Vibration analysis

A vibrating pipe rarely fails through a single overload. It fatigues. The stress amplitudes are small, but they repeat many times a second – within a few days, millions of cycles accumulate.

Damage occurs where the line is weakest: at small-bore branches for drains, vents and instruments, at weld toes and at threaded connections. Loosened supports, leaking flanges and damaged instruments are further consequences. Vibration analysis is meant to find these locations before a crack reveals them.

The first question is not how strongly the line vibrates but why. The remedy depends on the cause: a line excited by a machine needs a different measure from one in which the flow itself produces the motion.

The most important clue is the frequency. If it follows the running speed of a machine, the source is there. If it changes with flow rate or occurs only in certain operating conditions, the flow is the cause.

A modal analysis delivers the natural frequencies of the line and the associated mode shapes. If a natural frequency lies close to an excitation frequency, the motion builds up, limited only by damping. The aim of design is sufficient separation between the two; a margin of about 20 per cent is often sought, with the project specification governing.

The calculation model has to be more accurate than for static analysis: the masses of valves and actuators and the stiffness of supports and steelwork determine the result. A support that merely carries the line does not restrain it dynamically in the lateral direction.

On existing plants, measurement comes first: vibration velocity at defined points together with the frequency spectrum. Guidelines with frequency-dependent reference values exist for an initial classification – VDI 3842 in German-speaking countries and, internationally, the Energy Institute guidelines for the avoidance of vibration-induced fatigue failure in process pipework.

If a measuring point exceeds the reference values, a closer investigation follows: the dynamic stress at the location at risk, measured with strain gauges or calculated from the measured mode shape.

Effective measures change either the excitation or the stiffness of the line.