PIPELINE STRESS ANALYSIS OF PFTE EXPANSION JOINTS IN GLASS-LINED PIPING SYSTEM

A client required engineering support for a critical section of an existing piping system transporting methylene chloride between two pressure vessels operating at 180°C and 6 barg.

CLIENT ISSUE
The pipeline was lined with glass to withstand the environment, which imposed strict limitations on allowable stress. Adding to the complexity, the piping system incorporated Expansion Joints made of PTFE, a material chosen for its excellent corrosion resistance, but difficult to assess due to its non-linear mechanical behaviour and tendency to creep under elevated temperatures.

The initial intention was to define new design criteria for the replacement bellows based on an existing stress report. However, the documentation was found to be outdated and no longer reliable for current evaluation. As a result, Belman Design was engaged to start from first principles, carrying out a new stress analysis to establish the correct design criteria.

BELMAN DESIGN SOLUTION

FEA validation & stress analysis via ANSYS & ROHR2

The engineering analysis focused on two key elements influencing the pipeline system’s mechanical behaviour: the PTFE Bellows, and a critical junction where the main line connected to a vent line.

PTFE Bellows stiffness analysis

Standard catalogue stiffness values for PTFE bellows were insufficient for the accurate assessment of stress behaviour in the glass-lined pipeline at operating conditions. To obtain reliable mechanical properties, a non-linear Finite Element Analysis (FEA) was performed using ANSYS. Of the two Bellows sizes used in the pipeline (both of relatively close nominal diameters), only the larger was modelled, providing a conservative basis for the analysis by ensuring that the highest potential reaction forces were captured. The resulting stiffness values were incorporated into the ROHR2 model, enabling a realistic representation of the bellows’ influence on load distribution across the system.

Cross pipe evaluation

ROHR2 analysis revealed that the highest stress utilisation occurred at the cross pipe, a junction between the main line and a venting line, due to thermal expansion and temperature differences.

Given the limitation of the simplified ROHR2 model in representing this localised geometry, a detailed elastic FEA was conducted in ANSYS, supported by stress classification procedures according to EN 13445. This evaluation showed that stress levels were close to the allowable limits defined to avoid plastic deformation.

Consequently, a follow-up non-linear FEA was carried out, confirming that the stress levels remained within acceptable boundaries.

System-level load case evaluation in ROHR2

As part of the overall ROHR2 pipeline stress analysis, multiple operational scenarios were evaluated to reflect realistic loading conditions. These included variations such as liquid overfill and two phase flow, taking into account temperature fluctuations, pressure changes and fluid density shifts.