Piping Vibration

Piping fatigue failures due to vibration have been a major cause of downtime and fires in industrial plants across the world. As a result, it is important to evaluate the vibration levels of piping systems to ensure the levels are acceptable. If vibration levels are considered excessive, modifications to the piping and/or its support structure may be required. Alternately, the excitation mechanisms contributing to the vibration must be altered or eliminated. For modifications to be effective, it is necessary to understand the principles involved.

Common Piping Vibration Areas

• Long Pipe Spans
• Piping Appurtenances (vents, drains, gages, etc.)
• Large Masses (e.g., Valves and Components)
• Reciprocating Compressor Pulsation Bottles

Common Causes of Piping Vibration

• Excessive Pulsation
• Mechanical Resonance
• Inadequate Supports and/or Support Structure

Evaluation of the vibration characteristics during testing relative to operating parameters such as equipment loading, temperature, pressure, and gas composition are important to determining the root cause and source of the vibration, and to determine if mechanical resonance, acoustical resonance, or both, are related to the vibration.

Piping Design Guidelines

Piping fatigue failures caused by high vibration constitute a threat to the safety and reliability of gas compression and liquid pump systems.  Extra care should be given to the layout of a piping system to include vibration effects as well as pressure, weight, and thermal loading considerations.

To design safe, reliable piping systems free from excessive vibrations, the individual piping spans or piping components should not be mechanically resonant to system excitation forces.  In addition, all unnecessary bends should be eliminated since they provide a strong coupling point between pulsation excitation forces and the mechanical system. Pulsation control is paramount for control of piping vibration.

One of the most common failure is that of small auxiliary piping connections such as vents, drains, pressure test connections, etc.  Typically, the designs are such that relatively large valve and flange masses are cantilevered from the main piping.  This results in systems with large amplification factors and natural frequencies in the range of typical excitation forces.  The solution to this type of vibration problem is to design the connection such that the mass of the valve and flange can be effectively tied back to the main piping, thus eliminating all relative movement.

Structural computer programs, such as CAESAR II and ANSYS, can be used to calculate the natural frequencies and mode shapes of three-dimensional piping systems; however, their accuracy is highly dependent upon the assumptions of the end conditions.  Simplified tools or design procedures can also be used, with experience, to provide effective designs which will not experience excessive vibration or stress levels.  These techniques can be used in design or as aids in solving existing vibration problems in piping systems.