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

Part One of this article described the standard incident scenarios at tank farm fires and offered foam as probably the most appropriate extinguishing agent together with the firefighting equipment most commonly used. In Part Two we take a glance at foam focus proportioning applied sciences, cellular extinguishing methods and conclude with lessons realized.
NFPA eleven describes varied forms of foam concentrate proportioning tools. In the following, three systems are looked at that are most typical. Tight limits for the proportioning of froth concentrate apply to all of them.
The proportioning fee must not be lower than the permitted values – i.e. 3% for a 3% foam concentrate or 6% for a 6% foam concentrate.
The proportioning fee should not exceed 30% above the permitted value i.e. three.9% for a 3% foam focus or 7.8% for a 6% foam concentrate; respectively, the proportioning rate is allowed to be an absolute maximum of 1% above the permitted worth – i.e. 4% for a 3% foam focus or 7% for a 6% foam focus (the smaller value must be used respectively).
To guarantee correct proportioning, the proportioner, including the proportioning fee should be tested no less than annually and its correct functioning should be checked.
Fig. 1 Bladder tank with proportioner.
Bladder tank with proportioner
The bladder tank with a proportioner is a confirmed and cost-effective know-how. 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 foam focus from the bladder as required. The bladder is connected to a proportioner which operates using the venturi principle. When the fire pumps are activated, pressure is generated by the pump, inflicting supply of froth concentrate to the proportioner. The extinguishing water flows via the venturi proportioner. The resulting vacuum induces the foam concentrate into the extinguishing water move.
The benefits of this system are its easy design with out shifting components and its easy operation. No external energy is required, and the system is relatively cheap.
A disadvantage is that the system is a pressurized vessel topic to corresponding rules such as ASME Boiler & Pressure Vessel Codes. In order to refill foam focus, the system should be shut down and drained. The rubber bladder is sensitive; when broken, water will contaminate the foam focus. At a given proportioning rate, the system is suitable just for low variations in the extinguishing water circulate pressure and volume. Adding or changing particular person foam discharge devices is possible solely to a really restricted extent. The system is also unsuitable for proportioning extremely viscous foam concentrates.
To conduct any obligatory required annual testing, the system should be activated and premix generated at the venturi proportioner inside the extinguishing water line. The appropriate proportioning rate must be measured within the premix by laboratory evaluation. The generated premix should then be disposed of, and the consumed foam focus in the bladder tank needs to be replaced.
Fig. 2: Driven proportioning pump with move meter.
Driven proportioning pump with circulate meter
The system consists of an atmospheric tank for the froth focus, an electrical or diesel-powered foam focus pump with an electronically controlled valve and a move meter within the extinguishing water flow line. When the hearth pumps are activated, the froth concentrate pump drive and electronic management system have to be activated. The extinguishing water circulate fee is measured by the flow meter and the management system adjusts the proper foam concentrate amount via the management valve. The foam concentrate is injected into the extinguishing water move by the foam concentrate pump. If there’s a change in the flow rate, the amount of injected foam concentrate is regulated by the management valve.
The system’s benefit lies within the precise proportioning of the foam focus, independent of the extinguishing water stress or flow rate. Foam focus may be topped up during the extinguishing operation. The system is capable of proportioning highly viscous foam concentrates. For the aim of annual testing, the system must be activated; nevertheless, the delivered foam focus can be measured by way of a return line. The proportioning fee is calculated from the extinguishing water/foam concentrate circulate fee. No premix is produced; and as a result of the foam concentrate is passed again into the tank, no foam focus needs to be refilled.
Disadvantages are the requirement for an exterior interruption-free energy supply for the froth focus pump and the management system, in addition to the need for a sophisticated control system and the comparatively larger buying costs. Furthermore, it should be accepted that a delay occurs between the change of the extinguishing water circulate rate and the newly adjusted foam concentrate amount. The foam high quality may be compromised when continually altering operating circumstances 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 focus, a water motor installed in the extinguishing water flow line and a foam concentrate pump which is related on to the water motor. Water motor and pump type one compact unit. Upon activation of the fire pumps, rotation in the water motor starts. The direct coupling to the foam-concentrate pump gives immediate foam-concentrate injection into the extinguishing water. If the flow price adjustments, the amount of froth focus is tailored instantly.
The advantage of the system is its independence from exterior energy sources in addition to a precise and quick foam-concentrate proportioning regardless of the extinguishing water stress or move fee. 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 gadgets firmly related to each other. Foam focus refilling throughout operation is feasible. The system can additionally be capable of proportioning extremely viscous foam concentrates. The system should be activated for annual testing; nevertheless, the delivered foam concentrate can be measured through a return line. The proportioning fee is calculated from the extinguishing water/foam focus flow fee. No premix is generated; and if the foam concentrate is handed back into the tank, no foam concentrate needs to be topped up.
