Causative factors for vapour cloud explosions determined from past-accident analysis

EINEMANN

J. LossPrev. ProcessInd. Vol. 8. No. 6. pp. 355-358, 1995 Elsevier Science Ltd

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0950-4230(95)00042-9

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Causative factors for vapour cloud explosions determined from past-accident analysis A. Koshy, M. M. Mallikarjunan*

and K. V. Raghavan

Cell for Industrial Safety & Risk Analysis, Central Leather Research Institute, Madras 600 020, India

Past-accident analysis shows that most dangerous incidents are related to process operations. Often these operations are carried out under high pressures and/or high temperatures. The consequences, therefore, are significant. A scientific analysis of past accidents which led to vapour cloud explosion has been performed. The analysis has provided vital information for most probable accident scenarios for a new situation. Factors such as chemical characteristics, its release mode, time etc. show trends and relationships for the occurrence of vapour cloud explosions. Keywords: past accidents; vapour cloud explosions; causative factors

Extensive information on past accidents can also be obtained from data banks maintained by several international organizations. Table 1 lists some of the more significant VCE incidents in petroleum and petrochemical units. VCEs occur when energy arising from the ignition of a flammable cloud is converted into shock waves due to rapid equilibration of high-pressure gas with the environment. The heat release causes expansion of reaction products which push unreacted mixture in front of them. If the flame velocity is high enough, successive compression waves will overtake one another to form shock waves. A characteristic feature of explosions is blast. A number of VCE models are available to predict blast effects. They are based on TNT, self-similarity, fuel air cloud, piston and shock-wave concepts. The strengths and weaknesses of these models have been reported6.

Chemical accidents have occurred at regular intervals. After Flixborough, it was thought that the lessons from such accidents would have had worldwide impact, but the Mexico City and Bhopal accidents, both on a scale to dwarf Flixborough, suggest that there is still much to be learned systematically from past accidents if new technologies or chemical plants are to be designed and commissioned at a greater level of confidence with regard to safety. The progression and consequences of past accidents provide latent information which is difficult to generate or simulate either through laboratory or field investigations. A scientific analysis of past accidents can provide vital information to determine the most probable accident scenarios for new situations. This is highlighted in this paper for vapour cloud explosions. However, it should be noted that the hazard and risk assessment techniques have inherent limitations in fully reconstructing past-accident situations due to the complexities introduced by event interactions and multicomponent/ multiphase systems encountered in real situations.

Analysis of past accidents It is interesting to analyse past accidents in terms of chemical characteristics, release mode, time factor, ignition sources, and developmental aspects of vapour clouds. These are discussed below.

Vapour cloud explosions Plant equipment failures can lead to the sudden release of hazardous chemicals. The release consequences generally include heat radiation, explosions and toxic gas dispersions. Vapour cloud explosions (VCE) are the predominant cause of the largest losses in the chemical and petrochemical industries. More than hundred VCEs have been reported to have occurred from 192 1-199 11-5. * To

whom

correspondence

should

Nature of chemicals

Statistics for past VCE incidents (Figure I) show that hydrocarbons with three or four carbon atoms accounted for nearly 45% of known incidents. A number of VCE incidents have been reported involving hydrogen and other reactive chemicals such as acetylene and ethylene. The recent trend, due to techno-economic consider-

be addressed

355

356 Table 1

Causative VCE incidents

in the petroleum

Year

Location

1948

Ludwigshafen, Germany Jackass Nevada, USA Raunhein, Germany Port Hudson, Missouri, USA Lynchburg, Virgina, USA St Louis, Illinois, USA Flixborough, UK Enschede, The Netherlands Ufa, West Siberia, USSR Nagothane, Maharashtra, India

1964 1966 1970 1972 1972 1974 1980 1989 1990

factors

for VCEs: A. Koshy et al.

and petrochemical

industries

Chemical involved Dimethyl

ether

Spill quantity (tonnes)

Energy release (TNT equivalent, tonnes

30.4

20-60

Hydrogen

0.10

-

Methane

0.5

I-2.0

Propane

23.0

Propane

8.8

50

-

Propylene

n.a.

n.a.

