Failure Analysis of Fractionation Tower Top Line Elbow Rupture

Literature Overview

This paper by Jiang Jianxin from Shanghai Petrochemical Company was published in Chemical Equipment and Piping in 2012 (Vol. 49, Issue 4, pp. 72-74). It presents a detailed failure analysis of a rupture that occurred at an elbow fitting in the top overhead line of a fractionation tower at a petrochemical plant. The analysis employed chemical analysis, optical microscopy, and electron energy spectroscopy (EDS) to investigate the failure from multiple perspectives including material composition, microstructure, and corrosion product identification.

Failure Description and Investigation Methodology

The failure manifested as a leakage at the elbow fitting in the fractionation tower overhead draw line. The operating conditions at the time of failure included exposure to a process fluid containing sulfur and chlorine compounds, with unstable fluid flow conditions at the elbow location due to the geometric change in flow direction.

The investigation followed a systematic failure analysis approach:

Root Cause Analysis

The investigation identified two primary contributing factors:

Chemical Corrosion

The process fluid contained dissolved sulfur compounds (H₂S, COS, or organic sulfides) and chlorine species (HCl or organic chlorides). These aggressive species attack the steel surface through multiple mechanisms:

• Acidic corrosion: HCl dissolution creates localized acid films that accelerate metal dissolution, particularly at areas of high flow velocity where the protective oxide film is continuously stripped.
• Sulfide stress cracking (SSC): H₂S in the presence of hydrogen atoms can cause hydrogen embrittlement in susceptible microstructures, leading to intergranular cracking. The presence of sulfide-containing corrosion products confirmed active sulfide attack.
• Chloride-induced pitting: Chloride ions penetrate the passive film at localized sites, initiating pitting corrosion that can progress to through-wall penetration.

Flow-Induced Corrosion (Erosion-Corrosion)

The unstable fluid flow at the elbow location created conditions favorable for erosion-corrosion. At the inner bend radius, the flow separates and reattaches, creating high shear stress and turbulence. This mechanical action removes the protective corrosion products faster than they can reform, exposing fresh metal to the aggressive fluid. The combined effect of erosion and corrosion is synergistic, with the overall degradation rate exceeding the sum of the individual mechanisms.

The microstructural examination revealed intergranular attack patterns characteristic of sulfide stress corrosion cracking. The corrosion products identified by EDS included FeS, FeCl₂, and FeCl₃, confirming the involvement of both sulfur and chlorine species in the degradation process.

Standards and Prevention Measures

The failure highlights the importance of proper material selection for service environments containing H₂S and chlorides. The following standards and guidelines are relevant:

Preventive measures for similar applications include:

1. Material upgrade: Selecting a corrosion-resistant alloy such as duplex stainless steel (UNS S31803/S32750) or a higher-grade austenitic stainless steel (316L) for the elbow fitting.
2. Corrosion allowance: Increasing the wall thickness allowance to accommodate the expected corrosion rate over the design life.
3. Flow conditioning: Installing a flow straightener or increasing the bend radius to reduce flow-induced turbulence at the elbow.
4. Corrosion monitoring: Implementing electrical resistance probes or ultrasonic thickness mapping at the elbow location to detect wall thinning before failure.
5. Chemical treatment: Adding corrosion inhibitors to the process fluid where technically feasible.

Engineering Practice Integration

This case study is particularly instructive for engineers involved in designing or inspecting piping systems in refineries and chemical plants where sour service (H₂S-containing) conditions exist. The failure mechanism described—combined chemical corrosion and erosion-corrosion at a geometric discontinuity—is among the most common failure modes in process piping. The key lesson is that material selection must account for the full service environment, not just the bulk fluid composition. Localized flow conditions at elbows, tees, and reducers can create micro-environments that are significantly more aggressive than the bulk fluid.

The investigation methodology described in the paper—combining macroscopic examination, chemical analysis, microstructural analysis, and corrosion product identification—represents the standard approach to failure analysis in the petroleum and chemical industries. This multi-technique approach is essential because a single test method rarely provides a complete picture of the failure mechanism.

Study Insights and Reflections

One of the most valuable aspects of this paper is the clear demonstration of how flow instability at a geometric feature (the elbow) can dramatically accelerate corrosion. In many process piping systems, elbows are treated as simple mechanical fittings without consideration of their role in creating localized flow conditions. This case shows that the elbow is not merely a passive component but an active participant in the degradation process, creating the turbulence and flow separation that strip protective films and expose fresh metal to attack.

From a quality control perspective, this failure could have been detected through routine ultrasonic thickness testing at the elbow location. The asymmetric thinning pattern at the inner bend radius is a well-known indicator of erosion-corrosion, and monitoring programs should specifically target these locations. The paper serves as a reminder that inspection programs must be risk-based, with higher inspection frequencies assigned to locations where flow-induced degradation is expected.