Some problems in an assessment of the Some problems in an assessment of the consequences of a fire and an explosion during consequences of a fire and an explosion during

the multicomponent mixture unknown the multicomponent mixture unknown composition release.composition release.

    

  Melania Pofit-SzczepańskaMelania Pofit-Szczepańska

The Main School of Fire Service, Firefighting and Rescue Operation Department The Main School of Fire Service, Firefighting and Rescue Operation Department Słowackiego Street 52/54Słowackiego Street 52/5401-629 Warsaw, Poland01-629 Warsaw, Poland

      

The important part of the safety report is the analytical part in which fire, explosion and toxicity hazards are analyzed as well as the consequences of these releases to the atmosphere. If the released medium is the substance of a known composition and the parameters of process are known too, the calculation of the consequences of these releases is not difficult. The descriptions of the way of procedure can be found in the literature.If however, the mixture of unknown composition leaks through the rupture of the pipeline or vessel the assessment of the consequences of these releases is more complicated.

Consequences of hazardous Consequences of hazardous substances releasessubstances releases

TThe method of thermodynamic he method of thermodynamic substitutesubstitute

The method has been applied to the assessment of fire and explosion consequences during the rupture of the pipeline and the release of the slops mixture of unknown composition to the atmosphere in a Polish refinery.This method called “the method of thermodynamic substitute”, is one of the methods used in the calculations different parameters dealing with release of dangerous substances.

TThe method of thermodynamic he method of thermodynamic substitutesubstitute

The single component as a thermodynamic substitute is used very often. Of course, the most reliable use of a single component consequence model results when the single component simulates the behaviour of the multicomponent fluid over all potential conditions from storage conditions to ambient atmospheric conditions. Naturally, this involves an intimate knowledge of the thermodynamic behaviour of the mixture.

The single component models are used very often because they are much easier and generally run faster. Considering D.W. Johnson’s example : the release of methane – pentane vapour from a large vessel operating at 3 bars and 65oC and a release of pure propane in the same conditions. The release will escape through a 15 cm diameter hole made at the side of the vessel. The release is angled 45o above horizontal. The release rate will be relatively constant since pressure and temperature in the large vessel will change slowly with time. In table 1 the computed results are given and fig. 1 shows the LFL contours. With many uncertainties the agreement is good. Thus it appears possible to model release.

Single component modelSingle component model

Downwind Distance [m]

Hei

ght

abov

e G

rade

[m

]

- 15 0 15 30 45 60 75 90 105 120

90

75

60

45

30

15

0

Fig. 1 Dispersion of vapour releases to the lower flammable limit

Item Propane C1-C5Vapour production [kg/s] 15,7 15,8

Distance to LFL [m] 30 30

Height above grade at LFL [m] 30 30

Table 1.

road 1

road

A

32

1

Fig. 2 The arrangement of the installation and the location of vessels

Fig. 3 Visible damage on the pipeline which cause the realistic accidental spill

 •Problem: The refinery, division-slops installation. An arrangement of the installation and the cylindrical vessels 1, 2, 3 of a division are shown in the fig 2. The road A, road 1and cross-road A1 can be seen. Tank cars waiting for the loading of asphalt and their position when fire and explosion took place are marked in yellow. The area of an enveloped accident amounted to about 5000m2. The fill of vessels: 3 – 18%, 2 – 26%, 1 – 80%.•An assumed course of the incident: At first it was a small leak through a small hole. It was a long time before the slops were released through 0.1m diameter hole in the following conditions: t = 50oC, p = 0.5 bar. The part of slops according to their density were absorbed by the concrete bottom of pipelines duct or by the thermo-isolation of pipelines. The lightest fraction mixed with air generated the cloud of slops. Can be assumed that the released quantity of slops was larger than 150 tons (calculations). Data published in a literature indicate that the generation of cloud during the realistic accidental spill is possible if the flow out is above 100 kg of relative non-reactive fuel (hydrocarbons).

Case studyCase study

road 1

road

A

32

1

Case studyCase study

The following conditions were in the time of accidental spill:• night,• F - Pasquille class,• T = - 2,7oC• Vv = 2 m/s.

