fire accidents in process plants

7/28/2019 Fire Accidents in Process Plants

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FIRE ACCIDENTS IN PROCESS PLANTS:

MATHEMATICAL MODELLING, UNCERTAINTIES

AND RESEARCH NEEDS

Eullia Planas and Joaquim Casal

Centre for Studies on Technological Risks (CERTEC)

Department of Chemical Engineering

Universitat Politcnica de Catalunya

Summary

The accidents involving fire play an important role amongst major accidents, not only

because of their relatively high frequency (fire accidents are more frequent than

explosions and toxic releases) but also because of their effects. Although thermal effects

usually reach shorter distances than blast or toxicants atmospheric dispersion, they

often affect other equipment, thus originating a dangerous domino effect. Their

mathematical modelling is therefore essential in risk analysis. However, this modelling

is not yet correctly solved: some variables are poorly known, there are a number of

uncertainties and some of the equations widely applied should be improved. In this

chapter this situation is analyzed and a set of recommendations are suggested

concerning the research needs in this field.

Introduction

Major accidents have been defined as an occurrence such as a major emission, fire or

explosion resulting from uncontrolled developments in the course of the operation of

any establishment ... and leading to serious danger to human health and/or theenvironment, immediate or delayed, inside or outside the establishment, and involving

one or more dangerous substances (CCPS, 1999). Major accidents involve essentially

explosions, toxic releases and fires, and diverse possibilities exist among each one of

these phenomena. Furthermore, an accidental scenario can involve more than one of

these basic accidents; thus, typical sequences are constituted by an explosion followed

by a fire, a fire followed by an explosion, or a fire originating a toxic cloud.

A major accident in an industrial plant or in the transportation of a hazardous material is

always originated by a loss of containment. The loss of containment can be due to the

catastrophic collapse or the explosion of a tank, the rupture of a pipe, a leak trough a

flange, a hole or a safety valve, etc. After the initial release, the incident sequence canfollow different ways and diverse accidental scenarios can be reached depending on the


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circumstances and on the physical state of the released substance. If it is a liquid, a pool

can be formed. If the substance is flammable and is ignited, there will be a pool fire; if it

is not immediately ignited, the evaporation can give rise to a toxic or a flammable cloud

which, if ignited, will lead to a flash fire and possibly to an explosion. If a two phase

mixture (liquid plus vapour) is released, a cloud can again be originated (depending on

the meteorological conditions). If a gas or a vapour is released, a cloud can exist in thecase of low speed releases; at high (usually sonic) speed, the substance will probably be

quickly dispersed, but a jet fire is still possible. In any case, the final scenario will be a

fire, an explosion, a toxic cloud or no outcome (i.e. quick dispersion into the

atmosphere).

In order to perform a risk analysis, the effects of such major accidents must be

estimated. These effects are blast and missiles ejection in the case of explosion, thermal

radiation intensity in the case of fire and toxic dose in the event of a toxic cloud. They

can be estimated by using mathematical models of the involved phenomena. A

mathematical model is a set of equations which describe a given phenomenon. Diverse

models have been proposed for most of the aforementioned accidents, with differentdegrees of complexity. Simple models are easy to use, but can give significant errors;

complex models should provide more accurate predictions, but usually require detailed

information which often is not available.

The mathematical modelling of major accidents is a field in which there are still many

gaps and which requires a significant additional effort to improve our knowledge. An

essential aspect is the need to check the validity of any model, and this is only possible

through the comparison of the model predictions with real data. Liable data from real

accidents are therefore very important and with no doubt the best option but,

unfortunately, they are scarce. An alternative source of reference data is the

experimental work. However, to increase its significance, this experimental work should

be performed at a relatively high scale and this is complex, difficult and expensive; this

is why the data available are still relatively scarce.

Fire accidents

Fire accidents are relatively frequent amongst major accidents, being in fact the most

frequent one. A historical analysis concerning process plants and the transportation of

hazardous materials (Planas et al., 1997) showed that approximately 47% of all

accidents involved fire. Another recent survey (Gmez-Mares et al, 2008) found that

59% of these events were fires, followed by 35% for explosions and 6% for gas clouds.

There are several types of fire accidents, depending on the circumstances and on the

substances involved. Figure 1 is a simplified scheme of the diverse possibilities. The

essential fire types are pool fire, jet fire and flash fire.

