Part One of this text described the typical incident situations at tank farm fires and presented foam as essentially the most suitable extinguishing agent together with the firefighting equipment mostly used. In Part Two we take a glance at foam focus proportioning technologies, cellular extinguishing systems and conclude with lessons discovered.
NFPA eleven describes numerous forms of foam focus proportioning gear. In the next, three techniques are looked at which are most common. Tight limits for the proportioning of foam focus apply to all of them.
The proportioning fee should not be less than the permitted values – i.e. 3% for a 3% foam focus or 6% for a 6% foam concentrate.
The proportioning price must not exceed 30% above the permitted worth i.e. 3.9% for a 3% foam focus or 7.8% for a 6% foam concentrate; respectively, the proportioning price is allowed to be an absolute maximum of 1% above the permitted value – i.e. 4% for a 3% foam focus or 7% for a 6% foam focus (the smaller worth have to be used respectively).
To assure right proportioning, the proportioner, together with the proportioning rate should be tested at least every year and its correct functioning have to be checked.
Fig. 1 Bladder tank with proportioner.
Bladder tank with proportioner
The bladder tank with a proportioner is a proven and cost-effective know-how. The bladder tank is a pressurized vessel with a bladder inside which is crammed with foam concentrate. The tank is pressurized with water from the fire-extinguishing line and discharges the froth focus from the bladder as required. The bladder is related to a proportioner which operates utilizing the venturi precept. When the hearth pumps are activated, pressure is generated by the pump, inflicting supply of froth focus to the proportioner. The extinguishing water flows by way of the venturi proportioner. The resulting vacuum induces the foam focus into the extinguishing water circulate.
The advantages of this system are its easy design with out shifting elements and its straightforward operation. No exterior vitality is required, and the system is comparatively inexpensive.
A drawback is that the system is a pressurized vessel topic to corresponding rules similar to ASME Boiler & Pressure Vessel Codes. In order to refill foam concentrate, the system must be shut down and drained. The rubber bladder is sensitive; when damaged, water will contaminate the froth concentrate. At a given proportioning rate, the system is suitable just for low variations within the extinguishing water circulate strain and quantity. Adding or changing individual foam discharge devices is feasible only to a very limited extent. The system can be unsuitable for proportioning extremely viscous foam concentrates.
To conduct any mandatory required annual testing, the system must be activated and premix generated on the venturi proportioner within the extinguishing water line. The appropriate proportioning fee must be measured in the premix by laboratory analysis. The generated premix must then be disposed of, and the consumed foam concentrate in the bladder tank needs to be replaced.
Fig. 2: Driven proportioning pump with flow meter.
Driven proportioning pump with circulate meter
The system consists of an atmospheric tank for the foam concentrate, an electrical or diesel-powered foam concentrate pump with an electronically managed valve and a flow meter in the extinguishing water circulate line. When the fire pumps are activated, the froth concentrate pump drive and electronic control system have to be activated. The extinguishing water circulate rate is measured by the flow meter and the management system adjusts the correct foam focus amount via the control valve. The foam focus is injected into the extinguishing water move by the foam concentrate pump. If there is a change within the flow fee, the quantity of injected foam concentrate is regulated by the control valve.
The system’s advantage lies in the precise proportioning of the foam focus, impartial of the extinguishing water stress or circulate price. Foam concentrate could be topped up in the course of the extinguishing operation. The system is able to proportioning highly viscous foam concentrates. For the aim of annual testing, the system should be activated; however, the delivered foam concentrate could be measured via a return line. The proportioning fee is calculated from the extinguishing water/foam focus move price. No premix is produced; and as the foam concentrate is handed again into the tank, no foam concentrate needs to be refilled.
Disadvantages are the requirement for an exterior interruption-free energy supply for the foam concentrate pump and the management system, as properly as the necessity for a classy control system and the comparatively larger purchasing prices. Furthermore, it should be accepted that a delay happens between the change of the extinguishing water circulate price and the newly adjusted foam concentrate quantity. The foam high quality may be compromised when continuously changing operating circumstances as foam discharge devices are turned on or off or modified.
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 related 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 offers quick foam-concentrate injection into the extinguishing water. If the flow fee changes, the amount of froth focus is adapted instantly.
