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Gas systems, at relatively high flow velocities, can exhibit a form of tonal excitation which is generated when flow past the end of a 'dead leg' branch generates an instability at the mouth of the branch connection (refer to Figure 2-6), similar to blowing across the top of a bottle generating a tonal response. Process examples are a branch line with a closed end, such as a relief line or a recycle line with the valve shut. This leads to the generation of vortices at discrete frequencies which, if these frequencies coincide with an acoustic natural frequency of the branch, can generate high levels of pressure pulsation. The generation of the flow instability is heavily dependent on flow rate, and the highest flow rate may not be the worst case condition.
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