Learning from Tank Fire & Explosion on Non-Conducting Flammable Liquid
Mousom Some
Leading the Transformation in Petrochemicals | Advocate for Sustainable Energy | Lifelong Student of Progress |Driven by Growth and Nation First | Writer
Introduction: The fire incidents involving storage tanks for non-conducting flammable liquids serve as critical reminders of the risks associated with static electricity in industrial operations. These events often highlight how the accumulation of static charge, if not properly managed, can lead to catastrophic explosions or fires, particularly when flammable vapors are ignited by sparks. The dangers are particularly pronounced when handling non-conducting liquids, as they are more prone to static buildup due to their low conductivity. This paper captures the insights on better handling non-conducting flammable liquids.
Please note that the paper is intended for informational and awareness purposes. They should not replace or override any regulatory standards. Readers are advised to consult relevant codes, standards, and experts for specific applications and ensure compliance with applicable laws and regulations.
Tank Fire due to Static Electricity
1.??? Incident 1: On Jan 8, 1972, a full-surface fire occurred at a 10,000 kL tank of benzene at Refinery, Yokohama, Kanagawa, Japan on sampling through a gage hatch at the roof of the tank. The fire continued for 15 hours.
2.??? Incident 2: On April 7, 2003, 80,000 barrel capacity storage tank (tank 11) at ConocoPhillips Company’s Glenpool tank farm exploded and burned as it was being filled with diesel fuel delivered by pipeline. At the time of the explosion, tank 11 contained between 7,397 and 7,600 barrels of diesel. Tank 11 had been used to store gasoline, which was transferred to another tank (tank 12) earlier that day to make room for the diesel. The resulting fire burned for 21 hours and damaged two other storage tanks in the area.
3.??? Incident 3: On July 17, 2007, an explosion and fire occurred at the Barton Solvents facility in Valley Center, Kansas, as a tanker trailer was transferring VM&P (Varnish maker’s & Painter’s) Naphtha into a 15,000 gallon (above ground) storage tank. The force of the explosion blew the tank 130 feet into the air, and within moments two more tanks ruptured and released their contents, which ignited. The tank farm was destroyed.
4.??? CSB, USA investigated the Kansas Tank Explosion incident and detail case study is presented in the you tube video. The link of the video: CSB Safety Video: Static Sparks Explosion in Kansas.
Flammable Liquid & Static Electricity
1.??? Static electricity is the electrical charge produced on two dissimilar materials through physical contact and separation caused by the imbalance of positive and negative charges between the two.
2.??? As an electrostatic charge accumulates, the electric fields and voltages increase. If the charge is unable to bleed off to ground when the electric field exceeds the insulating properties of the atmosphere, a static discharge can occur.
3.??? The conductivity of refined petroleum liquids is very low, which allows static charges to accumulate.
4.??? Static electricity is generated as liquid flows through pipes, valves, and filters while being transferred. It can also be produced by entrained water or air, splashing or agitation, and when sediment in the bottom of the tank becomes suspended (Britton, 1999). Because nonconductive liquids, such as VM&P naphtha and other flammable liquids, dissipate (or “relax”) static electricity slowly, they pose a risk of dangerous static electric accumulation that can produce sparks inside tanks.
5.??? The rate of static charge generation during flow through pipe increases roughly with the square of the flow velocity. he VM&P naphtha involved in the Barton incident had a conductivity of 3 pS (pico Siemens)/m.
6.??? As per NFPA 77, the electrical conductivity of liquid of the chemicals relevant to Refinery and Petchem varies.
a)??? Conductive Liquid has Conductivity?>104?pS/m
Examples are Acetic?acid, Isobutyl?alcohol (IBA), secButyl?alcohol (SBA), tButyl?alcohol (TBA), Ethanol, Methanol, Ethylene?glycol (MEG, DEG, TEG) , Ethylene?oxide, Phenol, Sulfolane, HCl, H2SO4 etc.
b)??? Semi-conductive Liquid has Conductivity of 50-100 pS/m
Examples are Ethylene?dichloride (EDC), Gasoline (leaded), Sulfur?(130°C)
c)??? Non-conductive Liquid has Conductivity of <50 pS/m
Examples are as follows: Non-conductive pS/m
Benzene: 5?×?10?–3
Diesel ?0.1
Cyclohexane <2
Gasoline?(st.?run) ?0.1
Hexane 1?×?10?–5
Gasoline?(unleaded)?<50?(varies)?
