Abating Acoustical Noise in Piping

Last week, I shared how resonance can contribute to excessive machine vibration in the post, Spotting and Fixing Resonance in Plant Equipment. A colleague pointed me toward another true Emerson Expert, Dr. Allen Fagerlund. Al’s expertise is on noise measurement and prediction. He also serves on the ISA75, Control Valve Standards and its subcommittee, ISA75.07, Control Valve Noise Measurement and Prediction.

I came across a paper that he and colleagues had written a few years ago, Identification and Prediction of Piping System Noise. It describes how various components in a plant’s piping system can be sources of noise. The piping system forms a network through the facility, with sound waves radiating through this network. The authors describe this noise:

In an ideal sense, noise generated by any component or source will propagate in the fluid and cause the pipe wall to vibrate, with subsequent radiation from the outer surface to an observation point. In reality there also can be a direct structureborne path from the source to the pipe which adds to both the vibration level and to the radiated noise.

For flowing, incompressible liquids:

Generally liquid noise is not a problem unless cavitation [formation of vapor bubbles] occurs somewhere in a system. Since the noise from cavitation is an indicator of potential damage to piping and equipment, it has been more important to develop guidelines to prevent cavitation than to develop methods to predict the level of the noise.

I know from my days on offshore platforms that high-pressure natural gas piping was a large source of noise. Compressible fluid flows are potential sources of noise across a broad frequency range. The authors note that structural resonance is typically checked in the system design phase, but acoustic resonance may not be. The fix:

…is either to eliminate or change the frequency of the source tone or to decouple the resonance, whichever is most efficient and/or effective. Broadband noise can also excite system resonances which makes decoupling more difficult. Reducing the source noise levels are the main option.

Sources of noise include turbulent flow of compressible fluids, changes in pipe diameter, termination of piping at a manifold or vessel, and noise induced by equipment such as, “Compressors, valves, orifices, area expansions, spargers, etc…”

Some automation suppliers provide noise prediction for their equipment. Al is part of the Fisher Valve division, which has a noise prediction section in the Control Valve Handbook. Unfortunately, other piping components don’t have methods to predict noise levels. The authors reference the work, VDI 3733:

…a compendium of information on the noise generated by piping systems. The influence of piping components as well as piping configurations are examined and presented in a quick calculation style without having to work through a detailed explanation of the phenomenon involved. Its broad subject coverage makes it a unique reference.

The problem with broadband sources of noise is that their frequency range can include resonant frequencies of the piping. Noise at these resonant frequencies can cause acoustic fatigue, which is structural fatigue of the pipe wall from high-amplitude vibration. The authors point out:

Acoustic fatigue is generally not a concern for external noise levels below 110dB for unlagged piping (acoustically insulating the pipe to reduce noise levels below 110dB will not reduce the fatigue risk since the pipe is still vibrating with the same amplitude).

Al is applying his expertise in the new Marshalltown flow lab, where the team has built an extensive noise-abatement testing lab to continue to advance the science of ways to reduce acoustical noise in plants.

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Update:I received an email that the link to the article was not working. It is working for me. Here’s an alternative link to the article to a Google Docs version that worked for the person who emailed me. If you have troubles with the original link, let me know if this alternative link also does not work.

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