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Practical Solutions to Machinery and Maintenance Vibration Problems

Chapter 3, Detuning and Proving Resonance

Section 3, Further Considerations

At first glance, it might appear that a perfectly round pipe has the same resonant frequencies in both the vertical and horizontal directions. However, the stiffening effect of the elbows is very different in one direction as compared to the effect 90° from that direction. This can result in resonant frequencies that are very different from each other. Therefore, with sections of pipe, as well as with elbows, check for possible resonance in both directions (usually horizontal and vertical). When a pipe is resonant, there may be some curling mode shapes in both directions, but the direction that shows the most curl is the resonant direction.

When the troublesome vibration occurs at much higher frequencies than at 1 x rpm, such as at number of vanes x rpm, or number of gear teeth times rpm, or electrical hum frequency, then don't forget to check for possible resonances in such relatively rigid parts such as the adaptor from the pipe to the pump case. For very high vibration frequencies (usually well above 10,000 cpm or so), check for resonance on such easily ignored parts as short pipe nipples. For longer parts, look for several nodes and antinodes.

Plotting mode shapes was used to prove that a long pipe extending from an 1,800 rpm pump and suspected of resonating was actually not resonant. Instead, the short tapered adapter connecting the pump to the long pipe showed a characteristic first resonance mode shape. This section, which was rigid compared to the long pipe, was not resonating to the vibration at the pump's operating speed, but instead to the operating speed multiplied by the three impeller vanes (5,400 cpm). Plotting the mode shape indicated which part was actually resonant. (This section was stiffened with ribs, moving its natural frequency well outside the range of the 5,400 cpm vane vibration.)

 

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