Main Steam Pipe Research Articles

Test verification of an extensometer for deformation measurement of high temperature straight pipes

In order to measure deformations of high temperature pipes, an extensometer is designed and verified by experiments. Firstly, the structure and working principle of the extensometer are described. Then experiments both in the laboratory and on site are carried out to prove stability and accuracy of the extensometer. Results from experiments are compared with the calibrating values, and they are in good agreement. The operating condition of the main steam pipe is evaluated by the on-site test. Conclusions can be drawn that the designed extensometer has reliable stability and high accuracy and can be used for measuring deformations of high temperature pipes in power plants.

Evolution of carbide morphology and composition in Cr–Mo–V steel after service exposure

Low alloy Cr–Mo–V steels are often used in steam power generation units. The evolution of carbide often leads to embrittlement during elongated service. Therefore, the determination of carbide evolution mechanism during long time service is important to understand and prevent premature failures such as temper embrittlement. In this study, low alloy Cr–Mo–V steels used as main steam pipes in a thermal power plant were studied after various service times as well as in the as fabricated condition. Electron microscopic analyses were carried out on extraction replicas to observe and analyse the morphology and composition of the carbides. Predominant plate-like vanadium rich carbides were observed in the as fabricated condition. When exposed to high temperature as a result of the use, the V rich carbides transformed to Mo rich carbides, which have a typical H type morphology. The change in morphology and composition of the carbide is mainly due to the gradual depletion of Mo from the solid solution, and a non-destructive carbide extraction method was established for the examination of the precipitates in the working turbine rotor.

Application of Ultrasonic Guided Wave Method for Flaw Detection in Coal-Fired Power Plants

Ultrasonic guided wave method used for detection of cracks in the desuperheater header and main steam pipes of boiler in power plants has been reported. The dispersion curves of the guided wave for the main steam pipe were calculated and the wave modes used for the detection of notches were verified experimentally. One defect was found at the welded joint of the low temperature pipe on the furnace side. The time of flight diffraction (TOFD) was also carried out to verify the experimental results. The length, depth and height of the defect are 1200 mm, 18.9 mm and 5.5 mm, respectively. And one circular crack was found inside of the desuperheater header.

Radioactive Research on Main Steam Pipe Rupture Accident of Nuclear-Powered Equipment of Ship

Respectively considering both the condition of the main steam pipe rupture position at the reactor cabin and cabin behind the reactor cabin, the accident process of the main steam pipe rupture is analyzed, the mathematic and physical model about the transference, leakage and diffusion of the radioactive nuclide is established. Through the method of coupling the thermodynamic calculation result with the radioactive result analysis model, the calculation of the nuclide radioactive concentration in the cabin is realized. Further more the harm of the radioactivity has been analyzed. The research of the paper has provided a basis for the effective constitute of the radioprotection measure in the accident.

Reliability Analysis of Main Steam Pipe Containing Defects

Mathematical model of reliability of main steam pipe in power plant was founded by conducting thorough research on crack initiation and growth, considering uniaxial test constants and compact tension constants as random variables. An application example of reliability evaluation of main steam pipe containing initial defect was conducted in this paper using monte carlo method to confirm applicability of this model. The foundation of this model provide scientific basis for reliability evaluation for main steam pipe containing false dismissal detecting initial defect.

Creep‐fatigue analysis of the steel AISI type 316 component failure

AbstractThe purpose of this work is to extend a typical creep‐damage model in order to describe material behavior under variable thermal and mechanical loading in wide stress range. The model basis is creep constitutive law in form of hyperbolic sine stress response function proposed by Nadai. The constitutive law is extended to assume the damage process under creep and fatigue by the introduction of scalar damage parameters and appropriate evolution equations according to Kachanov‐Rabotnov concept. The material constants for model are identified by fitting the experimental creep and low‐cycle fatigue data for the steel AISI type 316 at the range of temperatures 500°C – 750°C. The development of such model is motivated by the well described failure case study of high‐temperature components at unit 1 of Eddystone power plant, which have operated during 130520 hours under creep‐fatigue interaction conditions. The main steam piping (MSP) from this power plant is selected for thermo‐mechanical creep‐fatigue analysis applying the proposed material model. The estimated values of damage parameters comply with the real location of the component failure and a scatter of experimental data on creep‐fatigue interaction diagram. (© 2011 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Hot deformation behaviour and constitutive modelling of P92 heat resistant steel

Hot compression tests are conducted in the present paper to investigate hot deformation behaviour of the newly developed heat resistant steel P92, which is used to fabricate main steam pipes for ultra supercritical power plants. Stress–strain curves at elevated temperatures and different strain rates are obtained. It is found that dynamic recrystallisation happens only when the temperature is above 1100°C and strain rate is below 0·1 s−1. Otherwise, dynamic recovery is the main softening mechanism. Constitutive modelling with the hyperbolic sine, including an Arrhenius term, is used to predict peak and saturated stresses. Material constants for this model are determined. Results show that the model can be used to predict peak and saturated stresses. However, the model would fail in predicting flow stress with respect to strain; thus, a model containing nine independent parameters and the complete form of Spittel equation are utilised to predict flow stress curves softened by dynamic recrystallisation and dynamic recovery respectively since there are no unified equations. The predicted stress–strain curves are in good agreement with experimental results, which confirmed that the models developed in the present paper are effective and accurate for P92 steel.

