The foam quality could additionally be compromised when continuously changing operating situations as foam discharge devices are turned on or off or changed.
Water motor with proportioning pump
The system consists of an atmospheric tank for the froth concentrate, a water motor installed within the extinguishing water flow line and a foam concentrate pump which is connected directly to the water motor. Water motor and pump type one compact unit. Upon activation of the hearth pumps, rotation in the water motor begins. The direct coupling to the foam-concentrate pump provides quick foam-concentrate injection into the extinguishing water. If the flow rate adjustments, the quantity of foam focus is adapted instantly.
The advantage of the system is its independence from external power sources in addition to a precise and quick foam-concentrate proportioning regardless of the extinguishing water strain or flow price. If a piston or plunger pump is used, adjustment or calibration after installation isn’t essential since the water motor and the pump are volumetric devices firmly linked to one another. Foam concentrate refilling throughout operation is possible. The system can be capable of proportioning highly viscous foam concentrates. The system must be activated for annual testing; nevertheless, the delivered foam focus could be measured through a return line. The proportioning rate is calculated from the extinguishing water/foam focus flow rate. No premix is generated; and if the foam concentrate is handed again into the tank, no foam focus must be topped up.
The larger design and the comparatively greater purchasing costs are a disadvantage of the system.
With any system, consideration ought to be taken into account for the annual testing prices, which could be appreciable by method of alternative foam concentrate, disposal of premix and labour prices.
Mobile extinguishing methods
As the stationary foam discharge gear may be broken in in depth fires in the tank or within the dyke area and thus lose effectiveness, cellular fireplace displays and foam pipes could additionally be used.
Foam pipes
Foam pipes are normally held by firefighters, making them very flexible. Yet they’ve solely limited extinguishing agent circulate rates and reaches.
Firefighting screens
Firefighting displays are discharge units mounted on vehicles or trailers and available in plenty of sizes. The extinguishing agent move price could be as a lot as 60,000 litres/min and the attain can be up to 180m if the stress of the fire pumps is sufficient. They are suitable to discharge foam, e.g., to extinguish a floor fireplace in a tank, or water, to cool down a neighboring tank or the tank wall of a burning tank so as to stop it reaching the important temperature for a boilover, or to maintain the flames from spreading. The accumulation of water inside the dyke space should at all times be noticed to avoid an overflow of the dyke.
Mobile hearth displays may be supplied either by the extinguishing water of the stationary hearth pumps or by mobile pumps. The injection of the froth focus often takes place through mobile proportioners. This clearly points in course of the benefit of power independence for water-motor-driven proportioning pumps.
The strategic choice for the sizes of cellular units available as back-ups is shown by the next instance for the location of monitors for fire extinguishing at tanks which are 45m in diameter and 15m excessive.
According to NFPA, 32,000 litres of premix per minute are required. This leads to a number of options for the displays. Generally, foam-concentrate proportioners for no much less than 32,000 litres/min are required, which should be ready to deal with varying move rates to ensure flexibility during extinguishing operations. Depending on the native situations, the monitors will want to maintain a minimum distance to the burning tank or may not be capable of be positioned near to the tanks due to debris. In addition, it won’t always be possible to place several displays across the tank. It have to be ensured the monitor has enough throwing peak in relation to the tank peak, to deliver foam into the within of the tank.
The dimensioning of the complete firefighting system is made following authorized regulations as well as recommendations by associations like NFPA and isn’t looked at more closely in the present article.
Fig. four: Trailer with monitor M4, water motor with proportioning pump FD8000 and foam concentrate tank.
Lessons realized
As mentioned within the introduction to Part One of this article, it seems that evidently many authorities and companies have not discovered the mandatory classes from disastrous fire incidents of past years. Tank farm fires within the oil and petrochemical trade don’t happen frequently. When they do, they normally have devastating consequences. Let us remember the tank farm hearth at Deer Park, Texas in March 2019 mentioned within the introduction.
The fire developed after over 30,000 litres butane-enriched naphtha had been leaking from a faulty valve for 30 minutes and caught fire for yet unknown reasons. The plant had no gas warning system and no remote-controlled valves to close off the leaking fluid. In addition, some areas did not have mounted extinguishing methods put in. All 15 tanks were surrounded by one single dyke. The proprietor had relied on the native fireplace service, which was on the spot in a short time however couldn’t take management over the fire with the tools obtainable, partially because flammable substance was continuously leaking from the tank. Thirty-six hours after the hearth had damaged out, a contract was made with an exterior firefighting company to do the extinguishing work. Mobilization of apparatus and foam focus in addition to the preparation of a plan of action took roughly thirteen hours. The hearth was finally extinguished after 14 hours of firefighting, 63 hours after it initially broke out. Extinguishing attempts were remodeled three days, with foam focus and water shortages occurring within the meantime. In complete, over 500,000 litres of froth focus have been used. Instead of a fire within the dyke area, 11 of the 15 tanks burnt down.
It may be very possible that the fire would have been extinguished quickly if the warning methods and valves had worked and a set hearth extinguishing system had existed. It can be probable that the hearth would have been extinguished rapidly if the extinguishing operation, which succeeded finally, had been started earlier. Both cases would have resulted in notably much less harm.
Summing up, the next factors must be realized as a minimum. As far as they haven’t yet been applied, or simply partly, they want to serve as a foundation for discussions about one’s personal fire-protection idea:
Have an acceptable fire-protection concept including different scenarios which adapt to the given scenario regularly.
Always have a sufficient variety of cellular extinguishing systems as a backup to fastened extinguishing techniques.
Stock an acceptable foam focus.
Ensure enough foam focus supply.
Ensure sufficient water supply.
Keep well-maintained, shortly and well-accessible, strategically positioned and functioning extinguishing tools out there in a adequate number.
Have educated personnel obtainable in a enough number.
Ensure a quick implementation of an appropriate plan of motion.
For more information, go to www.firedos.com
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