investigation of an lpg accident with different ... · abstract: an lpg (liquefied petroleum gas)...

340 Int. J. Risk Assessment and Management, Vol. 2, Nos. 3/4, 2001

Copyright © 2001 Inderscience Enterprises Ltd.

Investigation of an LPG accident with differentmathematical model applications

Fausto Zenier and Franco AntonelloARTES srl, via C. Battisti 2/A – 30035 – Mirano (VE) – ItalyFax: +39415700402 E-mail: [email protected]

Fabio DattiloComando Provinciale Vigili del Fuoco di Rovigo, via Ippodromo6 – 45100, Rovigo, Italy

L. RosaUniversità degli Studi Di Padova, Dip. di Ingegneria Meccanica,via Gradenigo 2 – 32444, Padova, Italy

Abstract: An LPG (liquefied petroleum gas) storage accident wasreconstructed through several simulation models, with the aims of the reportedand filmed effects and damage. The accident happened in Paese, near Treviso(Italy) in March 1996 and was characterized by complex phenomena due to achoked release of LPG from a tanker. There was a dispersion of light windsand, after about 40 minutes, a flash-fire throughout followed by a transientjet release and BLEVE (boiling-liquid-expanding-vapour explosion). Theevolution of these phenomena was studied by application of some availablemodels. The primary purpose of these applications was to check the possibleuse and the efficiency of the simulation models using comparisons andevidence on the dynamics of the accident. Another purpose was to get moredetailed information on the dynamics of the accident. The work was done incollaboration with the Fire Brigade of Rovigo, the University of Padova andARTES (consulting on risk analysis – Mirano – Venezia). Complex situations,such as the dispersion of heavy gas in a semi-confined area, with light windsand variable atmospheric stabilities, prolonged time of release with differenttypes of flow, various types of fire (flash fire, transient jet fire, BLEVE andfireball) were simulated with adaptations of models or input to obtain realisticresults.

Keywords: Liquefied petroleum gas; accident; simulation models; gasdispersion; BLEVE.

Reference to this paper should be made as follows: Zenier, F., Antonello, F.,Dattilo, F. and Rosa, L. (2001) ‘Investigation of an LPG accident with differentmathematical model applications’, Int. J. Risk Assessment and Management,Vol. 2, Nos. 3/4, pp.340–351.

Biographical notes: From 1991 to 1994 Fausto Zenier carried out occasionaljobs in the field of environmental pollution (emissions, writing of technicalrelations). In 1994 he began to collaborate with ARTES srl company (RisksAnalysis & Ecology and Safety Technologies) developing or modernizingsoftware programs for operability analysis and calculation models for risk

Investigation of an LPG accident with different mathematical applications 341

analysis. From 1997 he began to work on risk and reliability analysis (Hazop,fault-trees and events-trees), continuing the development, verification andvalidations of mathematical models.

Franco Antonello is presently the Safety Manager of ARTES srl, a consultingcompany which performs environmental and safety studies for Montedipe andother companies. He worked in the Safety Department of Montedipe from 1974to 1989, performing risk assessment and reliability analysis on several plantsand studying mathematical models to simulate the effects of accidents.

Fabio Dattilo is the Commander of the Provincial Fire Brigade of Rovigo, Italy.

Lorenzo Rosa is the associated professor at the University of Padova, Italy inthe Department of Mechanical Engineering.

1 Chronological sequence of the events related to the accident

At about 7 am the unloading of a 52m3 LPG tanker started. Shortly afterwards, probablybecause of a reduced flow, the operator interfered with the truck valve to increase theflow rate. For reasons not yet clarified, a leakage took place around the valves of thetanker, inside the valve protecting box. A two-phase propane jet filled the valve box,overflowing and forming a small pool of sub-cooled liquid on the asphalt on the ground.While the surrounding area was immersed in a thick low cloud of vaporized propane, theoperators responsible for carrying out the emergency plan, stopped the work, turned offthe electrical system, evacuated the area and called the National Fire Brigade (NFB).Some minutes after the arrival of the NFB, at 7.40 am, a spark caused a flash fire, whichcaused damage to several different utilities. Meanwhile, an explosion took place in abuilding some 10m from the source of the plume, seriously damaging part of thebuilding.

A fire broke out in the pool of liquid propane and the flames wound round the rear ofthe truck. The increased internal pressure and the mechanical stress due to the hightemperature, caused a wide rupture of the tanker wall with a release of gas, very similarto a fireball, followed by the combustion of the released gas like a flare. The initial flash-fire also caused damage to the loading arm connected to a 15m3 tanker nearby, containingLPG of no more than 10% of its volume.

