Vibration Reduction in a Process Piping Project

Introduction

The process piping running from a heat exchanger to a separator showed a high vibration level. The concern is that the dynamic stress level could cause fatigue failures specifically in the equipment nozzles in the future. The flow through the piping is a two-phase flow, a combination of liquid and gas, which causes slug formation. The slug flow causes dynamic loads of an unsteady nature. Due to space constraints, modification of the pipe supporting system was not desired. Modification of the separator vessel nozzle should also be avoided. The client requested to investigate the solutions to reduce the vibration level.

Analysis

Based on process data, an estimate was made on the most obvious excitation mechanism. Two-phase flow, or slug formation excitations, are characterized by the lack of a fixed excitation and show a large amount of randomness.

This type of excitation mechanism easily excited several mechanical natural frequencies/ mode shapes of the piping system.
The applicable piping systems were modeled including the connected vessels and supporting steel structures. The current situation was statically and dynamically analyzed to be used as a reference case. Slug flow calculations showed that avoiding the slug flow regime was not reasonably possible.

“Two-phase flow or slug formation excitations are characterized by the lack of a fixed excitation and show a large amount of randomness.”

Results

It was however apparent that the pipe diameter increase would lead to a decrease in the impact loads. The pipe diameter was increased from 400 to 500 mm. Dynamic analyses show that the vibration levels will be reduced by 35 to 59% in the nozzle flanges depending on the direction.

An alternative solution was to re-route the piping, this could change the load impact direction and the dynamic system response. The analysis showed a reduction of 16 to 83% in the nozzle flanges depending on the direction.

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