CYCLIC DEFLECTIONS AND CYCLE LIFE

Most deflections are repeated a number of times during the life of the piping system, since the deflections usually are produced by changes in temperature which occur each time the system is started and stopped, and from predictable variations in the way the system is used. Repetitions can also occur as a result of repetitive mechanical movements and from vibrations. Each time a deflection occurs it is a CYCLE. The number of cycles is important to assure the proper design of the expansion joint, since each design has a finite, but predictable life.

Vibrations which cause repetitive deflections can cause a premature failure of an expansion joint. Even though these deflections may be small in magnitude, they usually accumulate huge numbers of cycles in a short period of time. Since the bellows are metallic structures, they have specific and predictable resonant frequencies, like the pitch of a tuning fork. When driven by outside vibrations of the same frequencies (or harmonics of them), they can magnify the incoming deflections until they exceed the yield strength of the bellows material and induce early fatigue failure. When a piping system is known to have equipment which can produce vibrations, such as pumps, fans, and other motor or turbine driven devices, their rotational speeds or frequencies should be stated so that PT&P's designers can assure that the proper expansion joint does not have a resonant or harmonic frequency that is close to those.

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