What is a Detonation?

A detonation occurs where a flame travels along a pipe, usually at supersonic velocities and is combined with a shock wave. Typically this occurs as a result of turbulence-induced flame acceleration caused by roughness in the pipe walls or interruptions such as bends, valves or changes in section of the pipe. It can also occur simply by allowing the flame to continue to accelerate along a pipe for a sufficient distance. A shock wave is characterised by a step change in pressure and density through which the flame velocity changes from being subsonic to supersonic.

Development of a Flame Front

The chart below illustrates what typically happens to a flame front when allowed to burn unhindered down a straight pipe section with a diameter in the order of 200 mm filled with a hydrocarbon gas at ambient pressure and temperature.

Figure 1. Typical flame acceleration in a pipe.

It can be seen from Figure 1 that the flame begins as a slow deflagration (flame speed < 300 m/s), but accelerates into a fast deflagration (still at subsonic velocities ~ 500 m/s). It is recommended that a deflagration arrester is fitted for situations in which the ignition source is within 50 pipe diameters of the arrester for hydrocarbon gases (within 30 pipe diameters for hydrogen), although this distance is reduced for systems at pressures above atmospheric.

The flame then undergoes a rapid and sudden transition from deflagration to detonation. Under these conditions, the flame may accelerate to a velocity an order of magnitude higher than the initial slow deflagration (~ 2500 m/s). This represents the worst case conditions for the flame front and the associated pressure wave and is known as an overdriven detonation, or an unstable detonation. In such circumstances, a suitably designed unstable detonation arrester must be fitted.

More on detonations

1. What is a Detonation?
2. Stable and Unstable Detonations
3. Deflagration vs Detonation Arresters
4. Selecting a Detonation Arrester

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