Styrene 10?
Jet?fuel 0.01–50?
Toluene <1
Kerosene 1–50
Xylene 0.1?
As can be seen, from the list, Benzene and Hexane are one of the most non-conductive flammable liquids.
Bonding and Grounding
1.??? Bonding and grounding is a very effective technique for minimizing the likelihood of an ignition from static electricity. There is a difference between bonding and grounding, and best practice is to bond AND ground.
2.??? Bonding is the process of electrically connecting conductive objects, like tanker-trailers, to transfer pumps to equalize their individual electrical potentials and prevent sparking.
3.??? Grounding (earthing) means connecting a conductive object to the earth to dissipate electricity, like accumulated static, lightning strikes, and equipment faults, into the ground, away from employees/equipment and ignitable mixtures.
4.??? As per CSB, according to witnesses at Barton, the TT, pump, piping, and storage tank were bonded and grounded at the time of the incident. However, published safety guidance indicates that bonding and grounding measures applied to typical transfer and storage operations may not be enough if nonconductive flammable liquids are involved. Nonconductive liquids accumulate static electricity and dissipate (relax) it more slowly than conductive liquids, and therefore require additional precautions.
5.??? Chevron Safety Standard proposes the use Certified Clamps Only
a.??? Not Approved: ?Alligator clamps, automotive “jump-start” cable clamps and welding earth clamps.
b.??? Approved: FM- and ATEX-certified clamps with strong spring compression able to “bite” through rust, paint and deposits.
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Static Charge Accumulation in Kansas Incident - CSB
CSB identified the following key factors contributing to the Kansas incident:
1.??? Air Pocket Formation: During the transfer of VM&P naphtha from multiple compartments in the TT to the storage tank, air pockets were introduced into the fill piping. When these pockets transferred into the tank during hose reconnections, they exacerbated the conditions for static charge accumulation.
2.??? Static Electricity Accumulation: Studies highlight that the rapid accumulation of static electricity during pump startup can occur when nonconductive liquids like VM&P naphtha are transferred. In this incident, factors such as air bubbling and the likely presence of suspended sediment and water in the tank amplified the risk.
3.??? Tank Fill Level: At the time of the explosion, the VM&P tank was only 30% filled. This specific fill level created a liquid surface potential (voltage) close to the maximum expected during filling operations, increasing the likelihood of static discharge.
4.??? CSB estimates that the minimum ignition energy required for a spark to ignite the Barton VM&P naphtha was 0.22 mJ (plus/minus 0.02 mJ). Electrical testing of an exemplar tank level float indicated that a loose linkage could produce a spark with sufficient energy to ignite a flammable vapor-air mixture inside a tank.
5.??? To understand the intensity of 0.22 millijoules (mJ), let's compare it to some everyday examples:
a.??? Static Electricity: The spark we feel when we touch a doorknob after walking on a carpet can have an energy of around 1 mJ. So, 0.22 mJ is less than a quarter of that spark's energy.
b.??? LED Light: A small LED light uses about 1 mJ of energy per second. The energy required to ignite the Barton VM&P naphtha is about one-fifth of that.
Recommendation by CSB on Kansas Incident
To mitigate risks associated with static discharge and flammable liquids, CSB recommends several additional precautions can be implemented to ensure the safety of operations involving storage tanks:
1.??? MSDS:
The following to be included in MSDS of specially Non-Conducting Flammable liquid:
2.??? Inert Gas Usage: Employing an inert gas like nitrogen to displace oxygen in tank headspaces is highly effective in reducing the potential for ignition from static sparks.
3.??? Safe Level Measurement Practices:
For tanks equipped with conductive level floats that could generate sparks:
a.??? Inspect and replace conductive floats with spark-safe alternatives.
b.??? Bond and ground level floats to eliminate static discharge risks.
c.???? Remove slack in float mechanisms to prevent spark gaps from forming.
4.??? Anti-Static Additives:
Conductivity-enhancing additives can reduce static buildup by increasing liquid conductivity.