Kinetics of isochronal austenization in modified high Cr ferritic heat-resistant steel

Employment of high Cr ferritic steels as a main structural material is considered as a way to achieve economical competitiveness of main steam pipe and nuclear reactors in power plants. Differential dilatometry and microstructure observation were employed to investigate the isochronal austenitic transformation of the modified high Cr ferritic steel. The kinetics of the isochronal austenitic transformation were described by a phase-transformation model involving site saturation (pre-existing nuclei), diffusion-controlled growth, and incorporating an impingement correction. The experimental results and kinetic analysis indicate that an increase of the heating rate promotes the diffusion-controlled austenitic transformation. The dissolving degree of precipitates during the austenization process affects the activation energy for diffusion and the undissolved precipitates lead to an increase of the onset temperature of the subsequent martensite transformation upon cooling.

Effects of Two-Step Tempering Treatment on the Microstructural Formation of T91 Ferritic Steels

As a representative type of high Cr ferritic heat-resistant steels, T91 steels (ASME SA-213 T91/P91) has been recognized as the preferable materials and widely used in high-temperature structural components such as header and main steam pipe in advanced power plants. For the service condition is tempered martensites, its corresponding microstructure and mechanical performance are mainly adjusted by the tempering treatment. After exploring the size and number of MX and M23C6 precipitating particles and the width of martensitic lath as a function of tempering temperature, it is recognized that the high tempering temperature leads to an increase of secondary hardening effect, while the low tempering temperature brings a high dislocation density and a small martensitic lath. Hence, a two-step tempering treatment was developed after the traditional normalizing process, in which the T91 steels sample was firstly tempered at a low temperature in order to form some precipitates and then tempered at a high temperature. Those firstly-formed precipitates would pin the dislocations and martensitic laths on the subsequent tempering process, which finally leads to more precipitates, higher dislocation density and smaller martensitic lath width than that obtained from the traditional tempering process.

Estimating main steam piping circumferential weldment creep life consumption

A high energy piping (HEP) program is important for the safety of plant personnel and reliability of the piping systems. These piping systems have been designed considering allowable stresses with some safety margin at 100,000 hours. Unfortunately, many of the main steam (MS) piping systems evaluated by the author have operated beyond 350,000 hours, for which the ASME B31.1 safety margin may no longer exist. Since creep and fatigue is a typical failure mechanism, the probability of HEP failures increases with unit age.The MS piping system is one of the most critical HEP systems. Weldment failures are typically due to a combination of high temperature creep and fatigue. The circumferential weld stresses for MS piping systems operating in the creep regime should include a weldment performance factor that is both multiaxial and time dependent. Application of the actual field stresses, considering malfunctioning supports, to weldment life consumption has resulted in more accurate weldment creep – fatigue rankings and remaining life estimates. This paper demonstrates that the evaluation of actual piping stresses and variations in the actual pipe wall thickness can have a significant effect on the estimates of remaining lives of the girth weldments.

Metallurgical investigation and modelling of deterioration of creep rupture strength in heat affected zone of heat resistant ferritic steel

Heat resistant ferritic steels containing 9–12% chromium have been widely used as materials such as the main steam pipes for fossil fired boilers in thermal power stations. In these steels, creep rupture strength of welded joints deteriorates compared with base metals. The fracture due to this phenomenon is called Type IV cracking. Although this phenomenon occurs in the fine-grained heat affected zone (HAZ) in welded joints and causes limitations of design stress in fossil-fired boilers, its mechanism is not yet perfectly understood.To clarify the mechanism of deterioration of creep rupture strength in HAZ of heat resistant ferritic steel, we have surveyed the affect of weld thermal cycle on the creep rupture strength and the microstructural change at high temperature.As a result, it is found that the creep rupture strength deteriorated in HAZ heated to a peak temperature near 1273 K, at which M23C6 type carbides are partially dissolved and the growth rate of fine particles is faster. The mechanism to explain this phenomenon is discussed from the viewpoint of the growth of precipitates, such as M23C6 and MX, during long-term heating.

