Root Cause Failure Investigation of MSCV Drain Failures

Abstract

Main Steam Stop and Control Valves (MSCVs) are safety critical components in modern steam power plants including large, combined cycle gas turbine (CCGT) plants. Repeated failures of the after seat drain piping at a dissimilar metal weld have occurred at one large CCGT that is regulated by the ISO in AGC and therefore experiences few startups. This paper describes the results from a comprehensive evaluation of contributors to the repeated failures in one of two MSCV including: pipe stress analysis of the drain pipe layout, vibration testing of the MSCV and drain piping system using accelerometers and motion amplified video (MAV) technology and other inspections of the large bore pipe hanger system adjacent to the valves. Forensic investigations by a metallographic laboratory were conducted which provide important conclusions about the root cause of the drain weld failures, material condition and heat treatment procedures. Additional calculations of transient thermal stress were also conducted in accordance with the guidance that is available in ASME Code Section III NB-3650 to address additional stress that is present due to transient temperature conditions at the failed DMW weld where a substantial geometric discontinuity is present between the drain pipe and the downstream forged block valve. Results from plant operating data archives were used to establish process conditions during plant startup and indexing the measured vibration characteristics when the MSCV drains opened and closed. Field NDT (PMI, UT thickness testing) and hardness testing was used to confirm the material in the drain lines components which have not experienced failures. Recommendations were developed from this program that identify the principal cause of the high level of vibration in the affected MSCV, the benefits of modifications to the drain system pipe materials, dimension and layout and the improvement of oversight of heat treatment of future repairs.