Definitions

"Combustion terminology a disaster area" is the title of a paper by Bradley and Weinberg (1991). They pointed out that combustion nomenclature is in an unholy mess. Words like 'burning velocity', 'flame speed', 'flammable', 'inflammable', 'non-flammable', 'deflagration' and 'detonation' are often used wrongly. This is also our experience.

This lack of coherent nomenclature makes it very difficult for those who want to use the results from gas explosion research in practical safety work in the industry. Even the phenomenon that we are talking about has several names: 'Gas explosion', 'gaseous explosion', 'unconfined vapour cloud explosion', 'vapour cloud explosion' or 'fuel-air explosion'. In this handbook we have decided to use the term 'gas explosion'. We find this term the simplest, the least confusing, and the most general term for explosions caused by burning of premixed fuel-air or fuel-oxidiser in gas phase.

The objective of this chapter is to present the definitions used in this handbook. You may find different definitions in other literature. If you disagree with any of our definitions, please use our comment form.

2.1 Explosion

We define an explosion as an event leading to a rapid increase of pressure. This pressure increase can be caused by: nuclear reactions, loss of containment in high pressure vessels, high explosives, metal water vapour explosions, run-a-way reactions, combustion of dust, mist or gas (incl. vapours) in air or in other oxidisers.

2.2 Combustion

The burning of gas, liquid, or solid in which fuel is oxidised involves heat release and often light emission. Combustion of methane (CH₄) in air can be described by the chemical equation :

The chemical products from complete combustion of a hydrocarbon fuel are mainly CO₂ and H₂O (vapour). The combustion process will result in increased temperature due to the transformation of chemically bound energy into heat. It should be emphasised that the above equation constitutes a strong simplification of the real combustion process.

Combustion of gaseous fuel in air can occur in two different modes. One is the fire, where fuel and oxygen is mixed during the combustion process. In the other case the fuel and air (or another oxidiser) is premixed and the fuel concentration must be within the flammability limits. In general the premixed situation allows the fuel to burn faster, i.e. more fuel is consumed per unit time.

Figure 2.1.

Illustration of jet fire and gas explosion.

2.3 Gas Explosions

We define a gas explosion as a process where combustion of a premixed gas cloud, i.e. fuel-air or fuel-oxidiser, is causing rapid increase of pressure. Gas explosions can occur inside process equipment or pipes, in buildings or offshore modules, in open process areas or in unconfined areas.

The consequences of a gas explosion will depend on the environment in which the gas cloud is contained or which the gas cloud engulfs. Therefore it has been common to classify a gas explosion from the environment where the explosion takes place: i) Confined Gas Explosions within vessels, pipes, channels or tunnels. ii) Partly Confined Gas Explosions in a compartment, buildings or off-shore modules and iii) Unconfined Gas Explosions in process plants and other unconfined areas. It should be pointed out that these terms are not strictly defined. In an accidental event it may be hard to classify the explosion. As an example an unconfined explosion in a process plant may also involve partly confined explosions in compartments into which the gas cloud has leaked.

2.4 Confined Gas Explosions

Confined gas explosions are explosions within tanks, process equipment, pipes, in culverts, sewage systems, closed rooms and in underground installations. Confined explosions are also called internal explosions.

Figure 2.2.

Confined explosion within a tank.

Typical for this kind of explosion is that the combustion process does not need to be fast in order to cause serious pressure build-up. Chapter 9 covers gas explosions within vessels, pipes, channels and tunnels in more detail.

2.5 Partly Confined Gas Explosions

Partly confined explosions occur when a fuel is accidentally released inside a building which is partly open. Typical cases are compressor rooms and offshore modules. The building will confine the explosion and the explosion pressure can only be relieved through the explosion vent areas, i.e. open areas in the walls or light relief walls that open quickly at low overpressure. As discussed in Chapter 10 both size and location of explosion vent areas are important for the resulting explosion pressure.

Figure 2.3.

Gas explosion in a partly confined area with process equipment.

2.6 Unconfined Gas Explosions

The term unconfined was used to describe explosions in open areas such as process plants. Large scale tests have demonstrated that a truly unconfined, unobstructed gas cloud ignited by a weak ignition source will only produce small overpressures while burning (flash fire). The term unconfined gas explosions should therefore be used with care. In a process plant there are local areas which are partly confined and obstructed. In case of a deflagration it is these areas that are causing high explosion pressures.

Figure 2.4.

Gas explosion in a process area.