A pool fire occurs when a spill of liquid fuel is ignited. The size of the pool will be

determined by the ground features, by the eventual existence of a confining bund or by

the balance between the release rate and the evaporation rate. After a first step in which

the size of the flames increase, a stationary regime is reached during which the flames

size and shape remain approximately constant, with large fluctuations. The combustion

is rather bad and large amounts of smoke are produced. A significant part of the flamessurface are covered by non-luminous black smoke (smoke blockage effect) and this


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reduces the intensity of the thermal radiation; this intensity decreases quickly as the

distance from the flames increases. A similar scenario can occur when there is a fire in a

tank storing a flammable liquid; in this case, large inventories can imply large fires,

very difficult to be extinguished (BMIIB, 2008).

Fig. 1. Types of fire accidents (modified from Casal, 2008).

Jet fires occur when there is a release and ignition of a flammable gas/vapour or two-

phase flow trough a hole, a flange, etc., at a relatively high speed. The combustion is

much better than in pool or tank fires, and the thermal effect can be locally very intense,

especially if there is flame impingement, but their size is usually relatively reduced ascompared to pool fires.

When a flammable cloud usually generated from a liquid spill or a two-phase release

is ignited, the flames propagate through the flammable mixture and a flash fire occurs.

It is a quick and short phenomenon which can be accompanied by mechanical effects

(blast).

Finally, the fireball is usually associated to the sudden loss of containment of a

pressurized liquefied fuel, typically LPG. The two-phase cloud can burn only on its

outer surface as inside there is no oxygen. This phenomenon has a short duration, but

the thermal radiation intensity can be very strong.

Generally speaking, the effects of a fire (i.e., thermal radiation intensity) are limited to

relatively short distances as compared to those found in explosions or toxic clouds.

However, in process or storage plants fires can affect other equipments, especially if

there is flame impingement, thus increasing the scale of the accidental scenario through

the domino effect. Thus, in risk analysis the estimation of fire effects and consequences

as a function of distance can be very important.

Mathematical modelling of accidental fires

Different models have been proposed to predict the effects of a fire, i.e. the thermal

radiation intensity received by a given target located a certain distance from the flames.


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Amongst them the so-called solid flame model is probably the most widely used as,

although it is rather simple, it gives relatively good estimations of fire effects. It will be

used here to analyze the gaps and uncertainties associated to this mathematical

modelling.

According to the solid flame concept, the thermal radiation intensity reaching a givensurface can be calculated from the following expression:

I = F E (1)

where is the atmospheric transmissivity (-)

Fis the view factor (-) and

Eis the average emissive power of the flames (kW m-2).

, the atmospheric transmissivity, accounts for the absorption of radiated energy by the

atmosphere layer located between the flames and the target; its value is =< 1. Thermal

radiation is essentially absorbed by carbon dioxide and water vapour (humidity). As the

concentration of carbon dioxide is practically constant, is basically influenced by the

atmospheric humidity and the distance between the flames and the target. Expressions

have been proposed to calculate it as a function of the water vapour pressure (Casal,

2008; CPD, 1997).

F, the view factor, is the ratio between the amount of thermal radiation emitted by a

flame and the amount of thermal radiation received a given target not in contact with the

flame. Fdepends on the size and shape of the flame, on the distance between the flame

and the target and on the relative position of the flame and the target. As it is rather

complicated to calculate except for the case of fireball and the possibility ofcommitting an error is high, its values are tabulated for the most common situations

(cylindrical fire, rectangular fire, etc.).

Finally,Eis the radiant heat emitted per unit surface of the flame and per unit time (kW

m-2). As the value ofEcan change with the position on the flame, usually an average

value is used.

In the next paragraphs, the uncertainties associated to the determination of these

variables for the different types of accidental fires are commented.

Pool/tank fires

Pool or tank fires are the most frequent of the accidents involving fire. They often reach

a steady state and last a significant time.

Pool surface

In the event of a pool fire, the shape and size will depend on the area and shape of the

pool surface. Two possibilities exist: confined pool and unconfined pool.

If there is a bund and the amount of spilled liquid is enough to cover the whole bounded

surface, then the area of the pool surface is well established. However, if the amount is

smaller then the surface of the pool fire will be smaller than the bounded surface.


