TECHMAIL: Gas Explosions

It's not a pleasant subject, but when one deals with pressure piping it is always a possibility.

Of course, there are a bunch of other piping-related safety links at:

<a href="http://www.PipingDesign.com/safetydevices.html">http://www.PipingDesign.com/safetydevices.html</a>

This is a link to Christian Michelsen Research AS's Gas Explosion Handbook:

<a href="http://www.cmr.no/english/gexcon/Handbook/GEXHBchap1.htm">http://www.cmr.no/english/gexcon/Handbook/GEXHBchap1.htm</a>

<<We have all heard about accidental gas explosions and the destruction they can lead to. Fortunately most of us will not experience accidental explosions. But preventing them from happening requires a good understanding of what a gas explosion is and what one can do to reduce the frequency and consequences of such events.

The objective of this chapter is:

i) to give an introduction to the field of gas explosions ii) to give an overview of loss experience iii) to show how we can use our knowledge to improve safety.

This chapter covers these aspects of gas explosions briefly. It is intended to be a first introduction to the field, and should be read the first time the handbook is used.

1.1 What is a Gas Explosion

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 off-shore modules, in open process areas or in unconfined areas. When we are talking about a gas explosion as an event, it is a more general term. It is then common to include the events both before and after the gas explosion process, see the diagram below.

Figure 1.1. An event tree showing typical consequences of accidental releases of combustible gas or evaporating liquid into the atmosphere.

Figure 1.1 shows what can happen if combustible gas or evaporating liquid is accidentally released into the atmosphere. If the gas cloud, formed from the release, is not within the flammability limits or if the ignition source is lacking, the gas cloud may be diluted and disappear. Ignition may occur immediately, or may be delayed by up to tens of minutes, all depending on the circumstances. In case of an immediate ignition (i.e. before mixing with air or oxidiser has occurred) a fire will occur.

The most dangerous situation will occur if a large combustible premixed fuel-air cloud is formed and ignites. The time from release start to ignition can be from a few seconds up to tens of minutes. The amount of fuel can be from a few kilograms up to several tons.

The pressure generated by the combustion wave will depend on how fast the flame propagates and how the pressure can expand away from the gas cloud
(governed by confinement). The consequences of gas explosions range from no
damage to total destruction. The pressure build-up due to the gas explosion can damage personnel and material or it can lead to accidents such as fires and BLEVE's (domino effects). Fires are very common events after gas explosions.

When a cloud is ignited the flame can propagate in two different modes through the flammable parts of the cloud. These modes are:

i) deflagration
ii) detonation

The deflagration mode of flame propagation is the most common. A deflagration propagates at subsonic speed relative to the unburned gas, typical flame speeds (i.e. relative to a stationary observer) are from the order of 1 to 1000 m/s. The explosion pressure may reach values of several barg, depending on the flame speed (see Section 5.1).

A detonation wave is a supersonic (relative to the speed of sound in the unburned gas ahead of the wave) combustion wave. The shock wave and the combustion wave are in this case coupled. In a fuel-air cloud a detonation wave will propagate at a velocity of 1500-2000 m/s and the peak pressure is typically 15 to 20 bar.

In an accidental gas explosion of a hydrocarbon-air cloud (ignited by a weak source - a spark) the flame will normally start out as a slow laminar flame
(see sections 2.12 and 4.10) with a velocity of the order of 3-4 m/s. If the
cloud is truly unconfined and unobstructed (i.e. no equipment or other structures are engulfed by the cloud) the flame is not likely to accelerate to velocities of more than 20-25 m/s, and the overpressure will be negligible if the cloud is not confined.

Figure 1.2. Gas explosion in a partly confined area with process equipment.

In a building or in an offshore module with process equipment as shown schematically in Figure 1.2, the flame may accelerate to several hundred meters per second. When the gas is burning the temperature will increase and the gas will expand by a factor of up to 8 or 9. The unburned gas is therefore pushed ahead of the flame and a turbulent flow field is generated. When the flame propagates into a turbulent flow field, the effective burning rate will increase and the flow velocity and turbulence ahead of the flame increases further. This strong positive feedback mechanism is causing flame acceleration and high explosion pressures and in some cases transition to detonation.

In a confined situation, such as a closed vessel, a high flame velocity is not a requirement for generation of pressure. In a closed vessel there is no or very little relief (i.e. venting) of the explosion pressure and therefore even a slow combustion process will generate pressure (constant volume combustion, see section 4.9).

The consequences of a gas explosion will depend on:

type of fuel and oxidiser
size and fuel concentration of the combustible cloud location of ignition point
strength of ignition source
size, location and type of explosion vent areas location and size of structural elements and equipment mitigation schemes

Gas explosions may be very sensitive to changes in these factors. Therefore it is not a simple task to estimate the consequences of a gas explosion.>> Received on Sat Aug 05 08:02:00 2000