Crack Failure Analysis of a Cyclopentane Feed Line Tee Fitting

Literature Overview

This 2015 paper published in the journal Physical Testing and Chemical Analysis, Physical Section, by Wang Xi and colleagues from the PetroChina Dushanzi Petrochemical Research Institute, presents a detailed failure analysis of a crack that developed in a tee fitting used in a cyclopentane feed line. The analysis was conducted using a combination of macroscopic examination, wall thickness and hardness testing, chemical composition analysis, and metallographic examination. The study traces the crack to manufacturing defects that were subsequently propagated by installation stress and pipe system stress, and it also explains why the crack re-opened after repair welding.

Failure Analysis Methodology and Findings

The failure analysis followed a systematic approach that is representative of best practices in engineering failure investigation:

1. Macroscopic examination: Visual inspection of the tee fitting revealed the location and orientation of the crack. The crack originated at a specific location on the tee body and propagated in a direction consistent with the principal stress state in the component.
2. Wall thickness measurement: The wall thickness was measured at multiple locations to assess whether wall thinning due to corrosion or erosion had contributed to the failure. The measurements showed no significant wall thinning, indicating that corrosion or erosion was not a primary failure mechanism.
3. Hardness testing: Hardness measurements were taken across the cross-section of the crack to assess the material condition and to identify any heat-affected zones from the repair welding. The hardness profile revealed a hardened zone adjacent to the repair weld, indicating that the welding process introduced additional residual stresses.
4. Chemical composition analysis: The chemical composition was verified to confirm that the material conformed to the specified grade. The composition was found to be within the acceptable range for the specified material.
5. Metallographic examination: Metallographic analysis of the crack surface and the surrounding material revealed manufacturing defects, such as incomplete fusion or lack of penetration, that were present in the original weld or forged material. These defects acted as crack initiation sites.

Root Cause Determination and Failure Mechanism

The root cause analysis identified a sequence of events that led to the failure:

• Manufacturing defects: The tee fitting contained manufacturing defects, likely in the form of incomplete fusion or lack of penetration in a weld or in the forged material. These defects were present from the time of manufacture but were not detected during the original quality inspection.
• Stress-induced crack initiation: The manufacturing defects were subjected to installation stress and pipe system stress, which provided the driving force for crack initiation at the defect sites. The stress state in the tee fitting, arising from thermal cycling, pressure loading, and mechanical loads, exceeded the fracture toughness of the material at the defect locations.
• Crack propagation and leakage: The crack propagated through the wall thickness of the tee fitting, leading to a through-wall crack and leakage of the cyclopentane fluid.
• Repair welding and re-cracking: The repair weld was performed without adequately removing the original crack and without relieving the residual stresses from the original manufacturing defects. The welding process introduced new residual stresses that acted on the remaining crack tips, causing the crack to re-open.

Lessons for Engineering Practice

This failure analysis provides several important lessons for engineers involved in the design, fabrication, and maintenance of pressure piping and fittings:

• Manufacturing quality is paramount: The failure was ultimately traced to manufacturing defects that were not detected during the original quality inspection. This underscores the importance of rigorous non-destructive testing during fabrication, particularly for critical components such as tee fittings in high-pressure or high-temperature service. Techniques such as radiographic testing (RT), ultrasonic testing (UT), and magnetic particle testing (MT) should be applied according to the applicable code requirements, and the inspection acceptance criteria should be stringent for safety-critical components.
• Repair welding requires careful preparation: The repair weld in this case was performed without adequately removing the original crack, which is a common cause of repair weld failure. The repair procedure should include thorough crack termination drilling, complete removal of the cracked material, and stress relief welding to reduce the residual stresses introduced by the repair.
• Residual stress management is critical: Both the original manufacturing process and the repair welding process introduced residual stresses that contributed to the failure. Stress relief procedures, such as post-weld heat treatment (PWHT), should be applied according to the applicable code requirements to reduce the residual stress levels to acceptable values.
• FMEA and risk assessment should be applied: A formal Failure Mode and Effects Analysis of the tee fitting, considering all potential failure modes from manufacturing defects to stress corrosion cracking, can identify potential problems before they manifest in operation. The risk assessment should consider the consequences of failure, such as leakage of flammable or toxic fluids, and should drive the selection of appropriate inspection and testing strategies.

Summary

This failure analysis provides a clear and instructive account of how manufacturing defects, when combined with service stresses and inadequate repair procedures, can lead to the failure of a pressure piping component. The key takeaway is that the integrity of pressure piping components depends on a comprehensive approach that integrates manufacturing quality control, non-destructive testing, residual stress management, and careful repair procedures. Engineers should ensure that the fabrication and inspection of critical fittings, such as tee fittings in high-pressure cyclopentane service, are performed to the highest applicable code standards, and that repair procedures are designed to address the root cause of the failure rather than merely treating the symptoms.