Vapourcloudexplosionmodelling 151118121531-lva1-app6891

VAPOUR CLOUD EXPLOSION MODELLING

By: KUNDAN KUMAR

VAPOUR CLOUD EXPLOSIONS-:• A vapor cloud explosion occurs when a large quantity of flammable

vapor or gas is released, mixes with air and is subsequently ignited.

• The vapor or gas fuel is usually released due to the loss of process

containment. This could include the failure of a pipe, storage vessel or

a process reactor.

• The rapid discharge of flammable process material through a relief

system may result in a VCE.

CONDITIONS GENERALLY BE PRESENT FOR A VCE-:

• The released material must be flammable

•A cloud of sufficient size must form prior to ignition. If the cloud is too small, or is ignited early in the release, only a small fire ball will result without significant overpressures. A jet or pool fire may subsequently form.

CONTINUED-:

• The vapor cloud must mix with air to produce a sufficient mass in the flammable range of the material released. Without sufficient air mixing, a diffusion controlled fire ball may result with out significant overpressures developing.

• The speed of the flame propagation must accelerate as the vapor cloud burns. With out this acceleration, only a flash fire will result, which may pro duce significant damage due to thermal radiation and direct flame impingement

FAVOURABLE CONDITIONS FOR VCE CAN BE SHOWN AS-:

PHENOMENON OF EXPLOSION-:

1. INITIATION OF EVENT-:• Flammable vapor clouds may be ignited from a number of sources

that may be continuous (e.g., fired heaters, pilot flames) or occasional (e.g., smoking, vehicles electrical systems, static discharge).

• Clouds are normally ignited at the edge as they drift. The effect of ignition is to terminate further spread of the cloud in that direction.

CONTINUED-:

• Flash fires initially combust and expand rapidly in all directions. After the initial combustion, expansion is upward because of buoyancy.

•As the number of ignition sources increases the likelihood of ignition will generally increase correspondingly. Thus, a site with many ignition sources on or around it would tend to prevent clouds from reaching their full hazard extent, as most such clouds would find an ignition source before this occurs

2. FLAME ACCELERATION

• Flame acceleration is an important part of the vapor cloud explosion.

• The flame accelerates when turbulence stretches and tears the flame front increasing its surface area.

• The primary turbulence sources are flow turbulence established in the unburned gas as it flows ahead of the flame front, pushed by the expanding combustion products behind it; and turbulence caused by the inter actions of the gas with obstacles it encounters.

CONTINUED-:• As the turbulence increases, stretching the flame front, the

rate at which the fuel is combusted increases because the area of the stretched and torn flame front has increased.

• As the rate of combustion increases the push on the unburned gases increases, causing them to move even faster, increasing the turbulence further.

• A congested process area, populated with pipes, pumps, valves, vessels, and other process equipment is adequate to result in significant flame speed acceleration.

TNT Equivalency model-:TNT equivalency is a simple method for equating a known energy of a combustible fuel to an equivalent mass of TNT.The equivalent mass of TNT is estimated using the following equation:

W= --------(1)

where W = the equivalent mass of TNT (mass)

η = the empirical explosion efficiency (unitless) m = the mass of flammable gas (mass) Ec = the heat of combustion of the flammable gas (energy/mass) ETNT = the energy of explosion of TNT (energy/mass)(4437-4765 kJ/kg)

ηmE c𝐸𝑇𝑁𝑇

The procedure to estimate the damage associated with an explosion using the TNT equivalency method is as follows:1. Determine the total quantity of flammable material involved in the explosion. This can be estimated from the total quantity released or from a dispersion model.2. Estimate the explosion efficiency and calculate the equivalent mass ofTNT by Equation 1.

3. Use the scaling law given by Equation-

Z=

and Figure 1 to estimate the peak side-on overpressure.

ESTIMATING PEAK SIDE ON OVER PRESSURE-:

• For shock waves resulting from the detonation of high explosives, a well-defined relationship exists between overpressure and mass of TNT detonated. The figure shows the scaled peak side-on overpressure and scaled impulse correlated as a function of the scaled distance Z, which is defined as,

Z=

where Z is the scaled distance (distance/mass1/3), R is the distance from the explosion center (distance), W is the mass of TNT (mass).

The scaled overpressure is defined as-:

Ps=

where ps is the scaled overpressure, po is the peak side-on overpressure, pa is the absolute ambient pressure The term “overpressure” always refers to a gauge pressure.

Scaled overpressure and impulse curves for a TNT explosion on a surface

UNDERSTANDING GAS DYNAMICS AND OVERPRESSURE-:• An explosion results from the very rapid release of energy.

• This energy is then dissipated by a variety of mechanisms, including formation of a pressure wave, projectiles, thermal radiation, acoustic energy, or physical translation of equipment.

• If the explosion occurs in a gas, the energy causes the gas to expand rapidly, forcing back the surrounding gas and initiating a pressure wave which moves rapidly outward from the blast source.

IMPORTANT DEFINITIONS-:• BLAST WAVE-A pressure wave propagating in air is called a

blast wave.

If the pressure front has a very abrupt pressure change, it is called a shock wave or shock front.

• PEAK OVERPRESSURE-The maximum pressure over ambient is called the peak overpressure.

A typical shock wave at a fixed time Shock wave pressure at a fixed location.

CONTINUED-:

• SIDE-ON OVER PRESSURE- If the pressure transducer is at right angles to the blast wave, the overpressure measured is called the side-on overpressure and maximum value at fixed location is called PEAK SIDE-ON OVER PRESSURE.

• REFLECTED OVERPRESSURE- If the pressure transducer is placed in the middle of a large wall facing toward the oncoming shock wave, then the pressure measured is the reflected overpressure.

• DYNAMIC OVERPRESSURE-It is equal to ½ρu2, where ρ is thegas density and u is the gas velocity and maximum value of it is called PEAK DYNAMIC OVER PRESSURE.• BLAST IMPULSE- The change in momentum.

Overpressure curves at various times after the initial explosion

Side-on overpressure and dynamic pressure at a fixed location. For side-on overpressures of less than about 70 psi the dynamic pressure is less than the side-onoverpressure