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

Part One of this article described the everyday incident eventualities at tank farm fires and introduced foam as the most suitable extinguishing agent together with the firefighting equipment mostly used. In Part Two we take a look at foam concentrate proportioning technologies, cellular extinguishing methods and conclude with lessons realized.
NFPA 11 describes varied forms of foam focus proportioning tools. In the following, three methods are checked out which are commonest. Tight limits for the proportioning of foam concentrate apply to all of them.
The proportioning price should not be lower than the permitted values – i.e. 3% for a 3% foam focus or 6% for a 6% foam concentrate.
The proportioning rate should not exceed 30% above the permitted value i.e. three.9% for a 3% foam concentrate 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 concentrate (the smaller worth must be used respectively).
To guarantee right proportioning, the proportioner, including the proportioning price must be examined at least once a year and its appropriate functioning have to be checked.
Fig. 1 Bladder tank with proportioner.
Bladder tank with proportioner
The bladder tank with a proportioner is a confirmed and cost-effective expertise. The bladder tank is a pressurized vessel with a bladder inside which is crammed with foam focus. The tank is pressurized with water from the fire-extinguishing line and discharges the froth focus from the bladder as required. The bladder is connected to a proportioner which operates using the venturi principle. When the fireplace pumps are activated, pressure is generated by the pump, inflicting supply of foam concentrate to the proportioner. The extinguishing water flows through the venturi proportioner. The ensuing vacuum induces the foam focus into the extinguishing water circulate.
The benefits of this system are its simple design without moving elements and its easy operation. No external power is required, and the system is relatively inexpensive.
A drawback is that the system is a pressurized vessel topic to corresponding laws 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 damaged, water will contaminate the foam focus. At a given proportioning rate, the system is suitable only for low variations in the extinguishing water flow pressure and quantity. Adding or altering individual foam discharge devices is feasible only to a really restricted extent. The system can be unsuitable for proportioning highly viscous foam concentrates.
To conduct any obligatory required annual testing, the system must be activated and premix generated on the venturi proportioner throughout the extinguishing water line. The correct proportioning rate should be measured in the premix by laboratory evaluation. The generated premix must then be disposed of, and the consumed foam concentrate in the bladder tank must be changed.
Fig. 2: Driven proportioning pump with move meter.
Driven proportioning pump with flow 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 circulate line. When the fire pumps are activated, the foam focus pump drive and digital management system must be activated. The extinguishing water circulate rate is measured by the move meter and the management system adjusts the right foam focus quantity by way of the management valve. The foam focus is injected into the extinguishing water circulate by the foam focus pump. If there is a change in the move rate, the amount of injected foam focus is regulated by the control valve.
The system’s advantage lies in the exact proportioning of the foam focus, independent of the extinguishing water strain or circulate rate. Foam concentrate may be topped up during the extinguishing operation. The system is able to proportioning extremely viscous foam concentrates. For the aim of annual testing, the system have to be activated; however, the delivered foam focus may be measured by way of a return line. The proportioning rate is calculated from the extinguishing water/foam concentrate circulate price. No premix is produced; and as a outcome of the foam focus is passed back into the tank, no foam concentrate needs to be refilled.
Disadvantages are the requirement for an exterior interruption-free vitality supply for the foam focus pump and the management system, as nicely as the need for a classy management system and the comparatively larger buying costs. Furthermore, it have to be accepted that a delay happens between the change of the extinguishing water flow fee and the newly adjusted foam focus quantity. The foam high quality could additionally be compromised when continuously changing 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 concentrate, 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 fireplace pumps, rotation in the water motor starts. The direct coupling to the foam-concentrate pump provides instant foam-concentrate injection into the extinguishing water. If the flow fee adjustments, the quantity of foam focus is tailored instantly.
The advantage of the system is its independence from exterior energy sources as well as a precise and instant foam-concentrate proportioning regardless of the extinguishing water strain or move fee. If a piston or plunger pump is used, adjustment or calibration after set up isn’t essential for the reason that water motor and the pump are volumetric devices firmly linked to one another. Foam focus refilling during operation is feasible. The system is also able to proportioning highly viscous foam concentrates. The system should be activated for annual testing; nonetheless, the delivered foam concentrate may be measured by way of a return line. The proportioning fee is calculated from the extinguishing water/foam concentrate move rate. No premix is generated; and if the foam focus is handed again into the tank, no foam concentrate needs to be topped up.
The bigger design and the comparatively higher buying prices are a drawback of the system.
