Failure Analysis of Prehydrotreating Unit Pipe Elbow Rupture

Literature Overview

This paper by Li Hua and colleagues from the Guangzhou Institute of Special Pressure Vessel Inspection, published in Physical Testing and Chemical Analysis (2021, Vol. 57, No. 8), presents a comprehensive failure analysis of a pipe elbow rupture in a prehydrotreating unit of a catalytic reforming plant. The study employs multiple analytical techniques to determine the root cause of the failure and provides valuable insights into corrosion and erosion mechanisms in petrochemical service.

Failure Scenario and Analytical Methods

The elbow failure occurred in a prehydrotreating unit of a catalytic reforming process, which operates under conditions involving hydrogen, hydrocarbon vapors, and various acidic species. The investigation utilized a systematic multi-method approach:

Analytical Method	Purpose	Key Findings
Macroscopic observation	Visual inspection of failure area	Severe thinning on outer arc side
Wall thickness measurement	Quantify material loss	Significant reduction at outer arc
Chemical composition analysis	Verify material grade	Material met specifications
Mechanical property testing	Assess remaining strength	Reduced due to wall thinning
Metallographic examination	Microstructural analysis	No abnormal microstructural changes
Fracture surface analysis	Determine failure mode	Ductile failure due to loss of section
X-ray diffraction	Identify corrosion products	Iron sulfides and chlorides detected
Thickness calculation	Verify pressure capacity	Insufficient at thinned location

Root Cause Analysis

The investigation revealed that the elbow was subjected to a complex corrosion environment involving HCl, H2S, NH3, and H2O. The failure mechanism was not a simple corrosion process but rather a synergistic interaction between environmental corrosion and erosional damage from flowing media and detached corrosion products.

The outer arc side of the elbow experienced the most severe material loss due to:

Centrifugal forces directing the flow and suspended particles toward the outer wall
Higher velocity at the outer arc creating additional erosional forces
Accumulation and detachment of corrosion products that acted as abrasive particles
Continuous chemical attack by the acidic environment

This combined corrosion-erosion mechanism created a positive feedback loop: corrosion produced loose scale that was carried by the flow, which in turn abraded the pipe wall, exposing fresh metal to further corrosion.

Engineering Implications and Preventive Measures

The case study highlights several critical considerations for elbow design and maintenance in petrochemical service:

Wall thickness monitoring programs should be implemented for elbows in corrosive service, with particular attention to the outer arc side
Corrosion allowance calculations should account for the synergistic effects of corrosion and erosion, which may exceed the sum of individual mechanisms
Material selection should consider resistance to both chemical attack and erosional wear
Flow velocity limits should be established based on erosion-corrosion criteria, not just erosion alone
Regular ultrasonic thickness mapping should be performed at elbow locations in aggressive service environments

The findings underscore the importance of considering the combined effects of environmental degradation mechanisms in piping system design. Single-mechanism approaches to corrosion allowance determination may be insufficient for elbows in complex chemical environments where multiple degradation processes interact synergistically.

Study Insights

This failure analysis exemplifies the importance of systematic investigation methods in determining root causes of piping failures. The multi-technique approach allowed the investigators to distinguish between different contributing factors and identify the dominant failure mechanism. For engineers designing and maintaining piping systems in petrochemical facilities, this case study serves as a reminder that elbows are particularly vulnerable components where flow-induced effects and chemical environments can combine to accelerate degradation far beyond what might be predicted by considering either mechanism in isolation.