5.??? Reduced Flow Velocities:
Transfer nonconductive flammable liquids at reduced flow velocities to minimize static accumulation during pumping operations. NFPA 77 (2007); API 2003 (2008); and Britton (1999) recommend a flow (pumping) velocity of 1 meter per second when the risk of static ignition is high. Until the spark potential inside the tank is eliminated, use a pump flow velocity at (or near) 1 meter per second to transfer nonconductive flammable liquids.
ConocoPhillips Incident in Diesel Tank
Key Points of the Incident
1.??? Tank Capacity and Content: Tank 11 had an 80,000-barrel capacity and was being filled with diesel after being emptied of gasoline earlier the same day.
2.??? Sequence of Events:
o?? Gasoline transfer completed at 6:10 PM.
o?? Diesel pipeline delivery began at 8:33 PM with high flow rates of 24,000–27,500 barrels per hour.
o?? At 8:55 PM, an explosion occurred, likely due to static discharge igniting a flammable vapor-air mixture inside the tank.
o?? The fire burned for 21 hours, damaging nearby tanks and causing significant financial and environmental loss.
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Cause of Explosion
1.??? Residual Gasoline in the Sump: Approximately 55 barrels of gasoline remained in the sump after the transfer. The turbulence from diesel filling created a flammable vapor-air mixture.
2.??? High Flow Rates:
o?? Diesel filling exceeded the recommended flow velocity of 1 m/s (3 feet/second) as per API RP 2003, leading to significant static charge generation.
o?? The calculated fill velocity was 4 times higher than the recommended limit.
3.??? Static Electricity Buildup:
o?? Diesel is a static accumulator (conductivity < 50 pS/m), and the lab analysis found the diesel used had a conductivity of less than 1 pS/m.
o?? The combination of low conductivity, high turbulence, and residual gasoline vapors created conditions ripe for ignition.
Benzene Tank Fire Incident in Japan
Incident Details
Sequence of Events
1.??? January 5th, 1972: Tank heating began using steam-fed heating coils to prevent benzene freezing.
2.??? January 8th, Morning: Benzene transfer operations occurred: 38 kL loaded into a tank car, 251 kL received from another tank.
3.??? January 8th, Afternoon:
o?? Operators opened the gage hatch to measure the liquid level using a metal tape.
o?? Sampling commenced with a metallic sampling thief. Initial sampling from the top layer was successful.
o?? During sampling at the middle layer, a flame erupted, igniting the tank's vapor layer.
Cause of Explosion
1.??? Static Electricity Generation:
o?? Movement of the metallic sampling thief within the liquid created static charges.
o?? The string used for suspending the thief was non-conductive, preventing charge dissipation.
2.??? Explosive Vapor-Air Mixture Formation:
o?? Ambient temperature: 9.6 °C. Liquid temperature: 11–14 °C.
o?? Tank heating caused benzene vapor pressure to rise, creating a combustible concentration in the vapor layer.
o?? Fresh air entered the tank through the opened gage hatch, possibly forming a concentration gradient conducive to combustion.
3.??? Residual Static Charge: Static electricity from the earlier benzene transfer might have persisted in the tank.
Lesson to Learn and Leverage
The following lessons and practices can be leveraged to prevent accidents related to static electricity in operations involving non-conducting flammable liquids, such as benzene etc.:
1. Static Charge Occurrence in Daily Operations: Static charge is a common phenomenon during various industrial operations and should be proactively managed to prevent accumulation.
2. Substantial Energy Buildup from Static Charge: Even small amounts of static electricity can accumulate and cause significant energy buildup that may lead to sparks, capable of igniting flammable vapors.
3. Fire and Explosion Risks: Static discharge can cause substantial hazards, including fire and explosion, when it generates sparks with sufficient energy to ignite flammable vapors.
4. Preventive Measures: Effective bonding, grounding, relaxation times, and, where possible, controlling flow rates are crucial for preventing static electricity from causing sparks.
5. Non-Conductive Liquids as Static Accumulators: Non-conductive flammable liquids (e.g., benzene) are more prone to static charge accumulation compared to high-conductivity fuels, increasing the risk of static discharge.
6. Static Buildup Doesn't Require High Flow Rates: Static electricity can accumulate even at low flow rates, not just during high flow, which means careful attention is required across various flow conditions.
