Crack is noticed at cooling water pipe of a large generator which is suspected due to excessive vibration. Motion amplification (MA) is then use to measure vibration of pipe and associated structures as it is easy and convenient concerning the height and accessibility. In addition to motion amplification video, vibration data revealed predominantly at 100 Hz which is second harmonics of generator operating speed at 3000 rpm (or 50 Hz). This vibration frequency at 2X component is fairly common for two poles generator since its rotor has asymmetric stiffness as a result of cutting slots for installing bundle of copper coils. This means that the rotor is prone to deflect twice in one revolution.
However, excessive vibration amplitude at 100 Hz noticed at piping is due to structure resonance when 2X vibration generated from generator is coincident with natural frequency of the pipe. MA video shows clear evidence of pipe span mode in this case. In general, structure resonance can be prevented with separation margin at 15% between excitation source(s) and natural frequency. Measured vibration amplitude at discrete frequency is then compared to acceptance criteria of Energy Institute (EI)’s guideline. It was found excessive at some pipe sections, hence, this vibration amplitude is likely to cause fatigue failure. This can result in unplanned shutdown, hence, production lost.
Recommendations:
Dynamic stress prediction from measured vibration data using finite element analysis (FEA)
Carry out fatigue assessment from predicted dynamic stress and S-N curve of pipe material
Structure modification/adding pipe support and then predict dynamic stress again (new system) to ensure below cut-off limit of S-N curve, hence, fatigue failure is unlikely to occur.
This approach is to ensure accuracy of FEA model as measured vibration data is used, hence, accurate predicted results as well as proposed solution.
Machinosis specializes in advance vibration analysis of all types of machinery and associated structures (foundation and piping) plus machine condition monitoring and diagnostics. With extensive experience, we also use cutting edge technologies and tools to measure, analyze, and solve various assets problems encountered in the field.
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