Velocity and vibration 1

[MadieKerang]

Gents,

please advice if generally we cn say that high or excessive velocity is direct proportional to the vibration of the piping in the gas system?

 

[boilerone]

In general, the answer is 'not necessarily'.

For the typical ranges of velocities used in the flue gas systems, the major concern is the natural frequencies of the um-restrained tubes and pipes in the streams. When gas passes a tube it generates a vibration but only if the vibration frequency coincides with the tube's natural frequency then it will cause harmonic vibration and damage the tube or its support. If the gas velocity is higher or lower then the frequency might miss the tube's frequency then the tube can be safe.

The other concern in the flue gas design is the sound generated from gas passing tubes which can get bounced back-and-forth if the distance between the opposing duct walls matches or is certain times the wavelength of the sound generated by the gas then there will be a 'standing wave' and this can be also very destructive to the ducting structure. Also for this case, if the velocity is different then the problem might not happen at all.

Many times boilers reported vibration issues during start-ups or shut-downs but the problems mysteriously disappeared at full-loads which the gas velocities are at the highest. It is because typically boilers are designed (properly support the tubes and add sound-blocking baffles in the gas streams) to take care of vibrations during normal and continuous operations but it is hard to catch conditions at lower loads. Operators are told or have learned to avoid or quickly pass certain gas conditions (in addition to velocity, gas temperature, density, etc. also affect frequencies) to avoid damages.