Most engineers are familiar with static pipe stress analysis — checking that thermal expansion, weight, and pressure loads keep stress below code allowables. Dynamic vibration fatigue is a fundamentally different failure mechanism and requires a different analysis approach.
The Fatigue Mechanism
Vibration fatigue accumulates damage through repeated cyclic loading. A pipe running at 900 RPM experiences 900 stress cycles per minute, or over 470 million cycles per year. Even stress amplitudes well below the static yield strength can cause fatigue failure at this cycle count.
The critical factor is the stress range — the difference between the peak and minimum stress in each cycle. For high-cycle fatigue, the relevant limit is the endurance limit of the material, which for carbon steel piping is typically around 50% of the ultimate tensile strength. Stress ranges above the endurance limit will eventually cause fatigue failure regardless of how long the pipe has been running without incident.
Where Failures Concentrate
Vibration fatigue failures in compressor piping concentrate at stress risers — locations where the stress range is amplified by geometry. The most common failure locations are small-bore branch connections, socket welds, unreinforced fabricated tees, and the toe of support attachment welds.
Small-bore connections are particularly vulnerable because they are flexible relative to the main pipe run, experience high displacement amplitudes, and often have poor weld quality. The combination of high stress concentration factor and high displacement makes them the most common failure location in compressor piping systems.
The Analysis Approach
API 618 DA3 forced response analysis calculates dynamic stress ranges directly by applying the pulsation-induced forces to a finite element piping model. This is the only reliable method for verifying fatigue life — visual inspection and static stress analysis cannot identify fatigue risk in advance.
