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

Part One of this article described the typical incident situations at tank farm fires and introduced foam as essentially the most appropriate extinguishing agent together with the firefighting equipment mostly used. In Part Two we look at foam concentrate proportioning applied sciences, mobile extinguishing systems and conclude with lessons learned.
NFPA 11 describes numerous forms of foam focus proportioning gear. In the following, three methods are looked at which are commonest. Tight limits for the proportioning of froth focus apply to all of them.
The proportioning fee must not be lower than the permitted values – i.e. 3% for a 3% foam concentrate or 6% for a 6% foam concentrate.
The proportioning rate 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 price is allowed to be an absolute most of 1% above the permitted value – i.e. 4% for a 3% foam concentrate or 7% for a 6% foam concentrate (the smaller value should be used respectively).
To guarantee right proportioning, the proportioner, together with the proportioning fee must be tested no much less than once a year and its appropriate 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 full of foam concentrate. The tank is pressurized with water from the fire-extinguishing line and discharges the foam focus from the bladder as required. The bladder is linked to a proportioner which operates using the venturi precept. When the hearth pumps are activated, strain is generated by the pump, causing delivery of foam concentrate to the proportioner. The extinguishing water flows through the venturi proportioner. The resulting vacuum induces the froth focus into the extinguishing water circulate.
The advantages of this technique are its simple design without shifting elements and its easy operation. No external vitality is required, and the system is comparatively cheap.
A drawback is that the system is a pressurized vessel topic to corresponding laws similar to ASME Boiler & Pressure Vessel Codes. In order to refill foam focus, the system have to be shut down and drained. The rubber bladder is sensitive; when damaged, water will contaminate the foam focus. At a given proportioning fee, the system is suitable just for low variations within the extinguishing water flow pressure and quantity. Adding or changing particular person foam discharge devices is feasible only to a really limited extent. The system can be unsuitable for proportioning highly viscous foam concentrates.
To conduct any obligatory required annual testing, the system have to be activated and premix generated on the venturi proportioner within the extinguishing water line. The correct proportioning rate should be measured within the premix by laboratory analysis. The generated premix must then be disposed of, and the consumed foam focus within the bladder tank needs to be changed.
Fig. 2: Driven proportioning pump with move meter.
Driven proportioning pump with move meter
The system consists of an atmospheric tank for the foam concentrate, an electric or diesel-powered foam focus pump with an electronically managed 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 move price is measured by the circulate meter and the management system adjusts the proper foam focus quantity through the management valve. The foam focus is injected into the extinguishing water move by the foam focus pump. If there’s a change in the circulate price, the quantity of injected foam concentrate is regulated by the management valve.
The system’s benefit lies in the exact proportioning of the froth focus, impartial of the extinguishing water pressure or circulate 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 can be measured by way of a return line. The proportioning price is calculated from the extinguishing water/foam concentrate flow rate. No premix is produced; and as a outcome of the foam focus is handed back into the tank, no foam focus needs to be refilled.
Disadvantages are the requirement for an external interruption-free power supply for the foam focus pump and the management system, as nicely as the necessity for a complicated control system and the comparatively larger buying costs. Furthermore, it have to be accepted that a delay happens between the change of the extinguishing water flow fee and the newly adjusted foam concentrate amount. The foam high quality may be compromised when continuously altering working circumstances 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 circulate line and a foam concentrate pump which is related directly to the water motor. Water motor and pump type one compact unit. Upon activation of the fireplace pumps, rotation in the water motor begins. The direct coupling to the foam-concentrate pump provides immediate foam-concentrate injection into the extinguishing water. If the move price changes, the amount of foam concentrate is adapted immediately.
The advantage of the system is its independence from exterior power sources in addition to a precise and quick foam-concentrate proportioning regardless of the extinguishing water stress or circulate rate. If a piston or plunger pump is used, adjustment or calibration after set up isn’t essential because the water motor and the pump are volumetric units firmly linked to every other. Foam concentrate refilling throughout operation is possible. The system can also be capable of proportioning highly viscous foam concentrates. The system must be activated for annual testing; however, the delivered foam concentrate could be measured through a return line. The proportioning rate is calculated from the extinguishing water/foam concentrate move fee. No premix is generated; and if the foam focus is handed again into the tank, no foam focus must be topped up.
The bigger design and the comparatively larger purchasing prices are an obstacle of the system.
