PS6-7.1

A NEW UNIFIED INVESTIGATION INTO VAPOUR CLOUD FIRES

UNE NOUVELLE COLLABORATION INTERNATIONALE POURL’ETUDE DE FEUX DE NUAGES DE VAPEURS DE GNL ET GPL

N.C. DaishPrincipal Consultant

Cambridge Environmental Research Consultants Ltd.3, King’s Parade, Cambridge, CB2 1SJ, UK

P.F. LindenChief Executive OfficerCERC San Diego Inc.

4539, Ocean Valley Lane, San Diego, CA 92130, USA

V. VieillardResearch Engineer

D. NedelkaExpert EngineerGaz de France

Pôle Etudes Cryogéniques,BP 12417, 44024 Nantes, CEDEX 01, France

T.A. RobertsSection Head, Process Safety Section

C.J. ButlerSenior Scientist

Health and Safety LaboratoryHarpur Hill, Buxton, Derbyshire, SK17 9JN, UK

ABSTRACT

This paper describes an experimental and theoretical investigation into vapour cloudfires, including both flash fires and fireballs. Two new series of medium-scale tests havebeen carried out: LNG vapour clouds produced with deliberately high gas concentrationsin order to study fireball formation; and horizontal momentum releases of LPG, in whichignitability effects were clearly demonstrated. In parallel, theoretical investigationsfocused on the scientific evaluation of operational vapour cloud fire models. A frameworkwas developed to provide a unified approach to studying vapour cloud fires.

RESUMELe présent document décrit une étude théorique et expérimentale des feux de nuages

de gaz avec ou sans boules de feu. Deux séries d'essais à moyenne échelle ont été réalisées: des feux de nuages de vapeurs de GNL produits volontairement avec de fortesconcentrations en gaz pour étudier la formation de boules de feu, et des feux de jetshorizontaux de GPL pour lesquels les conditions d'inflammabilité ont été clairementétablies. En parallèle, une approche théorique a portée sur l'évaluation scientifique desmodèles de feux de nuages existants. Une méthodologie a été développée pour permettrece type d'évaluation.

PS6-7.2

A NEW UNIFIED INVESTIGATION INTO VAPOUR CLOUD FIRES

1. INTRODUCTION

When a cloud of flammable vapour, for example resulting from an accidental spill ofLNG, LPG or other flammable material, encounters an ignition source, the result may beno fire, a flash fire or a fireball. Although of relatively short duration, vapour cloud fires,or VCF’s, are capable of producing high levels of radiation and, furthermore, may providea possible pathway to escalated consequences in the nearby vicinity.

Yet, despite their importance, previous experimental and theoretical investigations intoVCF’s are sparse and there remain important unanswered questions concerning theirbehaviour, in particular the relationship between the concentration field in the cloud, theignition location and the resulting fire.

As a result, an international consortium, drawn from industry, regulatory bodies andconsultants, has carried out a project to gain an overview of the current state ofunderstanding of VCF’s, and to consolidate and extend that knowledge through acombination of experimental and theoretical programmes running in parallel.

The experimental programme comprised two series of medium-scale testsencompassing a broad range of conditions. One series used LNG and investigatedtransient releases from a high volume-flux vapour-rich area source; this is a rare situationin operational plants (where flow rates are limited and ignition of the relatively dilutecloud is far from the source), but was used to study the possibility of fireball formation invapour cloud fires. The other test series used LPG and considered steady, horizontal jetreleases from an orifice, and was particularly concerned with the ignition characteristicsand propagation of flame through the cloud. Extensive instrumentation was used to givehigh quality gas concentration, radiation and visual data. LNG and LPG were used asrepresentative flammable materials, but the results are expected to be of widerapplicability.

The principal aim of the accompanying theoretical programme has been to analysemodels used to simulate vapour cloud fires. Formal model evaluation techniques havebeen applied to a selection of operational vapour cloud fire models to establish thestrengths and weaknesses of current modelling techniques and highlight areas for furtherdevelopment.

Although vapour-rich clouds are a low probability occurrence in operational plants, amajor scientific outcome of this study has been to show that it is possible to generatefireballs from such clouds: a fireball was observed in more than half of the cases, and theireffects alter the burning characteristics completely. For clouds in which gas concentrationswere less vapour-rich, burning was more patchy and took place within the basic geometryof the unburned cloud. The large buoyancy forces associated with the fireball generatedstrong flows, which caused additional mixing of the gas with the air so that much more ofthe gas, even that above the UFL was burned in the resulting fire.

2. A UNIFIED TREATMENT OF VAPOUR CLOUD FIRES

One of the chief objectives of this project was to examine both existing experimentaland theoretical treatments of vapour cloud fires in some unified way. This was achieved

PS6-7.3

through scientific review of existing models (Section 6) together with review of existingVCF data sets (Section 3) and the new experimental programmes (Sections 4 and 5).

The scientific review of VCF models requires categorisation of the capabilities andformulation of these models, and this is achieved through the definition of a frameworkwith which to describe the basic physical problem. In the case of vapour cloud fires, thisframework is divided into two main parts, corresponding to “before and after” initiation ofthe fire:

(a) the source of the vapour cloud, the environment in which it forms, the vapourcloud formed and the ignition source

(b) the flame characteristics, radiation characteristics, possible escalation.

A broad framework such as this may be interpreted for both experiments, and theirassociated data sets, and for models, and this is indicated schematically in the outermostcolumns of Figure 2.1. Here we see that the aspects in (a) relate to the overall features ofthe experimental set-up (or real incident) on the one hand and to the input parameters tothe model on the other; while the aspects in (b) relate to the consequences of ignition ofthe vapour cloud in the experimental situation and to the output produced by the vapourcloud fire model.

In order to review data sets and to carry out a scientific review of models it isnecessary to make the structure more specific. We have, therefore, developed a detailedset of headings with which to structure reviews of data sets and another to review themodels, both based on the above general framework. The detailed headings are indicatedin Figure 2.1 for data sets, while Section 6 describes the approach for vapour cloud firemodels.

The same structure can be applied to the new data from the experimental programmesundertaken within this project. For reasons of limited space, it is not possible to use thefull data set review structure here, but nevertheless we have attempted to impose the broadstructure on the description of these experiments in Sections 4 and 5.

A further framework was also developed in order to review the scenarios leading tovapour cloud fires.