At 8.40 am the shell of this tanker, heated by the jet fire generated from the damagedloading arm, split and a BLEVE took place, followed by a fireball; the BLEVE destroyedthe second tanker. From about 9.00 am the efforts of the Fire Brigade were rewarded,putting the situation under control, limiting the area of the fire and leaving the gasflowing out of the first tank to burn out. The fire was completely put out at 5 pm.

2 Adopted criteria for the study of the event

The most important conditions and aspects related to the present study will be examinedaccording to the chronological sequence of the events.

342 F. Zenier, F. Antonello, F. Dattilo and L. Rosa

2.1 Initial outflow

The initial outflow can be considered as a two-phase flow in a half-enclosed environmentwith the starting point, the initial jet in the valve protecting box placed at the rear of thetanker. A dispersed spout created the presence of liquid propane in the box, (sub-cooledLPG due to isenthalpic flash) and spillage of liquid propane from the valve box to theasphalt on the ground. For model simulation purposes, the scenario described above canbe considered as a typical one, as the common options for modelling refer either to a freetwo-phase jet or to a single phase flow. However, the calculation was performed takinginto account the length of the pipe fixed in front of the valve (1m length and 25 mmdiameter).

2.2 Gas dispersion

With reference to the start of the events, the atmospheric conditions at the site wereobtained from the data supplied by two different weather stations, located few kilometresfrom the accident area (Table 1).

Table 1 Atmospheric conditions at the start of the event

first station second station

wind velocity 0.5 m/s 0.5 m/s

temperature 5 °C 6 °C

wind direction 100° 250°

relative humidity 80% 85%

The low wind velocity, compared to a standard altitude of 10m and therefore lower ifreferring to ground level, excludes the use of Gaussian models, which have some limits atlow velocity, particularly lower than 1 m/s [1,2]. On the other hand these conditionscause some problems in using all the common simulations models, by the lack of dataconcerning the choice of atmospheric stability class.

The first station recorded an sky overcast sky ratio of 5:8 – 7:8 with clouds above4000 ft; the second station recorded a ratio of <4:8 with base clouds at 3000 ft. Withreference to this situation and to the wind velocity, according to the Pasquill scheme [3],a stability class B can be considered, while referring to the standard deviation of the winddirection [4] and considering that no variation in the wind direction takes place at 0.2 m/svelocity at an altitude of 2m, a stability class F could be assumed.

2.3 Flash fire and fire

The reconstruction of this phase was not closely examined, due to several difficulties indefining the initial steps of the flash fire and the geometry of the cloud formation.

2.4 Burst of the first tank

A few minutes after the flash fire, an explosion wrecked the wall of the tanker, due tooverheating caused by the flame generated from the pool of propane and jet fire, lapping


on the surface of the tank. The typical pulsating effect of this kind of phenomenon, is notconsidered by most models (normally referring to a steady combustion with an averageheat emission divided among conduction, convection and radiation).

Considering that 30% of the developed heat is propagated by radiation [1], we canconsider 70% of the combustion heat of LPG to evaluate the overheating. Relevant to thefailure of the tanker construction material, it must first be noted that only a third of thetotal surface of the tanker was greatly affected by overheating by the flame action (asverified by a second trial). Of the remaining parts, as far as radiation is concerned, onlysuperficial effects have been found.

This calculation must take into account the irregular shape of the rupture area withdimensions of 40 x 60cm, located in the connection between the cylinder and thehemyspheric bottom of the tanker. The size of the opening is important for analysis of therelease of the burning gas: according to the above description, it cannot be classified as aclassic fireball, as related to BLEVE, which is normally generated by a very largeopening or by the collapse of the vessel.

2.5 Consequences of the fire in the second tank: BLEVE

The flash fire and the subsequent fire around the first tanker, damaged the loading armconnected to a nearby second tanker, causing the failure of gaskets and othercomponents. LPG flowing from the damaged connective device caught flameimpingement from the second tanker and brought about a final phenomenon that can beclassified as a BLEVE.

3 Analysis and reconstruction of the events

3.1 Study of the crack phenomenon in the first tank

Even if several models are available to study similar events, the peculiarity of theaccident described above, which took place with a partial rupture of the vessel instead ofcomplete destruction as in the classic BLEVE phenomenon, required some adaptation ofa mathematical model, particularly in carrying out a thermomechanical analysis relevantto mobile vessel fire damage and containing a two-phase flammable fluid. This wascarried out according to the calculation code ANSYS 5.3, allowing the study of structuresunder mechanical and thermal stress, also variable during the time. Due to the thicknessof the vessel wall (about 1cm), the theory of thick-walled pressure vessels was used. Themodel of the structure is explained in Figure 1.

