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Flame Acceleration and Deflagration to Detonation Within Pipelines
There are a number of factors which determine how a flame accelerates within a pipeline system. Which of these is most important can and will vary dependant upon the particular pipeline configuration.
Factors influencing flame acceleration include:
The chemical composition of the products within the system is, in most systems, the largest influence upon any flame acceleration.
Pressure within the pipeline has a large effect upon the flame acceleration and the likelihood of transition to detonation. Furthermore, since detonation cell size is directly related to pressure, the system pressure is an important parameter in determining whether a detonation will occur within a pipe.
Many of the pressure effects are as a direct consequence of the change of density and hence number of molecules and mean free path. Put simply the highe the pressure at a given temperature the more gas molecules there are to react and hence the greater potential energy release.
Temperature, like pressure, is important in determining the density of fluids within the pipe. The temperature also has an important effect in terms of heat transfer - cold pipes can under certain circumstances limit flame acceleration even to the point of preventing a relatively reactive mixture from running to detonation.
Turbulence is known to play an important part in flame acceleration. Generally greater levels of turbulence give rise to faster rates of turbulence and with a flame accelerating down a tube a positive feedback effect arises.
The diameter of a pipeline is fundamental to whether it is possible for a detonation to be established and sustained within a pipeline for any given flammable mixture. Each mixture, for a given temperature and pressure, has a characteristic detonation cell size. The larger the detonation cell size the larger the pipe diameter required to allow a detonation to sustain.
Potential ignition sources include the classics such as electrical sparks and hot surfaces. Other more exoteric ignition sources include laser ignition, exploding wires and chemical igniters.
The ignition source energy is an important factor in determining (a) if an ignition will occur and (b) consequent flame propagation prpoerties.
Normally in a pipeline a flame kernel will grow from the igntion point and the flame expand to fill the pipe diameter and subsequently propagating along the tube. The mode of propagation, in particular whether a transition to detonation will occur, will depend critically on the chemistry of the flammable mixture and also on pressure, temperature, tube diameter and roughness, and any bends or other tube fittings.
It is possible with a high energy ignition source to directly initiate a detonation.
A flow restriction in a pipe can have a considerable influence on the flame propagation.
Characteristics of Detonations
Mitigation techniques for pipeline explosions
Several mitigation techniques to minimise the probability of unwasnted pipeline explosions occurring which can be combined with methods of reducing the impact of any that do occur.