Use of froth for firefighting in tank farms of the oil and petrochemical industry – Part 2

Part One of this article described the standard incident situations at tank farm fires and presented foam as the most suitable extinguishing agent together with the firefighting equipment most commonly used. In Part Two we look at foam focus proportioning applied sciences, cellular extinguishing systems and conclude with lessons discovered.
NFPA eleven describes numerous kinds of foam focus proportioning equipment. In the following, three systems are checked out that are most common. Tight limits for the proportioning of foam concentrate apply to all of them.
The proportioning price must not be lower than the permitted values – i.e. 3% for a 3% foam focus or 6% for a 6% foam concentrate.
The proportioning fee must not exceed 30% above the permitted worth i.e. three.9% for a 3% foam concentrate or 7.8% for a 6% foam focus; 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 have to be used respectively).
To guarantee correct proportioning, the proportioner, together with the proportioning rate have to be examined a minimal of once a year and its right 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 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 froth concentrate from the bladder as required. The bladder is linked to a proportioner which operates using the venturi principle. When the hearth pumps are activated, pressure is generated by the pump, inflicting supply of froth concentrate to the proportioner. The extinguishing water flows through the venturi proportioner. The resulting vacuum induces the foam focus into the extinguishing water flow.
The advantages of this system are its easy design without shifting elements and its straightforward operation. No exterior power is required, and the system is relatively cheap.
A drawback is that the system is a pressurized vessel subject to corresponding rules such as ASME Boiler & Pressure Vessel Codes. In order to refill foam concentrate, the system have to be shut down and drained. The rubber bladder is delicate; when damaged, water will contaminate the foam concentrate. At a given proportioning rate, the system is appropriate just for low variations within the extinguishing water circulate strain and volume. Adding or changing individual foam discharge units is feasible only to a very limited extent. The system can be unsuitable for proportioning extremely viscous foam concentrates.
To conduct any necessary required annual testing, the system have to be activated and premix generated on the venturi proportioner throughout the extinguishing water line. The correct proportioning rate should be measured within the premix by laboratory analysis. The generated premix should then be disposed of, and the consumed foam focus within the bladder tank needs to be replaced.
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 concentrate pump with an electronically controlled valve and a move meter within the extinguishing water circulate line. When the fireplace pumps are activated, the foam focus pump drive and digital management system have to be activated. The extinguishing water move price is measured by the move meter and the management system adjusts the correct foam focus quantity through the management valve. The foam concentrate is injected into the extinguishing water flow by the foam focus pump. If there is a change in the move rate, the quantity of injected foam focus is regulated by the control valve.
The system’s advantage lies in the exact proportioning of the foam focus, impartial of the extinguishing water stress or move fee. Foam concentrate could be topped up during the extinguishing operation. The system is able to proportioning highly viscous foam concentrates. For the aim of annual testing, the system have to be activated; nevertheless, the delivered foam concentrate can be measured through a return line. The proportioning price is calculated from the extinguishing water/foam focus flow rate. No premix is produced; and as a result of the foam focus is handed back into the tank, no foam concentrate must be refilled.
Disadvantages are the requirement for an exterior interruption-free vitality supply for the foam focus pump and the management system, in addition to the necessity for a classy control system and the comparatively higher buying prices. Furthermore, it must be accepted that a delay happens between the change of the extinguishing water move price and the newly adjusted foam focus amount. The foam quality may be compromised when continually altering working conditions 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 foam focus, a water motor put in within the extinguishing water move line and a foam focus pump which is connected on to the water motor. Water motor and pump form one compact unit. Upon activation of the fireplace pumps, rotation in the water motor starts. The direct coupling to the foam-concentrate pump provides quick foam-concentrate injection into the extinguishing water. If the circulate rate modifications, the amount of froth focus is customized instantly.
The benefit of the system is its independence from external power sources as well as a exact and instant foam-concentrate proportioning whatever the extinguishing water stress or circulate rate. If a piston or plunger pump is used, adjustment or calibration after installation isn’t essential for the reason that water motor and the pump are volumetric devices firmly connected to one another. Foam concentrate refilling throughout operation is feasible. The system can be able to proportioning highly viscous foam concentrates. Certified must be activated for annual testing; nonetheless, the delivered foam concentrate could be measured through a return line. The proportioning price is calculated from the extinguishing water/foam concentrate circulate price. No premix is generated; and if the foam concentrate is handed again into the tank, no foam focus must be topped up.