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If there is no limiting barrier, then the pool diameter will increase up to the moment in

which the evaporation from the pool equals the release rate; this will correspond to the

maximum pool size. For unconfined pools, the following maximum values have been

proposed: 44 m for pools on land and 113 m for pools on water. As for the pool

thickness, a minimum value of 5 mm is usually assumed.

In the case of a tank fire, the maximum pool size will be the tank diameter (additional

pool surface can be due to the eventual spill onto the dike).

Fire shape

The shape of the pool will determine that of the flames. A square or rectangular pool

will imply an approximately parallelepipedic flames body, while a circular pool will

originate a cylindrical fire. However, these are only approximate shapes; due to the

turbulence of the phenomenon, the flames undergo a significant fluctuating movement

and, in the top, often fireballs are formed with the consequent oscillation in the shape

and size.

Flames height

The aforementioned fluctuation of flames implies a statistical approach to define an

average height. The most widely used criterion is that of intermittency proposed by

Zukoski et al. (1984). The intermittency i(L) is defined as the fraction of time during

which the length of the flame is at least greater than L; the average length of a flame is

defined as the length at which the intermittency reaches a value of 0.5 (Fig. 2).

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.40.0

0.2

0.4

0.6

0.8

1.0

(L/D)max

(L/D)av

Intermittency

L/D

Fig.2. Intermittency for a pool fire.

In practice, the height of flames in a pool fire is estimated by applying an empirical or

semiempirical expression. The most widely used is that proposed by Thomas (1963):

61.0

42

gD

m

D

H

a

(2)

where His the average height of the flames (m)


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D is the diameter of the pool (m)

m is the fuel mass burning rate (kg m-2 s-1)

a is the air density (kg m-3) and

g is the acceleration of gravity (m s-2).

However, even though this expression is often applied to hydrocarbon pool fires, it wasobtained from experimental data from wood cribs fire. Therefore, probably it gives a

significant error and a correction should be introduced.

Surface emissive power

The surface emissive power is in fact the radiant heat emitted from the flame per unit

surface and per unit time. It is a function of the substance burned and of the type of fire.

It can be expressed as a function of emissivity and of flame temperature; however, as

these two variables are rather difficult to calculate, the value ofEis usually established

empirically or semiempirically. Often approximate tabulated values are taken as average

values ofEfor the different fuels.

A better estimation can be made by applying the following expression:

sootlumlumlumav ExExE 1 (3)

where xlum is the fraction of the fire surface covered by the luminous flame and

Elum andEsootare the values ofEfor the luminous and non-luminous zones of the

fire, respectively (kW m-2).

Diverse values have been proposed forxlum,Elum andEsoot , which again depend on the

type of fuel, the type of fire and , in some cases, the size of the flames. The way inwhich the value ofE is usually established probably introduces some error in the

calculation of the effects of a fire.

Jet fire

Among the different fire accidents, jet fire direct effects are the least severe, due to their

relatively reduced size as compared to a pool or a flash fire. However, jet fires can

severely affect equipment, especially if there is flame impingement, thus leading to a

domino effect: among the accidents registered in the data bases, in approximately 50%

of the cases in which there was a jet fire at caused another event with severe effects

(Gmez-Mares et al., 2008). Nevertheless, the current knowledge of the main featuresand behaviour of jet fires is still rather poor.

Flame shape

A jet fire does not have in fact a well defined shape. Again, it is a turbulent

phenomenon and the concept of shape should be defined in statistical terms. This is

why diverse authors have assimilated jet fires to standard bodies: a frustum of a cone, a

cylinder, a spindle. The shape of a jet fire depends on the type of jet (low velocity or

high (often sonic) velocity) and on its direction: horizontal, inclined. The frustum of a

cone can describe fairly well low velocity flares or even an inclined jet fire, while a

spindle or a cylinder describe vertical jet fires. The cylindrical shape has clear

advantages: it is simply described and the view factor can be calculated in a relatively


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simple way. However, the shape (length to diameter ratio) has not been adequately

established yet.

Flame length

Diverse expressions have been suggested for calculating both the average and maximum

flame length of jet fires. Although these expressions have been usually obtained fromexperimental data, some of these data were obtained with relatively small jet fires, and

some of them with subsonic jets. Thus, again some uncertainty exists when calculating

the effects of a jet fire.

Lift-off

The lift-off is the centreline distance from the fuel release point to the start of the

detached and stabilized flame. It can be significant because, together with flame length,

it determines the position of the flame and the distance over which there can be flame

impingement on nearby equipment.

