Piping fatigue failures caused by high vibration constitute a great 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 vibration, 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. The stiffness of the clamp and pier needed at a bend to limit the vibrations to acceptable levels can be defined using acoustical analysis techniques developed by EDI to define the forces in conjunction with acceptable vibration amplitude criteria.
Another troublesome failure problem, which is commonly experienced in piping systems, is that of small auxiliary piping connections such as vents, drains, pressure test connections, etc. Historically, the design is 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, STARDYNE, ANSYS, etc., 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.