• Overall plant configuration• Mechanisms for and details of loss of containment• Environment into which release takes place• Results of interestThese categories are broadly in line with the headings outlined earlier; furthermore the

sub-categories also align with those in the treatment of data sets and models, e.g. “Results ofinterest” is further sub-divided into flame characteristics, radiative and convective heat flux.

3. REVIEW OF EXISTING DATA

The number of large-scale high quality field data sets for vapour cloud fires issomewhat limited. Table 3.1 lists the most extensive of these.

The data sets were reviewed according to the framework outlined in the previous section. Itis not the intention to give a detailed description here beyond that sketched above. Instead, wecomment on the overall features of these data sets and their relationship to the experimentalprogrammes in this project and to models in general. With regard to validation of models:

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Figure 2.1 Schematic representation of main features of vapour cloud fire data sets (events) and vapour cloud fire models.

DATA SETS MODELS

SOURCE, ENVIRONMENT, CLOUD• Vapour cloud source• Atmospheric conditions• Terrain and obstacles• Configuration of vapour cloud• Ignition source

OUTPUT• Flame characteristics• Heat flux• Escalation• Combustion products• Other

DESCRIPTION OF TRIALSList of trials

InstrumentationThermal radiationAerial overpressureVisual observation

Recording and processingDirect measurementsDerived data

Data reportedDirect measurements reportedDerived data reported

Summary of cloud fires

Other information

Thermal radiationAerial overpressureVisual observation

Flame characteristics• Geometry• Propagation• Correlation with

gas concentrationsSurface emissive power

EscalationCombustion productsOther COMPUTER IMPLEMENTATION

SOURCE, ENVIRONMENT, CLOUD• Vapour cloud source• Atmospheric conditions• Terrain and obstacles• Configuration of vapour cloud• Ignition source

PS6-7.5

Table 3.1 Summary of major vapour cloud fire data sets.

Maplin Sands Coyote MusselbanksReferences 4, 5 7 9Number of tests 11 5 Not givenMaterial LNG (7), C3H8 (4) LNG (4), CH4 (1) C3H8

Cont (C) / Inst (I) C and I C CSize (m3/min or m3) 2.1-5.6 (C); 5-12 (I) 13.5-17.1 ~4Wind speed (m/s) 4-8 4.6-9.7 Not givenSurface Water Desert/scrub Mud flatsObstacles - - üIgnition Edge Centre Centre (?)Flame speeds ü ü üFlame geometry ü ü -Thermal radiation ü ü -

1. The available data sets are limited. This type of experiment is expensive to carry outand is therefore necessarily limited in terms of the number of experimentalprogrammes and the cases studied in each one.• range of materials restricted to LNG (or methane) and LPG• no low wind conditionsFurthermore, the two main programmes, Maplin Sands and Coyote, are to someextent complementary, and so, for example, there is only one data set group dealingwith say vapour cloud fires over land or water. Not every parameter is available forevery data set.

2. The data are not necessarily independent of the data used in model correlations.Vapour cloud fire models of the type considered below (i.e. not general-purpose CFDmodels) require empirical input to parameterise various features of the fire, e.g. flamepropagation speed or geometry. Given the restricted data available it is likely thatmuch of the available data are used in formulating the model, so that it would bedesirable to have additional data available with which to validate models.

These limitations suggest recommendations for experimental scenarios andmeasurements where either there is sparse (or no) information or there is essentially onlyone group of data sets for a given feature.

Scenarios Measurements• Range of atmospheric conditions, • Flame surface emissive power including low wind conditions • Flame geometry• Edge and centre ignition • Flame propagation speed (and gas• Terrain types and obstructions velocity if possible)

• Concentration distribution in and next to burn area

The new experimental programmes serve both to consolidate existing data (data on heat fluxes,flame geometry and propagation speed) and to introduce novel features:

LPG releases LNG releases• Release momentum • Transient (puff-like)• Limited density effects • High concentrations• Obstacle effect • Low wind conditions

PS6-7.6

4. LNG EXPERIMENTS: LOW-MOMENTUM RELEASES

The first of the two new experimental programmes we describe is a series of originalexperiments featuring releases of LNG to generate large high-concentration vapour cloudswith low momentum. This situation has not been observed in the operation of existingplant, but rather is a theoretical extrapolation of a specific situation, such as the result of asudden (catastrophic) release of a quantity of LNG spilled on (warm) soil close to anignition source. Of primary interest were variations in the ignition location within thevapour cloud formed and the subsequent development and thermal impact of the fireresulting from ignition of the cloud, in particular the possibility of the formation of afireball when ignition takes place in the region where the average concentration is abovethe upper flammability limit (UFL). A novel technique has been developed to generatevapour clouds of significant size with a substantial volume of high concentrations abovethe UFL.

4.1 Overview of programme

The experimental programme was carried out by Gaz de France at the Saint Etienne deMontluc testing site near Nantes, France. In each experiment a quantity of LNG wasdischarged into a vapour generation device at a rate typically around 4kg/s for between30s and >100s, following which the vapour cloud generated was ignited at some locationwell within the cloud. Sensors were arranged in a pattern of circular arcs of radius up to70m and measured concentrations, temperatures and thermal radiation. Meteorologicaland release conditions were also monitored.

The vapour generation device had been developed by GDF in an earlier project andconsisted of a cylindrical pit sunk into the ground and filled with pebbles, into which theLNG was discharged. The pebbles provided a highly porous medium with a large thermalcapacity, enabling a large quantity of LNG to be vaporised in a short time, therebygenerating the desired vapour cloud with a significant volume above the UFL. The devicehad been shown to produce a steady vaporisation rate that was reproducible for a givendischarge rate and average pebble size; the vaporisation rate was not a linear function ofthe discharge rate, but could be inferred from the earlier experimental results.

Ten experiments were performed in total. In each test the objective was to generate acloud with a substantial volume above the UFL and then to ignite the cloud at variouslocations relative to the flammable volume. The ignition devices were located at fixedpositions, and so because the vapour cloud was unsteady it was necessary to monitor it inreal time in order to identify the time at which to trigger ignition.

4.2 Site layout

The Gaz de France test site is shown schematically in Figure 4.1 and is described inmore detail in the rest of this section.

