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

Part One of this text described the standard incident situations at tank farm fires and introduced foam as probably the most suitable extinguishing agent along with the firefighting equipment mostly used. In Part Two we take a look at foam concentrate proportioning technologies, cell extinguishing methods and conclude with classes realized.
NFPA eleven describes various types of foam focus proportioning tools. In the following, three methods are looked at which are most common. Tight limits for the proportioning of foam focus apply to all of them.
The proportioning rate should 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 worth i.e. 3.9% for a 3% foam concentrate or 7.8% for a 6% foam concentrate; respectively, the proportioning fee is allowed to be an absolute maximum of 1% above the permitted value – i.e. 4% for a 3% foam concentrate or 7% for a 6% foam concentrate (the smaller worth have to be used respectively).
To assure correct proportioning, the proportioner, together with the proportioning price should be examined a minimal of annually and its appropriate functioning must be checked.
Fig. 1 Bladder tank with proportioner.
Bladder tank with proportioner
The bladder tank with a proportioner is a confirmed and cost-effective technology. 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 froth focus from the bladder as required. The bladder is linked to a proportioner which operates using the venturi precept. When the fire pumps are activated, strain is generated by the pump, inflicting supply of foam concentrate to the proportioner. The extinguishing water flows via the venturi proportioner. The resulting vacuum induces the foam concentrate into the extinguishing water flow.
The advantages of this technique are its easy design with out shifting elements and its simple operation. No exterior energy is required, and the system is relatively inexpensive.
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 focus, the system should be shut down and drained. The rubber bladder is sensitive; when broken, water will contaminate the froth concentrate. At a given proportioning rate, the system is suitable just for low variations in the extinguishing water flow stress and quantity. Adding or changing particular person foam discharge units is possible only to a really restricted extent. The system is also unsuitable for proportioning highly viscous foam concentrates.
To conduct any mandatory required annual testing, the system should be activated and premix generated at the venturi proportioner throughout the extinguishing water line. The appropriate proportioning fee have to be measured in the premix by laboratory evaluation. The generated premix should then be disposed of, and the consumed foam focus within the bladder tank must 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 froth focus, an electrical or diesel-powered foam focus pump with an electronically controlled valve and a flow meter within the extinguishing water flow line. When the hearth pumps are activated, the froth focus pump drive and electronic control system must be activated. The extinguishing water flow price is measured by the move meter and the control system adjusts the right foam concentrate quantity via the management valve. The foam concentrate is injected into the extinguishing water move by the foam concentrate pump. If there is a change in the flow rate, the amount of injected foam concentrate is regulated by the management valve.
The system’s advantage lies within the precise proportioning of the froth concentrate, unbiased of the extinguishing water pressure or circulate rate. Foam concentrate can be topped up through the extinguishing operation. The system is capable of 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 by way of a return line. The proportioning fee is calculated from the extinguishing water/foam concentrate move rate. No premix is produced; and as the foam concentrate is passed again into the tank, no foam focus must be refilled.
Disadvantages are the requirement for an external interruption-free power supply for the froth concentrate pump and the control system, as properly as the need for a complicated control system and the comparatively larger purchasing prices. Furthermore, it must be accepted that a delay happens between the change of the extinguishing water move rate and the newly adjusted foam concentrate quantity. The foam quality may be compromised when continually changing working situations as foam discharge gadgets 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 move line and a foam concentrate pump which is related on to the water motor. Water motor and pump kind one compact unit. Upon activation of the hearth pumps, rotation within the water motor starts. The direct coupling to the foam-concentrate pump gives immediate foam-concentrate injection into the extinguishing water. If the circulate price modifications, the quantity of foam concentrate is tailored immediately.
The advantage of the system is its independence from exterior power sources as well as a precise and instant 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 just isn’t needed because the water motor and the pump are volumetric gadgets firmly linked to one another. Foam concentrate refilling throughout operation is possible. The system can additionally be capable of proportioning highly viscous foam concentrates. The system must be activated for annual testing; nonetheless, the delivered foam focus can be measured by way of a return line. The proportioning price is calculated from the extinguishing water/foam focus circulate price. No premix is generated; and if the froth focus is passed again into the tank, no foam focus needs to be topped up.
