Fitness for Service Assessments of Bulging and Out-of-Round Structures

This paper examines techniques for the evaluation of two kinds of structural imperfection, namely bulging subject to internal pressure, and out-of-round imperfections subject to external pressure, with and without creep. Comparisons between comprehensive finite element analysis and API 579 Level 2 techniques are made. It is recommended that structural, as opposed to material, failures such as these should be assessed with a structural model that explicitly represents the defect.

Design and Remnant Life Analysis of a Carbon Steel Containment Jacket Operating in the Creep Range

During routine inspection of a 35 year old steam generating plant, a large surface crack was found at a critical tee intersection. With the crack appearing to be close to a condition of sudden rupture, a probabilistic technique was used to assist in determining the likelihood of failure. This paper describes this technique and the design of a carbon steel containment jacket used to enclose the cracked area. The design of the jacket had to take into account its creep behaviour at elevated temperature. The advantage of this repair method was that it was able to be installed quickly and without having to completely de-pressure the steam system. It was later decided that the operating life of the jacket should be extended to defer a planned shutdown. A simplified remnant life analysis was then undertaken as detailed in this paper. Taking into account the operating history of the jacket, it demonstrated that the life of the jacket could be safely extended as required.

Pressure Equipment Integrity at a Large Chemical Process Facility

The paper provides an overview of an integrity program for pressure equipment implemented at a large chemical process facility. It addresses the challenges encountered and solutions achieved in the practical implementation of the ASME Boiler and Pressure Vessel Code and the National Board NB-23 Code.

An Estimator for the Influence of Service Temperature on Creep Rupture Life

The 2001 ASME B31.1 Code (Code) has a warning to the piping designer regarding materials susceptible to creep damage. However, the Code does not prescribe a methodology to determine the accelerated life reduction for a component due to events resulting in operating temperatures in excess of the design temperature. In general, the quantitative evaluation of the service life of a component subject to creep damage is very complex. Nevertheless, the amount of accelerated creep damage due to increased temperature can be approximately estimated. This paper is the technical basis for a recent modification to the Code. It provides an approximate relationship of operating temperature and time for equivalent creep damage of typical power piping materials. Piping designers, plant operators, and plant engineers may use this information as a rough idea of the relationship of temperature and time to maintain an equivalent safety margin on creep rupture life. This evaluation includes a discussion of tolerance to temperature increase for some low chrome ferritic, intermediate chrome martensitic, and austenitic stainless steel alloys.

Managing Flow Accelerated Corrosion in Carbon Steel Piping in Nuclear Plants

In 1996, Flow Accelerated Corrosion (FAC) was identified as a degradation mechanism affecting carbon steel outlet feeder pipes in CANDU® (CANadian Deuterium Uranium) reactors. The maximum rate of FAC was estimated to be <0.120 mm/year. In response, wall thickness inspection programs have been implemented to identify and measure the minimum wall thickness in outlet feeder pipes. These data are necessary to ensure fitness-for-service of the feeder pipe. These data, together with the thermalhydraulic and geometric parameters for the measured feeders, are also very useful for developing empirical wall thickness models. Such models can be used to enhance the understanding of feeder wall thinning leading to an improved capability to predict future wall thickness minima and their locations. The determined dependency of the wall-thinning rate on thermalhydraulic conditions can be used to quantify the potential benefits of maintenance activities, such as steam generator cleaning. Activities such as steam generator cleaning are generally viewed as beneficial in recovering lost thermal efficiency, thereby reducing the severity of the thermalhydraulic conditions by reducing the amount of quality (steam phase) exiting the reactor core. Finally, when wall thickness models are applied to data from different plants, there is the potential of identifying operating conditions that can lead to lower rates of wall loss. This paper addresses the aforementioned important issues associated with FAC of ASME PVP Class 1 carbon steel piping.

Analysis of a Bolted Joint Failure in a Hydraulic Press

Micrographic inspection, acceleration tests, Finite Element Analysis and detailed fatigue failure calculations for combined static and dynamic loadings, were used in determining the cause of failure, and an acceptable repair solution, for a critical bolted joint. The bolted joint holds together the piston and ram within a hydraulic press. The press had been in operation for eleven years when the failure occurred. As the press is used in a critical process operation, a repair solution and future maintenance strategy was required that would minimize the possibility of future failures and permit maintenance during planned outages. This paper discusses the method that was followed to arrive at an acceptable solution.