Test area. The overall test site comprised an area approximately 400m by 400m. Thesensors in the trials were confined to the circular area of approximately 70m radius shownin Figure 4.1. The cloud zone was the region into which the vapour cloud wouldpropagate. It consisted of an 80° sector originating at the source divided circumferentiallyinto 10° sub-sectors by radial axes; the zone was also traversed radially by arcs positionedat 5m, 10m, 20m, 30m, 40m, 60m, 80m and 100m from the source. The concentration

PS6-7.7

sensors, thermocouples and ignition systems were arranged at the intersections. Someoptimisation of the sensor distribution took place during the programme. The radiometerzone was superimposed on the cloud zone and comprised four sectors, namely north,south, east and west. There were 3 radiometers in the eastern sector and 6 in each of theothers. They were thus arranged all round the source, although concentrated to either sideof the cloud zone. The orientation of the radiometers, a key factor in calculating theradiation, was selected so that the normal line of the sensor surface pointed either towardsthe source (Trials 1-5) or 10m downstream of the source (Trials 6-10).

Figure 4.1 Layout of test site in GDF experimental programme.

Gas storage and release. The central discharge area contained a heat-insulated LNGstorage tank, the underground test bund (pebble pit) and scales to weigh the liquid in thetank. The storage tank held up to 1.8m3 and was mounted on a concrete slab supported onthe scales, thus allowing the discharge rate to be monitored. A discharge pipe led from thebase of the tank to the underground test bund via two valves, one manual and oneremotely-controllable pneumatic valve, and ended in a spray deflector to direct the LNGwidely over the surface of the bund. The test bund itself measured 1.8m in diameter and1.7m deep, and was filled with pebbles of average size 40-60mm to a depth of 1.5m.Thermocouples monitored conditions among the pebbles.

Ignition source. An electric sparking device was used in some tests, flares in others.

Pressurised cabinet and control bungalow. The site featured a pressurised cabinet,located close to the testing zone, to hold the data acquisition equipment (two HP3852systems), and a control bungalow located 150m from the testing zone where the datamonitoring and recording was carried out. The cabinet was located close to the testingzone to minimise deterioration of signals from the sensors, which was a potentially serious

PS6-7.8

problem due to the high acquisition rates. Wiring to the sensors was located in sand-filledtrenches underground: a single main radial artery connected each arc, with up to 7 oxygensensors and 2-4 thermocouples allowed on any given arc.

4.3 Instrumentation and measurements

Methane concentration. The concentration of LNG vaporisation products was measuredin two different ways. The majority of the sensors used measured oxygen concentrationsand then used these to infer “contaminant” concentrations assuming a constant proportionof oxygen in ambient air. These sensors were of the same 2FO CiTiceL variety as thoseused in the LPG experiments (see Section 5.3). Tests were performed on the response timeand robustness of these sensors, revealing a response time of approximately 1s as well asrelative immunity to the effects of a cold cloud or, given simple protection, to contact withflame. Flame protection was provided by inserting each one into a small stainless steelbox, which could be closed remotely just before ignition. It was necessary to take intoaccount the fact that each sensor had a different baseline response in deducing theconcentration from a given sensor. Information regarding this type of sensor was sharedbetween the two experimental programmes. The other type of concentration sensormeasured contaminant gas concentrations directly. They were pyrometer-type sensors,designed and installed by INERIS, in which the sensor is mounted inside a tube throughwhich gas is drawn by a vacuum pump.

Cloud temperature. Temperatures in the “cloud zone” were measured with type-Kthermocouples (Chromel-Alumel) inserted in a 1.5mm diameter Inconel 600 sheath.

Heat flux measurements. Thermal radiation was measured by Medtherm Series 64radiometers. These sensors were cooled continuously by a water circuit, which includedprovision to maintain the flow even for low temperature conditions. Each radiometer hadits own response curve.

Video recording. Four Sony DCR-TRV900E digital video cameras were arranged on eachside of the “cloud zone” for visual recording of the trials at a rate of 24 frames per second.Slow motion replay was effective for processing images of the trials, which can theneasily be saved in JPEG format.

Meteorological instruments. There was a 6m high weather mast equipped with windanemometers at 2m, 4m and 6m, a wind vane, temperature sensor and humidity sensor.The meteorological measurements were monitored closely so as to decide when to initiatea given test.

4.4 Trials performed

The chief characteristics of each of the trials performed are shown in the upper blockof Table 4.1.

A test could be carried out when the monitored meteorological conditions, in particularthe wind direction, were stable in time. An LNG road tanker was brought on-site and thestorage tank was filled, while final preparation of the rest of the site was undertaken –resetting of sensors, preparation of video cameras, pressurisation of data acquisitioncabinet and finally opening of manual valve in the LNG tank discharge circuit.

PS6-7.9

A trial began with the opening of the remotely-controlled pneumatic valve whichdischarged LNG into the test bund. The oxygen sensor data were used to give real-timeinformation on the progress of the cloud, while the concentration at specific sensorslocated near the igniter positions was also displayed in graphical form. Ignition wastriggered remotely when the concentration distribution was judged to be satisfactory, i.e.one of the ignition devices was near the UFL.

Table 4.1 Summary of trials carried out in LNG vapour cloud fires at GDF.

TRIAL

1 2 3 4 5 6 7 8 9 10

Date 23/11/99

23/11/99

02/12/99

10/12/99

14/12/99

07/03/00

09/03/00

09/03/00

30/03/00

30/03/00

Discharge rate (kg/s) 4.5 4.6 3.5 5.6 4.0 2.6 4.8 4.9 5.0 3.6

Discharge time (s) 49 105 50 75 48 107 37 33 30 34

Igniter typeSpar

kSpar

kSpar

kSpar

kSpar

kSpar

kSpar

k Flare Flare Flare

Igniter location (r(m), α(°)) (5,80) (10,8

0)(10,8

0)(10,8

0)(10,8

0)(20,9

0)(10,9

0)(20,9

0)(20,9

0)(30,9

0)Wind speed

(ms-1) 0.6 1.9 0.5 2.9 1.6 1.7 0.9 1.5 1.5 3.5Wind direction

(°) 110 103 121 113 94 107 163 124 175 129

Humidity (%) 60 50 48 48 57 40 58 54 46 43

Ambient temp. (°C) 7.3 12.0 10.0 10.0 4.4 16.3 16.0 23.0 12.0 27.0

Stability category C C B A C C B B C A

Distance to UFL (m) 7.5 6.5 5.0 5.0 7.5 5.0 5.0 5.0 5.0 5.0

Distance to LFL (m) 15.0 15.0 23.0 25.0 25.0 30.0 27.5 15.0 30.0 35.0Flammable cloud length

