Equipment: Crude Distillation Column Overhead Piping Vibration Flow Induced.

Problem: After a series of test runs were completed in the crude distillation unit on increasing the throughput from 225 KBPD to 260 KBPD, it was found that the existing overhead 24in piping was prone to flow induced vibrations due to increased flow rates and resulting over stressing piping components and subsequent fatigue impact on assessed piping.  The existing piping of was deemed not fit for continued service at unit throughputs above 225 KBPD based on the vibration spectral reports. Increased flow rate from 225MBD to 250MBD as a result of debottlenecking, resulting in higher flow velocities with a correspondingly greater level of turbulent energy in process systems is suspected to be the primary concern for the vibration behavior on the overhead piping system. Therefore, based on a proactive troubleshooting technique, the following approach recommended by Energy Institute publication on GUIDELINES FOR THE AVOIDANCE OF VIBRATION INDUCED FATIGUE FAILURE IN PROCESS PIPEWORK, was followed.
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Analysis:a comprehensive engineering assessment utilizing CAESAR-II software to simulate piping dynamic analysis along with CFD modeling of fluid flow patterns and FEA (ANSYS) modelling of pipe wall natural frequencies to establish the vibration characteristics of overhead piping system. The quantitative assessment consists of calculating the LOF value for each of the excitation mechanisms that have been identified in the qualitative assessment (section-3), as potentially being an issue;

Flow induced turbulence

Surge/Momentum Changes Due To Valve Operation

Flow induced excitation (pulsation)
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Results Conclusion: The vibration characteristics are STEADY-STATE (FORCED) VIBRATIONS  RANDOM. This process piping systems have traditionally been designed on the basis of a static analysis with little or no attention paid to vibration induced fatigue. This is principally because most piping design codes (ASME B31.3) do not address the issue of vibration in any meaningful way. This results in piping vibration being considered on an adhoc or reactive basis as we have done in this assessment. The existing design of overhead piping system has been found to be overly flexible due to the presence of the extremely low fundamental natural frequency and the subsequent first 10 modes (0.8 Hz to 3.8Hz). As a result of which the overhead piping system was found prone to low frequency excitations that typically cause lateral piping vibrations. New additional supports are proposed to be installed on the piping system which will result in the stiffening of the overhead piping system to an extent that the lower natural frequencies will disappear and the fundamental frequency in the dynamic design of overhead piping system is set at 4.051Hz. Hence it was recommended to add additional piping supports and to reinforce the pipe rack structure with additional brace members as a solution to overcome this problem
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Benefits: The CDU Overhead piping of distillation column was thus made more reliable in vibration service resulting from the increased flow rates. This enable the increase of the CDU capacity resulting in increased production and $$$.
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