A Fitness-for-Service (FFS) assessment is often a key exercise to support run/repair/replace decisions. If the assessment can show that the component can continue to operate safely for a significant period of time, costly replacements can be avoided or at least postponed. This article provides a case study of a recent FFS assessment for stop-check valves at a combined cycle plant. This case study provides background information, discusses the finite element analysis and fracture mechanics calculations performed, and finally provides conclusions and recommendations.
The FFS assessment was needed to evaluate cracking found during a field inspection for each of the three non-return valves at a combined cycle plant. It was recommended that more detailed analysis be performed to estimate crack growth rates and implications for remaining service life. The need was to evaluate unstable fracture and to estimate possible additional crack growth due to cyclic fatigue and long term creep at elevated temperature.
Based upon detailed numerical simulation and fracture mechanics calculations performed and Structural Integrity experience with similar valves, the mechanism of cracking present in the valve body adjacent to the guide rib was thermal fatigue. When subject to rapid temperature change, the relatively thick-walled valve body will respond slower to the temperature change than the internal cross member (guide rib) due primarily to the flow on both sides of the rib and its relatively small thickness compared to the valve body. This creates a temperature difference between the rib and the valve body, resulting in differential expansion and localized bending stress at the transition between the rib and the valve body. The largest thermal stresses are produced during the most rapid heating/cooling events which occur during start-up and at the very onset of cool-down events.
Original design data for the stop-check valves in the steam piping is provided below:
Design pressure: 2080 psig
Design temperature: 1085°F
Operating temperature: 1050°F maximum
Valve material: SA-217 Grade C12A (9Cr-1Mo-V-Nb cast)
Each of the three Heat Recovery Steam Generators (HRSGs) includes a 16-inch stop-check valve in the HP steam system. The valves are of identical geometry and comparable cracking was found in each valve. Dimensional information was used to develop Finite Element Analysis (FEA) models to simulate the valve thermal and structural response. Input was derived from design drawings, UT thickness data, and a 3D scanned image file provided by the valve manufacturer.
Visual examination of the ID of a typical valve identified linear indications on the bottom and top of the guide rib. The cracks were located on each end and both sides of the guide rib as shown in Figure 1.
PT examination verified the results of the visual inspection. The bleed-out from the cracks was extremely heavy and masked individual cracks indicating that the cracks were deep and/or wide. Cracks identified by PT were evaluated using Linear Phased Array (LPA) to determine the maximum crack depth along the length of the indication. The full length of the cracks on the bottom end of the guide rib were scanned, however the cracks on the top of the guide rib were scanned from the bore side only due to the inaccessibility and surface roughness in the top bore area. The largest crack depth on the bottom was 0.55” deep while at the top was 1.4”.
For the analysis of the valve it was necessary to establish the typical steady state loading as well as any significant load fluctuations that may contribute to crack growth. Structural Integrity’s experience with similar valves indicates that the primary mechanism of cracking is thermal fatigue due to start-up/shutdown cycles and possibly other significant temperature cycles. The important operating parameters that affect thermal fatigue are the frequency and severity of the temperature fluctuations.
Rates of temperature change were investigated for severity. Rates less than 100F/hr are not typically severe and do not lead to significant thermal gradients and stress in the valve. Rates that exceed approximately 1000°F/hr result in much more significant thermal gradients and higher stresses. Rates on the order of 3000°F/hr result in thermal/mechanical response that approaches an instantaneous step change in temperature, which is the bounding gradient case for a thermal shock.
A two-dimensional (2D) FEA model was used to simulate the thermal/mechanical response of the valve. Comparative 3D solid modeling was performed to validate this approach. Furthermore, the cracks were modeled explicitly in order to directly determine the crack tip driving force (also known as stress intensity factor and denoted as “K”) for use in subsequent crack growth calculations. A range of crack depths were analyzed to determine the stress intensity factors for various crack depths of concern (allowing subsequent calculation of crack growth by interpolation of stress intensity factor as a function of crack depth). Crack depths of 0.125, 0.375, 0.60, 1.0, 1.5, 2.0 and 2.6 inches were analyzed. Heat transfer and thermal-mechanical (temperature and pressure) stress analysis are performed together using a coupled-temperature displacement FEA approach. At various instants during the event being analyzed (e.g. start-up), the temperature distribution and the associated stresses and K values are determined due to the combined effects of temperature gradients and internal pressure.
