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

Part One of this text described the standard incident situations at tank farm fires and presented foam as essentially the most appropriate extinguishing agent along with the firefighting equipment mostly used. In Part Two we look at foam focus proportioning technologies, mobile extinguishing systems and conclude with classes realized.
NFPA eleven describes various forms of foam concentrate proportioning tools. In the next, three systems are checked out which are most typical. Tight limits for the proportioning of froth concentrate apply to all of them.
The proportioning price should not be less than the permitted values – i.e. 3% for a 3% foam concentrate or 6% for a 6% foam focus.
The proportioning fee should not exceed 30% above the permitted worth i.e. three.9% for a 3% foam focus or 7.8% for a 6% foam focus; 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 value should be used respectively).
To guarantee appropriate proportioning, the proportioner, together with the proportioning rate must be examined at least annually and its correct functioning must be checked.
Fig. 1 Bladder tank with proportioner.
Bladder tank with proportioner
The bladder tank with a proportioner is a proven and cost-effective expertise. The bladder tank is a pressurized vessel with a bladder inside which is full of foam focus. The tank is pressurized with water from the fire-extinguishing line and discharges the foam focus from the bladder as required. The bladder is connected to a proportioner which operates utilizing the venturi precept. When the fireplace pumps are activated, strain is generated by the pump, causing delivery of froth focus to the proportioner. The extinguishing water flows via the venturi proportioner. The resulting vacuum induces the foam focus into the extinguishing water move.
The advantages of this technique are its simple design without shifting components and its easy operation. No external power is required, and the system is relatively inexpensive.
A drawback is that the system is a pressurized vessel subject to corresponding rules similar to ASME Boiler & Pressure Vessel Codes. In order to refill foam focus, the system must be shut down and drained. The rubber bladder is delicate; when broken, water will contaminate the froth concentrate. At a given proportioning price, the system is suitable only for low variations in the extinguishing water move stress and quantity. Adding or changing particular person foam discharge devices is feasible solely to a really restricted extent. The system can additionally be unsuitable for proportioning highly viscous foam concentrates.
To conduct any mandatory required annual testing, the system must be activated and premix generated at the venturi proportioner inside the extinguishing water line. The correct proportioning price have to 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 needs to be changed.
Fig. 2: Driven proportioning pump with flow meter.
Driven proportioning pump with flow meter
The system consists of an atmospheric tank for the foam focus, an electrical or diesel-powered foam focus pump with an electronically controlled valve and a move meter within the extinguishing water circulate line. When the fire pumps are activated, the foam concentrate pump drive and digital control system should be activated. The extinguishing water flow fee is measured by the move meter and the management system adjusts the correct foam concentrate amount through the management valve. The foam concentrate is injected into the extinguishing water move by the foam concentrate pump. If there’s 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, independent of the extinguishing water pressure or move fee. Foam focus could be topped up during the extinguishing operation. The system is capable of proportioning extremely viscous foam concentrates. For the aim of annual testing, the system should be activated; nevertheless, the delivered foam concentrate may be measured through a return line. The proportioning price is calculated from the extinguishing water/foam concentrate flow rate. No premix is produced; and as a end result of the foam focus is passed back into the tank, no foam concentrate must be refilled.
Disadvantages are the requirement for an exterior interruption-free power provide for the foam focus pump and the management system, in addition to the need for a complicated 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 rate and the newly adjusted foam concentrate quantity. The foam high quality could additionally be compromised when continually changing working circumstances as foam discharge units 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 in the extinguishing water move line and a foam concentrate pump which is connected directly to the water motor. Water motor and pump type one compact unit. Upon activation of the fire pumps, rotation within the water motor starts. The direct coupling to the foam-concentrate pump offers instant foam-concentrate injection into the extinguishing water. If the flow price changes, the quantity of foam concentrate is adapted immediately.
The advantage of the system is its independence from exterior power sources as nicely as a exact and immediate foam-concentrate proportioning regardless of the extinguishing water pressure or move rate. If a piston or plunger pump is used, adjustment or calibration after set up is not necessary since the water motor and the pump are volumetric devices firmly connected to one another. Foam concentrate refilling throughout operation is possible. The system can additionally be able to proportioning highly viscous foam concentrates. The system must be activated for annual testing; nevertheless, the delivered foam concentrate could be measured via a return line. The proportioning rate is calculated from the extinguishing water/foam concentrate circulate fee. No premix is generated; and if the foam focus is handed again into the tank, no foam concentrate must be topped up.