With any system, consideration ought to be taken under consideration for the annual testing prices, which can be appreciable when it comes to substitute foam focus, disposal of premix and labour costs.
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As the stationary foam discharge equipment can be broken in intensive fires in the tank or within the dyke space and thus lose effectiveness, cellular fire screens and foam pipes may be used.
Foam pipes
Foam pipes are normally held by firefighters, making them very versatile. Yet they’ve solely limited extinguishing agent flow rates and reaches.
Firefighting displays
Firefighting displays are discharge units mounted on autos or trailers and obtainable in plenty of sizes. The extinguishing agent circulate fee could be as much as 60,000 litres/min and the reach could be up to 180m if the strain of the fire pumps is enough. They are suitable to discharge foam, e.g., to extinguish a floor hearth in a tank, or water, to cool down a neighboring tank or the tank wall of a burning tank so as to forestall it reaching the critical temperature for a boilover, or to maintain the flames from spreading. The accumulation of water inside the dyke area ought to all the time be noticed to keep away from an overflow of the dyke.
Mobile fire displays can be supplied either by the extinguishing water of the stationary fireplace pumps or by cellular pumps. The injection of the froth focus often takes place by way of cellular proportioners. This clearly points in the path of the advantage of power independence for water-motor-driven proportioning pumps.
The strategic determination for the sizes of mobile models available as back-ups is shown by the following example for the position of screens for fire extinguishing at tanks which are 45m in diameter and 15m high.
According to NFPA, 32,000 litres of premix per minute are required. This leads to several options for the displays. Generally, foam-concentrate proportioners for at least 32,000 litres/min are required, which ought to be able to deal with varying move rates to guarantee flexibility throughout extinguishing operations. Depending on the local conditions, the displays will want to maintain a minimum distance to the burning tank or may not be able to be positioned close to to the tanks due to debris. In addition, it is not going to always be attainable to position a quantity of screens across the tank. It should be ensured the monitor has sufficient throwing peak in relation to the tank top, to ship foam into the within of the tank.
The dimensioning of the entire firefighting system is made following legal regulations as properly as recommendations by associations like NFPA and is not looked at more intently in the present article.
Fig. four: Trailer with monitor M4, water motor with proportioning pump FD8000 and foam focus tank.
Lessons discovered
As mentioned in the introduction to Part One of this article, plainly many authorities and firms have not learned the necessary classes from disastrous fireplace incidents of past years. Tank farm fires in the oil and petrochemical trade do not happen regularly. When they do, they normally have devastating consequences. Let us keep in mind the tank farm hearth at Deer Park, Texas in March 2019 mentioned in the introduction.
The hearth developed after over 30,000 litres butane-enriched naphtha had been leaking from a defective valve for 30 minutes and caught fire for but unknown causes. The plant had no gas warning system and no remote-controlled valves to close off the leaking fluid. In addition, some areas didn’t have fastened extinguishing systems installed. All 15 tanks were surrounded by one single dyke. The proprietor had relied on the native hearth service, which was on the spot very quickly but couldn’t take control over the fireplace with the equipment obtainable, partially as a result of flammable substance was constantly leaking from the tank. Thirty-six hours after the hearth had broken out, a contract was made with an exterior firefighting company to do the extinguishing work. Mobilization of equipment and foam focus in addition to the preparation of a plan of action took approximately 13 hours. The fireplace was lastly 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 whole, over 500,000 litres of foam concentrate had been used. Instead of a hearth within the dyke area, eleven of the 15 tanks burnt down.
It is very probable that the fire would have been extinguished shortly if the warning techniques and valves had labored and a fixed fire extinguishing system had existed. It can be probable that the fireplace would have been extinguished rapidly if the extinguishing operation, which succeeded ultimately, had been began earlier. Both cases would have resulted in notably much less harm.
Summing up, the next factors ought to be realized as a minimum. As far as they have not but been carried out, or just partly, they want to serve as a foundation for discussions about one’s personal fire-protection idea:
Have an appropriate fire-protection concept together with various situations which adapt to the given scenario often.
Always have a adequate variety of cellular extinguishing techniques as a backup to mounted extinguishing systems.
Stock a suitable foam concentrate.
Ensure enough foam focus provide.
Ensure adequate water supply.
Keep well-maintained, rapidly and well-accessible, strategically placed and functioning extinguishing gear available in a adequate number.
Have trained personnel obtainable in a enough number.
Ensure a quick implementation of a suitable plan of motion.
For extra info, go to www.firedos.com
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