The fire around the rear of the tanker was simulated with a variable thermic flux,whose intensity was calculated by considering complete combustion of the gas flowingout of the tank and if there was a likelihood of an unburnt fraction of LPG. Thethermal load, computed according to the above criteria, was related to the part of thetanker wall damage caused by the flames of the pool-fire and its variation in dependenceof time corresponds to ‘LPG FIRE CURVE’ reported in UNI.ENV. 1991-2-2 standard(see Figure 2).


Figure 1 Model of the structure

Figure 2 Temporal temperature increase


Also in the hypothesis, in computing the simplification of isotropic behaviour of theconstruction material, two aspects related to the construction criteria of this kind of vesselmust be underlined. Taking into account that a spherical shape has a higher resistancethan a cylindrical one, the thickness of hemispheric bases is normally thinner than thethickness of the cylindrical walls. In a stress situation created by fire, the strains in thehemispheric part are different from the ones in the cylindrical part; one can say, tosimplify, that in the cylindrical part there are three components (radial, longitudinal,tangential) whilst in the hemispheric part there are only two components (radial andtangential). This causes a state of tension located at the interface of thecylinder-spherebecause the cylinder tends to ‘open’ more than the sphere but this deformation is blockedby the sphere itself. The spherical part transmitted a force moment and a shearing stressto the cylinder and vice versa and this new tension condition, caused by fire, must beadded to the ‘normal’ stress condition related to a vessel working under internal pressure.The simplified result is reported in Figure 3.

Figure 3 Tension state located at the interface cylinder-sphere

In spite of the simplified hypotheses adopted, the above considerations take into accountthe most important aspects related to the weakening and the rupture of the vessel. Theapplication of the program allowed the determination of the temperature distribution atthe moment of rupture, obtaining the stress configuration of the structure, relevant to thethermic and pressure loads, the value of the plastic deformation and the location of therupture point (Figure 4). The obtained results are in accordance with the reports of thepeople attending the accident.


Figure 4 Location of the rupture point

3.2 Analysis and reconstruction of the events by simulation models

The amount of gas still present in the first tanker after the explosion and the vapourrelease has been calculated on the basis of the combustion time and the vaporization rateof propane, provided that the controlled combustion of the contents lasted eight hours.Starting from this evidence we have reconstructed some of the events to verify if thecalculations performed would produce the same conclusion. The models listed belowwere utilized to simulate the events:

• ARCHIE (Automated Resource for Chemical Hazard Incident Evaluation) [5].

• DEGADIS 2.1 (DEnse GAs DISpersion – US Coast Guard, GRI and API – USEPA).

• HGSYSTEM 3.0 developed by Shell and distributed by API (Ed. 4636–1995 ).

• RMP*Comp (only for flow rate calculation in the alternative mode) US EPA [6].

• SIGEM-SIMMA developed by TEMA SpA. and used by the Italian National FireBrigade.

• STAR (Safety Techniques for Assessment of Risk – rel 3) developed by ARTES Srl.

The reference points for the study of the different phenomena, obtained by informationsupplied by those in attendance, can be summarized as follows:


1 The initial release of LPG, without fire, lasted about 50 minutes.

2 After the flash fire there was a mixed fire (pool and jet fire) with flames engulfingthe rear of the tank, causing the partial rupture of the shell of the tank with anintermediate phenomenon between fireball and jet fire (transient jet fire) [7].

3 Due to the damage caused by flash fire on the loading arm there was a fire involvinga second tanker which, after some ten minutes, collapsed, producing a BLEVE.

The flow rate has been determined by considering the presence of a deep tube, 1 metrelong and an orifice with an equivalent diameter of 25 mm. LPG temperature was 278 Kand equilibrium pressure about 5.4 bar (abs). The results of the calculations are reportedin Table 2.

Table 2 Results of calculations

Model Flow rate (kg/s) Flow condition

ARCHIE 11.6 (peak) two-phase

RMP*Comp (1) 8.87 –

SigemSimma 0.46 gas

STAR 2.25 two-phase

(1) The calculation was performed with an alternative scenario, vapour cloud fire optionand hole in liquid space; with the pipe release option the flow rate result is 11.7 kg/s

Since the flow, even if not in a steady condition for the probable presence of ice in theorifice, lasted about 50 minutes and a third of the content was emptied, the onlyconsistent result is the one referring to a flow rate of 2.25 kg/s. Taking into account theabove time (50 minutes), a total amount of 6.75 tons of LPG was ejected, that is aboutone third of the initial content (21.4 tons), as testified by people present at the site.

