Investigation Report

An explosion accident during tankrepair at a chemical plant

National Institute of Occupational Safety
and Health, Japan1Introduction
At a chemical plant, during work to attach a stiffening plate to a concentrated sulfuric acid tank,the tank suddenly exploded while the contact surface of the tank was being sanded. As a result,three persons were seriously injured and one was slightly injured, all of whom are workers of a
subcontractor. The explosion damaged the concentrated sulfuric acid tank, the blast broke theexterior wall of the adjacent ammonium sulfate storage building, and the top board of the tank,which was blown off, gave damage to the electric room.

2[Omission]

3

Outline of the accident
3.1 [Omission]
Figure 1:[Omission]

Outline of the workplace

3.2(1) - (2)[Omission](3) Category of business: Chemical industry(4) Outline of the workplace
The workplace primarily produces basic chemicals, including tar distillation products,
ammonium sulfate, and sulfuric acid, from coal tar and coke oven gas called C gas obtainedin the manufacturing process of coke for iron making and other materials. Figure 2 shows thematerial flow sheet for the entire workplace. Figure 3 shows the flow diagram of the sulfuric
acid facility in the C gas purification process. Figure 4 shows the layout of C gas purification
facilities. Figure 5 shows the concentrated sulfuric acid tank (044 tank) that caused theaccident. Note that this concentrated sulfuric acid tank was converted from the desulfur
tower.

3.3

Circumstances before the accident(1) Four days before the accident, signs of oozing were detected at two places by visualobservation (Figure 6). Oozing seemed to be occurring at a height of five meters justaround the liquid level. To prevent concentrated sulfuric acid from leaking, concentrated
sulfuric acid in the tank was discharged and its liquid level was lowered.
(2) At 8:40 on the day of the accident, arrangements for work were made and fire
prevention measures were taken.
(3) At 9:20 on the same day, work started.
(4) At 9:45 on the same day, gas-detecting operation was performed at the external parts of1

oozing, and it was confirmed that there was no leakage of inflammable gas or carbon
monoxide.
(5) At 9:50 on the same day, sanding started, and at 9:54 the explosion occurred.
Accident line
Sulfuric acidfacility

Desulfurization
facility

Sulfuric acid

(Coke oven gas)

Deammoniation
facility

Ammonium sulfate

Light oilrecovery device

BTX distillation
facility(Crude light oil)

BenzeneCoke ovenToluene

XyleneFuel gas(Coal tar)

Hydrogenseparation facility

Hydrogen gas

Phenolate
Tar acidfacility

Naphthalenefacility

95% naphthalene

Tar distillation
facility

Phthalic anhydride
facility

Phthalic anhydridePurifying
facility

Purifiednaphthalene

Surfactant
facility

Surfactant

Creosote oil
Pitch modification
facility

Electrode binder
pitch

Steel picklingprocess

Batch tardistillation facility

Impregnation pitch(Recoveredhydrochloric acid)

Spherulite
facility

Spherulite

Iron oxide
facility

Iron oxide

Figure 2: Material flow sheet for the entire workplace
Sulfuric acidproduction facility

Absorption tower

Desulfurization liquid(containing sulfur)

Converter

BoilerDrying tower

Burnerreactor

Chimney

042

044

043

Accident
facility
Concentrated
sulfuric acid

Odor combustion
facility

Odor

BurnerreactorConcentratedsulfuric acid

Chimney

BoilerFigure 3: Flow diagram of the sulfuric acid facility in C gas purification(during normal operation)
2

Figure 4:[Omission]

Vent
Vent pipepipe (100A)(100A)

Support forinternal filler

Explosion/rupture andoozing repair position
4.8 meters in height

16.2m

East

Worker B
Worker C

Worker A
Worker D
4.2-meter
scaffolding

2.7m

φ 6m

Figure 5: Schematic drawing of 044 Tank (Wokers’ location at the accident)	 

Figure 6: Oozing observed before the accident
3	 

3.4 Extent of the damage(1) [Omission](2) Material damage
Damage to the concentrated sulfuric acid tank, damage to the exterior wall of theadjacent ammonium sulfate storage building caused by the blast, damage to the electricroom brought by the top board of the tank that was blown off, and other damage.