Stress contour plots, highlighting hoop stress (crack opening stress direction) for internal pressure, start-up and thermal downshock were generated, with Figures 2 and 3 showing examples. Note that the magnitude of elastic stress at the crack tip is not relevant, since the crack tip driving force is calculated directly using a typical J-Integral approach and is provided as output (stress intensity factor) by the finite element analysis software.
Fracture mechanics calculations were performed to determine the critical flaw size considering the failure modes of unstable fracture and plastic collapse. The calculations were performed in accordance with the methodology outlined in Part 9 (Assessment of Crack-Like Flaws) of API 579, using the Failure Assessment Diagram (FAD) methodology.
A known flaw that is suitable for service (i.e., less than the critical crack size) at one inspection may subsequently grow during the operating period until the next inspection. It is therefore necessary to account for this potential flaw growth when dispositioning detected defects (i.e., run vs. repair) and establishing safe operating or re-inspection intervals as part of a fitness-for-service assessment approach.
The results indicate substantial margin relative to the FAD envelope for a 0.6 inch deep flaw in the bottom portion of the valve. The cracks in the upper portion of the valve were less limiting based on the amount of material present. Note that a lower bound toughness was used for the analysis. The critical flaw size was calculated and resulted in a through wall crack depth of 2.6 inches, indicating substantial margin relative to the current flaw depths.
To estimate the future rate of growth of the existing cracks and to assess inspection frequency, calculations were performed considering cyclic stresses and crack tip stress intensity factors calculated during operation. The frequency and severity of these cycles affect the rate of future crack growth.
Based on review of the operating data, the cold and warm start-up/ shutdown cycles were the dominant contributors to fatigue crack growth. The frequency and magnitude of smaller fluctuations in pressure and temperature during operation were considered typical and would not be expected to have a significant contribution to crack growth.
The fatigue crack growth analyses were performed using the pc- CRACK software developed by Structural Integrity. The software allows for definition of loading blocks, which can be used to define and combine various cyclic loading events during a specified operating period. Based on review of the operating data, a loading block representative of typical operation was specified. The material fatigue crack growth law used in the analysis was that given in API 579 for ferritic and austenitic steels exposed to non-aggressive service environments at temperatures between 100ºC (212°F) and 600ºC (1112°F). Results of the fatigue crack growth calculations indicate that if the plant continues to operate similar in the future as it has in the past, the cracks are not predicted to grow to the critical length of 2.6” for more than 2500 cycles or about 25 years of operation.
The valves are required to operate at high temperature (up to 1050°F) which is in the creep range for the C12A (Grade 91) material. Specifically, at such temperatures the long-term application of internal pressure can potentially result in creep damage. The time to creep failure is strongly influenced by the operating temperature and stress. The creep crack growth calculations were performed in accordance with Part 10 of API 579 and Structural Integrity Standard Operating Practices (SOPs).
Inputs to the assessment include component and crack geometry, operating conditions, applied stresses and material properties. Creep crack growth is based on a standard Paris law-like power law equation, but is based on the C* or Ct parameter rather than the K parameter for fatigue crack growth. The required creep crack growth constants are documented in our SOPs, and similar data is also included in API 579.
The detailed calculations are not provided here, but the time computed (remaining life) from creep crack growth analysis, for a 0.6 inch initial crack to grow to a 2.6 inch critical flaw depth is significantly large (millions of hours). The large thickness of the valve body translates to low pressure induced stresses to drive creep damage. Therefore, creep crack growth was not considered a significant factor affecting remaining life of the valves.
The FFS assessment indicated that the cracks identified do not appear to be an immediate threat to the operability of the valves, and future service can be realized with occasional inspections to verify that the cracks are not growing faster than predicted. The cracks are predicted to continue to grow as additional operational cycles are accumulated; however, that growth is predicted to occur at a relatively slow rate, and the rate decreases with crack depth due to decreased thermal stress away from the internal surface.
Based on the results of this analysis and Structural Integrity experience with similar valves, it was recommended to perform a follow-up inspection at the next scheduled opportunity (within the next three years) to measure crack depth to help verify predictions and provide some protection in the event that some cracking mechanism (e.g., environmental) or temperature excursions not considered in the predictions might be actively contributing to crack growth. Results of an inspection of this type should be carefully recorded to allow comparison of crack locations, surface lengths and depths between subsequent inspections.