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 scenarios at tank farm fires and introduced foam as the most appropriate extinguishing agent together with the firefighting gear mostly used. In Part Two we have a look at foam focus proportioning applied sciences, mobile extinguishing techniques and conclude with lessons realized.
NFPA eleven describes numerous kinds of foam focus proportioning tools. In the following, three techniques are looked at that are commonest. Tight limits for the proportioning of froth concentrate apply to all of them.
The proportioning rate should not be lower than the permitted values – i.e. 3% for a 3% foam focus or 6% for a 6% foam focus.
The proportioning price must not exceed 30% above the permitted value i.e. 3.9% for a 3% foam focus or 7.8% for a 6% foam focus; respectively, the proportioning price is allowed to be an absolute maximum of 1% above the permitted value – i.e. 4% for a 3% foam focus or 7% for a 6% foam concentrate (the smaller value should be used respectively).
To assure correct proportioning, the proportioner, including the proportioning rate should be examined at least 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 proven and cost-effective technology. 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 froth concentrate from the bladder as required. The bladder is linked to a proportioner which operates using the venturi principle. When the hearth pumps are activated, stress is generated by the pump, causing supply of foam concentrate to the proportioner. The extinguishing water flows by way of the venturi proportioner. The resulting vacuum induces the froth focus into the extinguishing water circulate.
The benefits of this system are its easy design without shifting elements and its simple operation. No exterior vitality is required, and the system is comparatively cheap.
A disadvantage 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 have to be shut down and drained. The rubber bladder is sensitive; when broken, water will contaminate the froth focus. At a given proportioning rate, the system is appropriate just for low variations within the extinguishing water circulate stress and volume. Adding or altering individual foam discharge devices is feasible solely to a very limited extent. The system can 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 throughout the extinguishing water line. The correct proportioning fee must be measured in the premix by laboratory analysis. The generated premix must then be disposed of, and the consumed foam concentrate in the bladder tank must 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 foam concentrate, an electric or diesel-powered foam focus pump with an electronically managed valve and a flow meter in the extinguishing water move line. When the fireplace pumps are activated, the foam concentrate pump drive and electronic management system must be activated. The extinguishing water circulate fee is measured by the move meter and the control system adjusts the proper foam focus quantity through the control valve. The foam focus is injected into the extinguishing water move by the foam focus pump. If there’s a change in the circulate rate, the quantity of injected foam focus is regulated by the management valve.
The system’s advantage lies in the exact proportioning of the foam focus, independent of the extinguishing water pressure or flow fee. Foam concentrate could be topped up during the extinguishing operation. The system is able to proportioning extremely viscous foam concentrates. For the purpose of annual testing, the system should be activated; however, the delivered foam focus could be measured via a return line. The proportioning fee is calculated from the extinguishing water/foam concentrate move price. No premix is produced; and as a result of the foam concentrate is passed again into the tank, no foam concentrate must be refilled.
Disadvantages are the requirement for an exterior interruption-free vitality provide for the foam concentrate pump and the control system, as properly as the necessity for a complicated control system and the comparatively larger buying costs. Furthermore, it should be accepted that a delay occurs between the change of the extinguishing water move fee and the newly adjusted foam concentrate amount. The foam quality may be compromised when continuously changing operating conditions 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 flow line and a foam focus pump which is connected on to the water motor. Water motor and pump form one compact unit. Upon activation of the hearth pumps, rotation in the water motor starts. The direct coupling to the foam-concentrate pump offers immediate foam-concentrate injection into the extinguishing water. If the circulate fee changes, the quantity of foam concentrate is adapted instantly.
The benefit of the system is its independence from exterior power sources in addition to a precise and immediate 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 just isn’t essential because the water motor and the pump are volumetric gadgets firmly linked to one another. Foam concentrate refilling during operation is possible. The system is also able to proportioning highly viscous foam concentrates. The system must be activated for annual testing; nevertheless, the delivered foam concentrate could be measured through a return line. The proportioning price is calculated from the extinguishing water/foam focus circulate fee. No premix is generated; and if the froth focus is handed back into the tank, no foam concentrate must be topped up.
The larger design and the comparatively greater buying costs are an obstacle of the system.
