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ably burned at relatively high temperatures.

The fire was localized and extinguished in less than 5 hr. It did not cause material dam-age outside the bitumen stordam-age tank nor any fatalities or injuries. Unburned bitumen had to be placed in barrels and reprocessed.

No anomalies were found in the unburned liquid during its removal and reprocessing.

Additional observations. The south-ern part of the tank roof, with vent nozzles, was within the scope of one camera belong-ing to the plant security system. Relevant record analysis provided interesting addi-tional information about what happened a few minutes before the accident. Records confirmed that emptying the filling line was performed within a 15-min time period, ending 25 min before the accident—per-formed in accordance with operational instructions. Records showed that a per-manent and stable steam discharge from tank vents occurred 3 hr 40 min before the accident and finished abruptly 47 s before the explosion. During the last 47 s, no outflows from vents were visible. Also, the course of the explosion was recorded. The lifting of the roof was visible, followed by a rising fireball and flames. Evidently, the tank roof was blown off by the explosion of a flammable mixture in the tank atmo-sphere. The investigations after the accident showed that the vents were free and that the steam pipeline to the steam-purging inlet was open and free, too.

Searching for accident causes. The fire triangle describes three requirements that have to be fulfilled for a fire/explo-sion of a gas mixture: an oxidant, a fuel and an ignition source.² Accident causes com-bine the three requirements. Identifying the direct accident cause was not possible without identifying the specific oxidant, fuel and ignition source that were present inside the bitumen storage tank.

Oxidant. For at least four days before the accident, steam purging had been inactive on the tank. Its gaseous volume commu-nicated freely with the atmosphere outside vent openings. Tank space above the liquid level undoubtedly contained mainly air at the start of steam purging 3 hr 40 min before the accident. Sweep steam purging was used to make the atmosphere of the tank inert, but it was not able to perfectly mix the whole gaseous volume when the liquid level was low.² Steam is much lighter than air (and hydrocarbon vapors). The arrangement of the steam inlet pipe did not make the steam move into the lower parts of the tank. Only the upper part of the tank (about 20% of its total volume, according to an estimation made by plant personnel) is believed to have been filled with a steam blanket. Lower parts of the gaseous volume probably still contained mainly air during the explosion.

Large volumes of bitumen in the storage tank were used from time to time as ter-minal volume to empty various connected pipes by air. These emptying operations brought additional oxidant into the tank.

Fuel. Flammable, gaseous substances had to be present in the atmosphere inside the tank in a concentration above the lower flammability limit for the explosion to occur. The bitumen itself releases a certain amount of light hydrocarbons, but mea-surements indicate that the total content above the bitumen level is one order below any conceivable lower flammable limit (LFL). The bitumen present inside the tank before the accident was of standard quality;

therefore, the fuel source for the explosion had to be found elsewhere.

The filling pipeline came into the tank from a manifold to which pipelines from a few other storage tanks were also con-nected. Analyzing operational records showed that asphalt varnish was pumped through a pipeline that was connected to the manifold, more than two days before the accident. The asphalt varnish repre-sented a mixture of bitumen (identical with the stored one) and lacquer diluents.

If a check valve in the pipeline that was used for pumping asphalt varnish had not worked properly, a certain amount of varnish would have entered the manifold.

Insufficiently closing the check valve in its closing direction is a rather frequent defect that cannot be excluded.

The liquid amount that would have entered the manifold in this case might have easily reached many liters.

Undesir-able liquid containing light hydrocarbons would then have been transported into the bitumen storage tank as soon as any of the pipelines connected to the manifold would have been emptied into it. Other potential sources of fuel such as catalytic cracking on steam heating pipes or steam reforming are not considered to be probable since tempera-tures inside the tank were not high enough.

Observed explosion outcomes enabled estimating the amount of light hydrocar-bons that had to be present in a flammable cloud inside the tank. The light hydrocar-bons originated from lacquer diluents, the boiling interval was 135°C–220°C, LFL is 0.8% vol. and upper flammable level (UFL) is 6.5% vol. To assess the minimum amount of light hydrocarbons necessary to have lifted the tank roof off, it was neces-sary to start with the overpressure, which could have caused it. Overpressure at 12 kPa is enough pressure to lift a storage tank roof off.³

The question is, how large does the flammable cloud have to be if it is capable of generating 12 kPa of overpressure inside the tank? From the state equation, it fol-lows that if the vapor space volume inside the tank is 989 m³, then an increase in the vapor volume should be equal to 117 m³ under normal pressure. This volume increase is caused by generating hot com-bustion products. The number of moles inside the tank should not change during combustion. Only the temperature differ-ence between the initial and final states could cause the volume increase. The sys-tem’s initial temperature was supposed to be equal to 478 K. The combustion prod-ucts’ temperature was estimated to be 1,500 K. This temperature is in accordance that the flame temperature at the LFL for meth-ane is 1,498 K and approximately 1,573 K for other lower paraffinic hydrocarbons.⁴ Comparing these final and initial states, an expansion factor equal to 3.14 was obtained. The equation for the volume of the explosive mixture capable of producing the given pressure increase is:

Vexpl + 117 = Vexpl × 3.14.