7. Safe Dissipation of Static: Static charges can be controlled and dissipated safely if allowed sufficient time and the right conditions (e.g., proper grounding and bonding).
8. Assumption of Static Accumulation: When in doubt, always assume a material to be a static accumulator and take necessary precautions to prevent the risk of static discharge.
9. Material Safety Data Sheets (MSDS) for Non-Conducting Liquids: Develop and emphasize MSDS that highlight the risk of static electricity accumulation in non-conducting flammable liquids and its potential to create ignitable vapor-air mixtures within tanks.
10. Avoid Air Pocket Formation: Prevent air pockets during the transfer of non-conducting flammable liquids to minimize the formation of dangerous vapor-air mixtures.
11. Periodic Tank Cleaning and Inspection: Regular cleaning and inspection of storage tanks help minimize sediments and water, which could elevate static electricity risks.
12. Inert Gas for Tank Headspace: Consider adding inert gases like nitrogen to the headspace of storage tanks to dilute vapors and prevent the formation of an explosive vapor-air mixture.
13. Modify or Replace Tank Level Floats: Replace or modify loose linkage tank level floats to minimize static charge buildup from moving parts.
14. Anti-Static Agents: Consider adding suitable anti-static agents to the liquid to reduce the potential for static charge accumulation.
15. Control of Flow Velocity (NFPA 77): Follow NFPA 77 guidelines by reducing the pumping velocity to 1 meter per second (or near that value) when transferring non-conductive flammable liquids to control static buildup.
16. Flow Velocity Compliance with API RP 2003: Strictly control flow velocities, particularly during switch loading operations, as per API RP 2003 standards to minimize static hazards.
17. Inerting Tanks Holding High-Volatility Products: Tanks previously holding high-volatility products (like gasoline) should be purged or inerted with nitrogen before introducing non-volatile liquids (e.g., diesel) to prevent residual flammable vapor risks.
18. Sufficient Time for Residual Static Dissipation: After fluid transfer, ensure sufficient time has passed to allow residual static charges to dissipate before conducting further operations.
19. Loading of Non-Conducting Liquids: Ensure compliance with API RP 2003 during loading:
·??????? Bonding and Grounding: Ensure bonding and grounding systems are in place and interlocked with the filling system. For top-loading, the fill pipe should form a continuous conductive path and be in contact with the tank bottom.
·??????? Initial Loading: Slow start (velocity less than 1 m/s) until the fill pipe inlet is submerged to prevent spraying and minimize turbulence.
·??????? Maximum Loading Rate: Limit fill pipe velocity to no more than 7 m/s or 0.5/d m/s (where d = inlet inside diameter in meters), whichever is less, to prevent excessive flow rates.
·??????? Safe Transition: Use loading regulators to control transition from slow start to normal pumping rate once submerged.
20. Alternative Sampling System for Benzene Tanks: Instead of taking samples from a dip hatch, design and use a closed sampling system from the pump discharge, minimizing the risk of static electricity buildup and ensuring safer sampling.
21. Gauging and Sampling: Gauging and sampling operations, including temperature measurement, can introduce spark promoters into storage tanks or compartments. To mitigate static discharge risks, the following precautions are essential:
·??????? Conductive Gauging Wells: Use conductive gauging wells for manual sampling to minimize static electricity or sparks.
·??????? Manual Operations: Ground personnel as per recommendations to reduce static discharge risks.
·??????? Materials: Use either fully conductive or fully non-conductive devices for gauging and sampling.
·??????? Automatic Gauging Systems: Prefer automatic systems with bonded floats or non-contact devices like radar and ultrasonic gauges. Avoid unbonded floats and isolated conductive components.
·??????? Waiting Period: Allow static charge to dissipate before gauging or sampling:
Sources
1.???? CSB Investigation report on Kansas: https://www.csb.gov/assets/1/20/csb_study_barton_final.pdf?13738
2.???? CSB investigation Video: CSB Safety Video: Static Sparks Explosion in Kansas
3.???? National Transportation Safety Board (NTSB) Investigation report on ConocoPhilllips incident:
4.???? Benzene Tank (Japan)? Fire incident report:
5.???? Chevron Static Electricity Hazards and Prevention:
6.???? NFPA 77
7.???? API: 2003