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Shell Transverse Acoustical (STA) Analysis

Vibration/Noise on Vessels and Piping Walls

Centrifugal compressor vibration study

Centrifugal and screw compressors generate pulsations at vane passing frequency (VPF). These pulsations can excite transverse acoustical modes of the gas and can also excite shell modes of thin-walled pipe and vessels. When these shell modes and transverse acoustical modes match the compressor VPF, severe shell wall vibration will occur. This vibration can cause fatigue failure of nearby small-bore connections and generate excessive noise problems.

Wood provides design and field services to identify and resolve shell mode vibration problems.   

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1 Background

Transverse acoustical waves travel perpendicular to the direction of flow and radiate outwards to the piping (shell) wall. If the shell mode natural frequency matches the transverse acoustical frequency, the wall will become resonant and amplify the vibration.

Thin-walled piping and vessels have many shell modes of vibration. The figure below shows a section of pipe with five (5) circumferential nodes and three (3) axial nodes. The figure also shows a cross-section of the pipe, with a representation of an acoustic mode of high and low pressure. If this acoustic mode matches the shell mode, there is the potential for high vibration. The excitation force that creates the high shell vibration is a result of the pressure pulsations generated by the compressor at VPF. If these frequencies line up – VPF, shell modes of the pipe, and acoustic modes of the gas – then the result is high pipe or vessel vibrations.

Acoustic mode of high and low pressure on centrifugal compressor Five Circumferential and Three Axial Nodes
Acoustic mode of high and low pressure      Five Circumferential Nodes                            Three Axial Nodes

As shown in the following chart, shell transverse acoustical (STA) vibration occurs at medium to high frequencies and can generate high amplitudes. Evaluating and mitigating STA for an existing facility (field service) or during the construction phase (design service) is an advanced study within Wood’s Piping Vibration and Integrity Assessment

-FIT (Flow-Induced Turbulence)

-FIV (Flow-Induced Vibration)

-AIV (Acoustic-Induced Vibration)

-STA (Shell Transverse Acoustical)

Excitation sources studied in Wood's Piping Vibration Assessment 
2 Related Information
3 Related Services
4 Key Words
  • Fluid structure interaction
  • Transverse modes
  • Shell modes

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