(m) 7.5 8.5 18 20 17.5 25 22.5 10 25 30Flammable cloud sector

angle (°) 70 60 40 30 60 30 60 80 30 40Estimated fuel mass

(kg) 13.3 14.4 12.8 8.2 19.2 8.6 11.5 11.7 11.2 5.1

Ignition time relativeto discharge 60 106 53 80 51 108 37 25 40 29Maximum

temperature (°C) 853 844 711 970 803 655 843 901 898 842

Fireball dimensions:Diameter (m)

Height (m)

Datanot

clear

30.020.0

5.017.0

14.026.0

22.024.0

10.020.0

10.026.0

Datanot

clear

Datanot

clear

Datanot

clear

4.5 Results

The second block of Table 4.1 contains information on the vapour cloud generated byeach release, as deduced from the concentration sensor record. This shows that ignitiontook place in most cases at a variety of locations between the UFL and LFL as well aswithin the UFL (Trial 1) and beyond the LFL (Trial 8). The length of the flammable zonevaried between 7.5m and 30m. The sector angle occupied by the fire also varied between

PS6-7.10

30° and 80°. The estimated fuel mass varied by almost a factor of 4 from 5.1kg up to19.2kg.

The third block of Table 4.1 summarises important characteristics of the fires in eachtrial, namely the maximum temperature observed and the characteristics of fireballsgenerated. The most significant fact is that some form of identifiable fireball event wasobserved in at least 6 of the 10 trials. Figure 4.2 shows snapshots of two typical examplesof fireballs occurring, one in Trial 6 and one in Trial 7.

The frequency of occurrence is a major outcome of these experiments. It appears thatignition of a cloud with a large vapour-rich volume leads to strong combustion and heatgeneration over a sufficiently localised area that a self-sustaining motion entraining fueland oxygen is generated and continues until most of the fuel is exhausted. This burningmode has a distinctive thermal-like character that distinguishes it from other burningmodes.

The temperatures generated by these fires were high, mostly in excess of 800°C andsome over 900°C, while the maximum heat flux generated varied between 5.6 and50kW/m2. Figure 4.3 shows the radiometer output from the instrument that recorded themaximum heat flux during Trial 7, located at a radius of 20m. The peak of almost 50kW/m2 was the largest recorded in any of the trials. Clearly, the value is dependent on theproximity of the sensor to the fire; nevertheless, this is a high value. Note that such heatfluxes are fugitive: for the example in the figure, the peak lasts a few seconds.

These tests have clearly shown that fireball formation is possible during a vapourcloud fire. The fireballs were small for these tests due to the limited size of the release andperhaps the fact that the LNG vapour became rapidly buoyant, limiting the formation of alarge concentrated cloud.

4.6 Conclusions

1. In the context of this multi-partner research project, Gaz de France performed a totalof ten ignition trials on LNG vapour clouds at the Saint Etienne de Montluc test site.

2. Weather conditions varied between the 10 trials: for example, the wind speed variedover the range 0.5-3.5m/s. These are relatively low wind speeds compared withprevious field-scale vapour cloud fire tests.

3. Different types of ignition were tested: in 7 of the 10 trials an electric sparking igniterwas used while in the remaining 3 a flare ignition source was used. The location ofthe igniter responsible for ignition was also varied between trials, being either 10m,20m or 30m for the electric igniters and 20m or 30m for the flares.

4. The fire produced varied between the trials: fire characterised by a high and wideflame, propagating over the entire cloud area and generating fireballs, or firecharacterised by a flame travelling from the middle to the borders of the cloud withoutthe generation of a fireball.

5. Flames, both with and without fireballs, were often impressive, with observed flameheights up to 45m. The fires were sometimes very intense with flame temperaturesexceeding 900°C and maximum heat fluxes ranging from 5.6 to 50kW/m2 dependingon the tests.

6. The data obtained in these trials will subsequently be further processed in order togain a better understanding of the ignition phenomena occurring in an LNG vapourcloud. These data sets already constitute a unique database on this type of cloud fire.

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0

10

20

30

40

50

60

0 20 40 60 80 100

Time (s)

Hea

t F

lux

(kW

/m2 )

Figure 4.2 Examples of fireballs observed during Trial 6 (upper left) andTrial 7 (lower left); heat flux measured at a radius of 20m in Trial 7 (right).

PS6-7.12

5. LPG EXPERIMENTS: CONTINUOUS JET RELEASES

The second new experimental programme, running in parallel with the LNG vapourcloud fires, was a series of trials featuring steady, horizontal jet releases of LiquefiedPetroleum Gas (LPG) with a co-flowing wind. Of primary interest were the ignitioncharacteristics and propagation of flame through the resulting vapour cloud, and so theprincipal variation between the trials was the ignition location. The effect of obstructionson the cloud development and subsequent ignition and flame propagation were alsoinvestigated.

5.1 Overview of programme

The programme was carried out by the Health and Safety Laboratory at their site atBuxton in Derbyshire, UK. The releases of commercial propane were made from a 50mmpipe 1.5m above the ground and measurements/observations made downwind over an areaapproximately 50m by 100m. Data were collected for gas concentrations, gas temperatureand heat flux from the vapour cloud fires as well as for the meteorological conditions andrelease conditions.

Ten different scenarios were examined, comprising 6 unobstructed releases and 4releases in the presence of a fence normal to the flow. The fence height of 1m was chosenso as to make a significant difference to the distribution of the flammable volume –experimentation with unignited releases was used to ascertain an appropriate height.

The main characteristic varied between each case was the point of ignition, which wasachieved by means of steady burning (ca. 0.2ms-1) pyrotechnic cord. In each case, threeparallel cords, spaced either 0.65m or 1.0m apart, were used to increase the chance ofignition and to reduce the distance to the nearest concentration sensor.

