Failure Analysis of Cooling Tower Fan-Arm
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TECHNICAL ARTICLE—PEER-REVIEWED
Failure Analysis of Cooling Tower Fan-Arm Basavaraj Padasale . J. K. Rakshan Kumar . Pavankumar R. Sondar Subray R. Hegde
. Sumanth Govindarajan .
Submitted: 18 February 2020 / Accepted: 8 July 2020 Ó ASM International 2020
Abstract This work presents failure investigation of cooling tower fan-arms commissioned in a chemical processing plant. The analysis aims at understanding the mechanism and root-cause of the failure. The investigation involves site visits, microstructural analysis, fracture surface analysis, hardness measurements, numerical stress analysis and experimental simulation. Work concludes that the fan-arms failed due to the lack of post-weld heat treatment, which caused localized stress-corrosion and pitting at critical locations that served as crack initiation sites. Fatigue loading condition and presence of residual stresses at the weld enabled easy propagation of cracks that led to recurring premature failure. Based on the root-cause and the mechanism identified in this analysis, proper preheating and post-weld heat treatment is recommended to relieve the residual stresses at the critical locations and thus to avoid/minimize such recurring failures in future. Keywords Fan-arm Fatigue Residual stress Galvanic corrosion Heat treatment
Introduction Most of the mechanical components operating in a dynamic loading condition fail due to fatigue. When corrosion combines with fatigue, failure may occur at lower loading and within shorter period of time. There are cases
B. Padasale J. K. R. Kumar P. R. Sondar (&) S. Govindarajan S. R. Hegde Department of Metallurgical and Materials Engineering, National Institute of Technology Karnataka, Surathkal 575025, India e-mail: [email protected]
where flawed design, improper fabrication, lack of preheating and post-weld heat treatments caused catastrophic failure of components that were serving in corrosive environments [1, 2]. In particular, several failure investigations of cooling water units are reported. A review on failure of metallic components suggested that surface defects such as scratches and localized corrosion pits act as crack initiation points that eventually cause fatigue failure of critical components [3]. Another study on failure of SS 316 N showed that formation of ammonia ions due to dissolution of nitrogen in stainless steel led to pitting corrosion. These corrosion pits acted as crack initiation sites and eventually led to fatigue failure [4]. Residual stresses formed during welding may also lead to failure of the components, and proper pre- and post-welding treatments become necessary to counter such failures [5]. A failure investigation reported that contact fatigue due to alternating loads during frequent start-up and shut-down cycles caused failure of gear box and lead to shutdown of the cooling unit [6]. Similar study on failure of cooling tower fan-blade indicated fatigue as the primary cause of failure [7]. The present work deals with recurring premature failure of cooling towe
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