The larger design and the comparatively higher buying costs are a drawback of the system.
With any system, consideration should be taken under consideration for the annual testing costs, which can be appreciable when it comes to alternative foam focus, disposal of premix and labour costs.
Mobile extinguishing methods
As the stationary foam discharge equipment could be damaged in in depth fires in the tank or within the dyke space and thus lose effectiveness, cell hearth screens and foam pipes could also be used.
Foam pipes
Foam pipes are usually held by firefighters, making them very flexible. Yet they’ve solely restricted extinguishing agent circulate charges and reaches.
Firefighting displays
Firefighting screens are discharge devices mounted on automobiles or trailers and out there in many sizes. The extinguishing agent circulate price can be up to 60,000 litres/min and the attain may be as a lot as 180m if the strain of the hearth pumps is enough. They are suitable 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 prevent it reaching the important temperature for a boilover, or to maintain the flames from spreading. The accumulation of water inside the dyke area ought to all the time be observed to keep away from an overflow of the dyke.
Mobile fire monitors could be provided both by the extinguishing water of the stationary fire pumps or by cell pumps. The injection of the froth focus normally takes place through cell proportioners. This clearly points in path of the benefit of vitality independence for water-motor-driven proportioning pumps.
The strategic choice for the sizes of mobile models available as back-ups is shown by the next instance for the location of screens for hearth extinguishing at tanks that are 45m in diameter and 15m excessive.
According to NFPA, 32,000 litres of premix per minute are required. This results in several alternate options for the screens. Generally, foam-concentrate proportioners for at least 32,000 litres/min are required, which should be able to handle varying flow rates to ensure flexibility throughout extinguishing operations. Depending on the local circumstances, the screens will want to keep a minimal distance to the burning tank or could not be able to be positioned close to to the tanks as a result of debris. In addition, it won’t at all times be possible to place several displays around the tank. It must be ensured the monitor has enough throwing peak 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 legal rules in addition to recommendations by associations like NFPA and isn’t checked out more closely in the current article.
Fig. four: Trailer with monitor M4, water motor with proportioning pump FD8000 and foam focus tank.
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As mentioned within the introduction to Part One of this text, it appears that many authorities and companies have not learned the necessary classes from disastrous fire incidents of previous years. Tank farm fires within the oil and petrochemical industry don’t happen frequently. When they do, they usually have devastating consequences. Let us bear in mind the tank farm hearth at Deer Park, Texas in March 2019 talked about in the introduction.
The hearth 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 close off the leaking fluid. In addition, some areas didn’t have fixed extinguishing techniques put in. All 15 tanks have been surrounded by one single dyke. The owner had relied on the local hearth service, which was on the spot in a quick time however could not take control over the fire with the tools obtainable, partially because 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 firm to do the extinguishing work. Mobilization of kit and foam focus as well as the preparation of a plan of action took approximately 13 hours. The fire was lastly extinguished after 14 hours of firefighting, 63 hours after it initially broke out. Extinguishing attempts had been revamped three days, with foam concentrate and water shortages occurring in the meantime. In total, over 500,000 litres of froth focus have been used. Instead of a fireplace within the dyke area, 11 of the 15 tanks burnt down.
It may be very possible that the fire would have been extinguished quickly if the warning techniques and valves had labored and a hard and fast fire extinguishing system had existed. It can also be probable that the fire would have been extinguished rapidly if the extinguishing operation, which succeeded ultimately, had been started earlier. Both cases would have resulted in notably less injury.
Summing up, the following factors should be learned at the least. As far as they have not but been applied, or simply partly, they want to function a foundation for discussions about one’s own fire-protection idea:
Have an acceptable fire-protection idea together with various scenarios which adapt to the given scenario frequently.
Always have a sufficient number of cellular extinguishing techniques as a backup to fastened extinguishing techniques.
Stock an appropriate foam focus.
Ensure enough foam focus supply.
Ensure sufficient water supply.
Keep well-maintained, rapidly and well-accessible, strategically positioned and functioning extinguishing tools out there in a enough number.
Have skilled personnel available in a enough number.
Ensure a fast implementation of a suitable plan of action.
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