However if an unconfined cloud detonates the explosion pressure will be very high, in the order of 20 barg and in principle independent of confinement and obstructions. Unconfined gas explosions are discussed further in Chapter 11.

2.7 Vapour Cloud Explosions (VCE)

There is no essential difference between a Vapour Cloud Explosion and a Partly Confined or an Unconfined Gas Explosion. In this handbook we will use the term gas explosion and we will not differentiate between vapour cloud explosions and gas explosions.

2.8 Flame Speed and Burning Velocity

Flame speed, S, is defined as velocity of the flame relative to a stationary observer i.e. the ground or an other fixed frame. The burning velocity, U, is the velocity of the flame front with respect to the unburned gas immediately ahead of the flame. The relation between flame speed, S, and burning velocity, U, is therefore:

where u is velocity of the unburned gas just ahead of the flame. For Stoichiometric (see section 4.3) hydrocarbon-air mixtures S is of the order of 8*U.

Figure 2.5.

Flame propagation in a tube. The flame speed, S, is defined as the velocity of the flame relative to the ground or another fixed frame. u is the velocity of the unburned gas ahead of the flame.

2.9 Burning Rate

The burning rate [kg/s] is the amount of fuel consumed by the combustion process per time unit. The burning rate is a measure of the rate of energy release in an explosion. The burning rate may also be defined as mass of fuel consumed per unit time and volume.

2.10 Deflagrations

A deflagration is defined as a combustion wave propagating at subsonic velocity relative to the unburned gas immediately ahead of the flame, i.e. the burning velocity, U, is smaller than the speed of sound, C, in the unburned gas. The velocity of the unburned gas ahead of the flame is produced by the expansion of the combustion products.

In an accidental gas explosion the deflagration is the common mode of flame propagation. In this mode the flame speed, S, ranges from order of 1m/s up to 500 -1000 m/s corresponding to explosion pressures between a few mbar and several bar.

For strong deflagrations, shock waves may propagate ahead of the deflagration (i.e., the flame).

2.11 Detonations

A detonation is defined as a combustion wave propagating at supersonic velocity relative to the unburned gas immediately ahead of the flame, i.e., the detonation velocity, D, is larger than the speed of sound, C, in the unburned gas.

In simple terms, a detonation wave can be described as a shock wave immediately followed by a flame (ZND theory). The shock compression heats the gas and triggers the combustion. However, an actual detonation wave is a three-dimensional shock wave followed by the reaction zone.

Figure 2.6.

A detonation wave can be described as a shock wave immediately followed by a flame (ZND theory)

For fuel air mixtures at ambient pressure the detonation velocity can be up to 2000 m/s and the maximum pressures produced are close to 20 bar.

A detonation can either: i) be initiated directly by detonating a high explosive charge, or ii) be produced when a deflagration accelerates due to obstacles and confinement and transits into a detonation.

2.12 Turbulence

In fluid dynamics we divide the flow into laminar and turbulent regimes. Laminar flow means that the fluid flows in laminars or layers, while turbulent flow is characterised by an irregular random fluctuation imposed on mean (time-averaged) flow velocity. Figure 2.7 shows the trajectory of a particle in laminar and turbulent flows.

Figure 2.7.

Illustration of particle trajectories in laminar and turbulent flows.

Whether the flow is laminar or turbulent depends mainly on flow velocity u, characteristic dimension of the geometry L, and kinematic viscosity u. The Reynolds number Re, is defined by:

and is a dimensionless parameter characterising whether the flow regime is laminar or turbulent.

Figure 2.8 shows the flow field around a cylinder in a crossflow for different Re. The characteristic length scale, L, for this geometry is the diameter of the cylinder. For a low Re and low flow velocity, the flow around the cylinder is laminar. For higher Re vortices develop in the wake of the cylinder and the flow in the volume will be turbulent.

Figure 2.8.

Cylinder in a crossflow at different Reynolds number Re.

The turbulence is very important for how fast the flame can propagate in a premixed gas cloud. The turbulence will wrinkle the flame front and increase diffusion of heat and mass and thereby cause higher burning rate.

2.13 Hydrodynamic Instability

The interface between a light and a heavy gas is stable if the fluid is accelerated in the direction of the positive density gradient. However, if the fluid is accelerating in the other direction, the interface is unstable.

Figure 2.9.

Acceleration of density gradient illustrating the Taylor instability. Left is stable and right is unstable.

This hydrodynamic instability phenomenon also occurs in gas explosions. If the flame front is exposed to a compression wave propagating from the heavy gas (i.e. fuel-air) into the light gas (i.e. burnt gas) or a rarefaction wave propagating in the opposite direction, the flame front becomes wrinkled (unstable) and the burning rate increases.