Main Steam Piping Girth Weldment Stresses and Life Consumption Considering Malfunctioning Supports

A high energy piping (HEP) program is important for the safety of plant personnel and reliability of the generating units. HEP weldment failures have resulted in serious injuries, fatalities, extensive damage of components, and significant lost generation. Since creep/fatigue is a typical failure mechanism, the probability of HEP failures increases with unit age. The main steam (MS) piping system is one of the most critical HEP systems. Weldment failures are typically due to a combination of high temperature creep and fatigue. Industry best practices include (1) the evaluation of historical operating conditions; (2) examinations of critical weldments to reveal nondestructive examination (NDE) indications, microstructural material damage, and detailed geometry data; (3) hot and cold walkdowns to document the field piping system behavior and anomalies; (4) simulation of as-found piping displacements to estimate actual stresses; (5) ranking of critical weldments; (6) recommendations for support repairs and adjustments; (7) recommendations for future examinations; and (8) remaining life estimates at critical weldments. Appropriate examinations, condition assessments, and recommendations for corrective actions are provided as a cost-effective life management process to maintain the piping system integrity. This paper provides examples demonstrating that the girth welds ranked below the top five to six welds are subject to significantly less applied stress and have substantially more creep/fatigue life than the top ranked welds. Hanger adjustments, along with selective identification, NDE, and possible repairs of top ranked welds provide substantially greater life to MS piping systems. Some fitness-for-service and risk-based programs for MS piping system girth weldments recommend a stress evaluation using typical pressure vessel or boiler tube calculations, in which the hoop stress is the principal stress. In some cases, the effective weldment stresses can be more than 50% above the hoop stress, resulting in the estimated remaining lives less than 15% of the life estimates using the hoop stress methodology. Some HEP life management programs may vaguely discuss using the principal stress based on a finite element analysis of the piping system. These principal stress values may be based on a conventional as-designed piping stress analysis. In the majority of the as-found piping stress analyses performed by the author, the maximum as-found stresses are substantially greater than the maximum conventional as-designed piping stresses. In the example case study, the maximum effective weldment stress was more than three times greater than the estimated as-designed piping stress at the same location. This paper illustrates than an as-designed piping stress analysis will typically overestimate the life of an HEP system and typically not predict the locations of maximum creep/fatigue damage.

Safety Evaluation of the Uprated BWR Using the Large Assembly with Small Pins Concept

An advanced design of a Large Assembly with Small Pins (LASP) has been proposed at the Massachusetts Institute of Technology to increase the power density of boiling water reactors (BWRs) while keeping most of the operating conditions of current BWRs. LASP is based on replacing four traditional assemblies and the large water gap regions with a single large assembly having a 22 × 22 square lattice. In-assembly water rods accommodate control rods as well as provide help to the moderation of neutrons. Previous steady-state analysis showed that the LASP core allows for operation with 20% higher power density than the core with traditional 9 × 9 fuel assemblies. However, the void reactivity coefficient of the LASP core is 25% more negative and the steam flow rate is 20% higher than that of the reference core. In this study, the performances of the LASP core and reference core are compared for selected design-basis accidents and transients. Generally, the LASP design is found to behave in a manner similar to the traditional assemblies. First, the clad peak temperature during a large-break loss-of-coolant accident analysis satisfies regulatory criterion, and it is possible to preserve peak cladding temperature margin of the reference design if the capacity of the low-pressure core injection system is increased by 20%. Second, the generator load rejection with bypass failure and feedwater controller failure analyses show a decrease in dryout margin for the LASP core because of the combination of more negative void coefficient and increased steam load. However, this problem could be remedied by increasing the steam line flow area or allowing an additional flow restrictor in the steam line to attenuate the back propagating pressure wave in the main steam pipe following the turbine stop valve closure. Finally, the LASP core preserved the same level of margin to dryout as the reference core in the cases of four other evaluated events.

Reduction of Solid-Particle Erosion on the Control-Stage Nozzle of a Steam Turbine through Improved End-Wall Contouring

The iron oxide scales exfoliated from the inner wall of a boiler tube and a main steam pipe is known to cause solid-particle erosion on the control-stage nozzle. A combined experimental and numerical investigation was conducted to explore the optimization method of end-wall contouring for reducing the nozzle's erosion damage most effectively. The results indicate that increasing the end-wall contraction ratio and (or) decreasing the distance between the starting point of end-wall contouring and the trailing edge can significantly reduce the erosion-induced weight-loss of the nozzle, and can slightly improve the nozzle efficiency, irrespective of the variation in the particles size distribution and the aerodynamic parameters of a steam turbine. A main reason of erosion reduction is that the movement of loading towards the rear of the nozzle cascade caused by these contoured end walls has reduced the incident velocity of particles. In this study, the weight-loss of the nozzle was reduced by 40—50 per cent, and the nozzle efficiency was improved by 0.4—0.5 per cent by improving the end-wall contouring of the nozzle according to the methods mentioned above.

