Investigation Report
Fire in a
Petroleum Tank Being Cleaned

Japan National Institute ofOccupational Safety and Health1. Overview
A fire occurred when seven workers were cleaning a floating roof
petroleum tank (diameter 75 m, height 25 m, capacity 100,000 kL). The
cleaning, part of a legal inspection, removed the solid precipitate (sludge) on
the bottom of the tank. The workers entered the tank and began scraping thesludge using scrapers. Two of them escaped from a nearby manhole,suffering minor injuries. The others (five workers), who were working near
the fire, were killed (see Figure 1). The surviving workers testified that in
the beginning of the afternoon, they heard a sound, and found that a
projector had fallen into the sludge. One survivor said, “After the fire started,a pale blue flame began to spread rapidly.”
Two of the five workers who were killed might have escaped from thetank, but they had whole body burns. The others (three workers) died in thetank because interior metal heating tubes, electrical wiring, and air hoses forgas masks were in the way of their escape.
The sludge was softened with solvent oil (light oil or heavy oil A) before
cleaning. The combustible vapor from the solvent oil was exhausted from two
manholes using explosion-proof electric blowers that were just outside thetank. There is a company (local) rule that workers cannot enter a tank when
the concentration of flammable vapors is more than 1,400 ppm (the reference
value). At 8:30 am on the day of the disaster, 700 ppm of the concentration
was measured near the manhole when the workers entered the tank. It
seems that the concentration was not measured again after the firstmeasurement. The explosion-proof projectors were sitting on a temporary
table in the tank to light the inside. All workers wore waterproof antistatic
raincoats and gas masks with air hoses. The temperature was 10.7°C andthe relative humidity was 48%. At the first investigation, it was thought thatradiant heat from a light bulb or a spark caused by the fall of the projector
were the ignition sources, but there was no proof for this.
From disaster case analyses, we have a good understanding of the risk ofstatic electricity being released from metal with an insulating coating, andwe investigated the possibility that a scraper was the ignition source.

Quay
Tank truck

Two workers cameback alive
Operationarea at the fire
Projector

Header box(Branch box)

Five workers
killedTank
Switchboard(a) Location of workers when the fire started(b) Tank manhole(c) Projector (explosion-proof)

Figure 1. Diagram of the accident
2. Estimation of the ignition source
A photograph of the same type of scraper as the ones used at the disasteris shown in Figure 2. The metal shaft of the scraper is coated with
polypropylene plastic. Grabbing the shaft with gloves can cause a charge
separation between the gloves and the coating (Fig. 3 (a)). Some of the charge
remains in the coating after the shaft is released. This residual charge

causes an induction charge on the metal shaft. The induction charge
increases electric potential and distributes a charge on the shaft
heterogeneously (Fig. 3 (b)). Moving the scraper close to ground metal
produces a spark discharge (Fig. 3 (c)). Charge separation is caused not only
by grabbing, but also by the shaft rubbing against an object (clothing, duster,etc.).

Grip

Shaft

Head

Rubberblade

Figure 2. The same type of scraper used

GloveStatic by friction
Plastic coating (insulating)
Shaft

Metal rod(a) Friction between glove and plastic coating causes static.

Static remains

Induction charge(b) A static charge remains in the coating on the metal rod after theshaft is released (induction charge).

Ground metal
produce

Spark(c) Moving the rod close to ground metal produces a spark
discharge.

Figure 3. Principle of electrostatic charge on metal parts with aninsulating coating (shaft)3. Samples and Experimental Methods
For our tests, the sample oils used were the sludge collected at thedisaster site, as well as petroleum and solvent oils (light oil and heavy oil A).
The petroleum was the same quality as the oil in the tank. The same type ofscrapers (used and new), the same type of clothing (unused), and the gloves(used) of the victims were obtained.
3.1. Liquid sample
The measured values of the flashpoint, auto-ignition point and the
conductivity of the sludge, petroleum, light oil and heavy oil A are shown in
Table 1. These values were measured in accordance with the Fire Service Act
test manual for hazardous materials, ASTME-659, and the guide forelectrostatic safety. Ignition energy of the petroleum vapor was measured byan original method devised by our institute. The result of the experiment: at
13˚C the vapor concentration reached 3.7 vol%. The minimum value (0.8 mJ)of ignition energy of this concentration was measured. The concentration of
the sludge vapor was not enough to measure ignition energy.
Table 1. Characteristics of the materials
Flashpoint