4Investigation into the cause of the accident
4.1

Substance that caused the explosion

C gas, composed mainly of hydrogen and carbon monoxide, is highly inflammable by its nature.
However, since this workplace is a facility that processes sulfur content extracted from gas andhas the burner reactor in the previous stage, C gas is unlikely to flow into the tank.
The 044 tank that exploded was made of SUS304 and SS41 (previous name per JIS: SS400)
and has not undergone a process of surface protection inside, including lining, since the time it
was used as a desulfur tower. Therefore, a reaction with sulfuric acid can generate hydrogen. In
the case of concentrated sulfuric acid, however, the amount of water contained is too small tofacilitate the generation of ionized hydrogen, and generally the dissolution rate of iron is
sufficiently low in concentrated sulfuric acid. For this reason, SS400 and other similar materials
are widely used for concentrated sulfuric acid tanks. Nevertheless, if concentrated sulfuric acidis diluted with water for some reason, the generation of hydrogen can be facilitated.
As a possible cause of dilution with water, the leakage of rainwater is considered. There is little
record on the inspection of the inside of the tank, with the only record being about opening amanhole when the facility was converted. And since the top board part (Figure 7) was blown
off by the explosion and reached the adjacent electric room (Figure 4, upper right), it is notpossible to examine if any leakage had occurred just before the explosion. If, meanwhile,rainwater comes into the tank through the top, mixing is only by diffusion because the specificgravity of concentrated sulfuric acid is heavier than that of diluted sulfuric acid and thus
convection is not generated. In this case, the layer of diluted sulfuric acid is formed in theuppermost part of the concentrated sulfuric acid in the tank and can lead to a disaster like thisaccident if the diluted sulfuric acid erodes the tank.
In addition, as concentrated sulfuric acid is hygroscopic in itself, even if there was no rainwaterleakage, sulfuric acid concentration may decrease at a portion near the liquid level if left under4

high humidity. The inside of the 044 tank comes in contact with the outside air via the vent
100A pipe that was bent into a J-shape, reversed, and attached to the top board (Figure 7). So
the sulfuric acid inside the tank could have been diluted if the tank was left under the outside airwith humidity high enough to develop a dense fog for a long time. However, the 100A pipe isthin with respect to the tank capacity, which accordingly limits the amount of moisture thatcomes in and out, suggesting that outside air humidity is unlikely to have decreased theconcentration.
Although the possibility of the above two causes cannot be eliminated, the investigation
revealed a higher probability of dilution as described in 4.3. Therefore, it can be considered thatthe primary cause was dilution (as described in 4.3) and the explosion was caused by themixture of hydrogen and the air.

Figure 7: The 044 tank top board (blown off and separated from the tank, the original position was upside down)

54.2 Ignition source
The absence of inflammable gas was confirmed according to work instructions by using a
4-point gas detector (inflammable gas, carbon monoxide, hydrogen sulfide, and oxygen) at thescaffolding on the east side just before the explosion. However, as detecting operation wasperformed only at the outside of the container, the concentration of the inside is considered tohave been undetectable. Figure 8 shows a ruptured section of the 044 tank. The ruptured partshowed apparent thinning that ranged from several centimeters to several dozen centimeters
heightwise. This part is considered to have appeared as oozing four days before the accident,
opened wide at the time of marking-off or sanding, and leaked the gas mixture out of the tank.
It is highly likely that this leaked gas or internal gas was ignited by sparks generated by thesander. Note that after oozing was found, the 044 tank was used mostly for discharge and its
liquid volume was continuously decreased to prevent the leakage of sulfuric acid (Figure 9);
therefore, the back side of the oozing area was a gas phase.

Figure 8: Ruptured part showing apparent thinning6Volume in the tank[t]Figure 9: Sulfuric acid volume in the 044 tank

4.3

Cause of sulfuric acid dilution

The facility that caused the accident is mainly used to obtain high-purity sulfuric acid byburning desulfurization liquid coming from the desulfurization facility but sometimes also used
to remove odors of exhaust gas and other substances as shown in Figure 3. There is anotherodor treatment facility, but the investigation found that this odor treatment facility had been
undergoing the annual inspection since one week before the accident, and thus the facility thatcaused the accident was used as an alternative to the odor treatment facility. During that period,
the facility only treated odor, which accordingly did not make sulfuric acid obtained at thedrying tower sufficiently concentrated. As a result, sulfuric acid that was more dilute than usual
was transferred to the 044 tank on November 26. This is the primary cause of the accident.
Examining sulfuric acid that was left in the drying tower confirmed that its concentration wasapproximately 20%. This concentration is low enough to erode iron. In addition, the facility
was converted from the desulfur tower and had a shelf to put catalysts and other substances
inside, the aspect which is different from regular tanks. Therefore, the surface of the wall has
many welded parts that can be easily eroded, and the shelf itself also has many parts that can beeasily eroded.