Ignition location was varied on the basis of

(a) direction of entry into the cloud: in some cases the ignition sources lay in the jetcentre plane and entered the cloud horizontally (running up the length of the vapourcloud formed from a downwind direction) or vertically (burning downwards intothe cloud), while in others the ignition sources entered from the side of the cloud

(b) position relative to the flammable region: three different positions were considered,namely as far downwind as possible, near to the centre of the cloud and close to therelease point (where fireball effects may result)

The ignition positions for the 10 scenarios were as follows:

• unobstructed releases: 3 cases with centre-plane ignition sources (both upstreamhorizontal and vertical downward entry directions) , while the other 3 cases usedside-entry ignition sources

• releases with fence: as for the unobstructed releases, the effect of the ignition sourceposition was varied between cases

5.2 Site layout

The layout of the trials site is shown in Figure 5.1. The site comprised an LPG storagefacility, an LPG release system and a discharge area in which the vapour clouds aregenerated and monitored using the sensor array, both before and after ignition. The trialssite is located on a remote area of the HSL Buxton site.

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Weather mast

Control room portacabin

LPG Storage tanks

Concrete test pad

Telescopicvideo tower

Protective wall

Sensor array

N

26°

Exclusion zone / safetyperimeter

Concrete pad

28

2

5

4

17 18 21 24 253

1

6

7

8

9

10

11

13

14

15

100 m37 m

Oxygen-freenitrogen supply

Releasenozzle

Figure 5.1 Layout of trials site.

Test area. The area is located within a gently sloping grassed area of approximately 100 mwide by 200 m long, aligned with the prevailing wind. For the trials studied, sensors wereplaced over an area of 60 m by 100 m.

Gas storage and release. LPG was stored in two 1-tonne tanks. Each tank had a 50mmliquid outlet pipe, which was fitted with a manually operated valve. The outlets from thetwo tanks were manifolded together into a single 50 mm pipe, which led across the storagearea to the discharge point, where the LPG was released through an orifice plate with19mm diameter. The discharge was controlled with a pneumatic valve operated from thecontrol building. The pressure and temperature of the liquid were measured by a pressuretransducer and a thermocouple located just after the release valve. The discharge pipe wasmounted horizontally at a height of 1.5 m above the ground and was oriented along thelength of the field.

Ignition source. The source of ignition for the vapour cloud was supplied using apyrotechnic fuse, consisting of a pyrotechnic composition within a plastic coating. Thesecords are designed to burn at a relatively constant rate and are not extinguished by wateror wind. For each test, three parallel lengths of igniter cord were used. The three cordswere joined at the ignition end (so that they all lit at the same time) and connected to anelectric match head, which was operated from the control building.

Obstructed releases. A 1 m high fence was chosen to be a suitable obstruction. The top ofthe fence lay approximately in the middle of the gas cloud, allowing a significant volumeof gas to flow unobstructed, while at the same time providing an obstruction for part of the

PS6-7.14

cloud. The fence was constructed using 2m by 1m steel sheets. A 20 m long fence waspositioned 15 m from the release nozzle, perpendicular to the flow of gas.

Control building. The trials were controlled and monitored from a control buildinglocated on the south-west edge of the test site, approximately 100 m from the gas releasepoint. All instrumentation sited in the field was wired back to the control building. Thedata generated during the tests were recorded on a PC.

5.3 Instrumentation and measurements

A variety of instrumentation was deployed to measure properties of both the unignitedcloud and the burning cloud and resulting radiation.

Propane concentration. The concentration of gas in the unignited cloud was determinedby measuring the oxygen concentration within the cloud. It was assumed that any decreasein the concentration of oxygen was caused by displacement of oxygen by LPG vapour.The concentration of oxygen in the vapour cloud was measured using 2FO OxygenCiTiceL sensors. The devices produce an electrical current, which is proportional to theconcentration of available oxygen. The sensors were calibrated against an oxygenanalyser. 30 sensors were used in the trials. These were positioned in the discharge area asindicated in Figure 5.1. The sensors were located at heights of 0.2, 0.8 or 1.5 m above theground, depending on the location. Sensors not on the centre line of the sensor array werelocated at 0.2 m only. The sensors were mounted inside a copper tube housing to protectthe sensors from heat from the vapour cloud fires produced. The front end of the tube wascovered by a fine mesh gauze, which allowed gas to diffuse to the sensor, but at the sametime acted as a flame trap to protect the sensor from the flame and heat flux.

Cloud temperature. The temperature of the gas cloud was measured using thin, reducedtipped (0.5 mm) type K thermocouples. These were located at a height of 1.5 m at thepositions along the centre line. Reduced tipped thermocouples were used in order to give arapid response to changes in the temperature.

Heat flux measurements. Heat flux measurements from the resulting vapour cloud fireswere made using Vatell Heat Flux Microsensors. These are essentially a combination of athermopile heat flux sensor (HFS) and a pure platinum thin film resistance temperaturesensing (RTS) element. The HFS signal gives a measure of the heat flux impinging ontothe sensor. This signal has to be adjusted using the temperature data measured from theRTS.

Video recording. Three Panasonic NV-DX100EG digital video cameras were used toproduce visual records of the trials. These cameras were positioned to the side and eitherupwind or downwind of the release point. In addition, a single VHS video recorder andcamera was used in each of the trials to produce a visual record of the release, viewedfrom an elevated position from behind the release point.

Thermal imaging. A LAND CYCLOPS Ti35Plus thermal imaging system operating at awavelength of 3.9 ± 0.005µm was used to produce a thermal image of each of the ignitedreleases. This camera has a relatively narrow field of view, so was positioned about 200 m(east) from the release point, and was arranged to view the resulting fire from one side.

Meteorological measurements. The wind speed and wind direction at 1.5m and 10mabove the ground, together with relative humidity and temperature, were measuredupstream of the instrumented test area.

PS6-7.15

Mass flow rate. The mean flow rate was determined by observing the change in mass ofthe storage tanks during the gas release, as given by the change of fill level.

5.4 Trials performed

A small number of preliminary unignited releases were undertaken in order tocharacterise the release and identify the best position for various sensors and equipment.These were followed by the ignited releases.

In the ignited releases, the flow was allowed at least 30s to settle down into a quasi-steady state before ignition, and in conditions where there were significant variations inthe wind direction the delay was longer to allow the cloud to align with the sensor array.In general, the trials undertaken at HSL were very weather-dependent, in that releasescould only be performed if the wind was in the correct direction, not too strong andconditions were dry. Trials were undertaken on days when the weather conditions wereacceptable.

A selection of the best trials from the programme is given in Table 5.1.

Table 5.1 Summary of trials and results obtained.