This results in 54.7 m³ of the explosive mixture, with a concentration equal to LFL.

The light hydrocarbons may be represented by C9 fraction with a mean molecular weight of 148.4 g/mol. Using the molecu-lar weight, the evaporated flammable vapor amount is 1.66 kg. It is certainly the lowest possible amount, not taking into account the product cooling and venting through the two vent nozzles. Higher amounts of

S N

Bitumen storage tank; an aerial view with selected openings.

FIG. 2

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evaporated flammable vapors (e.g., 5 kg and 10 kg) would lead to higher values of the calculated overpressures (36.2 kPa and 72.4 kPa, respectively). Since these values are well above the 12 kPa, neither cooling nor venting through two vent nozzles would have prevented the roof from lifting off.

Such an amount of flammable vapors could have easily originated in the asphalt varnish that entered the tank via the manifold. The flammable cloud could have been formed after the temperature increase during the inflow of hot bitumen between –3:40 and –0:40 hr. Hydrocarbon vapors are heavier than air so the operation of sweep steam purging would not have removed them from the tank with a low bitumen level.

Fig. 3 illustrates the situation that is supposed to have been established inside the tank after adding hot bitumen.

Possible ignition sources. Some igni-tion sources may include hot work, static electricity, hot surfaces, pyrophoric iron sul-fides, pressure (compression ignition), fric-tion and mechanical sparks, sudden

decom-pression and catalysts.⁵ Some alternatives may be excluded immediately.

There was no hot work carried out at the tank weeks before the accident. A com-pression or decomcom-pression sharp enough to ignite the flammable vapors is not conceiv-able under conditions inside the tank. No moving mechanical parts that would be able to cause friction or sparks were present inside the tank.

Movement of nonconductive liquid into the tank finished at least 25 min before the accident; hence collection and discharge of static electricity are not considered to be probable to ignite the explosion. Hot sur-faces, in the usual meaning of this term, were not present inside the tank. However, a layer of coke sediments were found on the south–southwestern wall, burning intensely after the explosion. Suspicion arose—the coke sediments had been smoldering even before the accident and they ignited the flammable cloud. The possible presence of catalysts (e.g., coke particles with large active surfaces) in liquid bitumen was considered, too. The presence of catalytic surfaces could

cause a decrease in the auto-ignition tem-perature of flammable vapors and lead to ignition after an induction period.² Without a catalyst, auto-ignition of lacquer diluent vapors is not possible under 240°C.

Pyrophoric iron sulfides form when iron is exposed to hydrogen sulfide (H2S), or any other compound that contains sulfur, in an oxygen-deficient atmosphere. Pyrophoric iron sulfide may form in heated bitumen storage tanks as the result of a reaction between H₂S given off from the bitumen surface and iron in the form of rust on the tank roof.⁶ H2S was present inside the tank.

Hence, the area of new bitumen circulation inlet welds seemed to fulfill all conditions for pyrophoric iron sulfide formation.

Examining the facts and formu-lating hypotheses. Three possible ignition sources were identified: smol-dering coke, auto-ignition catalysts and pyrophoric iron sulfides. Examination of these three hypotheses with known facts is necessary, and Table 1 represents the fact/

hypothesis matrix.⁷

TABLE 1. Fact/hypothesis matrix. Legend: (+) compatible with hypothesis; (×) not likely

Temperatures inside tank No anomalies (coke particles, Abrupt interruption of Tank roof fell beside between 140°C and 205°C, hot spots) were found in steam discharge 47 sec tank in south–

Fact or condition/hypothesis steam blanket under roof unburned bitumen before the explosion southwest direction

Light hydrocarbon vapors + + × ×

ignited by smoldering coke on south–southwest wall

Light hydrocarbon vapors + × × +

auto-ignited after induction period Light hydrocarbon vapors ignited by pyrophoric iron

sulfides from new weld × + × +

Air with hydrocarbon vapors Steam blanket, 140°C

Situation inside the tank, 30 min before the accident.