Auto-ignition(˚C)

point (˚C)

-6.0

343

SludgeNot

Not(bottom layer, mud)measuredmeasuredPetroleum

-20

233

2.7×10-8

Heavy oil A

68.5

230

1.3×10-12

Solvent oil

Light oil

69.0

221

8.3×10-10

Solvent oil

Material
Sludge(top layer, liquid)

Conductivity

Remarks

1.0×10-8

Sample in the tank

7.7×10-5

Sample in the tank
Equivalent fromthe tank depot

	  The volatile flammable vapors from the sludge and residue on the dirty
scraper shaft were analyzed. The dirty shaft was simulated by wiping it with

sludge and then wiping this off with a cloth. The results of our analysis areshown in Figures 4 (a) and (b). These results show that the hydrocarbons (upto C8) are volatilized from the sludge, and that this formed an explosive
atmosphere with the ignition energy (0.8 mJ) at the same temperature asthat during the disaster. Also, the existence of petroleum wax (C14 ~ C35) on
the shaft was suggested.
Incidentally, industrially purified petroleum wax is used as an electricalinsulating material due to its very high resistivity: 1013 ~ 1017 Ωm.

(a) Composition of the sludge vapor(b) Composition of non-volatile components of the sludge (petroleum wax)
Figure 4. Composition of sludge3.2. Static characteristics of the outfits
The results of measuring the volume resistivity and surface resistivity of
the coating on the shaft (peeled from the rod), the gloves and the clothing(antistatic raincoats) are shown in Table 2. Two types of samples of coatingwere prepared. One of them was cleaned ("clean"). Another was dirtied withsludge and wiped with a cloth ("polluted"). It should be noted that surface
resistivity of the "polluted" coating was higher than that of the "clean"coating. Figure 4 (b) shows that the difference in resistivity is caused by theremains of the sludge, which contains much petroleum wax with highinsulating properties. This result was difficult to anticipate from sludge
conductivity (Table 1). In addition, the amount of the charge generated wasmeasured by the tumbler method defined in the standard for antistatic work
wear (JIS T 8118). It was 0.26~0.38 µC. This value is less than the reference
value (0.60 µC) in this standard. Therefore, the antistatic performance of theraincoats was confirmed, despite the fact that they did not pass the test in
the standard.
The rubber blade of the scrapers is made of chloroprene. A conductive
coating was painted on the rubber tip and the shaft to measure theresistance of the scraper. It was 2×1012 Ω or more. Therefore, the scrapers
were electrically insulated from the floor of the tank.
Table 2. Static characteristics of the non-conductive components
Component
Shaft coating(clean)Shaft coating(polluted)
GlovesRaincoat(antistatic)

Thickness

VolumeSurface(mm)

resistivity (Ω·m)

resistivity (Ω)