The advantage of the system is its independence from exterior power sources as nicely as a precise and immediate foam-concentrate proportioning regardless of the extinguishing water stress or flow price. If a piston or plunger pump is used, adjustment or calibration after installation isn’t necessary since the water motor and the pump are volumetric gadgets firmly related to one another. Foam concentrate refilling during operation is possible. The system can be able to proportioning highly viscous foam concentrates. The system should be activated for annual testing; nevertheless, the delivered foam concentrate may be measured via a return line. The proportioning rate is calculated from the extinguishing water/foam concentrate move rate. No premix is generated; and if the foam focus is handed back into the tank, no foam concentrate must be topped up.
The bigger design and the comparatively greater purchasing prices are a disadvantage of the system.
With any system, consideration should be taken into consideration for the annual testing costs, which can be considerable in phrases of alternative foam focus, disposal of premix and labour prices.
Mobile extinguishing systems
As the stationary foam discharge equipment could be damaged in extensive fires in the tank or within the dyke space and thus lose effectiveness, cell fire 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 restricted extinguishing agent flow rates and reaches.
Firefighting displays
Firefighting displays are discharge devices mounted on automobiles or trailers and available in plenty of sizes. The extinguishing agent circulate fee could be up to 60,000 litres/min and the attain can be as a lot as 180m if the pressure of the hearth pumps is enough. They are appropriate to discharge foam, e.g., to extinguish a floor fire in a tank, or water, to cool down a neighboring tank or the tank wall of a burning tank in order to forestall it reaching the important temperature for a boilover, or to keep the flames from spreading. The accumulation of water inside the dyke area should always be noticed to keep away from an overflow of the dyke.
Mobile fire screens may be supplied either by the extinguishing water of the stationary fireplace pumps or by cell pumps. The injection of the foam concentrate usually takes place via mobile proportioners. This clearly points in the course of the benefit of power independence for water-motor-driven proportioning pumps.
The strategic choice for the sizes of cell items available as back-ups is shown by the next instance for the placement of screens for fireplace extinguishing at tanks that are 45m in diameter and 15m excessive.
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 a minimal of 32,000 litres/min are required, which ought to be capable of handle various circulate rates to guarantee flexibility throughout extinguishing operations. Depending on the local conditions, the screens will want to keep a minimal distance to the burning tank or might not be in a position to be positioned near to the tanks as a end result of particles. In addition, it will not at all times be possible to position several monitors across the tank. It have to be ensured the monitor has enough throwing height in relation to the tank peak, to deliver foam into the within of the tank.
The dimensioning of the entire firefighting system is made following authorized rules in addition to suggestions by associations like NFPA and is not checked out more intently in the present article.
Fig. 4: Trailer with monitor M4, water motor with proportioning pump FD8000 and foam concentrate tank.
Lessons learned
As mentioned in the introduction to Part One of this article, it seems that many authorities and firms haven’t realized the mandatory lessons from disastrous fire incidents of previous years. Tank farm fires in the oil and petrochemical industry do not happen regularly. When they do, they usually 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 defective valve for half-hour and caught fire for yet unknown reasons. The plant had no gas warning system and no remote-controlled valves to shut off the leaking fluid. In addition, some areas didn’t have fastened extinguishing techniques put in. All 15 tanks had been surrounded by one single dyke. The proprietor had relied on the local hearth service, which was on the spot very quickly but could not take management over the hearth with the equipment out there, partially because 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 motion took roughly 13 hours. The fire was finally extinguished after 14 hours of firefighting, 63 hours after it initially broke out. Extinguishing attempts have been remodeled three days, with foam focus and water shortages occurring in the meantime. In whole, over 500,000 litres of froth concentrate have been used. Instead of a fire in the dyke space, eleven of the 15 tanks burnt down.
It may be very probable that the fire would have been extinguished quickly if the warning techniques and valves had labored and a hard and fast hearth extinguishing system had existed. It is also probable that the fireplace would have been extinguished shortly if the extinguishing operation, which succeeded finally, had been started earlier. Both cases would have resulted in notably less harm.
Summing up, the next factors ought to be realized at the least. As far as they have not yet been carried out, or simply partly, they should serve as a basis for discussions about one’s own fire-protection idea:
Have an acceptable fire-protection concept including alternative eventualities which adapt to the given situation frequently.
Always have a sufficient variety of cell extinguishing methods as a backup to mounted extinguishing techniques.
Stock an acceptable foam focus.
Ensure sufficient foam concentrate provide.
Ensure ราคาpressuregauge provide.
Keep well-maintained, rapidly and well-accessible, strategically positioned and functioning extinguishing tools available in a sufficient number.
Have trained personnel out there in a adequate quantity.
Ensure a quick implementation of an acceptable plan of action.
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