Dust Explosions in the Process Industries Second Edition phần 4 doc

Case histories 189

Figure 2.25 Sequence of nine impact energy pulses from nine successive explosions in the Harbin

Linen Textile Plant, Harbin, P. R. China, 15th March 1987, postulated on the basis of a seismic record

of the event (From Xu Bowen et al., 1988)

190 Dust Explosions in the Process Industries

2.9.3

EXPLOSION INITIATION AND DEVELOPMENT, SCENARIO 2

This alternative scenario originates from the investigation of Zhu Hailin (1988), who

found evidence of an initial smouldering dust fire caused by a live 40 W electrical portable

light lamp lying in a flax dust layer of 6-8 cm thickness in a ventilation room. He also

found evidence of flame propagation through the underground tunnels for the dust

collection ducting. On the basis of his analysis, Zhu suggested that the explosion was

initiated in the eastern dust collectors (5 in Figure 2.24) from which it transmitted to nine

units of the central dust collecting plant (1 and 2 in Figure 2. 24) via the ducting in the

underground tunnels. Severe room explosions were initiated when the ducting in the

tunnel ruptured, and the resulting blast dispersed large quantities of dust in the

workrooms into explosible clouds that were subsequently ignited. From the eastern dust

collectors the explosion also propagated into the underground flax stores.

It is not unlikely that even this scenario could be developed further in such a way as to

agree with the evidence from the seismic recording.

2.9.4

ADDITIONAL REMARK

The investigation of the Harbin disaster exposed the great difficulties in identifying the

exact course of events of major explosions creating massive damage. In addition to causing

pain and grief, loss of life also means loss of eye witnesses. Besides, the immediate need

for fire fighting and rescue operations, changes the scene before the investigators can

make their observations. Also, the explosion itself often erases evidence, e.g. of the

ignition source.

This problem was also shared by the experts who investigated the Harbin explosion, and

it seems doubtful that the exact course of events will ever be fully resolved.

However, the Harbin disaster unambiguously demonstrated the dramatic consequences

of inadequate housekeeping in industrial plants where fine dust that can give dust

explosions, is generated.

2.1 0

FIRES AND EXPLOSIONS IN COAL DUST PLANTS

2.1 0.1

METHANE EXPLOSION IN 17000 rn3 COAL SILO AT ELKFORD, BRITISH

COLUMBIA, CANADA, IN 1982

As mentioned in Section 1.5, handling and storage of coal can, in addition to the dust

explosion hazard, also present a gas explosion risk, due to release of methane from some

types of coal. An account of such an explosion was given by Stokes (1986).

The silo of height 48 m and diameter 21 m that exploded, was used for storage and

load-out of cleaned, dried metallurgical coal. The capacity of the silo was 15000 tonnes.

Case histories 19 1

Prior to the explosion accident, a methane detector had been installed in the roof of the

silo. The detector activated a warning light in the silo control room when a methane

concentration of 1% was detected, and an alarm light was activated when detecting 2%

methane. A wet scrubber was located in the silo head house to remove dust from the

dust-laden air in the silo during silo loading. A natural ventilation methane stack was also

located in the silo roof to vent any build-up of methane gas from the silo.

The explosion occurred early in the morning on 1st May, 1982, devastating the silo roof,

head house, and conveyor handling system. Witnesses stated that a flash was noticed in

the vicinity of the head house, followed seconds later by an explosion which displaced the

silo top structures. This was followed by an orange-coloured fire ball that rolled down the

silo walls and extinguished prior to reaching the base of the silo. Fortunately, neither

injury nor death resulted, and damage to surrounding structures was minimal, although

large blocks of concrete and reinforcing steel had been thrown several hundred metres

from the silo. However, the plant itself had suffered substantial damage.

