Part One of this text described the typical incident scenarios at tank farm fires and introduced foam as probably the most suitable extinguishing agent together with the firefighting equipment most commonly used. In Part Two we look at foam concentrate proportioning applied sciences, cell extinguishing techniques and conclude with classes realized.
NFPA 11 describes numerous types of foam concentrate proportioning equipment. In the following, three methods are looked at that are most typical. Tight limits for the proportioning of foam focus apply to all of them.
The proportioning rate must not be less than the permitted values – i.e. 3% for a 3% foam focus or 6% for a 6% foam focus.
The proportioning rate should not exceed 30% above the permitted value i.e. 3.9% for a 3% foam concentrate or 7.8% for a 6% foam focus; respectively, the proportioning fee 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 value must be used respectively).
To assure right proportioning, the proportioner, including the proportioning fee must be examined at least annually and its correct functioning should 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 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 related to a proportioner which operates using the venturi precept. When the fireplace pumps are activated, stress is generated by the pump, causing supply of foam concentrate to the proportioner. The extinguishing water flows via the venturi proportioner. The ensuing vacuum induces the foam focus into the extinguishing water flow.
The benefits of this technique are its simple design without moving elements and its simple operation. No exterior vitality is required, and the system is comparatively cheap.
A drawback is that the system is a pressurized vessel subject to corresponding laws similar to ASME Boiler & Pressure Vessel Codes. In order to refill foam concentrate, 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 fee, the system is appropriate only for low variations in the extinguishing water circulate stress and volume. Adding or altering individual foam discharge units is feasible 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 inside the extinguishing water line. The appropriate proportioning rate must be measured in the premix by laboratory evaluation. The generated premix should then be disposed of, and the consumed foam concentrate within the bladder tank needs to be replaced.
Fig. 2: Driven proportioning pump with circulate meter.
Driven proportioning pump with flow meter
The system consists of an atmospheric tank for the froth concentrate, an electrical or diesel-powered foam focus pump with an electronically controlled valve and a move meter within the extinguishing water move line. When the fire pumps are activated, the foam focus pump drive and electronic management system must be activated. The extinguishing water move price is measured by the flow meter and the control system adjusts the proper foam concentrate quantity via the control valve. The foam focus is injected into the extinguishing water flow by the foam focus pump. If there is a change in the circulate rate, the amount of injected foam concentrate is regulated by the management valve.
The system’s benefit lies in the precise proportioning of the foam concentrate, independent of the extinguishing water strain or circulate rate. Foam concentrate can be topped up in the course of the extinguishing operation. The system is capable of proportioning extremely viscous foam concentrates. For the aim of annual testing, the system must be activated; 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 price. No premix is produced; and because the foam focus is passed again into the tank, no foam concentrate needs to be refilled.
Disadvantages are the requirement for an exterior interruption-free vitality supply for the foam concentrate pump and the management system, as nicely as the need for a classy control system and the comparatively larger buying prices. Furthermore, it have to be accepted that a delay occurs between the change of the extinguishing water move price and the newly adjusted foam concentrate amount. The foam quality may be compromised when constantly changing operating circumstances as foam discharge units are turned on or off or changed.
Water motor with proportioning pump
The system consists of an atmospheric tank for the froth focus, a water motor installed within the extinguishing water circulate line and a foam concentrate pump which is linked on to the water motor. Water motor and pump type one compact unit. Upon activation of the fireplace pumps, rotation within the water motor begins. The direct coupling to the foam-concentrate pump provides instant foam-concentrate injection into the extinguishing water. If the circulate fee modifications, the amount of froth concentrate is adapted immediately.
The benefit of the system is its independence from exterior vitality sources in addition to a precise and instant foam-concentrate proportioning whatever the extinguishing water strain or flow rate. If a piston or plunger pump is used, adjustment or calibration after installation is not needed because the water motor and the pump are volumetric devices firmly linked to each other. Foam concentrate refilling during operation is possible. The system can additionally be able to proportioning extremely viscous foam concentrates. The system must be activated for annual testing; however, the delivered foam focus may be measured through a return line. The proportioning fee 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 concentrate needs to be topped up.
