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

Part One of this article described the standard incident scenarios at tank farm fires and introduced foam as the most appropriate extinguishing agent together with the firefighting equipment most commonly used. In Part Two we have a glance at foam focus proportioning applied sciences, mobile extinguishing methods and conclude with lessons learned.
NFPA 11 describes various types of foam concentrate proportioning tools. In the following, three techniques are checked out that are most common. Tight limits for the proportioning of froth concentrate apply to all of them.
The proportioning rate should 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. three.9% for a 3% foam focus or 7.8% for a 6% foam focus; respectively, the proportioning rate 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 worth have to be used respectively).
To guarantee right proportioning, the proportioner, including the proportioning price have to be tested no less than once a year 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 proven and cost-effective know-how. 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 connected to a proportioner which operates utilizing the venturi precept. When the fire pumps are activated, strain is generated by the pump, inflicting delivery of froth concentrate to the proportioner. The extinguishing water flows via the venturi proportioner. The ensuing vacuum induces the froth focus into the extinguishing water move.
The advantages of this technique are its simple design with out moving elements and its easy operation. No external energy is required, and the system is comparatively cheap.
A disadvantage is that the system is a pressurized vessel topic to corresponding laws such as ASME Boiler & Pressure Vessel Codes. In order to refill foam focus, the system must be shut down and drained. ที่วัดแรงดันน้ำ is delicate; when damaged, water will contaminate the foam concentrate. At a given proportioning rate, the system is appropriate only for low variations in the extinguishing water move pressure and quantity. Adding or altering particular person foam discharge gadgets is possible solely to a really restricted extent. The system can additionally be unsuitable for proportioning highly viscous foam concentrates.
To conduct any obligatory required annual testing, the system should be activated and premix generated at the venturi proportioner throughout the extinguishing water line. The right proportioning rate should be measured within the premix by laboratory analysis. The generated premix should then be disposed of, and the consumed foam focus in the bladder tank must 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 froth focus, an electrical or diesel-powered foam focus pump with an electronically controlled valve and a flow meter within the extinguishing water circulate line. When the fireplace pumps are activated, the foam concentrate pump drive and digital control system have to be activated. The extinguishing water move fee is measured by the flow meter and the control system adjusts the right foam focus quantity by way of the management valve. The foam concentrate is injected into the extinguishing water move by the froth concentrate pump. If there’s a change in the move rate, the quantity of injected foam focus is regulated by the management valve.
The system’s benefit lies within the exact proportioning of the foam focus, independent of the extinguishing water pressure or move fee. 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 have to be activated; nevertheless, the delivered foam focus may be measured via a return line. The proportioning price is calculated from the extinguishing water/foam focus circulate fee. No premix is produced; and as the foam concentrate is passed back into the tank, no foam concentrate must be refilled.
Disadvantages are the requirement for an external interruption-free power supply for the foam concentrate pump and the management system, as nicely as the necessity for a classy control system and the comparatively greater buying prices. Furthermore, it have to be accepted that a delay happens between the change of the extinguishing water circulate rate and the newly adjusted foam concentrate quantity. The foam 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 concentrate, a water motor installed in the extinguishing water flow line and a foam focus pump which is connected on to the water motor. Water motor and pump kind one compact unit. Upon activation of the fireplace 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 flow rate modifications, the quantity of froth concentrate is tailored instantly.
The advantage of the system is its independence from exterior vitality sources as nicely as a exact and instant foam-concentrate proportioning whatever the extinguishing water stress or move price. If a piston or plunger pump is used, adjustment or calibration after installation is not necessary because the water motor and the pump are volumetric gadgets firmly linked to each other. Foam concentrate refilling during operation is feasible. The system can additionally be capable of proportioning extremely viscous foam concentrates. The system must be activated for annual testing; nonetheless, the delivered foam focus could be measured by way of a return line. The proportioning price is calculated from the extinguishing water/foam focus move price. No premix is generated; and if the froth concentrate is handed back into the tank, no foam concentrate needs to be topped up.
The larger design and the comparatively greater buying prices are a drawback of the system.
With any system, consideration ought to be taken into consideration for the annual testing costs, which could be appreciable in phrases of alternative foam focus, disposal of premix and labour costs.
