Failure Analysis of Turboexpander
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CASE HISTORY—PEER-REVIEWED
Failure Analysis of Turboexpander A. Al-Meshari • Gys van Zyl • S. Al-Shahrani A. Al-Sahli
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Submitted: 11 August 2014 / in revised form: 2 February 2015 ASM International 2015
Abstract This article describes an investigation into a failure of a turboexpander wheel driving a compressor. Some of the wheel blades were completely cracked and detached, while others had cracks of varying lengths running parallel to the root of the blade. Detailed failure investigation using different characterization techniques such as visual examination, chemical analysis, fractography, metallography, SEM/EDX, hardness testing, modal impact testing, and FEA showed that the cracks and fractures in the turboexpander wheel were caused by fatigue which progressed from multiple origins on concave blade side surfaces, near the hub. The fatigue performance of the turboexpander wheel was degraded significantly by the presence of the anodized layer on the wheel surface. Corrective measures to avoid similar failures are suggested and discussed.
factors in order to establish adequate corrective action(s) to prevent similar failures in future. The turboexpander operating conditions are given in Table 1, below. The turboexpander wheel has an open design type milled from a solid ingot. The wheel was made of aluminum alloy type 7075-T6. The turboexpander wheel surfaces were anodized in order to impart a better protection against mercury-induced attack. The turboexpander wheel failure occurred during start up after a scheduled plant shutdown. Postmortem inspection of dismantled mechanical center assembly revealed extensive damage in the turboexpander wheel. No damage was seen on the driven compressor impeller. Moreover, the inspection of turboexpander inlet strainer revealed no damage or deposit accumulation.
Keywords Turboexpander Aluminum alloy Fatigue Anodizing FEA
Investigation Visual and Stereoscopic Inspection
Introduction A turboexpander wheel driving a compressor failed after 3 years of service. Some of the wheel blades were completely cracked and detached, while others had cracks of varying lengths running parallel to the root of the blade. The turboexpander wheel did not experience previous failures. Therefore, a failure investigation was launched to understand the wheel failure mechanism and possible contributing
A. Al-Meshari (&) G. van Zyl S. Al-Shahrani A. Al-Sahli Materials, Corrosion, Static Equipment Domain, SABIC Technology Centre, P.O. Box 11669, Jubail Industrial City 31961, Saudi Arabia e-mail: [email protected]
Five blades of the turboexpander wheel, which were covered with a golden coat, were completely cracked and detached (Fig. 1). Cracks in different sizes were also seen on some of the other, attached blades. Two fracture regions could be visually distinguished on the blade fracture surfaces, i.e., smooth, flat region and rough, darker region suggesting that the initiation and propagation of the cracks were caused by two different mechanisms. It appears that the cracks we
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