High Frequency Acoustic Excitation (HFAE)

The presence of sonic flowing gas gives rise to high levels of noise. This noise, generated at pressure let-down points, is mainly broad-band but may have additional energy at frequencies defined by pipe, or pipe element, dimensions.

The generated noise comprises two elements, a shock component which contains some periodicity and a turbulent component which is more broad-band.

The calculation of High Frequency Acoustic Excitation (HFAE) can be undertaken using a number of methods, and Spectrum uses in-house field proven algorithms for detailed studies. These give accurate predictions in octave bands 63 Hz-8 kHz, useful for analysis of pipe breakout noise. Procedures relating to VIFIPP however tend to incorporate simpler calculation methods, such as those described in the Energy Institute Guidelines.

Effect on Pipework
The high frequency, and short wavelengths associated with HFAE are generally unable to excite flexing modes in piping. They do however have the capability of exciting ring and shell modes, especially in large diameter thin walled pipework such as found in flare collection headers. At junctions with small bore pipework, dynamic differential movements and high stress concentrations can lead to Vibration Induced Fatigue failure.

Mitigation
Shell modes in thin walled pipes can be detuned by circumferential stiffening applied to the mainline close to the small bore connection. Highest quality fittings, such as welded tees, should be used, where necessary, to minimise the risk of fatigue failure.