Pipe supports are often treated as a construction detail rather than an engineering deliverable. In reciprocating compressor systems, this approach consistently produces problems. Support design directly controls the natural frequencies of the piping system and therefore determines whether the system will vibrate excessively in operation.
Supports and Natural Frequency
A pipe span between two supports behaves as a beam with a specific bending natural frequency. That frequency depends on the span length, pipe size, wall thickness, and the stiffness of the supports themselves. If the natural frequency of any pipe span falls close to a compressor excitation frequency, resonance occurs and vibration amplitudes increase dramatically.
The relationship is straightforward: shorter spans have higher natural frequencies. Adding a support in the middle of a span effectively creates two shorter spans with higher natural frequencies, moving them away from the problematic excitation frequency.
Common Support Design Errors
The most common error is over-spanning — placing supports too far apart and creating low-frequency spans that coincide with compressor running speed or its harmonics. This error is almost always made when support locations are set by construction convenience rather than by dynamic analysis.
The second common error is insufficient support stiffness. A clamp that appears substantial may still allow significant pipe movement if the supporting structure is flexible. The effective support stiffness is the stiffness of the entire load path from the pipe to the foundation — a stiff clamp on a flexible beam provides little restraint.
Getting It Right
Piping dynamic analysis per API 618 calculates the natural frequencies of the piping system with the proposed support configuration and identifies spans where adjustment is needed. The output is a specific support location and stiffness recommendation, not a general guideline. When supports are installed per the analysis recommendation, the system’s natural frequencies fall where they need to be.