Cyclohexane

30.0

15-45

Propane Natural

0.1 gas

C2-C3 Hydrocarbon

n.a.

-

-

16.0

IO-20

PO01 fir. powibility

Llauid

and

Ne”,r*, diaperriot! VCE

Figure 1

Checklist

for identification

of credible

scenario

ations, is to establish petrochemical plants with larger capacity, higher working pressures, higher process temperatures and greater inventory and hold-up. Accordingly, chemical accident losses have increased in both frequency and severity. It is interesting to note that nearly 75% of the VCEs that have occurred in various parts of the world occurred in petrochemical units.

*..

wilh remote chances

its normal boiling point contains a large quantity of latent heat which can be instantaneously released in a flash vaporization. Most mathematical models do not precisely predict the mode and rate of energy release in chemical accidents. Such information from past accidents is invaluable in hazard analysis. The factors involved in energy release are complex. It is interesting to note that, as the size of a leak decreases, the probability of explosion diminishes and the chance of a fire event increases. In addition, most

Causative

factors

for VCEs: A. Koshy

25.3

XYLENE

H2 SYNGAS ISOPRENE DIMET

ETH 0

Figure 2

5

10

15

I

/

20

25

30

Nature of chemicals versus percentage of VCE incidents

unconfined vapour cloud explosions (UVCE) have occurred in clouds generated due to turbulent momentum jet-type chemical releases. Notable examples include the Flixborough and Port Hudson accidents. Ignition sources An analysis of past incidents shows that vapour cloud ignitions predominantly resulted from (a) sparking electric devices, (b) hot surfaces such as extruders and hot steam lines, (c) friction between moving parts of machines, and (d) open fire and flame in furnaces, heaters and other high-temperature equipment. Their relative contribution is shown in Figure 2. In most cases, the ignition sources were fairly close to the source of the leak, as indicated in Table 2. Time factor for hazard development The time factor for hazard development is important both with regard to the rate of energy/material release and the warning time available to emergency teams for

Table

2

Explosions involving hydrocarbons Distance from release point (m)

VCE incident

ignition source

Abqaiq, Saudi Arabia, 15 April 1978

Flare (low level)

460

Commerce City, Colorado, USA, 3 October 1978

Heater

40

Enschede, The Netherlands, 26 March 1980

Heater

Near by

Pampa, Texas, USA, 14 November 1987

Boiler

50 (upwind)

Baton Rouge, Louisiana, USA 24 December 1989

Furnace

300

et al.

357

taking counter-measures. Past accidents provide vital information on the time factor. An analysis of VCEs (Figure 3) shows that a large number of VCE incidents occurred within 3 min after the release. A closer look at those VCEs which occurred in less than a minute shows two distinct characteristics, i.e. (a) most of the chemicals involved are reactive in nature, and (b) the mode of chemical release was either as pressurized gas or as a two-phase mixture. The relatively large number of VCEs involving hydrogen/air and other reactive chemical is due to their wide flammability range. This explains their tendency to instantaneously ignite on release, precluding the need for a dispersion process. For example, when diluted with inert gas, hydrogen (flammable limits between 4% and 75.6%) can bum with less than 5% oxygen. A comparison of burning velocities of flammable chemicals shows that hydrogen has a higher rate compared with hydrocarbon vapours (Table 3). It can be ignited by low-energy sparks as it requires only about one-tenth of the energy that is required by hydrocarbon vapours. In the case of VCEs occurring later than 2 min after chemical release, the chemicals are generally non-reactive and flammable. Dispersion to the atmosphere is therefore most likely and the ignition process has to occur before the cloud drops below its flammable concentration limits. Dispersion calculations for dense gas clouds reveal that the lower flammability limits can be reached within the first few minutes. Assuming a wind speed of 2-3 m SK’, the cloud would have travelled 120180 m before ignition. It is therefore expected that the ignition sources were within this distance. The previous section has shown that ignition sources do occur within these distances. Effect of release quantity on spread of the vapour cloud Most chemical releases are confined within the plant installation due to the presence of obstacles. Accordingly, the cloud dimensions and contours are determined by the prevailing configuration at plant installations. The normal depth of clouds varies from 1-2 m. However, gas release in open terrain, as in the case of accidents during transportation and in storage terminals, causes cloud to spread to very large areas. Past incidents have shown that these vapour clouds can travel several hundreds of metres, and flash fires are more probable in these cases. ‘:E_1IC_E/_OCOMOTlVtS