The situation with lift-off is similar to that found with flame length. There are diverseexpressions available to estimate it, usually as a function of Froude or Reynolds

number:

for subsonic flow

bFra

d

L (4-a)

for any type of flow:

ec

d

LRe (4-b)

Fireball

Fireballs, usually associated to the explosion of a vessel (often a BLEVE), release large

amounts of thermal energy in a short time, originating very strong thermal radiation

intensities with severe potential effects.

The thermal effects of a fireball can be estimated by applying the solid flame model. To

do this, the size, position and duration of the fireball are required. These variables canbe calculated with rather simple expressions:

diameter:

nMmD

height at which the centre of the fireball is located:

pDH

and duration time:


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r

Mqt

The problem is that at least twelve expressions have been proposed by diverse authors,

with different values for the constants m, n, p, q andr. Another variable which is still

subjected to uncertainty is the fraction of the energy released which is emitted asthermal radiation. All these values require a validation from experimental work, which

in this case is rather complex, expensive and difficult to perform.

Flash fire

This is the type of fire accident which has been less studied and, from the point of view

of mathematical modelling, it is practically unknown. There is only a semiempirical

model proposed by Raj and Emmons (1994) to estimate the height of the flames.

Although usually a simplifying assumption is applied in risk analysis (those people

inside the flash fire dye, those outside do not undergo any damage), some experimental

work would be quite interesting, although it seems rather complex to perform it.

Experimental work: general considerations

A set of variables have an influence on the thermal intensity reaching a target. Amongst

them, the following ones are the most significant:

- pool surface, fuel mass flow rate or fuel mass involved- burning velocity- flames size-

flames shape- flames location- flames temperature- surface emissive power- radiant heat fraction- view factor- atmospheric transmissivity.

Of course, these variables depend on the type of fire, on the substance involved and on

some meteorological conditions. For example, the emissive power has not the same

value for a pool fire than for a jet fire, the flames shape depend on the existence of wind

in the case of a pool fire and on the direction (horizontal, inclined or vertical) in the caseof a jet fire, etc.

Predicting the radiative characteristics of large flames is still subject to considerable

uncertainty, because some parameters associated with large turbulent diffusion flames

cannot be determined accurately for a given fire.

Thus, experimental research should be and, generally, has been devoted to allow a

better prediction of these variables taking into account these relationships.

There is a considerable literature describing experimental studies on thermal radiation

from flames. However, a number of these studies have focused on small-scale pool firesor jet fires, which differ significantly from large turbulent fires (Gritzo et al, 1998). Pool


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fires of less than 1 m diameter can not be considered representative of real full-scale

fires occurring in process or storage plants. The same can be stated about jet fires with a

length less than approximately 0.5 m. Concerning fireballs, experimental work has been

restricted to few experiments performed at rather small scale. Finally, as for flash fire

experimental data probably the most difficult to be obtained as far as we know no

data are available.

Experimental work on pool fires

Experimental data obtained from large pool fires of different fuels (crude oil, kerosene,

heptane, JP4, etc.) have been published by Koseki (1989, 2000). Hayasaka et al (1992)

measured the emissivity for heptane pools with a diameter of 3 m. Planas et al (2003)

measured also the emissivity from hydrocarbon pool fires by using infrared

thermography.

Experimental work on jet fires

Experimental work on fireballs

Research needs

Conclusions

References

BMIIB. The Buncefield incident. The final report of the Major Incident Investigation

Board. OPSI. Richmond, 2008.

Casal, J. Evaluation of the Effects and consequences of Fire Accidents in Industrial

Plants. Elsevier. Amsterdam, 2008.

CCPS. Center for Chemical Process Safety. Guidelines for Consequence Analysis of

Chemical Releases. AIChe. New York, 1999.

CPR14E. Methods for the calculation of physical effects (Yellow Book). Ministerie van

Verkeer en Waterstaat. The Hague, 1996.

Gmez-Mares, M., Zrate, L., Casal, J. Fire Safety Journal (2008).

Gritzo, L. A., Sivathanu, Y. R., Gill, W. Combustion Science and Technology 139

(1998) 113-136.

Planas, E., Montiel, H., Casal, J. Trans. IChemE, 75, Part B (1997) 3.

Thomas, P. H. The size of flames from natural fires. 9th Symposium on Combustion.

Academic Press. New York, 1963.

fire accidents in process plants

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