With any system, consideration ought to be taken into account for the annual testing costs, which can be considerable when it comes to alternative foam concentrate, disposal of premix and labour prices.
Mobile extinguishing methods
As the stationary foam discharge gear may be broken in extensive fires within the tank or within the dyke space and thus lose effectiveness, mobile hearth monitors and foam pipes could also be used.
Foam pipes
Foam pipes are usually held by firefighters, making them very flexible. Yet they’ve solely limited extinguishing agent circulate charges and reaches.
Firefighting displays
Firefighting displays are discharge units mounted on automobiles or trailers and out there in many sizes. The extinguishing agent move price can be as a lot as 60,000 litres/min and the reach can be up to 180m if the stress of the hearth pumps is sufficient. They are suitable to discharge foam, e.g., to extinguish a surface fireplace in a tank, or water, to cool down a neighboring tank or the tank wall of a burning tank to be able to stop it reaching the crucial temperature for a boilover, or to maintain the flames from spreading. The accumulation of water contained in the dyke area should always be observed to keep away from an overflow of the dyke.
Mobile hearth monitors could be provided either by the extinguishing water of the stationary hearth pumps or by mobile pumps. The injection of the foam concentrate usually takes place via cell proportioners. This clearly factors in the path of the advantage of vitality independence for water-motor-driven proportioning pumps.
The strategic determination for the sizes of mobile models obtainable as back-ups is shown by the following example 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 leads to a number of alternatives for the displays. Generally, foam-concentrate proportioners for at least 32,000 litres/min are required, which should have the ability to deal with varying move rates to guarantee flexibility throughout extinguishing operations. Depending on เกจวัดไนโตรเจนราคา , the monitors might want to hold a minimum distance to the burning tank or may not have the flexibility to be positioned near to the tanks as a outcome of particles. In addition, it is not going to all the time be possible to place several screens across the tank. It should be ensured the monitor has sufficient throwing top 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 laws as properly as suggestions by associations like NFPA and is not looked at extra carefully in the current article.
Fig. 4: Trailer with monitor M4, water motor with proportioning pump FD8000 and foam focus tank.
Lessons realized
As mentioned within the introduction to Part One of this article, it seems that evidently many authorities and corporations haven’t realized the required classes from disastrous fire incidents of past years. Tank farm fires in the oil and petrochemical business do not occur regularly. When they do, they usually have devastating penalties. Let us remember the tank farm fire at Deer Park, Texas in March 2019 mentioned in 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 causes. The plant had no gasoline warning system and no remote-controlled valves to shut off the leaking fluid. In addition, some areas did not have mounted extinguishing techniques installed. All 15 tanks were surrounded by one single dyke. The owner had relied on the local fire service, which was on the spot very quickly but couldn’t take control over the fire with the equipment available, partially as a result of flammable substance was continuously leaking from the tank. Thirty-six hours after the fireplace had broken out, a contract was made with an external firefighting firm to do the extinguishing work. Mobilization of kit and foam concentrate as nicely as the preparation of a plan of motion took roughly thirteen hours. The hearth was lastly extinguished after 14 hours of firefighting, 63 hours after it initially broke out. Extinguishing makes an attempt were remodeled three days, with foam focus and water shortages occurring within the meantime. In total, over 500,000 litres of froth focus have been used. Instead of a fire within the dyke space, eleven of the 15 tanks burnt down.
It could be very possible that the fire would have been extinguished quickly if the warning systems and valves had worked and a onerous and fast fire extinguishing system had existed. It can additionally be probable that the fireplace would have been extinguished quickly if the extinguishing operation, which succeeded ultimately, had been began earlier. Both cases would have resulted in notably less damage.
Summing up, the following factors must be discovered at the least. As far as they have not yet been applied, or simply partly, they should function a basis for discussions about one’s own fire-protection concept:
Have an appropriate fire-protection concept together with alternative situations which adapt to the given situation regularly.
Always have a sufficient number of cell extinguishing methods as a backup to fastened extinguishing methods.
Stock a suitable foam concentrate.
Ensure enough foam focus supply.
Ensure sufficient water supply.
Keep well-maintained, shortly and well-accessible, strategically positioned and functioning extinguishing gear available in a sufficient quantity.
Have skilled personnel available in a sufficient quantity.
Ensure a fast implementation of a suitable plan of action.
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