The larger design and the comparatively greater purchasing prices are an obstacle of the system.
With any system, consideration should be taken under consideration for the annual testing costs, which can be considerable when it comes to alternative foam concentrate, disposal of premix and labour prices.
Mobile extinguishing systems
As the stationary foam discharge gear may be broken in intensive fires within the tank or in the dyke space and thus lose effectiveness, cell fireplace displays and foam pipes may be used.
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Foam pipes are normally held by firefighters, making them very flexible. Yet they have solely restricted extinguishing agent circulate charges and reaches.
Firefighting displays
Firefighting displays are discharge devices mounted on autos or trailers and out there in plenty of sizes. The extinguishing agent move rate could be as a lot as 60,000 litres/min and the reach can be as much as 180m if the strain of the fire pumps is adequate. They are suitable to discharge foam, e.g., to extinguish a surface hearth in a tank, or water, to chill down a neighboring tank or the tank wall of a burning tank in order to stop it reaching the crucial temperature for a boilover, or to maintain the flames from spreading. The accumulation of water inside the dyke area should at all times be noticed to avoid an overflow of the dyke.
Mobile fireplace displays can be supplied both by the extinguishing water of the stationary fireplace pumps or by cell pumps. The injection of the foam focus usually takes place via cellular proportioners. This clearly factors in the direction of the advantage of power independence for water-motor-driven proportioning pumps.
The strategic choice for the sizes of cellular items out there as back-ups is shown by the following instance for the placement of displays for fireplace extinguishing at tanks which are 45m in diameter and 15m high.
According to NFPA, 32,000 litres of premix per minute are required. This results in a number of alternate options for the monitors. Generally, foam-concentrate proportioners for no much less than 32,000 litres/min are required, which should be ready to handle various circulate charges to ensure flexibility during extinguishing operations. Depending on the native circumstances, the monitors will need to keep a minimal distance to the burning tank or might not be able to be positioned close to to the tanks due to debris. In addition, it won’t always be attainable to place several monitors around the tank. It should be ensured the monitor has adequate throwing height in relation to the tank height, to deliver foam into the within of the tank.
The dimensioning of the complete firefighting system is made following authorized laws as nicely as suggestions by associations like NFPA and is not looked at more carefully within the current article.
Fig. four: Trailer with monitor M4, water motor with proportioning pump FD8000 and foam focus tank.
Lessons learned
As talked about within the introduction to Part One of this article, plainly many authorities and firms have not discovered the required classes from disastrous hearth incidents of previous years. Tank farm fires within the oil and petrochemical trade don’t occur regularly. When they do, they often have devastating consequences. Let us keep in mind the tank farm fire at Deer Park, Texas in March 2019 mentioned within the introduction.
The hearth developed after over 30,000 litres butane-enriched naphtha had been leaking from a defective valve for half-hour and caught fireplace for yet unknown causes. The plant had no fuel warning system and no remote-controlled valves to shut off the leaking fluid. In addition, some areas did not have fastened extinguishing systems put in. All 15 tanks have been surrounded by one single dyke. The proprietor had relied on the native hearth service, which was on the spot in a quick time however could not take control over the hearth with the equipment obtainable, partially as a end result of flammable substance was repeatedly leaking from the tank. Thirty-six hours after the fireplace had broken out, a contract was made with an exterior firefighting firm to do the extinguishing work. Mobilization of equipment and foam concentrate as nicely as the preparation of a plan of action took roughly thirteen hours. The fireplace was finally extinguished after 14 hours of firefighting, 63 hours after it initially broke out. Extinguishing attempts were made over three days, with foam concentrate and water shortages occurring within the meantime. In complete, over 500,000 litres of foam concentrate had been used. Instead of a hearth in the dyke area, eleven of the 15 tanks burnt down.
It is very probable that the fireplace would have been extinguished rapidly if the warning methods and valves had worked and a hard and fast fire extinguishing system had existed. It can additionally be probable that the fire would have been extinguished shortly if the extinguishing operation, which succeeded finally, had been began earlier. Both circumstances would have resulted in notably less injury.
Summing up, the following factors should be realized as a minimum. As far as they have not but been applied, or simply partly, they should serve as a foundation for discussions about one’s own fire-protection concept:
Have an appropriate fire-protection concept including alternative situations which adapt to the given state of affairs frequently.
Always have a sufficient number of mobile extinguishing techniques as a backup to fixed extinguishing systems.
Stock an acceptable foam focus.
Ensure enough foam focus supply.
Ensure adequate water provide.
Keep well-maintained, shortly and well-accessible, strategically placed and functioning extinguishing gear available in a sufficient quantity.
Have skilled personnel out there in a adequate quantity.
Ensure a quick implementation of a suitable plan of action.
For more data, go to www.firedos.com
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