Flow-Excited Acoustic Resonance in Industry

The excitation mechanism of acoustic resonances has long been recognized, but the industry continues to be plagued by its undesirable consequences; manifested in severe vibration and noise problems in a wide range of industrial applications. This paper focuses on the nature of the acoustic resonance excitation mechanism for the case of closed side branches because of its relative importance to industrial applications. Design charts are presented for the Strouhal number at the onset of acoustic resonance and for the acoustic source strength representing the integral effect of the shear layer at the mouth of the side branch. Because these design charts are developed from tests of cylindrical pipes conveying turbulent flow at high Reynolds numbers, they can be used in industrial applications to predict the onset flow velocity and the intensity of acoustic resonances in side branches. Two industrial examples involving flow-excited acoustic resonance of closed side branches are presented. The first example deals with acoustic fatigue failure of the steam dryer in a boiling water reactor due to acoustic resonance in the main steam piping system, whereas the second example considers acoustic resonances in the roll posts of the short take-off vertical lift (STOVL) joint strike fighter. In both examples, effective means to alleviate the acoustic resonance mechanism are discussed.

Estimating main steam piping circumferential weldment creep life consumption

A high energy piping (HEP) program is important for the safety of plant personnel and reliability of the piping systems. These piping systems have been designed considering allowable stresses with some safety margin at 100,000 hours. Unfortunately, many of the main steam (MS) piping systems evaluated by the author have operated beyond 350,000 hours, for which the ASME B31.1 safety margin may no longer exist. Since creep and fatigue is a typical failure mechanism, the probability of HEP failures increases with unit age.The MS piping system is one of the most critical HEP systems. Weldment failures are typically due to a combination of high temperature creep and fatigue. The circumferential weld stresses for MS piping systems operating in the creep regime should include a weldment performance factor that is both multiaxial and time dependent. Application of the actual field stresses, considering malfunctioning supports, to weldment life consumption has resulted in more accurate weldment creep – fatigue rankings and remaining life estimates. This paper demonstrates that the evaluation of actual piping stresses and variations in the actual pipe wall thickness can have a significant effect on the estimates of remaining lives of the girth weldments.

Effects of prior damage on the creep failure behaviour of similar and dissimilar welded CrMoV main steam pipes incorporating a partial repair

Creep damage finite element analyses, with the inclusion of “prior damage”, were performed for partially-repaired circumferential welds in a thick-walled, main steam, CrMoV pipe. The repair consists of aged parent material, weld metal and one HAZ region being partially excavated and replaced by new weld metal. The pipe welds were subjected to realistic internal pressure and uniform axial loading, the magnitude of the latter being up to that allowed by design codes. The material properties used are related to those of a CrMoV weldment at 640 °C. It is assumed that a full post-weld heat treatment has been carried out and that the effects of welding induced residual stresses reduce to negligible levels. The results obtained are used to examine the subsequent performance for “similar” and “dissimilar” welds with a range of “repair times” (defined as prior damage levels), magnitudes of axial (system) load, etc. From these results, the failure behaviour of this particular partial repair case was evaluated and discussed.

Estimation of material properties and maintenance of pressure parts of thermal power plant boilers under long-term service of 300,000 hours

Tensile and creep properties were investigated of 2 1/4 Cr–1Mo steel (A335 P22) taken from a main steam pipe operated for 300,000 hours for examining the effect of long-term operation. The test specimens in this study were taken from the circumferential welded joint of a Y-piece having been operated as a main steam pipe in a power plant for 302,188 hours at 839 K under 293 start–stop cycles. Creep test of a welded joint taken from this component was carried out and the remaining life prediction was performed considering Type IV damage. In this prediction, finite element creep analysis was combined with a microscopic damage simulation considering the effect of increase of number density of creep voids in the fine-grained heat-affected zone. Thus, the fracture time of the welded joint was predicted.

Damage and Residual Life Assessment of Bends for X20CrMoV12.1 Main Steam Pipe after Long-Term Service

The bent sections from a main steam pipe in a thermal power plant in Shanghai were examined after 165,000 h service at 550 °C under 13.73 MPa pressure. The residual life of the bend sections is determined by evaluation of the service stresses and testing to obtain creep rupture data. Metallographic analysis and tensile, impact, and hardness tests are also conducted. These combined tests show that the properties of the steel deteriorated during service, displaying embrittlement tendencies; the corresponding microstructures exhibit grain boundary weakening and creep damage characteristics. However, considering no evidence of localized damage in the form of creep cavitation or surface cracks was observed in the examined parts, considering the residual life of the bends at service condition, they are adequate for an additional 44,000 h of operation. It is recommended that a health assessment should be taken after 25,000 h service exposure for safety reasons.

PSRDの熱水過渡応答特性解析 - 主給水喪失、主蒸気管破断 -

PSRDの熱水過渡応答特性解析 - 主給水喪失、主蒸気管破断 -