In these conditions, the direction of wind had less influence on thecloud propagation than the buoyant forces. The flammable cloud haveencountered an ignition source probably somewhere in the vicinityof the tank car 1 /fig. 2/ in the form of low energy source /damagedelectric installation of tank car 1/.In order to analyse the development offire and explosion, two variants of the procedure have been discussed.In the basis on the information received from the rafinery was assumedthat the released mixture had the C1 to C8 composition and components

were in equivalent concentrations.

For C 1 to C 5 For C 6 to C 8

n-butane could be the substitute C 1 to C 5

of the fraction 

n-heksane could be the substitute C 6 to C 8

of the fraction /liquid fraction/

from the following causes: 

• the relative density of n-butane ≈ 2.0• the relative density of released

gases mixture ≈ 1.68• Lower flammability limit

of n-butane ≈ 2.21%• Lower flammability limit

for the mixrure ≈2.36%

• the relative density of n-heksane ≈ 3.0• the relative density of released

liquid mixture ≈ 3.4• Lower flammability limit

of n-heksane ≈ 1.52%• Lower flammability limit

for the mixrure ≈1.61%

Case studyCase study

Case studyCase study

Assumptions applied in calculations:

- fuel-air mixture burns in the way that no damaging overpressure is generating /flash fire/

- generated explosion is the deflagration

- dispersion of the released mixture occurs in two types of surroundings:

a) in obstructed environment, the vapour cloud is located in space between dike area and vessels and between the vessels 3 and 2 and pipelines ducts /fig. 2/, where the ruptured pipeline /fig. 3/ was situated

b) in open space – the flammable mixture covers the area about 5000m2: the area of pipelines duct – cross-road A1 – a space outside of the cross-road – the space round tank cars waiting for the loading of asphalt. 

road 1

road

A

32

1

The generated cloud was spreading down from SE direction to NW.

- The area of the tank car was 162m2 (18m x 3m x 3.5m).

-- The volume: 567m3.

- The area of pipelines duct was about 460m2 (46m distance from the realistic accidental leak to cross-road marked A1/fig 2/.

- The open area (non-built) of accident was 1500m2.

In tables 2-7 can be seen some results of the calculations: an overpressure, positive-phase duration of explosion and the energies of combustion at different

Case studyCase study

Case studyCase studyDistances from accidental leak : 10m, 30m, 48m, 68m and 100m were considered and

they made calculations of the explosion parameters possible in the following places:-

- 10m – the nearest distance from an accidental leak to the dike area

- 30m – the distance from an accidental leak to the vessel 3

- 48m – distance from the accidental leak to the cross-road A1

- 68m – distance from the accidental leak to the place where probably the piloted ignition occurred

- 100m – the distance from the accidental leak to the place faraway 30m outside of the cross-road A1 /near tank car 1/ /fig. 1/

On the basis of the technical documentation the mass of the accidental leaks was determined:

- 6,000 kg

- 10,000 kg

- 20,000 kg

EXPLOSION PARAMETERS OF RELEASED SLOPS IN A EXPLOSION PARAMETERS OF RELEASED SLOPS IN A FUNCTION OF THE DISTANCE FROM AN ACCIDENTAL FUNCTION OF THE DISTANCE FROM AN ACCIDENTAL

LEAKLEAK

Table 2. N-butane – thermodynamic substitute, obstructed environment.

Real distance from accidental leak [m]

Overpressure [bar] Positive-phase duration

of overpressure [s]

Energy of combustion during

of explosion [MJ] leak 6,000 kg

10 0.20 0.12 10,000 30 0.18 0.13 10,000 48 0.12 0.13 10,000 68 0.08 0.13 10,000 100 0.06 0.13 10,000

leak 10,000 kg 10 0.17 0.11 16,600 30 0.10 0.10 16,600 48 0.08 0.09 16,600 68 0.06 0.09 16,600 100 0.04 0.09 16,600

leak 20,000 kg 10 0.19 0.13 23,320 30 0.19 0.12 23,320 48 0.12 0.11 23,320 68 0.02 0.11 23,320 100 0.01 0.10 23,320

Table 3. N-hexane – thermodynamic substitute, obstructed environment.