Trial No. (VCFn)n 3 6 7 8 9 11 14 15 16 17 18 20 23

Date 22/2 15/5 15/5 16/5 16/5 8/6 20/6 20/6 20/6 18/8 18/8 18/8 21/8Mass flow rate

(kgs-1) - 4.9 3.2 2.5 2.7 3.4 3.8 3.0 2.6 3.0 2.4 3.2 2.6

Duration of release(s) - 66 59 131 78 141 82 41 116 160 143 148 51

Wind speed(ms-1) 2.7 5.5 6.0 3.0 3.0 5.0 7.0 2.5 5.0 2.0 2.0 3.0 2.0

Wind direction(°)

170-210

185-235

180-225

195-225

180-215

110-225

200-220

180-220

200-225

160-215

130-210

150-225

145-210

Humidity (%) 75 58 50 63 69 63 51 90 85 75 77 84 84

Ambient temp. (°C) 4.0 21 20.5 14.5 14.5 17.5 26.0 16.0 17.0 14.0 14.0 14.0 11.5Ignition

direction* L V S V V L S S V L L L V

Ignitor position(m) 70 20 20 30 5 50 14.7 15.3 17.5 50 50 50 25

Obstructed trials - - - - - X X X X - - - -

Repeated trials * † - - - - - - - * * * †

Time to ignition(s) - 53 41 110 62 125 71 30 105 145 129 125 38

Ignition position(m) 29 20 20 30 5 15 14.7 15.3 17.5 25 30 30 25

Conc. of gas atignition (vol.%) 2.0 2.5 1.5 2.0 >3.

02.0 2.0 2.5 2.0 2.5 2.0 2.5 2.5

Burning velocity(ms-1) - 9.7 7.3 7.3 - 12.2 10.2 11.2 11.2 10.3 9.1 12.8 13.4

Ignition direction: L: Ignitors ran along length of vapour cloudV: Ignitors positioned vertically in vapour cloudS: Ignitors burn into cloud from the side (perpendicular to flow)

Repeated trials: trials with the same symbol, either * or †, are repeats for the same nominal conditions.

PS6-7.16

5.5 Results

A summary of the results from the tests is given in Table 5.1. Figures 5.2 and 5.3 showa map of the concentration of propane, obtained from the oxygen measurements, and aprofile of the gas concentration with distance from the release point, respectively.

The following general observations were made.

1. The concentration of gas at a given position varies with cloud position and time andalso varies significantly with wind velocity.

2. Higher gas concentrations are generally measured closer to the ground.3. When the cloud was ignited the flame generally burnt back to the gas source. The

burning velocity (calculated as the velocity of the flame front across the ground,adjusted to account for the wind velocity and direction) varied greatly from test to test.

4. The presence of gas was generally visible as a white cloud. In many cases, when thecloud first ignited, no flame was visible. Instead, ignition was only visible as gaps inthe cloud, where the gas was consumed. This was due to the low gas concentration andlean burning.

5. Pockets of gas within the cloud could ignite without the flame propagating back to thesource. In this case, the pyrotechnic cord continued to burn and provide a furthersource of ignition for the unignited cloud. Often, there was repeated ignition ofisolated pockets of gas, without propagation to the whole cloud.

6. The fence prevented a lot of gas from flowing any further downstream. Theconcentrations measured beyond the fence were always low.

7. The visible cloud could engulf the ignition source without ignition occurring. Ignitiononly occurred when the gas concentration was sufficiently high.

8. The concentration of gas within the cloud was more evenly distributed throughout thecloud in light wind conditions.

0 7010 20 30 40 50 60

12 m0.5 %

1.5 %2.0 %2.5 %5.0 %

Figure 5.2 Map of propane concentration contours before ignition and without a fence (VCF 3).

PS6-7.17

0

2

4

6

8

10

12

0 10 20 30 40 50 60 70 80 90

Distance from release point (m)

20 cm

85 cm

150 cm

Height above ground

Lower flammability limit (2.2 vol.%)

Upper flammability limit (10 vol. %)

Figure 5.3 Concentration of propane at different heights, with increasing distance from release point(VCF3).

Figure 5.4 shows a series of pictures to illustrate the propagation of a flame throughthe vapour cloud. In this example, ignition occurred at 25m from the release point (just offthe bottom of the pictures). The flame propagated to the nozzle within 5s of the cloud firstigniting. In this case, isolated pockets of ignition were not observed.

2 secs 38 secs 39 secs

40 secs 41 secs 43 secs

Figure 5.4 Video images of an ignited release (VCF 23).

PS6-7.18

At the time of writing, data from the trials were still being analysed. This includesanalysis of the heat flux measurements and thermal images, neither of which are presentedhere.

5.6 Conclusions

From the results obtained so far, the following conclusions are drawn.

1. Vapour clouds could be ignited at concentrations below the lower flammability limit(LFL) of propane of 2.2% volume concentration.

2. If the gas in the cloud was not evenly distributed, isolated pockets of gas could ignite,but would not lead to propagation of the flame through the vapour cloud.

3. The concentration of gas in the cloud was generally low. The flames produced wereoften invisible and were only detected as a results of secondary effects (e.g. the gascloud being consumed, grass burning, etc.).

4. Under the conditions tested, no fireballs were produced either when the gas burnt backto the source or when the cloud was initially ignited.

5. For a release at 1.5 m above the ground, a 1 m high fence at 15 m from the releasepoint greatly reduced the concentration of gas in the downwind cloud. Ignition of thecloud downwind of the fence did not occur easily.

6. The data provide valuable information on the behaviour of LPG vapour clouds.

6. MODELLING ACTIVITIES

In parallel with the experimental programmes there has been a complementarymodelling programme. The major part of this programme has been model evaluationthrough the scientific assessment or review of a number of operational vapour cloud firemodels.

6.1 Model evaluation

Scientific model evaluation is a relatively new technique for the formal review oftechnical models (Ref. 1). The evaluation of a model may be taken to consist of threeseparate elements:

• assessment – the systematic, objective review of a model according to its scientific basis, itsuser-oriented aspects, etc. with the aim of identifying strengths and weaknesses of a modeland highlighting its limitations when applied to (in this case) vapour cloud fire problems

• verification – checks that the computer implementation of a model is an accuratetranslation of the mathematical algorithms

• validation – comparisons of model predictions against experimental data, oftenusing quantitative, statistical techniques.

