Turbomachinery piping can be subjected to various types of dynamic loads and excitations. These piping systems are normally checked for their natural frequencies and modal shapes to prevent resonance due to vibratory or dynamic frequencies. For any such piping, the layout and supports are provided with special attention to rigidity and damping effects.
Excessive vibration is a problem frequently encountered in turbomachinery piping systems. Long-term excessive vibration can lead to fatigue crack propagation and, consequently, may cause piping system failure. The sources of vibration could be categorized as low frequency (<20 Hz), medium frequency (20 Hz to 200 Hz), and high frequency (>200 Hz). These categories can better enable one to identify the excitation mechanisms.
How piping is excited
It is important to study how a section of piping can be excited by a source of dynamic excitation. For the excitation of piping mode by the unbalance (excitation frequency equal to machinery rotational frequency) or the misalignment (excitation frequency equal to two times the machinery rotational frequency), the match of natural frequency and excitation frequency is not usually enough, the forcing pattern should be in such a way as to excite the concerning mode.
Piping support scheme and modal analysis
The modal analysis of a piping is closely related to its route and support scheme. In general, stiff piping layout and conservative/restrictive support schemes, particularly those supports which restrict side-to-side (lateral) and up-down piping movements and located in short spans, can increase natural frequencies and prevent easy-to-vibrate modes forming in the piping.
If a section of a piping is not supported conservatively, ie, for instance, if a considerable span of it is left unsupported (even if this is allowed by stress analysis), the natural frequency(s) of that section is relatively low. This section of piping might be acceptable from allowable span check, stress check and load check, but it can move up/down and laterally (side-to-side) and there are at least two vibration modes with low natural frequencies associated with it. Therefore, it is susceptible to vibration.
Another example is when resting support is used instead of the more restrictive gripping supports such as U-bolts, clamps, and so on. Such a section of piping can move laterally (side-to-side) and there is a low natural frequency mode associated with it. Again, such piping is susceptible to vibration.
Many sections of piping cannot be conservatively supported due to thermal movements (thermal stresses), required access (support cannot be located because it restricts access), lack of support point and so on. These are often the points where vibration modes with low natural frequencies are observed.
Many modes of piping vibration are indeed combined modes where two or even three connected piping systems are involved. A piping system may be provided with stiff and conservative support scheme, but if another piping connected to it vibrates, then this vibration can be transmitted through the connection.
Thermal movements vs vibration control
A piping stress analysis mitigates thermal movements and stresses and typically involves selectively providing flexibility with a mixture of resting supports, guides, line stops, hangers, spring supports, and hold-downs. Controlling vibration typically involves restraining the piping using only hold-down and gripping supports such as clamps. It requires closer spacing of piping supports to raise the mechanical natural frequency of the piping to avoid resonance. Vibration control standards require that piping supports have enough stiffness to stop vibration at the support and caution against the use of hangers, guides or similar.
Piping configuration and their supports should be in a way that make them flexible enough for thermal movements. But, such a flexibility and support scheme should not excessively reduce natural frequencies and make the piping vulnerable to dynamic and vibrational excitations. In other words, provision for more flexibility such as minimum number of supporting elements, flexible piping layout, reduced wall thickness etc can lead to vibration problems due to a lower set of natural frequencies. Therefore, optimization is needed. Flexibility should be added only as required and too much flexibility is as bad and damaging as the lack of flexibility.
Some minimum flexibility is needed considering thermal movements of the piping. However, proper hold-down supports, clamps and similar arrangements should be provided to keep natural frequencies high enough and control vibrational modes in a way that no dynamic excitation is expected. This is a combination of engineering and art to meet all the thermal, operational and dynamic requirements while achieving an optimum and cost-effective piping system.
The importance of modal analysis
The first step to battle any vibrational or dynamic problem is the modal analysis. The natural frequencies and modes should be checked first. In theory, the piping natural frequency should not be coincident with the excitation frequencies. If the natural frequency levels and modes are judged as vulnerable, the piping support scheme, piping configuration, piping span length and others should be modified to make the system acceptable.
A key point is sensitivity of the modal status to different parameters and factors. The results of a piping modal analysis might be sensitive to many different model parameters such as friction factors, support types, gaps, clearances, etc. As many of these factors and parameters in the piping model are just some rough estimates, the actual piping modal behaviour can be different from calculated ones.