By Vincent A. Carucci
To screen vibration problems, answer the following questions: How long has it lasted? How fast is it moving? How loud is it? How much is it moving? The following are some screening guidelines that may be used.
If a component lasts to its endurance limit without failure, it is unlikely to ever fail by fatigue unless conditions change. For example, consider an endurance limit of 10 x 106 cycles. At a frequency of 5 Hz, it takes 24 days to reach the endurance limit, at 100 Hz it takes 1.2 days. For equipment that sees only a few cycles a day, the endurance limit will never be reached within the useful life of the equipment.
A useful screening limit for maximum vibration velocity is 2 in./sec. Stress is proportional to velocity for frequencies at or above the first natural frequency.
In some cases the noise level is an indication of a vibration problem. The sound power level inside the pipe can be calculated from a measured level outside the pipe and the calculated pipe transmission loss.
The displacement of the pipe can be checked to determine if the level is dangerous. The stress can then be calculated and compared to fatigue data or the endurance limit (about 3000 psi for steel considering a stress concentration factor of 5). Figure 1 provides general screening criteria for allowable peak-to-peak vibration amplitude as a function of frequency.