The silo was full of coal 24 hours prior to the explosion. During the evening before the

explosion, 10 OOO tonnes of coal were discharged. At the same time, conveying of

deep-seam coal into the silo commenced and continued until the explosion occurred. At

the time of the explosion, there were approximately 12 300 tonnes of coal in the silo, of

which 7600 tonnes were deep-seam coal. Testing had shown that this quality of coal has a

high methane emission rate and produced a low volatile coal dust. Clouds in air of this dust

could not be ignited unless the air was mixed with methane.

The ignition source was not identified, but the following three possible sources were

considered:

0 Spontaneous combustion of the stored coal.

An electrical or mechanical source.

Hot coal from the thermal dryer.

During ten years of operation, with coal being stored in different environments for

varying lengths of time, spontaneous combustion had never presented a problem, and

consequently was not considered to be a probable source of ignition. During demolition of

the damaged silo, all electrical and mechanical components were recovered and inspected

and did not show any evidence of being the ignition source. Stokes (1986) did not exclude

the remaining possibility that hot coal from the thermal dryer was the source of ignition.

2.1 0.2

METHANVCOAL DUST EXPLOSION IN A COAL STORAGE SILO AT A

CEMENT WORKS AT SAN BERNARDINO COUNTY, CALIFORNIA, USA

This incident was reported by Alameddin and Foster (1984). A fire followed by an

explosion occurred inside a coal silo of 900 tonnes capacity while the silo was nearly

empty, and the remaining 85 tonnes of coal were being discharged. Prior to the explosion,

a hot-spot of 0.6 m X 1.0 m had been detected on the lower part of the silo wall by means

of an infrared heat detector. The hot-spot originated from smouldering combustion in the

coal in the silo. This process liberated methane, carbon monoxide and other combustible

gases from the coal. The explosion probably resulted from ignition of a mixture of

combustible gas and airborne coal dust in the space above the bulk coal by the

192 Dust Explosions in the Process industries

smouldering fire or glow when it reached the surface of the coal deposit. (See Figure 1.9 in

Chapter 1. )

It was concluded that the supply of carbon dioxide from the top, which was used for

suppressing the fire and preventing explosion, was insufficient to prevent the development

of an explosible atmosphere in the space above the bulk coal.

In order to prevent similar accidents in the future, it was recommended that a carbon

dioxide system be installed in both the top and bottom of the coal silo. Sufficient inerting

gas should be added for development of a slight positive pressure inside the silo. The

inerting gas must be of sufficient quantity to insure a nonexplosible atmosphere above the

coal and sufficient pressure to prevent a sudden inrush of fresh air into the silo.

2.1 0.3

GAS AND DUST EXPLOSION IN A PULVERIZED COAL PRODUCTION/

COMBUSTION PLANT IN A CEMENT FACTORY IN LAGERDORF IN

F. R. GERMANY, IN OCTOBER 1980

According to Patzke (1981), who described this explosion accident, the explosion occurred

while coal of about 30% volatiles was milled at a rate of 55 tonnes per hour. The start-up

of the cement burner plant followed a compulsory break of at least 20 minutes of the

milling operation to allow all airborne dust to settle out. A few seconds after the main gas

valve had been opened, there was a violent explosion. The probable reason was a failure

in the system for electric ignition of the gas. Within the period of six seconds before the gas

valve was reclosed automatically, about 1 m3 of gas had been discharged to the

atmosphere of the hot combustion chamber and become mixed with the air to an

explosible gas cloud. The temperature of the walls of the chamber was sufficiently high to

ignite the gas, and a gas explosion resulted. The blast and flame jet from this comparat￾ively mild initial explosion was vented into the milling system where a large, turbulent dust

cloud was generated and ignited, resulting in a violent secondary dust explosion.

Various parts of the milling plant, some unvented and some vented, had all been

designed to withstand the pressure generated in an extensive dust explosion. Furthermore,

a passive device for explosion isolation of the type shown in Figure 1.82 in Chapter 1 had

been installed upstream of an electrostatic dust filter.

