A 16” Line on a Chemical Plant

Introduction

A strong vibration of the main 16” line is observed on a chemical plant. It is assumed that the vibration is caused by slug flow in the 16” line. The 16” pipe connection between a low-temperature separator and a heat exchanger. DRG was requested to investigate the vibration.

Analysis

The two-phase flow data analysis indicated the possibility of slug flow in the horizontal pipes of the system. Two different profiles were used for analysis: 1 second in between the different, small, slugs and 10 seconds in between the somewhat larger slugs. The slug profiles were used to analyze the existing system with its current support arrangement.

The resulting loads and moments on nozzle 3 of the separator of the original system were observed to be too high as a result of the slug flow. In the case of the 10-second slug profile, the stresses were outside the allowable at several points. The various analyses are performed using the software package CAESAR II.

Results

An improved support arrangement was introduced consisting of the following modifications:

  • Introduction of an axial stip in addition to the existing guide and +Y- support
  • Introduction of an axial stop in addition to the existing guide and +Y- support. The +Y-support might have a 20 mm gap.
  • Introduction of an additional vertical support on the header.
  • Introduction of a guide with a 1 mm gap. A vertical hanger was included to reduce the static load on nozzle 3.
  • A steel structure with side bracing was introduced including a snubber between the steel structure and the piping, to reduce the rapid dynamic movement of the system.

“A steel structure with side bracing was introduced.”

With these modifications applied to the system, all stresses remained within the allowable margins.

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