Mobile extinguishing techniques
As the stationary foam discharge gear could be damaged in in depth fires in the tank or in the dyke area and thus lose effectiveness, cell fireplace displays and foam pipes may be used.
Foam pipes
Foam pipes are normally held by firefighters, making them very flexible. Yet they have solely restricted extinguishing agent circulate charges and reaches.
Firefighting screens
Firefighting screens are discharge gadgets mounted on autos or trailers and available in lots of sizes. The extinguishing agent circulate rate can be up to 60,000 litres/min and the reach can be as much as 180m if the pressure of the fireplace pumps is sufficient. They are appropriate to discharge foam, e.g., to extinguish a surface fireplace in a tank, or water, to chill down a neighboring tank or the tank wall of a burning tank to be able to forestall it reaching the crucial 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 avoid an overflow of the dyke.
Mobile hearth monitors may be provided both by the extinguishing water of the stationary fireplace pumps or by cell pumps. The injection of the foam focus normally takes place by way of cell proportioners. This clearly points in the course of the benefit of power independence for water-motor-driven proportioning pumps.
The strategic determination for the sizes of cellular models obtainable as back-ups is proven by the following instance for the placement of screens for fireplace extinguishing at tanks which are 45m in diameter and 15m excessive.
According to NFPA, 32,000 litres of premix per minute are required. This leads to a quantity of options for the displays. Generally, foam-concentrate proportioners for no much less than 32,000 litres/min are required, which ought to have the power to deal with various move rates to guarantee flexibility during extinguishing operations. Depending on the native conditions, the displays will want to maintain a minimal distance to the burning tank or might not have the flexibility to be positioned close to to the tanks due to particles. In addition, it will not always be potential to place a quantity of screens around the tank. It must be ensured the monitor has adequate throwing peak in relation to the tank height, to ship foam into the inside of the tank.
The dimensioning of the entire firefighting system is made following legal laws as properly as suggestions by associations like NFPA and is not checked out extra closely within the present article.
Fig. 4: Trailer with monitor M4, water motor with proportioning pump FD8000 and foam concentrate tank.
Lessons discovered
As mentioned within the introduction to Part One of this article, it appears that evidently many authorities and companies have not realized the required classes from disastrous hearth incidents of past years. Tank farm fires within the oil and petrochemical business don’t 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 in the introduction.
The fire developed after over 30,000 litres butane-enriched naphtha had been leaking from a faulty valve for 30 minutes and caught fire for but unknown causes. The plant had no fuel warning system and no remote-controlled valves to shut off the leaking fluid. In addition, some areas didn’t have fastened extinguishing methods put in. All 15 tanks were surrounded by one single dyke. The owner had relied on the local fire service, which was on the spot in a brief time but could not take management over the fireplace with the tools available, partially as a result of flammable substance was repeatedly leaking from the tank. Thirty-six hours after the hearth had broken out, a contract was made with an exterior firefighting company to do the extinguishing work. Mobilization of apparatus 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 have been remodeled three days, with foam concentrate and water shortages occurring in the meantime. In total, over 500,000 litres of foam concentrate were used. Instead of a fire within the dyke area, 11 of the 15 tanks burnt down.
It could be very probable that the fireplace would have been extinguished quickly if the warning methods and valves had labored and a fixed fireplace extinguishing system had existed. It can be probable that the fire would have been extinguished shortly if the extinguishing operation, which succeeded eventually, had been began earlier. Both cases would have resulted in notably much less harm.
Summing up, the next factors should be realized at least. As far as they haven’t but been implemented, or just partly, they need to serve as a basis for discussions about one’s personal fire-protection concept:
Have a suitable fire-protection concept together with various eventualities which adapt to the given state of affairs often.
Always have a enough variety of mobile extinguishing techniques as a backup to fixed extinguishing methods.
Stock a suitable foam concentrate.
Ensure enough foam focus provide.
Ensure enough water supply.
Keep well-maintained, rapidly and well-accessible, strategically positioned and functioning extinguishing equipment obtainable in a sufficient number.
Have trained personnel available in a enough number.
Ensure a quick implementation of an acceptable plan of action.
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