Many readers will be familiar with validation and, indeed, this term is often usedinterchangeably with “evaluation”; however, we use it here in a precise way to representone of the three elements of the more comprehensive activity of evaluation.

The evaluation of models in this project was confined to the assessment element,partly for reasons of limited resources but also because this would not have been carriedout before, whereas validation and verification should already have formed part of thedevelopment of a model. (In fact, if the information is available, we include sections in theassessment that analyse previous validation and verification.)

PS6-7.19

We have confined ourselves to operational models that are in current use and wereavailable through the participants in the project. Specifically, we considered the followingmodels.

(i) Raj & Emmons (Ref. 6) This is described in more detail below, but broadly it is asimple analytical model to predict the height and width (dimension in direction oftravel) for a planar flame front propagating through a uniform constant-thicknessground-level vapour cloud. This model is also cited in the CCPS Guidelines book(Ref. 3).

(ii) CLOUDF (Ref. 2) This is a computer-based proprietary model developed by ShellResearch and calculates not only cloud parameters (geometry and propagation) butalso the thermal radiation received at a given point. Both flash fires and fireballs areconsidered.

(iii) HSE flash fire model (Ref. 8) A new model developed by WS Atkins for the UKHealth & Safety Executive to calculate the flame geometry, propagation andresulting thermal radiation at a specified point. The model has not been implementedyet, but a specification has been defined.

(iv) Dispersion model methodology Finally, we consider the methodology basedprimarily on the vapour cloud configuration: a dispersion model is used to obtain theconcentration distribution, in particular the location of the LFL (or possibly half-LFL) contour; it is then assumed that any personnel within the contour are fatalitieswhile those outside the contour survive. This is a common methodology for thosewithout direct access to a dedicated flash fire model.

In order to carry out the scientific assessment of these models, it was first necessary

(a) to obtain information on the models, and(b) to have a documented procedure or protocol.

The information on the models was obtained from project participants.

Production of a formal protocol document was beyond the scope of this project, butnevertheless a template for the evaluation report (see below) was developed, whichpresents a clear structure for the assessment. Figure 6.1 outlines the categories used.

Each model has undergone a scientific assessment, with the results recorded in adocument called the Model Evaluation Report or MER, which is a document agreed withthe model developer or the participant supplying the model information.

As an example of the results of such an assessment, we consider briefly the Raj &Emmons model. This is the only generally-available vapour cloud fire model cited in theCCPS Guidelines. The general set-up is sketched in Figure 6.1. The model considers anidealised problem, in which a planar flame front propagates with speed Uf through ahorizontally infinite uniform vapour cloud of finite depth Hc and molar concentration φ atrest on the ground. The flame front defines the leading edge of a burning zone of width Win the direction of propagation, while the trailing edge of this zone is where combustionhas ceased; above this zone there is a plume of height H rising above the cloud, entrainingair over its bounding surface that reacts with the fuel in the plume to release heat andcause the gases to expand. The fire is taken to be uniform in the direction perpendicular tothe plane of the section shown, and its properties vary with height but are taken to beuniform (averaged) over each horizontal plane.

PS6-7.20

Figure 6.1 Some of the categories in the scientific assessment of a vapour cloud fire model.

The assessment of this model (and the others in this exercise) is carried out by workingthrough each category in turn. The specific conclusions from performing this exercise forthe Raj & Emmons model included the following.

• The assumption of uniform concentration needs to be reconciled with application toa real cloudIn a real cloud, the concentration varies continuously through the cloud. It might bepossible to take this into account for a general variation through a suitableimplementation of the model by applying it to instantaneous/local conditions.

• Empirical data are limited and may be inappropriate.Closure of the mathematical model relies heavily on empirical input, in particular tohelp parameterise the flame geometry and entrainment into the fire. Field-scale dataat the time (1975) were very limited; while laboratory-scale data, although moreplentiful, may not be directly applicable since they relate to combustion experimentsusing pure fuel rather than fuel-air mixtures.

• Non-Boussinesq effects may affect the predictions.The entrainment into the hot plume is modelled in Boussinesq form, as if the gasdensity in the plume were not greatly different to the ambient air density, whereas inreality the heat of the fire causes the plume density to be significantly less than theambient value, making it more appropriate to use an entrainment formula thatreflects this.

The Raj & Emmons model is potentially in widespread use (through its inclusion inRef. 3) and users should be aware of the limitations of the model. Hence the MER is adocument that could usefully be made publicly available. It also highlights the main areasof uncertainty and therefore focuses attention for further development of the model.

GENERAL MODELDESCRIPTION

SCIENTIFIC BASISOF MODEL

USER-ORIENTEDASPECTS

VERIFICATIONPERFORMED

VALIDATIONPERFORMED

SUMMARY

• SPECIFICATION OFVAPOUR CLOUD

• SPECIFICATION OFENVIRONMENT

• MODEL PHYSICS ANDFORMULATION

• SOLUTION TECHNIQUE

• RESULTS OR OUTPUTFROM MODEL

• SOURCES OF MODELUNCERTAINTY

• LIMITS OF APPLICABILITY

• SPECIAL FEATURES

• Equations,initial/boundaryconditions

• Flame types• Flame propagation• Flame geometry• Combustion

modelling• Surface emissive

power• Escalation• Terrain effects• Obstacle effects• Radiation

• Flamecharacteristics

• Flame propagation• Flame geometry• Thermal radiation• Incident flux/thermal

dose

PS6-7.21

Figure 6.2 Model configuration proposed by Raj & Emmons. The figure shows a section throughthe vapour cloud fire, which is uniform in the direction perpendicular to the plane of the section.

6.2 Scaling

The purpose of this section is to assess the validity of the results of the medium scaletrials when applied to vapour releases at different scales. In particular, we are concernedwith VCF’s caused by larger releases than have been tested in these experiments.

We restrict our discussion here to the case of a vapour rich cloud that generates afireball. Assuming that the vaporization rate is Q, the volume of gas available for ignitionat time t is Qt. In the absence of wind this cloud will spread under its own buoyancy g ′ a(radial) distance

4/34/1 tBR ≈

where B g Q= ′ is the buoyancy flux of the vaporizing gas. Note that B is conserved withinthe cloud independent of mixing processes either near the source or elsewhere, but theconcentration does depend on mixing with the ambient air. (For the case of jet releases,both the mixing and the initial motion of the gas are determined by the momentum flux ofthe jet source.)