The larger design and the comparatively larger buying prices are a drawback of the system.
With any system, consideration must be taken into account for the annual testing costs, which can be appreciable in phrases of replacement foam focus, disposal of premix and labour prices.
Mobile extinguishing methods
As the stationary foam discharge gear may be broken in in depth fires in the tank or within the dyke area and thus lose effectiveness, mobile fireplace monitors and foam pipes could also be used.
Foam pipes
Foam pipes are normally held by firefighters, making them very flexible. Yet they have only limited extinguishing agent circulate rates and reaches.
Firefighting displays
Firefighting displays are discharge devices mounted on vehicles or trailers and out there in plenty of sizes. The extinguishing agent move rate can be up to 60,000 litres/min and the reach may be as much as 180m if the strain of the hearth pumps is adequate. They are suitable to discharge foam, e.g., to extinguish a surface fire in a tank, or water, to cool down a neighboring tank or the tank wall of a burning tank in order to prevent 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 at all times be noticed to avoid an overflow of the dyke.
Mobile hearth displays may be equipped both by the extinguishing water of the stationary fire pumps or by cell pumps. The injection of the froth concentrate often takes place by way of cell proportioners. This clearly points in path of the benefit of energy independence for water-motor-driven proportioning pumps.
The strategic decision for the sizes of cell units obtainable as back-ups is shown by the next example for the position of monitors 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 monitors. Generally, foam-concentrate proportioners for at least 32,000 litres/min are required, which ought to have the ability to handle various circulate charges to ensure flexibility throughout extinguishing operations. Depending on the native conditions, the monitors will want to keep a minimal distance to the burning tank or could not be succesful of be positioned close to to the tanks as a outcome of debris. In addition, it won’t always be attainable to place several screens around the tank. It have to be ensured the monitor has sufficient throwing peak in relation to the tank peak, to deliver foam into the inside of the tank.
The dimensioning of the whole firefighting system is made following legal laws in addition to recommendations by associations like NFPA and isn’t looked at extra closely in the present article.
pressure gauge ดิจิตอล : 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 text, it appears that many authorities and firms have not learned the mandatory classes from disastrous hearth incidents of past years. Tank farm fires in the oil and petrochemical business don’t occur frequently. When they do, they usually have devastating penalties. Let us remember the tank farm hearth 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 hearth for yet unknown causes. 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 methods installed. All 15 tanks were 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 control over the hearth with the equipment available, partially as a outcome of flammable substance was continuously leaking from the tank. Thirty-six hours after the fire had damaged out, a contract was made with an external firefighting company to do the extinguishing work. Mobilization of apparatus and foam focus in addition to the preparation of a plan of motion took roughly 13 hours. The fireplace was lastly extinguished after 14 hours of firefighting, 63 hours after it initially broke out. Extinguishing makes an attempt have been revamped three days, with foam focus and water shortages occurring within the meantime. In complete, over 500,000 litres of froth concentrate have been used. Instead of a fire within the dyke area, eleven of the 15 tanks burnt down.
It may be very possible that the fireplace would have been extinguished rapidly if the warning techniques and valves had labored and a exhausting and fast fire extinguishing system had existed. It can also be probable that the fireplace would have been extinguished quickly if the extinguishing operation, which succeeded ultimately, had been started earlier. Both cases would have resulted in notably less harm.
Summing up, the next factors should be learned at the least. As far as they haven’t but been implemented, or simply partly, they should serve as a foundation for discussions about one’s own fire-protection idea:
Have an acceptable fire-protection concept together with different scenarios which adapt to the given situation frequently.
Always have a sufficient number of cell extinguishing methods as a backup to fastened extinguishing techniques.
Stock a suitable foam focus.
Ensure adequate foam focus provide.
Ensure sufficient water provide.
Keep well-maintained, rapidly and well-accessible, strategically placed and functioning extinguishing equipment available in a adequate quantity.
Have trained personnel available in a sufficient number.
Ensure a fast implementation of an appropriate plan of action.
For extra data, go to www.firedos.com
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