Apart from deformation of some explosion vent doors, the dip tubes of two cyclones,

and the coal feeder upstream of the mill, the plant had been able to withstand the

explosion without being damaged. The passive explosion isolation device effectively

protected the electrostatic filter from becoming involved in the system.

2.1 0.4

FURTHER EXPLOSION/FIRE INCIDENTS INVOLVING COAL

Anderson (1988) gave a step-by-step account of the process of extinction of a smould￾ering fire in a 50 m3 coal dust silo in Arvika in Sweden, in August 1988. It was necessary to

pay attention to the risk of explosion of combustible gases driven out of the coal by the

heat from the fire.

Case histories 193

First gaseous carbon dioxide was loaded into the silo at the top to build up a lid of inert

atmosphere immediately above the coal deposit. Then all the coal was discharged carefully

through the exit at the silo bottom. In this particular case, supply of carbon dioxide at the

silo bottom was considered superfluous.

Wibbelhoff (1981) described a dust explosion in a coal dust burner plant of a cement

works in F. R. Germany, in March 1981. Prior to the explosion, an electrical fault had

caused failure of an air blower. The explosion occurred just after restart of the repaired

blower. During the period in which the blower was out of operation, dust had accumulated

on the hot surfaces inside the furnace and ignited, and as soon as the blower was restarted,

the glowinghurning dust deposits were dispersed into a dust cloud that exploded

immediately.

Pfaffle (1987) gave a report of a dust explosion in the silo storage system of a pulverized

coal powder plant in Dusseldorf, F. R. Germany, in July 1985. The explosion occurred

early in the morning in a 72 m3 coal dust silo. The silo ruptured and burning material that

was thrown into the surroundings initiated a major fire, which was extinguished by means

of water. Fortunately no persons were killed or injured in this primary accident. However,

during the subsequent cleaning-up process, a worker was asked to free the damaged silo of

ashes by hosing it down with water. It then appeared that a glowing fire had developed in

the dust deposit that was covered by the ashes. The worker had been warned against

applying the water jet directly to the smouldering fire, but for some reason he nevertheless

did this. The result was an intense dust flame that afflicted him with serious third degree

burns. The smouldering fire was subsequently extinguished by covering its surface with

mineral wool mats, and subsequently soaking the whole system with water containing

surface-active agent.

2.1 1

DUST EXPLOSION IN A SILICON POWDER GRINDING

PLANT AT BREMANGER, NORWAY, IN 1972

In this serious explosion accident, five workers lost their lives and four were severely

injured. The explosion that occurred in the milling section of the plant, was extensive,

rupturing or buckling most of the process equipment and blowing out practically all the

wall panels of the factory building. Figure 2.26 gives a flow chart of the plant. Figure 2.27

shows the total damage of the entire grinding plant building, whereas Figure 2.28 gives a

detailed view of the extensive damage.

Eye-witnesses reported that the flame was very bright, almost white. This is in

accordance with the fact that the temperature of silicon dust flames, as of flames of

aluminium and magnesium dust, is very high due to the large amounts of heat released in

the combustion process per mole of oxygen consumed. (See Table 1.1 in Chapter 1.).

Because of the high temperature, the thermal radiation from the flame is intense, which

was a main reason for the very severe burns that the nine workers suffered.

The investigation after the accident disclosed a small hole in a steel pipe for conveying

Si-powder from one of the mechanical sieves to a silo below. An oxygedacetylene cutting

torch with both valves open was found lying on the floor about 1 m from the pipe with the

194 Dust Explosions in the Process Industries

Figure 2.26 Flow chart of dry part of plant for production of refined silicon products at Bremanger,

Norway. The grinding plant that was totally damaged in the explosion in 1972 is shown to the right in

the chart

Figure 2.27 Totally destroyed milling section of

silicon powder production plant at Bremanger, Nor￾way, after the dust explosion in October 1972