Characteristics of Repair Welds on Aged 2.25Cr-1Mo Steel for Creep and Creep-Fatigue Applications

The creep and the creep-fatigue properties of full repair welds (FRW) and partial repair welds (PRW) were evaluated in this study. Since the PRW contained the service-aged girth weld which was the weak link of the cross weld, the PRW was a shorter creep strength than the FRW. Moreover, the PRW showed a remarkably shorter creep-fatigue life compared to that of the FRW. In order to consider the poor creep-fatigue properties of PRW, finite element (FEM) analysis was conducted with experimentally measured material constants using service-aged base metal, aged weld metal, simulated coarse-grained HAZ, simulated fine-grained HAZ and repair weld metal. The analysis revealed that the strain concentrated on the aged and softened base metal or girth weld metal of the repair-welded cross weld specimen and not on the virgin cross weld specimen. The failure locations in creep-fatigue tests were close to the strain concentrated zone. Thus, the strain concentration is considered to work as a significant role and dominate the creep-fatigue properties of repair welds. In addition, the ductility of the weld metal was much less than that of the base metal. Consequently, the interaction of the strain concentration and the lack of ductility induced the lesser creep-fatigue properties of the PRW.

Assessment of a Damaged Piping System Utilizing Laser Surface Scanning and Finite Element Analysis

The application of laser surface scanning technology in conjunction with finite element analysis to evaluate the fitness for service of a damaged section of piping is presented in this paper. The difference between the results obtained using a simplified dimensional model produced using hand measurements and the laser scan is also discussed. A section of NPS 18 (DN 450) diameter piping that carries vapors generated by a coke drum in an oil refinery delayed coking unit had been damaged in service from contact with a nearby structural steel member. The damaged area was deformed nearly 1.2 in. (30 mm) deep over an area measuring approximately 10 in. × 12 in. (250 mm × 300 mm) and included a pronounced crease along the line of contact with the steel. The piping was also slightly out of round outside of the area of gross distortion. Laser scanning was employed to obtain an accurate dimensional representation of the surface geometry and to develop a finite element analysis model. The decision to allow the piping system to continue to operate until the next scheduled outage was then based upon the operating stresses, maximum strain in the deformed area and fatigue life determined for the damaged section.

Determination of Corrosion Layers and Protective Coatings on Steels and Alloys Used in Simulated Service Environment of Modern Power Plants

The development of modern power generation systems with higher thermal efficiency requires the use of constructional materials of higher strength and improved resistance to the aggressive service atmospheres. In this paper the following examples are discussed: 1) The oxidation behavior of 9% Cr steels in simulated combustion gases: The effects of O2 and H2O content on the oxidation behavior of 9% Cr steels in the temperature range 600–800°C showed that in dry oxygen a protective scale was formed with an oxidation rate controlled by diffusion. In contrast, that in the presence of water vapor, after an incubation period, the scale became non-protective as a result of a change in the oxidation mechanism. 2) The development of NiCrAlY alloys for corrosion resistant coatings and thermal barrier coatings of gas turbine components: The increase of component surface temperature in modern gas turbines leads to an enhanced oxidation attack of the blade coating. Considerable efforts have been made in the improvement of the temperature properties of MCrAlY coatings by the additions of minor elements like yttrium, silicon and titanium. The experimental results show the positive, but different influence of the oxidation behavior of the MCrAlY coatings by the addition of these minor elements. 3) The development of light-weight intermetallics of TiAl-basis: TiAl-based intermetallics are promising materials for future turbine components because of the combination of high temperature strength and low density. These alloys, however, possess poor oxidation resistance at temperatures above 700°C. The experimental results showed that the oxidation behavior of TiAl-based intermetallics can be strongly improved by minor additions of 1–2 at.-% silver. 4) The oxide dispersion strengthened (ODS) alloys provide excellent creep resistance up to much higher temperatures than can be achieved with conventional wrought or cast alloys in combination with suitable high temperature oxidation/corrosion resistance. The growth mechanisms of protective chromia and alumina scales were examined by a two-stage oxidation method with 18O tracer. The distribution of the oxygen isotopes in the oxide scale was determined by SIMS and SNMS. The results show the positive influence of a Y2O3 dispersion on the oxidation resistance of the ODS alloys and its effect on growth mechanisms.

Failure Analysis of Weld Repaired Turbine Rotor

The first stage of an IP turbine rotor failed after approximately eight years of service. The rotor had been weld repaired using a 360 degree all weld metal buildup. The rotor material was specified as ASTM A-470 Class 8, 1Cr-1Mo-1/4V forging material, and the weld metal was Modified 9Cr-1Mo known as Grade 91 in the wrought product forms. The weld repair procedure used a controlled weld bead deposition process to refine the grain size of the HAZ. A failure analysis was performed including metallurgical examination and stress analysis. For the failure analysis, the primary features of interest were the fracture appearance, cracking/failure location, and failure mechanism. The fracture appearance was “lack of fusion” where the fracture surface nominally conforms to the weld bead shape. The primary cracking and failure was located at the fusion interface of the weld repair. The failure mechanism was creep rupture. Based on the thermal and stress analysis, the probable root cause of failure was improper location of the weld fusion line during the design stage. Both the operating stress and temperature were higher than design. The operating temperature was higher than the design temperature because of the lower cooling of the rotor experienced in service than predicted. Thus, the HAZ was located at a radial position having higher than allowable temperatures and stresses for the creep weak HAZ.