A Systematic Study to Determine the Remaining Life of a 60 year old Westinghouse-design Steam Chest

Transcription

1 A Systematic Study to Determine the Remaining Life of a 60 year old Westinghouse-design Steam Chest Sazzadur Rahman, Ph.D. Waheed Abbasi, Ph.D. Thomas W. Joyce Siemens Energy, Inc., 4400 Alafaya Trail, Orlando, FL , USA Abstract: Siemens Westinghouse steam turbines were designed for approximately 150k equivalent operating hours (EOH) of reliable life assuming that the OEM provided operating guidelines are followed. However, in recent years there has been a strong push from electric utilities to assess the condition of their power generating equipment beyond the usual inspections for fitness. Steam chest is the most affected component as it experiences a wide variation of stresses and loads during transient events and steady-state operation which affect the material properties degradation and influence the reliability of the steam chest. Siemens has taken a systematic approach to assess a remaining life of a sixty year old Westinghouse-design steam chest as part of Siemens Lifetime Management program. Results from this assessment are discussed in this paper. Introduction: Fossil steam turbines are generally designed for reliable operation for a finite life based on a normal duty cycle. Majority of the turbine components are made of steels with addition of different alloying elements such as chromium, vanadium, nickel, molybdenum, titanium, etc. During operation, these materials undergo different metallurgical degradation processes due to high stress, creep, fatigue etc. Lifetime Assessment of these components is essential for continued reliable and safe operation of the units [1]. Factors that affects the lifetime of the turbine components are creep, high cycle and low cycle fatigue, corrosion and material aging [2]. As the components undergo operation, these damage mechanisms limit the remaining life of the service components. As a result these components go through cyclic thermal and mechanical loading and which can lead to creep and fatigue. Among the entire turbine components steam chests are most vulnerable as they see a wide variation of temperature and stress during start up and shut down. In addition during start ups, if controls are given to the governor before first stage temperature is reached, water induction could occur and can create significant thermal stresses within the steam chest. These factors can strongly influence the metallurgical condition and overall reliable life of steam chests. As steam chests are subjected to cyclic loads and prolonged exposure to high temperature steam, the material degrades through mechanisms of low cycle fatigue and material creep. Areas of high operating stress and with high stress concentration factors due to geometry will be most affected by these mechanisms. A lifetime assessment study was performed on the approximately 60 year old Westinghouse design steam chest. The objective of this study was to estimate the remaining life of the steam chest with respect to several failure mechanisms. A systematic approach was taken which included on-site inspection with nondestructive methods for flaw detection, hardness and microstructural investigations for material degradation such as creep; and finally, an analytical - 1 -

2 approach was taken to determine the remaining life based on the unit s actual operating data and material properties. Unit Description: The unit is a Westinghouse steam turbine simple cycle that operates at 3600 RPM and has a MW rating of 75 MW. The inlet steam conditions for this unit are 1250 psig and 950 F. The unit is composed of a combined HP-IP turbine and a LP turbine. The unit frame is HT646 and went into commission in The unit has had a reported total of 286,824 operating hours. The unit has a steam chest integrated to the HP-IP cylinder and has two separate throttle valve bodies (Figure 1). Figure 1: (a) Steam Chest Overview; (b) Throttle Valve Body Overview LTA Methodology: Lifetime assessment of power plant components involves a variety of approaches to obtain comprehensive knowledge of the service condition of the component [3]. Siemens LTA process involves Field Inspections (Materials and NDE) and Engineering Analysis. Several NDE inspection methods are used at various locations on the inside and outside of the steam chest and throttle valve bodies as per engineering recommendation. These methods include Penetrant Testing (PT), Magnetic Particle Testing (MT) and Ultrasonic Testing (UT). Materials inspections are performed by means of replica investigation and hardness testing on the inside and outside of the steam chest and throttle valve bodies per engineering recommendations. The number and positions of test locations are defined by consultation with the engineering department based on service loads and experiences. Finally, analytical approaches are taken for remaining life analysis based on the operational history and inspection results. Remaining life quantifies the remaining time period until the functionality and integrity of the component can no longer be validated. The FEA evaluated the material life consumed from past service. The consumed life was broken down into valve locations and operational events. The events include cold, warm, and hot start-ups, shut-down, load change, and steady-state. The past life consumption results were then extrapolated assuming unvarying operation procedures to estimate the remaining life in the valve bodies