With any system, consideration must be taken under consideration for the annual testing costs, which may be considerable by means of alternative foam focus, disposal of premix and labour prices.
Mobile extinguishing techniques
As the stationary foam discharge gear can be broken in intensive fires within the tank or within the dyke area and thus lose effectiveness, cellular 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 limited extinguishing agent move rates and reaches.
Firefighting monitors
Firefighting screens are discharge devices mounted on vehicles or trailers and available in many sizes. The extinguishing agent circulate price may be as much as 60,000 litres/min and the reach may be as much as 180m if the stress of the hearth pumps is adequate. They are appropriate to discharge foam, e.g., to extinguish a surface fire in a tank, or water, to chill down a neighboring tank or the tank wall of a burning tank in order to forestall it reaching the crucial temperature for a boilover, or to keep the flames from spreading. The accumulation of water contained in the dyke area ought to at all times be noticed to avoid an overflow of the dyke.
Mobile hearth displays may be supplied both by the extinguishing water of the stationary hearth pumps or by cellular pumps. The injection of the froth concentrate often takes place by way of cell proportioners. This clearly factors towards the benefit of power independence for water-motor-driven proportioning pumps.
The strategic choice for the sizes of cellular models available as back-ups is proven by the next instance 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 alternatives for the monitors. Generally, foam-concentrate proportioners for a minimum of 32,000 litres/min are required, which ought to be capable of handle varying move charges to ensure flexibility during extinguishing operations. Depending on the local conditions, the displays might want to keep a minimal distance to the burning tank or might not have the power to be positioned close to to the tanks due to debris. In addition, it is not going to always be potential to place a number of monitors around the tank. It have to be ensured the monitor has adequate throwing peak in relation to the tank peak, to deliver foam into the within of the tank.
เกจวัดความดันแก๊ส of the whole firefighting system is made following authorized rules in addition to recommendations by associations like NFPA and isn’t checked out more closely in the present article.
Fig. 4: Trailer with monitor M4, water motor with proportioning pump FD8000 and foam concentrate tank.
Lessons realized
As talked about within the introduction to Part One of this text, plainly many authorities and firms haven’t discovered the necessary lessons from disastrous fire incidents of previous years. Tank farm fires in the oil and petrochemical business do not happen frequently. When they do, they usually have devastating consequences. Let us remember the tank farm hearth at Deer Park, Texas in March 2019 mentioned in the introduction.
The fireplace developed after over 30,000 litres butane-enriched naphtha had been leaking from a defective valve for 30 minutes and caught hearth for but 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 fixed extinguishing techniques installed. All 15 tanks were surrounded by one single dyke. The owner had relied on the local fireplace service, which was on the spot in a brief time however could not take control over the fire with the tools obtainable, partially as a 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 firm to do the extinguishing work. Mobilization of kit and foam focus in addition to the preparation of a plan of motion took approximately thirteen hours. The fire was finally extinguished after 14 hours of firefighting, sixty three hours after it initially broke out. Extinguishing makes an attempt were made over three days, with foam concentrate and water shortages occurring within the meantime. In total, over 500,000 litres of froth concentrate were used. Instead of a hearth within the dyke area, eleven of the 15 tanks burnt down.
It could be very possible that the fireplace would have been extinguished shortly if the warning techniques and valves had labored and a set fire extinguishing system had existed. It can be probable that the hearth would have been extinguished quickly if the extinguishing operation, which succeeded ultimately, had been started earlier. Both circumstances would have resulted in notably less harm.
Summing up, the next points must be learned at least. As far as they haven’t but been applied, or simply partly, they should serve as a basis for discussions about one’s personal fire-protection idea:
Have an acceptable fire-protection concept together with alternative eventualities which adapt to the given state of affairs frequently.
Always have a sufficient variety of cellular extinguishing techniques as a backup to fixed extinguishing methods.
Stock an acceptable foam concentrate.
Ensure enough foam concentrate supply.
Ensure sufficient water supply.
Keep well-maintained, rapidly and well-accessible, strategically positioned and functioning extinguishing equipment available in a adequate quantity.
Have skilled personnel obtainable in a enough quantity.
Ensure a fast implementation of an appropriate plan of action.
For extra information, go to www.firedos.com
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