The quantity of LPG evaporated during the flash can be obtained by adding theamount of LPG evaporated inside the tube and the amount of LPG evaporated during theisenthalpic flash, that is about 23% of the total flow. To evaluate the concentration in theair, several models have been utilized, each one associated with the relevant flow rate; itmust be underlined that the atmospheric conditions (stability class F and wind velocity0.51 m/s) are of limited acceptability for most models. The results are reported in Table 3(reference concentrations: Lower Flammability Limit – LFL – and 50% of LFL):

Table 3 Results of computations

Model Flow rate LFL 50% LFL Flammable mass

ARCHIE peak 11.6 kg/s 140 m 203 m 1800 kg

Degadis 2.25 kg/s 160 m 205 m 1030 kg

HGSystem 2.25 kg/s 33 m 55 m n.c.

RMP*Comp 8.87 kg/s <160 m n.c. n.c.

SigemSimma 0.46 kg/s (gas) n.r. n.r. n.r.

STAR 2.25 kg/s 38 m 55 m 157 kg

n.c. = not calculated n.r. = not reached


Regarding the ARCHIE model, it is clear that the vapour flow rate adopted for dispersionis lower than the peak value indicated in the results, but this datum is not supplied. Theresults of the DEGADIS model, in accordance with the ARCHIE model, seems affectedby the limit related to the wind velocity (minimum value suggested 1 m/s).

A similar limit (1.5 m/s) is required by the HGSystem; however, the final result isconsiderably different in comparison with the result of the ARCHIE model.

The results obtained with RMP*Comp are interesting from the viewpoint that arelevant flow rate produces the same LFL distance of DEGADIS, which uses a lowerflow rate. The SigemSimma model lacks a calculation routine for dispersion of heavyvapours and aerosols and the evaluation of the flow rate refers to the gas phase only.

The STAR model (box model developed from [8]), which considers the presence ofobstacles or buildings in the propagation of the gas, offers an indication of gasaccumulated in the area in front of the building, without, in substance, modifying thedistances of the flammability limits. Disregarding the simulation of the initial flash fire,the radiation caused by the burning gas released from the first tanker and the fireball andBLEVE related to the second tanker, will be analysed.

The phenomenon related to the first tanker in concomitance with the burst orstructural rupture can be regarded as a ‘transient jet release’, which occurs when thesubstance in the vessel is not overheated [9]. According to relations obtained fromdifferent sources [7,10], the required time for the collapse of a vessel is a function of thegeometry and characteristics of the flame engulfing the vessel; with the model ofturbulent flame on the top of the vessel, a time of seven minutes can be estimated, whilefor pool flames a time of 15 minutes can be estimated.

On the basis of the survey, the opening of the first tanker corresponds to a hole 50cmin equivalent diameter; according to this indication the following criteria were adopted toevaluate the effects of the phenomenon:

• simulation of a jet fire with flow rate equal to the initial flow rate, with pressure 17bar and temperature 323 K,

• evaluation of the jet fire duration by calculation of the amount of the released gasfor the time necessary to reach the atmospheric pressure inside the tanker,

• evaluation of the radiated energy.

The initial flow rate was calculated by two models, with very similar results (503 kg/s bySigemSimma; 523 kg/s by STAR). The duration of the release was calculated accordingto the STAR model (about three seconds) and by evaluating the amount to be released toreach the atmospheric pressure in the tank starting from the initial pressure 17 bar.Considering the quantity of LPG released before the burst (about 13m3) and the volumeof the tanker (52m3) filled by 80%, the volume occupied by the gas, before the burst, was28m3 with a pressure of 17 bar, equivalent to 456m3 of gas, that is 12700 kg. With a flowrate equal to 520 kg/sec, about 24 seconds are necessary to decrease the internal pressureto the atmospheric value. The indications supplied by jet fire models were considered totake into account the limited duration of the phenomenon by transforming the radiationinto energy, as indicated in Table 4.


Table 4 Results of computations

Model 350 kJ / m2 250 kJ /m2 125 kJ / m2

ARCHIE 49m - 106m

SigemSimma n.r. n.r. n.r.

STAR n.r. n.r. 110m

The results seem to be overestimated because people present at less than 100m were notinjured. We must underline that the evaluation of the phenomenon is affected bysimplification, mainly because the decrease of the flow rate and gas density was notrelated to the decrease of pressure.