ELECTRICAL

1: 5

Figure3 Relative percentage past VCE incidents

BOILER

?

i

of ignition sources involved

in

Causative factors for VCEs: A. Koshy et al.

358

Table 3 Explosion properties of hydrogen and reactive compounds eric conditions

in comparison

with other saturated

hydrocarbons

Auto-ignition temperature (“C)

Laminar burning velocity (m s-l)

Minimum energy

Gas or vapour

Flammability limits (vol%)

Hydrogen

4.0-75.6

560

3.25

0.019

1.5-100 2.7-34 2.0-11.7 1.7-36

305 425 455 170

1.55 0.735 0.512 0.486

0.007

515 450

0.476 0.464

0.252 0.25

Hydrocarbon

ignition

(MJ)

(reactive)

Acetylene Ethylene Propylene Diethylether Hydrocarbon

at atmosph-

(non-reactive

but flammablel

Ethane Propane

3.0-I 5.5 2.1-9.5

% OF EVENTS

LI

o-1

0.190

2-3

4 - 10

24 -35

TIME (MTS) Figure4 Percentage of VCE occurrences as a function of time available before ignition

Effect of barricades on explosion The presence of obstructions such as vessels, pipe racks, supporting structures, buildings etc. enhances the overpressure of an explosion’. Very severe damage has been reported within a short distance from the release point. Control rooms, piping vessels, and, in extreme cases, the entire plant installation were destroyed.

Devising a checklist Based on the above analysis, it is possible to devise a checklist with appropriate logic drawings to help to identify possible routes by which VCEs can take place. Two probable routes have been identified in Figure 4. In the first route, a release of the chemical from pressurized storage as gas or a two-phase mixture is assumed. The result of such a release is a pre-mixed vapour cloud capable of immediate ignition. Depending

on the reactivity of the chemical and the temperature of its release, it may instantly ignite and result in an explosion if released in semi-confinement such as pipe racks, equipment, etc. For the second route, the pressurized or refrigerated liquids spill and then form a vapour cloud. Dispersion of such clouds takes a finite time and they may travel some distance before the concentration falls to within its flammable limits. If the cloud encounters an ignition source (which in this case needs to be a high-energy source such as furnaces, boilers and welding points etc.), then ignition capable of sustaining combustion will results. The combustion may result in an explosion if there are obstacles in its path. These conditions are also indicated in Figure 4. Past incidents have shown that the time taken for an explosion to occur by this route may be longer than 2 min.

Conclusion Analysis of past accidents has shown that factors such as chemical characteristics, release mode, time etc., show definite trends and relationships for the occurrence of vapour cloud explosions.

References Lenoir, E. M. and Davenport, J. A. Proc. Safety Prog. 1993, 12, 12-33

Kletz, T. E. ‘Learning from Accidents in Industry’, Butterworths, Tonbridge, UK, 1989 Pietersen, C. M. IChem E Loss Prev. Bull. 1985, 64 Marshal, V. C. ‘Major Chemicals Hazards’, Wiley, London, 1987 Lees, F. P. ‘Loss Prevention in Process Industries’, Butterworths, London, UK, 1980 Harrison A. J. and Eyre, J. A. Cornbust. Sci. Technol. 1986, 52, 121-137 Zecuwen, J. P., Van Wingerden, C. J. M. and Dauwe, R. M. in ‘Proceedings of the 4th Symposium (International) on Loss Prevention and Safety Promotion in the Process Industries, Harrogate, UK, pp D20-D29 Giesbrech, H., Hess, K., Leuckel, W. and Maurers, B. German Chem. Eng. 1981, 4, 305-325