In the absence of wind, entrainment into the cloud is given by the velocity of the cloudand the area of its surface. Thus the cloud will dilute so that the volume increases at a rate

4/94/3 tBQ ∝ ,

with a corresponding decrease in the concentration. This calculation ignores the stabilizingeffect of density at the top of the cloud, which is expected to reduce the volume changewith B. Thus we conclude that at larger scales than the present trials, high concentrationswill still be achieved and fireballs are likely to be generated.

The motion caused by the fireball is related to its buoyancy. Assuming that thetemperature of the flame is constant, then the buoyancy is proportional to the volume of

BURNING BURNEDUNBURNED

W

HEntrained air

ρ0, u0

φ Uf Hc

PS6-7.22

ignited gas within the fireball. For larger scale releases, this volume scales with thevaporization rate. The rise velocity of the fireball is that of a turbulent thermal

2/14/1 tBw ≈ ,

and so is only a weak function of the buoyancy contained within it. This dependence is thesame as for the speed of growth of the cloud and so the geometric characteristics will staythe same. Thus for larger clouds the fireball will show the same ability to entrain fluidfrom the cloud to feed the fire within it.

7. CONCLUSIONS

In this paper we have presented a framework for studying vapour cloud fires, bringingtogether both the experimental data on these phenomena and the operational models usedto simulate them.

Operational vapour cloud fire models are structured along similar lines to one another,all relying heavily on observation to describe the overall features of vapour cloud fires andempirical data to parameterise the complex processes occurring. Such zonal modelstypically calculate the flame geometry (burn area) and flame propagation first and thencombine this information with a standard radiation model to give the thermal radiationreceived at a specified location.

Model evaluation techniques have been applied to these models in an effort to identifytheir strengths and weaknesses and their limitations, and hence where further developmentor revision would be beneficial. One widely-used example, namely the Raj & Emmonsmodel, is discussed, and a number of areas of uncertainty have been identified for thismodel, suggesting that a model evaluation document would be a useful accompaniment tothe model. Some of the uncertainties relate to the assumptions in the model formulation,while others are associated with the data used.

We have seen that high quality field-scale data sets are sparse, and although theyprovide much valuable information on vapour cloud fire behaviour, they are necessarilylimited in the range of conditions examined, e.g. flammable materials, atmosphericconditions. There are also important aspects that have not been investigated in this type ofexperiment, in particular issues concerned with the relationship between the concentrationfield in the cloud and its ignition. Two sets of medium-scale field experiments have beencarried out. One set concerns LPG and represents a very common situation relevant tohazard analysis, namely a continuous release from a hole, sheared pipe, etc. The other,dealing with a vaporising LNG area source, represents a very unlikely situation but wasconsidered necessary for a better understanding of the phenomenon of fireball generation.Extensive data have been collected in both sets of experiments on vapour concentrations,flame development and thermal radiation.

The experiments have shown that very different fires result from a jet release and froma high volume flux area release. The reason for this difference is the concentration field inthe two gas clouds.

In the jet release there is strong mixing with the ambient air and the concentrations arerelatively small, compared with the deliberately vapour-rich area release where mixing isreduced and a significant fraction of the plume is above the UFL.

PS6-7.23

In the jet release the fire burns back from the ignition point to the source, and producesrelatively little change in the jet as it does so. In contrast, the area release, which wasspecifically designed to reduce mixing with the environment, often generated a fireball onignition. The fireball generated a strong vertical flow that entrained gas from thesurrounding cloud away from the ignition point that sustained the fireball. This is anonlinear process, using the buoyancy generated by the fire to drive more fuel and air intothe ignited gas. The characteristics of the fireball suggest that it is a self-similar structurewhose size scales with the size of the initial vapour cloud. It causes much more of the gas,including the majority of that above the UFL, to be burnt in a VCF.

The data from these experiments will undergo further analysis and it is highly likelythat this will yield further interesting and revealing insights into the development ofvapour cloud fires, and will prove equally valuable in the development and validation ofvapour cloud fire models.

Acknowledgement

The financial support of Enagas, S.A., Gaz Métropolitain, Mitsubishi HeavyIndustries, Total, S.A. and the UK Health & Safety Executive and in-kind contributionsfrom Advantica, Shell Research Ltd. and INERIS are gratefully acknowledged.

8. REFERENCES

[1] Britter, R.E. 1993 The evaluation of technical models used for major-accident hazardinstallations. Reference EUR 14774EN.

[2] Carsley, A.J. & Cracknell, R.F. 1997 A computer program for the assessment ofcloud fire hazards. RTS Report OP.97.47000.

[3] Center for Chemical Process Safety 1994 Guidelines for Evaluating theCharacteristics of Vapor Cloud Explosions, Flash Fires and BLEVEs. 387 pp.

[4] Eyre, J.A., Hirst, W.J.S. & Mizner, G.A. 1982 Spill tests of LNG and refrigeratedpropane on the sea: Maplin Sands 1980. Section VII, analysis and interpretation ofcombustion trials: Part 1 - background to the combustion trials. Report TNGR.82.008.

[5] Mizner, G.A. & Fearnett, M.P. 1982 Spill tests of LNG and refrigerated propane onthe sea: Maplin Sands 1980. Section VII, analysis and interpretation of combustiontrials: Part 2 - thermal radiation. Report TNGR.81.164.

[6] Raj, P.P.K. & Emmons, H.W. 1975 On the burning of a large flammable vaporcloud. Paper presented at the Joint Technical Meeting of the Western and CentralStates Section of the Combustion Institute, San Antonio, Texas.

[7] Rodean, H.C., Hogan, W.J., Urtiew, P.A., Goldwire, H.C., Jr., McRae, T.G. &Morgan, D.L., Jr. 1984 Vapor burn analysis for the Coyote series LNG spillexperiments. Lawrence Livermore National Laboratory UCRL-53530.

[8] WS Atkins Consultants Ltd. 1999 HSE flash fire model specification. AM5222-R1/Issue2, September 2000.

[9] Zeeuwen, J.P., van Wingerden, C.J.M & Dauwe, R.M. 1983 Experimentalinvestigation into the blast effect produced by unconfined vapour cloud explosions.IChemE Symposium Series No. 80, 4th International Symposium on Loss Preventionand Safety Promotion in the Process Industries Vol. 1, pp. D20-D29.