Introduction to Piping Vibration Analysis
From our experience piping vibration problem is increasing demand for analysis and field troubleshooting which can be due to increasing productivity (power density, flow rate, etc.), reducing manufacturing cost (thin wall pipe is used), and/or dynamic behaviors are not considered during design phase as it is not specified in the design code.
Vibration induced fatigue failures of pipework are a major concern due to the followings.
Safety (hazardous and flammable)
Downtime and corrective action cost
Any piping system will exhibit a series of natural frequencies which depend on the distribution of mass and stiffness throughout the system. Each natural frequency will have a unique deflection shape associate with it, which is called mode shape. The response of pipework to an applied excitation is dependent upon the relationship between the frequency of excitation source and system's natural frequencies. The excitation vibration frequency can be tonal (narrow band) or broadband depending its generating sources. If the frequency of the excitation is coincident with a natural frequency, hence, resonant condition with excessive vibration as a consequence.
Turbulence-induced vibration (up to around 100 Hz)
Mostly experience at major flow discontinuities in the system
The dynamic directly load the pipework connected to the machine
Acoustic natural frequencies can amplify low levels of pressure pulsation in the system
Flow over a body causes vortices to be shed at specific frequencies
High frequency acoustic
Generated from pressure reducing device
The time to failure is short due to the high frequency response
Surge/momentum from valve operation
High transient forces can be generated by the rapid change in fluid momentum caused by the suddenly closing or opening of a valve
Cavitation and flashing
Cavitation (bubble suddenly form and collapse) can occur where there is a localized pressure drop within the process fluid.
Flashing is in case when the pressure in the pipe becomes less than the vapor pressure of the fluid
Piping Vibration Mitigation
Generally, the mitigation process start with vibration measurement and severity screening (roughly compare with history record, first developed by J. C. Wachel, Engineering Dynamics, Inc.) with the applicable guidelines/standards i.e. Energy Institute (EI) Guidelines for the avoidance of vibration induced fatigue failure in process pipework or VDI 3842, etc. If vibration amplitude falls in concern or problem zone then there is potential to fatigue failure. Mitigation procedure is required. Root cause of excessive vibration amplitude can then investigated which might be involved multiple disciplines e.g. FEA, CFD, process pattern analysis, modal/operating deflection shape (ODS) analysis, acoustics and vortex frequency assessment, fatigue assessment, etc. before simulation can be done to find the most effective solution.
Fatigue is the progressive, localized, permanent structural change that occurs in material subject to fluctuating stresses (dynamic stress) and strains that may result in cracks or fracture after a sufficient number of fluctuation. Dynamic stress which is required for the fatigue analysis/assessment cannot be obtained directly from vibration measurement data, because of the pipe complexity. Finite element analysis which capable for complex structure calculation is required to calculate the dynamic response (stress/strain) and assess against the material fatigue curve. For many conditions, fluid turbulence can generate broadband dynamic load rather than tonal load at certain frequency. The predictive technique (e.g. CFD) is required in order the predicted broadband and analyses together with fluid acoustic characteristic. System modification can be made based on the appropriate separation margin between predicted broadband load and structure natural frequencies.
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