Small-bore connections — instrument taps, vent connections, drain lines, and sample points — are the most common source of fatigue failures in reciprocating compressor piping. They are also the most frequently overlooked in both design and inspection programs.
Why Small-Bore Connections Fail
Small-bore connections attached to a vibrating main pipe act as cantilevered beams. The main pipe vibrates, the connection vibrates with it, and the bending stress concentrates at the base of the connection where it meets the main pipe wall. Socket welds at this location have a stress concentration factor of approximately 2.1 — meaning the actual stress at the weld toe is more than double the nominal calculated stress.
At typical compressor vibration frequencies and amplitudes, the fatigue life of an improperly supported small-bore connection can be measured in months rather than years. The failure mode is typically a circumferential crack at the socket weld, sometimes accompanied by a visible leak before complete separation.
Design Practices That Reduce Risk
The most effective design practice for small-bore connections on compressor piping is to brace them against the main pipe or to a rigid support structure. A brace reduces the unsupported length of the connection and increases its natural frequency, moving it away from the compressor excitation range.
Where bracing is impractical, minimizing the unsupported length of the connection is the next best option. Connections should be kept as short as possible, with valves and instruments located close to the main pipe rather than at the end of a long extension.
Butt welds should be specified in place of socket welds where fatigue risk is identified. The lower stress concentration factor of a full-penetration butt weld significantly extends fatigue life.
What the Analysis Should Cover
A complete piping dynamic analysis for a reciprocating compressor system should specifically evaluate small-bore connections, not just the main pipe runs. Tech Transfer includes small-bore connection assessment as a standard element of our piping dynamic analysis scope.