Real distance from accidental leak [m]

Overpressure [bar] Positive-phase

duration of overpressure [s]

Energy of combustion during of explosion

[MJ] leak 6,000 kg

10 0.20 0.11 7,500 30 0.17 0.07 7,500 48 0.15 0.07 7,500 68 0.07 0.07 7,500

100 0.04 0.07 7,500 leak 10,000 kg

10 0.20 0.13 12,495 30 0.18 0.09 12,495 48 0.12 0.08 12,495 68 0.05 0.08 12,495

100 0.03 0.07 12,495 leak 20,000 kg

10 0.23 0.17 24,990 30 0.18 0.15 24,990 48 0.12 0.12 24,990 68 0.09 0.11 24,990

100 0.03 0.10 24,990

Table 4. N-butane – thermodynamic substitute, open, non-built environment.

Real distance from accidental leak [m]

Overpressure [bar]

Positive-phase duration of

overpressure [s]

Energy of combustion during of explosion [MJ]

leak 6,000 kg 10 30 48 68 100

0.001 0.94 122,186

leak 10,000 kg 10 30 48 68 100

0.001 1.11 203,728

leak 20,000 kg

10 30 48 68 100

0.001 1.40 407,456

Table 5. N-hexane – thermodynamic substitute, open, non-built environment.

Real distance from accidental leak [m]

Overpressure [bar] Positive-phase duration

of overpressure [s]

Energy of combustion during of explosion

[MJ]

leak 6,000 kg 10 30 48 68 100

0.0015 0.96 91,720

leak 10,000 kg 10 30 48 68 100

0.002 1.20 152,791

leak 20,000 kg 10 0.019 30 0.018 48 0.008 68 0.008 100 0.008

1.58 305,582

Table 6. Summary comparison some results of the calculations of the range cloud vapour explosion /ST – n-hexane/

Table 7. Summary comparison some results of the calculations of the range cloud vapour explosion /ST – n-butane/

Type of environment Energy of combustion

[MJ] Leak [kg]

Range of explosion [m]

7,500 6,000 42.03

12,495 10,000 49.83 obstructed

/ with obstacles/ 24,990 20,000 62.79

91,720 6,000 96.84

152,791 10,000 114.79 open

/non-built/ 305,582 20,000 144.63

Type of environment Energy of combustion

[MJ] Leak [kg]

Range of explosion [m]

10,000 6,000 46.26

16,600 10,000 54.78 obstructed

/ with obstacles/ 23,320 20,000 61.35

122,186 6,000 106.55

203,728 10,000 126.35 open

/non-built/ 407,456 20,000 159.19

In the basis of the technological data and the analysis of the run of fire which had taken place before the explosion, you can explain the relationship: “fire–explosion–consequences”. The fire of slops cloud have started at about two o’clock at night when the cloud had already propagated for about 70 m towards the three tank cars. About 2 min after fire the first explosion took place which almost completely damaged the vessel no.3 filled only in 18 % /photo 1-2/.

The analysis of resultsThe analysis of results

Photo 1. Deformation of vessel no.3 after the explosion with the

visible displacement

Photo 2. Deformation of vessel no.3, visible detachment from the foundations

as a result of an explosion

The vessel no.2 /filled 26 %/ was damaged by the second explosion after 45s later /photo 3/.

Probably, this time was needed to form explosive mixture because the hydrocarbons have the narrow flammability limits /about 1–10%/. The vessel no.3 filled with benzol recovery oil in 80 % was less damaged /photo 4/.

The analysis of resultsThe analysis of results

Photo 3. Deformation of vessel no.2, visible detachment from the foundations and

displacement outside concrete wall

Photo 4. Deformation of vessel no.1

ConclusionsConclusions

The safety reports and firefighting-rescue plans should contain: • characteristics of hazardous materials• indentification of the threat sources• the probable scenarios of the accidents • the quantitative evaluation of the potential results for both people and the environment

To calculate the results of the hazardous occurrences the knowledge of the input data is needed

Simulation via the use of pure component consequences makes it possible to predict more or less accurately conditions when a liquid or gases is released.

Thank you very much for your Thank you very much for your attentionattention