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

Part One of this article described the typical incident eventualities at tank farm fires and introduced foam as probably the most suitable extinguishing agent along with the firefighting equipment most commonly used. In Part Two we look at foam concentrate proportioning applied sciences, cellular extinguishing systems and conclude with lessons discovered.
NFPA eleven describes various kinds of foam focus proportioning equipment. In the following, three techniques are checked out that are commonest. 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 concentrate 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 fee is allowed to be an absolute most of 1% above the permitted worth – i.e. 4% for a 3% foam focus or 7% for a 6% foam focus (the smaller worth should be used respectively).
To assure correct proportioning, the proportioner, including the proportioning fee should be tested at least every year and its right functioning should 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 filled with foam focus. The tank is pressurized with water from the fire-extinguishing line and discharges the foam concentrate from the bladder as required. The bladder is linked to a proportioner which operates utilizing the venturi precept. When the hearth pumps are activated, pressure is generated by the pump, causing delivery of froth focus to the proportioner. The extinguishing water flows through the venturi proportioner. The ensuing vacuum induces the froth focus into the extinguishing water flow.
The benefits of this system are its simple design without transferring parts and its straightforward operation. No external energy is required, and the system is relatively inexpensive.
A disadvantage is that the system is a pressurized vessel topic to corresponding rules corresponding to ASME Boiler & Pressure Vessel Codes. In order to refill foam focus, the system must be shut down and drained. The rubber bladder is sensitive; when broken, water will contaminate the froth focus. At a given proportioning price, the system is appropriate just for low variations in the extinguishing water circulate strain and quantity. Adding or altering individual foam discharge gadgets is feasible only to a very restricted extent. The system can also be unsuitable for proportioning extremely viscous foam concentrates.
To conduct any obligatory required annual testing, the system have to be activated and premix generated on the venturi proportioner inside the extinguishing water line. The correct proportioning rate must be measured within the premix by laboratory evaluation. The generated premix must then be disposed of, and the consumed foam focus in the bladder tank must be changed.
Fig. 2: Driven proportioning pump with flow 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 circulate meter within the extinguishing water move line. When the hearth pumps are activated, the foam concentrate pump drive and electronic control system should be activated. The extinguishing water move fee is measured by the move meter and the management system adjusts the correct foam focus amount via the management valve. The foam focus is injected into the extinguishing water move by the foam concentrate pump. If there’s a change in the move rate, the amount of injected foam focus is regulated by the control valve.
The system’s advantage lies within the exact proportioning of the foam concentrate, independent of the extinguishing water stress or move price. Foam concentrate could 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 concentrate may be measured by way of a return line. The proportioning fee is calculated from the extinguishing water/foam focus flow price. No premix is produced; and because 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 provide for the foam focus pump and the management system, in addition to the necessity for a classy management system and the comparatively higher purchasing prices. Furthermore, it should be accepted that a delay occurs between the change of the extinguishing water circulate fee and the newly adjusted foam concentrate amount. The foam high quality may be compromised when continually altering working conditions 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 foam focus, a water motor installed within the extinguishing water circulate line and a foam concentrate pump which is related directly to the water motor. Water motor and pump kind one compact unit. Upon activation of the fireplace pumps, rotation within the water motor starts. The direct coupling to the foam-concentrate pump offers quick foam-concentrate injection into the extinguishing water. If the flow rate modifications, the quantity of foam focus is adapted immediately.
The benefit of the system is its independence from external power sources in addition to a exact and quick foam-concentrate proportioning whatever the extinguishing water pressure or move fee. If a piston or plunger pump is used, adjustment or calibration after set up isn’t necessary since the water motor and the pump are volumetric gadgets firmly connected to one another. Foam focus refilling throughout operation is feasible. The system can be able to proportioning highly viscous foam concentrates. The system must be activated for annual testing; however, the delivered foam concentrate can be measured through a return line. The proportioning rate is calculated from the extinguishing water/foam focus move rate. No premix is generated; and if the foam focus is passed again into the tank, no foam concentrate must be topped up.
