Stable and Unstable Detonations

Unstable to Stable Transition

An unstable detonation will degrade to a less severe stable detonation as the flame proceeds further down the pipe.

Galloping Detonations

However, such flame fronts may suddenly undergo further transitions to unstable conditions. These events are unpredictable, and are caused by anything that may increase the turbulence of the system, including roughness in the inside surface of the pipe, or a protruding gasket or instrument port as well as bends or constrictions due to valves. If the flame front continues to undergo transitions to unstable conditions it is referred to as a 'galloping' detonation, and can cause severe damage to plant if unchecked.

Risks Associated with Assuming a Detonation will Remain Stable

Because of this unpredictability, most flame arrester manufacturers recommend that an unstable detonation arrester is fitted in the pipe. A few manufacturers will take the risk of fitting a stable detonation arrester under such circumstances, but this is not recommended by Elmac Technologies^®.

Elmac's Policy on Stable Detonation Arresters

Elmac Technologies has a clear policy on stable detonation arresters which can be downloaded from the Data Centre.

In summary it concludes:

• Unstable detonations are much more destructive events than stable detonations
• The location of unstable detonations cannot be predicted with any certainty
• A stable detonation may undergo sudden transition to an unstable detonation
• It follows that use of a stable detonation arrester in isolation is not sufficient to protect a system from transmission of a flame and exposes the user to significant risk.
• In any system where detonation events are a possibility, an appropriately designed and tested unstable detonation arrester device should 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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