The phenomenon regarding the second tanker approaches the BLEVE theory, with atotal collapse of the tanker and fragments and metal splinters found 500m from theirorigin. By application of the simulation models and considering the collapse conditionssimilar to the conditions of the first tanker (p = 17 bar; T = 323 K; M = 800 kg LPG), theresults plotted in Figure 5 were obtained.

Figure 5 Simulation results

In comparison with the previous calculation, the results seem to be more homogeneous,but, also in this case, over estimated. It must be considered that for small quantities themodels supply conservative estimates which are often not in accordance with thepractical results; the results are more accurate for phenomena of great magnitude.

With regard to the projectiles, two of them, of considerable size, were found at adistance of about 500m; the first was a part of the tanker shell with dimensions of 1.2m


by 1.2m and thickness of 10mm, the second was part of a pipe (1m long), probably partof the deep tube.

The simulation of the fragmentation and projectiles is affected by great uncertaintyrelated to the original shape of the fragments and to the angle of attack. With theexception of the STAR model, based on NASA studies and publications [11,12], it is notpossible to consider correctly the shape and size of the fragments; for this reason theresults obtained by using models other than STAR look quite unrefined. In fact, themodel SigemSimma gives a range of 314–1800m, while the results of the STAR model,considering an angle of attack of 35° on the basis of the reported evidence, are: 670m forthe first piece and 520m for the second. The Birk simplified model [13] giving a distanceof 610m referred to a fragment of significant size from a 15m3 tanker BLEVE.

4 Conclusions

With reference to the historically registered accidents, the one described in this study,related to a particular sequence of a great number of events and circumstances, cannot beconsidered as a single case. Most accidents are constituted by different events generatingcomplex phenomena: one major problem with the description of these phenomena bysimulation models, is that their main purpose is to give an idealization of reality throughcalculation algorithms, not to describe it in all of its complexity. So, the use of thesemodels appears unsuitable for the detailed reconstruction of complicated cases as the oneunder consideration and in particular for the simulation of the diffusion of heavy gases inthe presence of obstacles; in this case the use of 3D models is suggested, particularly inthe case of the absence of wind.

Nevertheless, with reference to the content of this study, it is clear that the answersupplied by the models adopted for risk analysis appears prudent, but, however, useful forforecasts and predictions to use in safety reports. Finally, it must be emphasized that theindications which come from simulations models should always be analysed in relation toexperience, comparing the output result with reference situations (for example, historicalcases).

References

1 A Guidance Manual for Modeling Hypothetical Accidental Releases to the Atmosphere (1996)API publication No. 4629.

2 ‘Methods for the calculation of the physical effects of the escape of dangerous material’(‘Yellow Book’) (1979) Report of the Committee for the Prevention of Disasters, TheDirectorate General of Labour Ministry of Social Affairs, The Netherlands.

3 Lees, F.P. (1996) ‘Pasquill’s stability categories (Pasquill, 1961)’, Loss Prevention in theProcess Industries - Tab. 15.14 - 15/ pp.89–94, 2nd edition.

4 Lees, F.P. (1996) ‘Relation between Pasquill stability categories and parameters of sometyping schemes (after Sedefian and Bennet, 1980)’, Loss Prevention in the Process Industries– Tab. 15.23 – 15/ pp.94, 2nd edition.

5 ARCHIE – Federal Emergency Management Agency, US DOT, US EPA.

6 RMP*Comp – Ver 1.06 – EPA Risk Management Planning – RCRA Superfund andEPCRA – US.


7 Birk, A.M. (1997) ‘BLEVE research’, Queen’s University of Kingston, Ontario, Canada,(http://conn.me.queensu.ca/birk).

8 ‘CRUNCH – a dispersion model for continuous release of a denser than air vapour into theatmosphere’ (1983), SRD R229, UKAEA.

9 ‘Le esplosioni BLEVE: rischi e misure preventive’(1987), Rivista Antincendio, Agosto.

10 ‘Guide for pressure relieving and depressuring system’(1997), API RP521.

11 ‘Workbook for estimating effects of accidental explosions in propellant ground handling andtransport system’ (1978), NASA – Report 3023.

12 ‘Workbook for predicting pressure wave and fragment effects of exploding propellant tanksand gas storage vessels’ (1977), NASA – Report 134906.

13 Birk, A.M. (1996) ‘Hazard from propane BLEVE: an update and proposal for emergencyresponders’, Journal of Loss Prevention in the Process Industries, Vol. 9, No. 2.

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