FIG. 3

Injection of water

Position of new weld Bitumen circulation inlet pipe

Hydrocarbon vapors

Situation inside the tank during water injection.

FIG. 4

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Coke smoldering can develop slowly and gradually, so there would have been no reason for an abrupt interruption of steam discharge. If the explosion was ignited by smoldering coke at the south–southwestern wall, then the final position of the blown roof would have been expected on the opposite side of the tank. Catalyzed auto-ignition would require the presence of a catalytic surface on the bitumen. However, no corresponding anomalies were indicated in the liquid. Auto-ignition develops slowly and gradually, thus giving no explanation for the abrupt interruption of steam dis-charge. Pyrophoric iron sulfides spontane-ously ignite after they dry out and come in contact with air, but there was no indica-tion that the steam blanket was replaced by air at new welds. Again, the abrupt inter-ruption of steam is not compatible with the hypothesis.

Water injection. None of the consid-ered ignition sources were able to explain satisfactorily the abrupt interruption of steam discharge 47 sec before the explo-sion. Evidently, the interruption did not result in any of the conceivable ignition

processes. It resulted from an additional cause and probably contributed to the ignition process. Closing or plugging the steam pipeline or the outage of the steam supply system would have caused a slow decrease of steam discharge, not an abrupt interruption. An event must have occurred that caused an immediate pressure decrease inside the gaseous volume of the tank. Such an event could have been the steam pipeline plugging with water.

Steam lines need to be equipped by steam traps. A steam trap is a device used to discharge condensate and non-condensable gases while not permitting live steam escap-ing. If the steam trap is not present or if it fails, then a water plug may form inside the pipeline. There are indications that a steam trap was not present in the lower part of the steam line to the steam purging inlet, so a water plug formation seemed possible.

If one liter of water had penetrated into the steam purging line it would have cre-ated a water plug about 2 m long. The plug would have been transported into the end piece of the steam purging pipe. The water would have been injected into the tank’s vapor space through tiny holes in the end

piece at 4.5 bar. The injection would have abruptly cooled steam and gases inside the space, causing the gases to shrink. This results in under pressure inside the space, reversing the flow through the vent nozzles.

The situation is illustrated in Fig. 4.

In Fig. 4, superheated water leaves the holes in the end piece and some of it evaporates immediately (approximately 7.9%). Flash evaporation of water creates an expanding zone around the end piece inside. The temperature decreases to the boiling point of water (100°C). The volume of this zone is relatively small since one liter of water creates 0.14 m³ of flash evaporated steam. Tiny droplets of boiling water fly away from the expanding zone into much warmer steam and/or air around and below.

The droplets are heated and evaporated. The tank atmosphere cools down and shrinks.

Globally, 1.7 m³ of steam will emerge from 1 L of water. Simple calculations show that, in evaporation and balancing temperatures in 100 m³ of steam, the steam is cooled by 18.2°C and shrinks to 4.4 m³. Analogously, 100 m³ of air would be cooled by 24.8°C and shrunk to 6.2 m³.

This process leads to a movement inside the tank atmosphere towards the expanding zone around the end piece of the steam-purg-ing pipe. Possible evaporatsteam-purg-ing droplets on the tank walls make this movement even stron-ger. If some droplets fell on bitumen liquid level, they would evaporate and raise hydro-carbon vapors above the liquid surface.

The mixture of water aerosol and cold steam is relatively heavy and tends to sink into the air and hydrocarbon mixture.

Expansion in the steam-aerosol area and turbulences caused by water evaporation make the steam-aerosol area less permeable for downward flowing gas, especially in the vicinity of the end piece.

TABLE 2. Modified fact/hypothesis matrix. Legend: (+) compatible with hypothesis; (×) not likely

Temperatures inside tank No anomalies (coke particles, Abrupt interruption of steam Roof of the tank fell between 140°C–205°C, hot spots) were found in discharge 47 s before beside tank in south to Fact or condition/hypothesis steam blanket under roof unburned bitumen the explosion south-west direction

Light hydrocarbon vapors ignited + + + ×

by smoldering coke on south–

southwest wall after the introduction of a small amount of water

Auto-ignition of light hydrocarbon + × + +

vapors occurred after the introduction of a small amount of water

Light hydrocarbon vapors ignited + + + +

by pyrophoric iron sulfides from new weld after introducing small amounts of water

Welding of the

120 t of bitumen added, level

Development of the accident.

FIG. 5

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