3 Results and Discussion: Non Destructive Evaluation (NDE): All inspected areas were thoroughly cleaned with sand blasting. Inspection was performed from top of the turbine cylinder to the steam chest on 100% accessible outside surfaces and all accessible inside surfaces with different inspection methods including VT, PT and MT. All the governor valve seats were inspected with PT and did not show any indications. For throttle valve bodies the inspection area covered the overall bodies up to the weld between the inlet pipe and throttle valve bodies. Inspection methods such as VT, MT and PT were used to perform the inspection. None of the locations had shown any indications. Inspected areas for the steam chest and throttle valve bodies are highlighted in Figure 2 (a) and (b). Figure 2: (a) Steam Chest NDE Location; (b) Throttle Valve NDE Location Materials Inspection: Materials investigation was performed to monitor the creep damage in the material. Replications of the microstructure were used to evaluate the creep damage with proper interpretation. This method can identify the material degradation by identifying the void formations and carbide precipitation at the grain boundaries. Assessment of the microstructure was performed based on the VGB-Guidelines [4]. Replications were performed at different critical locations both inside and outside of the steam chest identified by the Engineering (Figure 3). The microstructure showed a bainitic microstructure with carbide precipitation at the grain boundary. A few locations showed advanced creep damage (2a) according to the guidelines. In other locations no creep damage was observed, but it could not be excluded that individual creep voids were present. Figure 3: (a) Microstructure of Replica inside the Throttle Valve Body; (b) Replica Location - 3 -

4 Hardness values also can provide valuable information on the materials degradation. The hardness of any material is reduced as it undergoes different degradation modes caused by creepfatigue as a result of high temperature exposure for a long period of time and high stress condition during start-ups and shut-downs. It was first proposed by Goldhoff et. al [5] that hardness value can be used as a nondestructive method to evaluate creep damage. There is a correlation between the hardness values to the remaining creep life of any material. Thus, by measuring the hardness of any component it is possible to extract information that assists in evaluating the remaining life of the component. Hardness measurements were taken at all replica locations and did not show any significant differences among measured locations. Remaining Life Analysis: The steam chest body was analyzed using a three dimensional transient and a steady-state finite element method. Due to the symmetry, only half the steam chest body was analyzed. The three dimensional model of the steam chest body is shown in Figure 4(a) and section view showing the steam chest internals is presented in Figure 4(b). Based on the customer provided operation data, the operation events of the steam chest body were simulated in a transient and steady-state finite element analysis. The transient and steady-state steam heat transfer coefficients and pressures were applied to the inside of the steam chest body. The outside of the steam chest body was insulated for all transient and steady-state events. Figure 4: (a) Full 3D Model of Steam Chest Body; (b) Section View The damage results are given for specific valve body locations that are susceptible to low cycle fatigue damage from thermal cycling and time-dependent creep damage, according to Siemens experience. These locations were extensively examined for damage from the transient operational events: cold, warm, and hot start-ups, shut-down, load change, and steady-state profiles. These events were used to calculate stresses and temperatures in the valve bodies using transient and steady-state finite element analysis. The worst combination of stress and temperature were used to calculate the damage fraction associated with each operational event. The accumulated damage fractions were summed to calculate the overall damage per location

5 Figure 5: (a) Steam Chest Inlet and Exit Creep; (b) Throttle Valve Creep The throttle valve body has experienced the most damage in the steam inlet cavity area and in the area of the bushing area within the valve elbow. The inlet area experiences a higher percentage of creep damage and the bushing area experiences roughly the same LCF and creep damage. The steam chest has experienced the most damage in inlet area closest to the initial exit and the governor valve exit areas. The inlet area has experienced a higher percentage of creep damage and exit areas have experienced similar LCF and creep damage results. In the end, each valve body had an estimated remaining life greater than 23 years. Conclusions: The clear objective of the Lifetime Assessments (LTA) process is to assist users in evaluating the potential for continued safe and efficient operation of the power plant by detecting and addressing potential debilitating damage before it may occur. Lifetime Assessment depends on actual component inspection data and an estimation of the damage based on modeling for damage accumulation. Proper inspection planning (including the locations of the expected damage and the methods of inspection) is important for an accurate assessment. Remaining life estimate calculation depends on the material properties and actual operating information, such as temperature, pressure during start-up and shut-down, total operating hours etc. By leveraging the expertise of the Siemens Lifetime Assessment Program, engineering know-how, and analysis methods, Siemens was able to provide the customer with the knowledge that reliable operation of the steam chest is possible for a discrete time period. Permission for Use The content of this paper is copyrighted by Siemens Energy, Inc. and is licensed only to ASME for publication and distribution. Any inquiries regarding permission to use the content of this paper, in whole or in part, for any purpose must be addressed to Siemens Energy, Inc. directly

6 References: 1. Alexander S. Leyzerovich, Steam Turbines for Modern Fossil-Fuel Power Plants, 2008, Fairmont Press 2. Siegel, M., et. al, Remaining Life Analysis of Highly Stressed Steam Turbine Components and Total Operating Enhancement. Power-Gen Europe 2007, Madrid, Spain, June 26-28, Abbasi Waheed, Rahman Sazzadur, Metala Michael, Lifetime Assessment of a Westinghouse-Design Aged Steam Turbine Based on Field Inspections and Engineering Analyses PowerGen 2010, December 14-16, Orlando, FL. 4. VGB-Guideline TW507, Assessment for the Microstructure and damage development of creep exposed materials 5. R. M. Goldhoff, D. A. Woodford, The Evaluation of Creep Damage in a Cr-Mo-V Steel, Testing for Prediction of Materials Performance in Structures and Components, STP-515, ASTM,