The bigger design and the comparatively higher buying costs are a disadvantage of the system.
With any system, consideration ought to be taken under consideration for the annual testing costs, which may be appreciable in terms of replacement foam concentrate, disposal of premix and labour prices.
Mobile extinguishing methods
As the stationary foam discharge gear can be damaged in intensive fires in the tank or within the dyke area and thus lose effectiveness, mobile hearth 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 circulate charges and reaches.
Firefighting displays
Firefighting monitors are discharge devices mounted on autos or trailers and available in plenty of sizes. The extinguishing agent circulate price may be as a lot as 60,000 litres/min and the reach can be as much as 180m if the strain of the fireplace pumps is adequate. They are suitable 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 to have the ability to prevent it reaching the critical temperature for a boilover, or to keep the flames from spreading. The accumulation of water inside the dyke area should at all times be observed to keep away from an overflow of the dyke.
Mobile fire screens can be supplied either by the extinguishing water of the stationary fireplace pumps or by mobile pumps. The injection of the froth concentrate usually takes place by way of cell proportioners. This clearly factors in the path of the advantage of vitality independence for water-motor-driven proportioning pumps.
The strategic choice for the sizes of cellular units available as back-ups is proven by the following instance for the placement of displays 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 several alternate options for the screens. Generally, foam-concentrate proportioners for at least 32,000 litres/min are required, which ought to have the ability to deal with varying flow charges to guarantee flexibility throughout extinguishing operations. Depending on the local circumstances, the monitors might need to maintain a minimum 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 all the time be attainable to position a number of monitors around the tank. It should be ensured the monitor has sufficient throwing top in relation to the tank peak, to deliver foam into the inside of the tank.
The dimensioning of the complete firefighting system is made following legal laws in addition to recommendations by associations like NFPA and is not looked at more carefully within the current article.
Fig. 4: Trailer with monitor M4, water motor with proportioning pump FD8000 and foam concentrate tank.
Lessons realized
As mentioned in the introduction to Part One of this text, it appears that many authorities and corporations have not discovered the mandatory classes from disastrous fireplace incidents of past years. Tank farm fires in the oil and petrochemical industry don’t happen regularly. When they do, they often have devastating penalties. Let us bear in mind the tank farm fire 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 systems put in. All 15 tanks had 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 could not take control over the fire with the tools out there, partially as a end result of flammable substance was constantly leaking from the tank. Thirty-six hours after the fireplace had damaged out, a contract was made with an exterior firefighting company to do the extinguishing work. เครื่องมือที่ใช้วัดความดันเลือด of apparatus and foam concentrate as properly as the preparation of a plan of motion took roughly thirteen hours. The hearth 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 total, over 500,000 litres of froth focus were used. Instead of a fire in the dyke area, eleven of the 15 tanks burnt down.
It may be very probable that the hearth would have been extinguished quickly if the warning systems and valves had labored and a fixed hearth extinguishing system had existed. It can be possible that the fireplace would have been extinguished quickly if the extinguishing operation, which succeeded eventually, had been started earlier. Both circumstances would have resulted in notably much less injury.
Summing up, the next factors ought to be discovered at the least. As far as they haven’t yet been implemented, or simply partly, they want to serve as a basis for discussions about one’s personal fire-protection concept:
Have an appropriate fire-protection concept including different eventualities which adapt to the given scenario frequently.
Always have a enough number of mobile extinguishing systems as a backup to mounted extinguishing methods.
Stock a suitable foam focus.
Ensure adequate foam concentrate supply.
Ensure enough water provide.
Keep well-maintained, quickly and well-accessible, strategically positioned and functioning extinguishing tools out there in a sufficient quantity.
Have skilled personnel out there in a enough quantity.
Ensure a quick implementation of an acceptable plan of action.
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