The larger design and the comparatively greater buying costs are a drawback of the system.
With any system, consideration must be taken under consideration for the annual testing prices, which could be considerable when it comes to alternative foam focus, disposal of premix and labour prices.
Mobile extinguishing systems
As the stationary foam discharge gear could be broken in intensive fires in the tank or within the dyke area and thus lose effectiveness, cellular hearth monitors and foam pipes could additionally be used.
Foam pipes are usually held by firefighters, making them very flexible. Yet they have only restricted extinguishing agent move rates and reaches.
Firefighting displays
Firefighting monitors are discharge units mounted on automobiles or trailers and out there in many sizes. The extinguishing agent move rate can be up to 60,000 litres/min and the reach can be up to 180m if the strain 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 so as to stop it reaching the important temperature for a boilover, or to keep the flames from spreading. The accumulation of water contained in the dyke space ought to always be noticed to keep away from an overflow of the dyke.
Mobile hearth displays may be supplied either by the extinguishing water of the stationary fire pumps or by mobile pumps. The injection of the froth focus often takes place via mobile proportioners. This clearly points towards the advantage of vitality independence for water-motor-driven proportioning pumps.
The strategic decision for the sizes of cellular items out there as back-ups is shown by the following example for the placement of monitors for hearth extinguishing at tanks which are 45m in diameter and 15m excessive.
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 ought to have the ability to handle varying circulate charges to ensure flexibility throughout extinguishing operations. Depending on the native conditions, the displays might need to keep a minimal distance to the burning tank or could not be capable of be positioned close to to the tanks as a result of debris. In addition, it won’t at all times be possible to place a number of monitors across the tank. It have to be ensured the monitor has adequate throwing peak in relation to the tank height, to deliver foam into the inside of the tank.
The dimensioning of the whole firefighting system is made following authorized regulations in addition to recommendations by associations like NFPA and isn’t checked out extra carefully in the current article.
Fig. four: Trailer with monitor M4, water motor with proportioning pump FD8000 and foam focus tank.
Lessons discovered
As talked about within the introduction to Part One of this text, it seems that evidently many authorities and corporations haven’t realized the required lessons from disastrous fire incidents of previous years. Tank farm fires in the oil and petrochemical trade do not occur incessantly. When they do, they normally have devastating penalties. Let us remember the tank farm hearth at Deer Park, Texas in March 2019 mentioned within 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 but unknown causes. The plant had no fuel warning system and no remote-controlled valves to close off the leaking fluid. In addition, some areas didn’t have fixed extinguishing systems put in. All 15 tanks have been surrounded by one single dyke. The proprietor had relied on the local fireplace service, which was on the spot in a quick 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 hearth had broken out, a contract was made with an external firefighting company to do the extinguishing work. Mobilization of equipment and foam concentrate in addition to the preparation of a plan of action took roughly 13 hours. The fireplace was finally extinguished after 14 hours of firefighting, 63 hours after it initially broke out. Extinguishing makes an attempt were revamped three days, with foam focus 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 space, eleven of the 15 tanks burnt down.
It could be very probable that the fireplace would have been extinguished rapidly if the warning systems and valves had labored and a onerous and fast hearth extinguishing system had existed. It can be possible that the fire would have been extinguished quickly if the extinguishing operation, which succeeded eventually, had been started earlier. Both cases would have resulted in notably much less damage.
Summing up, the next points must be realized at least. As far as they have not yet been carried out, or just partly, they should serve as a basis for discussions about one’s own fire-protection idea:
Have an acceptable fire-protection idea together with different eventualities which adapt to the given scenario frequently.
Always have a adequate number of cell extinguishing systems as a backup to fastened extinguishing systems.
Stock an appropriate foam focus.
Ensure sufficient foam focus supply.
Ensure sufficient water supply.
Keep well-maintained, quickly and well-accessible, strategically placed and functioning extinguishing gear out there in a adequate number.
Have educated personnel obtainable in a enough quantity.
Ensure a quick implementation of an acceptable plan of action.
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