Sounds Dangerous – Analyzing Sound Power Level in AFT Arrow

As gases flow through pressure reducing devices, the pressure drops and the gas expands, experiencing irreversible energy losses. At these locations, the gas undergoes turbulent mixing and deflects off the pipe walls at high velocities. If the flow is sonically choked due to the restricting device, the flow must also pass through a standing shockwave immediately downstream of the restriction. All these processes contribute to production of high frequency acoustic energy (500 to 2000 Hz) in and around the pipe and vibration in the pipe wall. This type of noise is tonal in nature, occurring in finite frequency bands, so noise levels can be impacted by resonance.

Nearby geometric discontinuities in the pipe wall from welded supports, measurement instruments, and other welds can be subject to high levels of stress due to these vibrations and associated wall flexture, known as acoustic fatigue. Extreme sources of high frequency acoustic excitation can lead to a system failure within hours or even minutes.

Beyond weld failures, persistent vibration can damage seals or electrical equipment, among other system components, as well as presenting a hazard to human hearing and a nuisance to the surrounding community and environment. Needless to say, it is important to identify and resolve potential acoustic problems in the design phase and to do so as early as possible so they can be avoided or mitigated.

The Energy Institute provides guidelines for assessing and avoiding piping failure due to vibration using a methodology of calculating likelihood of failure (LOF) due to high frequency acoustic excitation, flow induced vibration, flow induced pulsation, mechanical excitation, and surge or momentum changes. More information about using AFT Arrow to analyze flow induced pulsation and vibration or LOF from high frequency excitation can be found in our blog The Opposite of Good Vibrations - Evaluating FIV, AIV, FIP Guidelines from AFT Fathom and AFT Arrow. If you are interested in analyzing surge forces, or excitation by reciprocating compressors and other pulsation sources that interact with natural system frequencies, AFT xStream should be used.

AFT Arrow can automatically calculate the sound power level from high frequency acoustic excitation (AIV) for junctions with a pressure loss using the Energy Institute approach (Energy Institute 2008, pg 60) and can identify noise sources that potentially require a more detailed LOF analysis. Not only does this highlight potential problems quickly, but it also allows for rapid LOF analysis and ensures your focus is on the correct parts of the system for noise safety considerations. Sound power level is increased by the presence of sonic choking, which is accounted for automatically.

To see sound power levels in your Arrow model, add the parameter Sound Power Level to the junction output parameters within Output Control as shown in Figure 1.

Sound power level results are shown in the Output window within the Applied Standards tab in the general section and any tabs within the junctions section as shown in Figure 2 below.  

Sound power level results are available for junctions that have flow, a pressure loss across them, and one upstream and one downstream pipe. These results for the noise levels at the source and additional calculations would be required to determine the noise perceived by an observer at a given location and do not include the noise produced by other junctions. Sound power level can be used as a parameter for Design Alerts and Workspace Layers labels, or as a goal for the Goal Seek and Control Module. These capabilities can be used in powerful ways such as creating design alerts for occupational noise exposure limits or identifying minimum Cv values for valves to keep noise below some limit.

The relevant threshold for Energy Institute LOF analysis is 155 dB, so there is a built-in warning message when junctions exceed this value. This warning can be shown if the sound power level is selected in Output Control prior to running the model and appears as shown in Figure 3.

When you receive this warning on a junction that has no noise mitigation measures, a more detailed LOF analysis will be necessary, and there is a spreadsheet with instructions in the Arrow installation folder to complete the next stages of the LOF study. If the device in question has some sort of noise abatement technology (e.g. low noise valve trim), the noise mitigation rating should be subtracted from the Arrow sound power results and compared again with the 155 dB boundary.

If the likelihood of failure analysis ultimately concludes that there is unacceptable risk of failure at a small-bore discontinuity due to high frequency noise, extra design consideration may need to be given to the connection to the parent pipe (weldolet etc.) and noise mitigation may be necessary at the noise source. Pipes may require noise insulation and piping supports may also require vibration-isolating pads.

Sound power level results in AFT Arrow are crucial for identifying these needs before they can lead to a safety hazard.

References

Energy Institute, Guidelines for the Avoidance of Vibration Induced Fatigue Failure in Process Pipework, 2nd Edition, Energy Institute, London, UK, 2008.