polypropylene

0.5

5.3×1014

6.4×1012

polypropylene

0.5

5.3×1014

1.2×1013

vinyl chloride

1.0

3.9×108

3.9×1012

nylon

0.12

5.8×1012

1.6×1012

Material

3.3. Experimental method for electrostatically charging the
scrapers
Based on interviews with workers, the work using these scrapers was
reproduced and the potential of the metal shaft of the scraper was measured.
For scraping, brushing (Figure 5(a)) motions and rest (Figure 5(b)) wererepeated alternately. To reproduce the atmosphere at the time of thedisaster, experiments were conducted at 10˚C with a relative humidity (RH)of 50%.
3.4. Ignition experimental method
The ability of a spark to cause an ignition near petroleum was examined(Figure 6). Petroleum was poured into a metal pan (60 cm × 40 cm × 10 cm).
A scraper was charged from a DC high voltage power source. A spark was
created on the inner wall of the oil pan near the liquid surface. This
experiment was carried out at 8.2˚C at 21% relative humidity (RH).
4. Results and Discussion
4.1. Potential changes and electrostatic energy in the operation
Six pairs of gloves (used), eight scrapers (two were used; six others wereunused) were tested. The scrapers were tested under both the "clean" and"polluted" conditions. The potential change during these operations wasmeasured.
A typical example of the potential change is shown in Figure 7. During
scraping, the potential barely increased, but the potential did rise themoment there was pause in scraping and the scraper was released from one
hand. As discussed in Section 2, an electrostatic charge was generated
between the gloves and the coating on the scraper’s shaft, and anelectrostatic induction occurred in the metal rod of the shaft from the charge
remaining in the coating when it was released. Charge relaxation was slow
because the resistance of the rubber is high.
A plot of the maximum value of the potential obtained in eachcombination is shown in Figure 8. Items 6 and 7 are used scrapers. Figure 8
shows clearly that a "polluted" scraper had much higher potential than a"clean" one. This was an effect from the petroleum wax components. The
surface resistivity increase from the wax was a factor in the difference in thepotential (as described above).

Grasping the grip with one
hand and the shaft with theother hand. Scrubbing floor
back and forth.

Glove
Grip
Shaft (covered)Electrostatic
meter
Lead wire

Antistatic
shoes
Stainless floor

Scrubbing floor
back and forth(a) Brushing

Pausing in the brushing.
Grasping only the grip with
one hand.

Electrostatic
meter
Lead wire
Antistatic
shoes
Stainless floor(b) Rest

Figure 5. Experiment to determine static electricity generated
during scrapingTemperature:7.5~8.2 ˚C
Humidity:21~24 %RH

Charging
electrode

ChargeDischarge
Shaft
Gloves(vinyl chloride)

Oil panPetroleum
Spark

Figure 6. Ignition test of petroleum with charged scraper

Potential (kV)Rest

Brushing

Figure 7. Potential change of the scraper during scraping

Gloves
GlovesGlovesMaximum potential (kV)GlovesGlovesGloves

C

P

C

P

C

P

C

P

C

Scraper ID

P

C

P

C

P

C

P

C: clean
P: polluted

Figure 8. Difference in potential between "clean" and "polluted"
shafts
The charge generated was 0.26~0.36 µC. The electrostatic energy was
0.51~0.91 mJ. These values were calculated from the capacitance 52~70 pFof the scraper at rest (Fig. 5 (b)) and the maximum potential 4.7 ~ 5.5kV
obtained for the "polluted" scraper. Based on the supposition that theeffective contact area between the shaft and the gloves was 100 cm2, theaverage surface charge density was estimated to be 36 µC/m2 at the
maximum. In theory, the charge density of the conductor surface is 27 µC/m2
at the maximum because it is limited by discharge into the air. (Thedielectric breakdown field of air is 3 MV/m.) However, in this case, thininsulation on the surface of the metal rod suppressed the potential and theelectric field at the surface of the insulating layer. This can cause higher
surface charge density.
4.2. Ignition test
Scrapers with charging potentials of 5 kV, 6 kV, 6.5 kV and 7 kV were
tested for their ability to cause an ignition. The potential of 5 kV or 6 kV
could not ignite petroleum. The potential of 6.5 kV and 7 kV ignited andcaused the petroleum to burst into flames, as shown in Figure 9. Dischargeenergy at ignition was estimated to be 1.3 mJ and 1.5 mJ, respectively.
These values were slightly higher than the minimum ignition energy (0.8mJ) shown in Section 3.1. The reason for this difference could be theinstability of vapor concentrations in an open space. In Section 4.1, themaximum electrostatic energy obtained by simulating the operation was
0.91 mJ, which is higher than the minimum ignition energy (0.8 mJ).
It is notable that in the testimony (see Section 1) of a survivor he said, "Apale blue flame began to spread rapidly.” In this test, after ignition a pale
blue flame on the liquid surface was observed. During cleaning in the tankthere would have been a flammable premixed gas on the sludge surface.