4.4

Mechanism of the occurrence of the accidentThis accident is a gas explosion accident caused by the explosion of a mixture of hydrogen andthe air. Hydrogen was generated by reaction between sulfuric acid and the tank, which was7caused by the unintended dilution of sulfuric acid, and the ignition source was sparks generated
by friction/impact at the time of sanding.
The explosion of the mixture of hydrogen and the air can occur with hydrogen of 4 vol.% to 75vol.%, and detonation, a type of explosion in which the flame propagation velocity exceeds thevelocity of sound, can occur with hydrogen of around 30 vol.%. If there is an obstruction in thedirection of flame propagation, turbulence is caused in the front part of the flame flow. Then,the flame becomes wrinkled in the turbulence, increasing the substantial flame area and thereaction velocity, and accelerating the propagation velocity. Repeating this acceleration toexceed the velocity of sound is the mechanism leading to detonation. In this accident too, theflame is considered to have accelerated as the shelf to put catalysts for desulfurization
corresponds to the obstruction. If the accident leads to detonation, the energy of the explosion,
which can escape from an aperture area if it exists, cannot escape due to the flame velocity thatexceeds the velocity of sound and concentrates in the flame front, generating an extremely highpressure of more than 1 MPa. During this process, many parts of the tank would be ruptured. In
this accident, the whole tank maintains its shape, suggesting the accident did not lead todetonation. It can be considered that the top board part was ruptured and separated by thepressure that increased in a certain part due to the reflection of the pressure wave generated byflame propagation, and by the stress that was concentrated due to the board’s shape.
One explosion in a sealed container can generate the absolute pressure of about 400 kPa; thetank is destroyed unless it was specially designed to withstand high pressure. In this accident,
pronounced thinning was observed at the part where the layer of dilute sulfuric acid isconsidered to have formed, indicating reduced strength. Actually, the rupture occurred at thatparticular part.

5

Recurrence prevention measures
5.1

Improvement of work instructions

In this accident, there are work instructions for tank repairs, records regarding the results of theinflammable inspection and the inspection of surrounding conditions, and the instruction sheet
for fire prevention measures that has been filled out. The instruction sheet for fire prevention
measures has items of “Removal of internal inflammables” and “Detection of internalinflammable gas,” but they were not filled out maybe because they did not apply to the work inquestion. This indicates no operation was performed to detect internal hydrogen gas, which led
to the accident. Operations for detection need to be mandatory not optional. In addition, specific8detection methods need to be considered not only for the tank that caused the accident but for
facility repairs in general, and work instructions need to be improved accordingly.

5.2

Thorough impact assessment at the time of process change

A remote cause of this accident is the regular inspection of the odor treatment facility. This
accident was caused because the impact of the difference in the concentration of sulfuric acidgenerated in the two processes (sulfuric acid production and odor treatment) was not recognizedsufficiently when the impact of the regular inspection of the odor treatment facility wasexamined. The principle that the reaction of a chemical substance varies according to theconcentration must be assumed.
Generally, when a change was made to a process, the change history needs to be recorded, andthe impact of the change needs to be estimated every time and made known to workers not onlyin the previous and following processes but also adjacent facilities. Particularly in chemicalplants, where multiple changes may occur simultaneously, if the estimated impact of the change
is not shared by all related workers, even a known reaction may not be foreseen and can lead toan accident.
The regular inspection of the odor treatment facility is conducted once a year, meaning theinspection was not the first one even after the introduction of the 044 tank. Hydrogen gas that
may have been generated at the previous regular inspection is considered to have leaked out,
but there may be similar conditions with respect to thinning of the tank. It is therefore desirable
to identify the current conditions by conducting an inspection or other means.

5.3

Monitoring of the concentration of liquid-phase sulfuric acid

The concentration of sulfuric acid, which was the direct cause of this accident, needs to bemonitored in some way. The concentration of the liquid level portion in particular must be
monitored since the possibility of rainwater and humidity diluting the liquid level portion
cannot be denied. Although partly due to the impact of the explosion, the vent pipe shown in
Figure 7 was corroded and also the height of the tank was more than 16 meters high, it ispractically difficult to continue monitoring the rainwater. Because it took only two days todetect oozing after the tank was supplied six days before the accident, at least one inspectionneed to be conducted daily. This time interval varies depending on the tank shape without
question, so continuous monitoring is recommended. Since it is necessary to detect a drop in
sulfuric acid concentration before hydrogen gas is generated at substantial amounts, it isnecessary to use a method that allows the detection of changes at relatively high concentrations.
9Examining the concentration of discharged sulfuric acid can be an alternative to the monitoringof the concentration of the liquid level portion, if the uniformity in the tank can be secured bystirring or other means. However, in the case where the tank has been converted as in this
accident, stirring is extremely difficult and the monitoring of the liquid level is more realistic.

5.4

Monitoring of the concentration of gas-phase hydrogen

Considering that the eventual explosion was caused by the mixture of hydrogen and the air,detecting hydrogen gas around the vent pipe will make it possible to take measures before an
explosive mixture forms. Since hydrogen gas is light, it can be satisfactorily detected byinstalling a monitoring device at the vent pipe area, for example. The detection of internal
inflammable gas during repair work can be achieved automatically with this monitoring device.

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