This instability phenomenon is known as a Taylor instability.

2.14 Flash Fires

A flash fire is the term for a slow deflagration of a premixed, truly unconfined, unobstructed gas cloud producing negligible overpressure.

Thermal effects are the main hazard.

2.15 BLEVEs

Acronym for Boiling Liquid Expanding Vapour Explosion. The BLEVE is an explosion due to flashing of liquids when a vessel with a high vapour pressure substance fails. The failure of the vessel is often caused by an external fire as shown in Figure 2.10.

Figure 2.10.

A situation that can lead to a BLEVE.

If the substance released is a fuel, the BLEVE can result in very large fire balls. Rocketing vessels are also hazards related to BLEVEs. Figures 2.10 and 2.11 show a BLEVE and a fire ball in a tank car accident, but BLEVEs can also happen in process areas or in offshore modules.

Figure 2.11.

Fire balls and rocketing vessels are often the main hazards of a BLEVE.

2.16 Shock Wave

A shock wave in a gas can be defined as a fully developed compression wave of large amplitude, across which density, pressure, and particle velocity change drastically (McGraw-Hill, 1978).

The thickness of a shock wave is of the order of the mean free path and may be treated as a discontinuity.

Figure 2.12.

A shock wave followed by a rarefaction wave.

A shock wave propagates at supersonic velocity relative to the gas immediately ahead of the shock, i.e. the gas ahead is undisturbed by the shock. The propagation velocity of the shock wave depends on the pressure ratio across the wave. Increasing pressure gives higher propagation velocity.

2.17 Blast Wave

A blast wave can be defined as the air wave set in motion by an explosion (McGraw-Hill,1978).

Figure 2.13.

Free field blast wave.

The term blast wave includes both sonic compression waves, shock waves and rarefaction waves. Figure 2.14 illustrates in principle different types of blast waves. We can have i) a shock wave followed by a rarefaction wave, ii) a shock wave followed by a sonic compression wave and then a rarefaction wave, iii) a sonic compression wave and a rarefaction wave. The type of blast wave depends on how and when the energy is released in the explosion and the distance from the explosion area. For strong explosions category i) is typical. Weak explosions gives initially category iii), but the wave can be shocking up and end as category i) when it propagates away from the explosion.

Figure 2.14.

Blast waves.

Blast waves from TNT-explosions and other military tests are often divided into ranges depending on the peak overpressure. In order to avoid confusion we should use the same classification for blast waves from gas explosions. The classification is given in Table 2.1. It should be noted, however, that this classification is not fully consistent, since by the definition below far-field pressures can be said to occur inside a gas explosion of for instance 0.5 bar overpressure. One should therefore ensure that the range classification is applied only to sufficiently strong gas explosions and only outside the cloud.

Table 2.1.

Classification of close-in, mid distance, and far field blast waves (Shepherd et al.,1991).

In this handbook we will use the term free field blast as a definition of a propagating hemispherical blast wave outside the exploding cloud.

2.18 Pressure

Pressure is a type of stress which is exerted uniformly in all directions; its measure is the force exerted per unit area (McGraw-Hill,1978).

In fluid dynamics we often use the terms i) static pressure, ii) dynamic pressure and iii) stagnation pressure.

Static pressure is what we normally call the pressure. The strict definition of static pressure is: a) the pressure that would exist at a point in a medium if no sound waves were present, or b) the normal component of stress, the force per unit area, exerted across a surface moving with the fluid, especially across a surface which lies in the direction of the flow (McGraw-Hill,1978).

Dynamic pressure is the pressure increase that a moving fluid would have if it was brought to rest by isentropic flow against a pressure gradient (McGraw-Hill,1978) The dynamic pressure can also be expressed by the flow velocity, u and density, r:

Stagnation pressure is the pressure that a moving fluid would have if it was brought to rest by isentropic flow against a pressure gradient (McGraw-Hill,1978). The stagnation pressure is the sum of the static and the dynamic pressures.

For blast waves and shock waves we use the terms side-on pressure and reflected pressure. The side-on pressure is measured perpendicular to propagation direction of the wave. Side-on pressure is the static pressure behind the shock wave. The reflected pressure is measured when the wave hits an object like a wall head-on. Since reflection is not an isentropic process there is a difference between stagnation pressure and the reflected pressure. These definitions of side-on and reflected pressures are illustrated in Figure 2.15.

Figure 2.15.

Side-on pressure and reflected pressure.