1: Just before ignition
2-4: Flame propagation in premixed gas (blue flame)
5-8: Diffusion combustion (orange flame)

Figure 9. Ignition of petroleum with charged scraper
4.3. Discussion of the high number of fatalities
The number of people killed in this disaster (five) is high. Two of thosekilled could have escaped from the tank but all of their clothing was burned
and they were almost naked when they were found. One of the raincoats ofthe victims is shown in Figure 10. The victims wore these raincoats overtheir work clothes (cotton coveralls). Their clothing was considered to be bothantistatic and nonpolluting. Sample pieces (320 mm × 50 mm) of theseclothes were tested by the A-4 method for the combustibility of fiber products(JIS L 1091). The results of the test showed that they were "gutted" within10 seconds. Therefore these clothes were highly flammable. Theircharacteristics made these clothes easy to burn and caused the spread ofdamage in this disaster. To prevent a reoccurrence of this disaster, antistatic
coverall should be incombustible or flame retardant where there is a risk offire.

Figure 10. Raincoats that the victims wore (antistatic)
5. Prevention of a similar disaster
This disaster was probably caused when volatile components ofpetroleum or sludge formed a flammable mixture and a spark ignited that
mixture. (There is a high possibility that the spark came from a static
electricity discharge from a scraper.) Based on this belief and theinformation gathered about this accident, the prevention of the same kind ofdisaster is discussed below, from the point of view of safety management.
5.1.

Organizing

problems

from

the

viewpointofsafetymanagement
Two points were the main focus: problems with safety management andpreventing a similar disaster. These points are summarized below.
a) This fire occurred during the cleaning of a petroleum tank (a part ofmaintenance).
•

Could the fire have been predicted?

•

If it could not, why could the fire not be predicted?

b) This operation was outsourced (subcontracted).
•

Were the workers aware of the risks?

5.2. Measuring management problems (by factor)
Below is a list of items —human factors, equipment management, work
environment, and management factors— that need to be addressed toprevent a reoccurrence of this type of disaster.
(1) Human factorsa) The workers’ understanding and awareness of the risks•

All workers must be told at a briefing about hazardous materials, ignition
sources, and safe handling methods, etc.

b) Information exchange between workers•

Information about the sources of risk (hazards) and found risks caused by
mistake must be shared with each other.

c) Confirmation of emergency response•

To be prepared for emergencies, all workers must confirm and be certainof their respective roles (evacuation routes, communication procedures,etc.).

d) Compliance with regulations•

All workers must wear regulation clothes and shoes (if necessary,antistatic wear).

(2) Equipment management factorsa) Safe handling of equipment (support equipment: the projectors in thisdisaster)•Equipment must be used according to the manual (compliance with theconditions of use).

•

Equipment must not be modified unnecessarily (in particular, any
modifications that affect explosion protection are strictly prohibited).

•

Equipment must be installed to prevent objects from falling. (Theprojectors were not fixed in place. They must be secured to prevent themfrom falling even if it interferes with work.)•

It is necessary to prevent equipment from igniting, even after a fall(isolated from the sludge, etc.).

•

A substitute for the equipment should be used (Find another equipment
for this objective).

b) Understanding the characteristics of the equipment•

Workers must understand the characteristics of all equipment (especiallyto prevent heating by particular use or misuse).

•

Explosion prevention and fire protection must be considered separately.
These measures must be implemented without fail.

(3) Work environment factorsa) Improvements must be made to the work environment (dark, small or
narrow spaces, unstable scaffoldings, etc.).
b) Securing evacuation routes (including location of projectors)c) Characterization and risk assessment of materials to use•

A prior risk assessment (and confirmation) of the work must be carriedout (with workers).

•

Workers must understand the characteristics of the substances(including the flashpoint, and the risk factors to reach it, etc).
Understanding these characteristics is necessary to anticipate fires, etc.
due to the fall of projectors.

(4) Management factorsa) Safety guidance for workers (not only the confirmation of the work
procedures but the understanding and attention to sources of risk)•

Daily safety management (such as confirmation before work) must not beroutinized.
-­‐

Routine daily meetings decrease awareness of risks.

