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Piping Vibration with 2-Phase Flow

Problem: A plant was experiencing excessive piping vibration on a line that was downstream of a heat exchanger train. Several existing pipe supports had been broken, and plant personnel were concerned about the safety of the line. Due to process conditions two-phase (gas and liquid) flow was likely present in this line, particular downstream of the pressure control valve. Vibration issues are common in piping with two-phase flow. This is because transient unbalanced forces can develop in such piping at flow direction change points, such as tees or elbows. These transient forces regularly excite, or “ring”, the piping, which will then vibrate at its natural frequencies.


Solution:
An assessment confirmed initial suspicions that the root cause of the vibration was indeed two-phase flow. KHE employed field services to measure and characterize the vibration. KHE found a generally broadband and low frequency vibration. This pattern is consistent with two-phase flow conditions, where significant displacement will typically only occur at lower frequencies.

Fig. 1 Example of Measured Broadband, Low Frequency Vibration.

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Spectrum
Time Waveform
Ideally, two-phase flow induced vibration is addressed by eliminating the two-phase flow from the system. However, in some cases including this one, process requirements are such that two-phase flow is unavoidable. Therefore, KHE developed a modified pipe support design that removed all low frequency natural frequencies from the system. Since this approach results in a piping system that is much stiffer than typical piping designs, special care must be taken in the pipe stress analysis of such designs. KHE employed a highly iterative approach, studying and addressing individual mode shapes, while maintaining adequate thermal expansion flexibility.
Result: The existing pipe support design was found to have twenty-six natural frequencies under 10 Hz. KHE’s proposed pipe support design eliminated all natural frequencies below 7 Hz, which was the threshold that most of the measured vibration was found to be below. KHE confirmed their proposed design still provided adequate flexibility by ensuring the design passed an ASME B31.3 code assessment.
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