The bigger design and the comparatively greater purchasing prices are an obstacle of the system.
With any system, consideration should be taken into consideration for the annual testing prices, which may be considerable when it comes to substitute foam focus, disposal of premix and labour prices.
Mobile extinguishing methods
As the stationary foam discharge equipment may be broken in extensive fires within the tank or in the dyke space and thus lose effectiveness, mobile fireplace monitors and foam pipes may be used.
Foam pipes
Foam pipes are normally held by firefighters, making them very versatile. Yet they’ve solely restricted extinguishing agent flow charges and reaches.
Firefighting screens
Firefighting displays are discharge devices mounted on vehicles or trailers and available in many sizes. The extinguishing agent move fee may be as a lot as 60,000 litres/min and the reach may be as much as 180m if the pressure of the hearth pumps is sufficient. They are appropriate to discharge foam, e.g., to extinguish a floor fire in a tank, or water, to chill down a neighboring tank or the tank wall of a burning tank so as to prevent it reaching the critical temperature for a boilover, or to maintain the flames from spreading. The accumulation of water inside the dyke space ought to always be noticed to avoid an overflow of the dyke.
Mobile hearth screens can be equipped both by the extinguishing water of the stationary fire pumps or by cell pumps. The injection of the froth focus normally takes place via cellular proportioners. This clearly factors towards the benefit of energy independence for water-motor-driven proportioning pumps.
The strategic determination for the sizes of cellular items available as back-ups is proven by the following instance for the placement of monitors for fireplace 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 quantity of alternate options for the displays. Generally, foam-concentrate proportioners for at least 32,000 litres/min are required, which ought to be ready to handle varying circulate rates 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 could not be able to be positioned close to to the tanks because of particles. In addition, it is not going to at all times be potential to position several displays around the tank. It must be ensured the monitor has enough throwing top in relation to the tank top, to ship foam into the inside of the tank.
The dimensioning of the whole firefighting system is made following authorized rules as nicely as suggestions by associations like NFPA and is not looked at extra closely 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 in the introduction to Part One of this text, it seems that many authorities and corporations haven’t learned the necessary classes from disastrous fire incidents of past years. Tank farm fires within the oil and petrochemical business don’t happen incessantly. When they do, they usually have devastating consequences. Let us remember the tank farm fireplace at Deer Park, Texas in March 2019 talked about in the introduction.
The fireplace developed after over 30,000 litres butane-enriched naphtha had been leaking from a faulty valve for 30 minutes and caught fireplace for yet unknown reasons. The plant had no gasoline warning system and no remote-controlled valves to shut off the leaking fluid. In addition, some areas didn’t have fastened extinguishing methods installed. 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 short time however couldn’t take management over the hearth with the equipment out there, partially as a outcome of flammable substance was continuously leaking from the tank. Thirty-six hours after the hearth had broken out, a contract was made with an exterior firefighting firm to do the extinguishing work. Mobilization of kit and foam concentrate as properly as the preparation of a plan of motion took roughly 13 hours. The fire was lastly extinguished after 14 hours of firefighting, sixty three hours after it initially broke out. Extinguishing makes an attempt had been revamped three days, with foam concentrate and water shortages occurring within the meantime. In total, over 500,000 litres of foam focus were used. Instead of a hearth in the dyke area, eleven of the 15 tanks burnt down.
It could be very possible that the fire would have been extinguished rapidly if the warning methods and valves had labored and a onerous and fast hearth extinguishing system had existed. It is also probable that the hearth would have been extinguished shortly if the extinguishing operation, which succeeded ultimately, had been started earlier. Both cases would have resulted in notably less damage.
Summing up, the next points must be learned at the least. As far as they haven’t yet been implemented, or simply partly, they should function a foundation for discussions about one’s own fire-protection concept:
Have a suitable fire-protection concept including various scenarios which adapt to the given state of affairs often.
Always have a enough variety of cell extinguishing methods as a backup to fastened extinguishing systems.
Stock an appropriate foam focus.
Ensure adequate foam focus supply.
Ensure adequate water provide.
Keep well-maintained, quickly and well-accessible, strategically positioned and functioning extinguishing tools obtainable in a sufficient number.
Have skilled personnel obtainable in a sufficient number.
Ensure a quick implementation of a suitable plan of motion.
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