Failure Analysis: Sulfide Stress Corrosion Cracking and Hydrogen-Induced Cracking of A216-WCC Wellhead Flow Control Valv
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TECHNICAL ARTICLE—PEER-REVIEWED
Failure Analysis: Sulfide Stress Corrosion Cracking and Hydrogen-Induced Cracking of A216-WCC Wellhead Flow Control Valve Body S. M. R. Ziaei • A. H. Kokabi • J. Mostowfi
Submitted: 31 August 2013 / in revised form: 23 November 2013 / Published online: 27 March 2014 Ó ASM International 2014
Abstract The wellhead flow control valve bodies which are the focal point of this failure case study were installed in some of the upstream facilities of Khangiran’s sour gas wells. These valve bodies have been operating satisfactorily for 3 years in wet H2S environment before some pits and cracks were detected in all of them during the periodical technical inspections. One failed valve body was investigated by chemical and microstructural analytical techniques to find out the failure cause and provide preventive measures. The valve body alloy was A216-WCC cast carbon steel. During investigation many cracks were observed on the inner surface of the valve body grown from the surface pits. The results indicate that flow control valve body failed due to combination of hydrogen-induced corrosion cracking and sulfide stress corrosion cracking. According to HIC and SSC laboratory tests and also with regard to cost of engineering materials, it was evident that the best alternative for the valve body alloy is A217-WC9 cast Cr–Mo steel. Keywords Flow control valve A216 A217 SSC HIC Anodic polarization
S. M. R. Ziaei (&) A. H. Kokabi Department of Materials Science and Engineering, Sharif University of Technology, Tehran, Iran e-mail: [email protected] J. Mostowfi Technical Inspection Department, East Oil and Gas Production Company, Khangiran, Iran
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Introduction The number of sour (H2S containing) oil and gas fields being produced worldwide is increasing, as sweet (CO2 containing) fields are being depleted. A concern in the production sour oil and gas is the corrosion caused by the acid gas H2S [1]. Even though corrosion resistant alloys (CRA) have long been available as a material selection option that mitigates H2S corrosion, carbon steel is in general more cost-effective for oil and gas facilities [2]. The most important element in the production process of upstream facilities is the control valve. The control valve manipulates a flowing fluid, such as sour gas, steam, or chemical compounds to compensate for the load disturbance and keep the regulated process variable as close as possible to the desired set point [3]. Scheduled and unscheduled shutdowns for repairing corrosion damage or replacing corroded equipments in upstream facilities can be very expensive and anything that can be done to reduce these shutdowns will be of great benefit. Selection of materials of construction has a significant impact on the efficiency of the wellhead facilities. Among the many metals and alloys that are available, a few can be used for the construction of process equipment such as control valve bodies. A216 carbon steel (the common material for wellhead flow control valve bodies), is probably used
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