-­‐

Reconfirm guidance daily. (Absence of a problem yesterday cannot
mean that today is safe.)•

Make certain every worker, regardless of experience, understands theoperation (purpose, intent, and risks of the work).

•

Inform workers of the characteristics of the work environment (safety
equipment, the purpose of the work clothes, etc.) (e.g., reason for wearing
antistatic clothing).

b) Risk confirmation onsite (understanding of the sources of risk)•

Not only a briefing in the office, but onsite “toolbox” meetings should be
held. In those meetings, every worker should confirm and share
information on risk factors (hazards).

•

Supervisors must guide, educate and monitor workers at the operationsite (also related to the revised Industrial Safety and Health Act).

c) Confirmation of supervisor's location•

Supervisors must monitor the work onsite (in order to discover early on
the risks of the work in progress).

•

Supervisors must not leave the monitoring location during important
operations on safety management (or delegate this responsibility, such asappointing an assistant supervisor).

•

One supervisor must not be responsible for multiple areas that requiresafety management. (It is easy to miss administrative issues).

d) Discussion of emergency action•

Securing evacuation routes, procedures for escape, firefighting, and therescue of workers must be confirmed.

5.3. Support based on the Industrial Safety and Health Act
Article 31(2) of the revised Industrial Safety and Health Act took effect inApril 2006, and prescribes the duties of an “orderer.” The duties(responsibilities) are that when an order is given for modification, renovation
or cleaning of equipment or product or to handle hazardous chemicals, theorderer must notify the contractor about occupational safety and healthissues to prevent industrial accidents involving these chemicals (informing
by ordering party, management duties). For reference, an accident that
occurred before the act was revised, shows the importance of the duties, as
follows. The duties are the agreement of the importance of informing thecontractor and ensuring that these are carried out.
Reference: Revised Industrial Safety and Health Act (Article 31(2))
The orderer of works for alteration of facilities manufacturing or handlingchemical substances or preparations containing chemicals and defined by
the Cabinet Order, or other works as provided for by the Ordinance ofMinistry of Health, Labour and Welfare, shall take necessary measuresconcerning said substances to prevent workers of contractors of the said
works from industrial accidents.
5.4. Problems clarified and future issues
The problems clarified by this investigation and future issues are
summarized as follows.
1) Problems clarifieda) The importance of finding and identifying risks in the workplace (hazards
overlooked in the planning).
b) The necessity of prior risk assessment for maintenance work.
2) Future issuesa) Setting up methods to identify hazards liable to be overlooked in the risk
assessment of normal operations.
In this disaster, the risks during cleaning were overlooked.

•

Risks during maintenance operations are hard to identify at the design(or planning) stages.

•

The risk surrounding operational mistakes were assumed (and discussed)
during the design, but risks in abnormal situations, such as maintenance(cleaning in this case), were overlooked and safety measures were notdiscussed.

b) Introduction of standards for risk identification•

Considering deviations from standard procedures (for example, misuse of
equipment, operational mistakes) is a good starting point for risk
analysis (identification and assessment).

•

Risks could be identified by using guide words, as follows:"Equipment”, “Operation”, “State” changed by the operationc) The importance to draw up a safety check procedure at the workshop•

The hazard to identify by inspections must be considered on the risk
assessment.

•

Explosion prevention must be distinct from fire protection.

6. Summary
The ignition source of the fire disaster in a petroleum tank undercleaning was investigated. As a result, there is a possibility that the fireoccurred because the scrapers that were used had an insulating coating thatwas charged electrostatically by the simple gesture of gripping the handle.
The metal parts of the scraper received a high potential by electrostaticinduction. Finally, an electrostatic spark generated from the metal parts
ignited the volatile components of the sludge.
To prevent a reoccurrence of this kind of disaster, it is important to avoid
using cleaning tools that have an insulating coating. In addition, to mitigate
potential harm to people, clothing should be flame retardant orincombustible.
Before beginning this kind of operation, an appropriate risk assessment
is important. This will be useful when considering the items listed in Section
5 (above).