Search Results for reheaters

Advances in materials for power plants include not only new materials with higher-temperature capabilities, but also the use of current materials at increasingly higher temperatures. This latter activity builds on extensive experience of the performance of the various alloys, and provides a basis for identifying changes in alloy behavior with increasing temperature as well as understanding the factors that ultimately determine the maximum use temperatures of the different alloy classes. This paper presents results from an effort to model the exfoliation processes of steam-side oxide scales in a manner that describes as accurately as possible the evolution of strains in oxides growing inside small-diameter tubes subjected to large thermal gradients and to thermal transients typical of normal steam boiler operation. One way of portraying the results of such calculations is by plotting the evolving strains in a given oxide scale on an ‘Exfoliation Diagram’ (of the type pioneered by Manning et al. of the British Central Electricity Research Laboratory) to determine the earliest time at which the trajectory of these strains intersects a criterion for scale failure. Understanding of how such ‘strain trajectories’ differ among different alloys and are affected by the major variables associated with boiler operation has the potential to suggest boiler operating strategies to manage scale exfoliation, as well as to highlight the mode of scale failure and the limitations of each alloy. Preliminary results are presented of the strain trajectories calculated for alloys T22, T91, and TP347 subjected to the conditions experienced by superheaters under assumed boiler operating scenarios. For all three alloys the earliest predicted scale failures were associated with the increased strains developed during a boiler shut-down event; indeed, in the cases considered it appeared unlikely that scale failure would occur in any practically meaningful time due to strains accumulated during operation in a load-following mode in the absence of a shut down. The accuracy of the algorithms used for the kinetics of oxide growth appeared to be a very important consideration, especially for alloy TP347 for which large effects on oxide growth rate are known to occur with changes in alloy grain size and surface cold work.

Sanicro 25 material is approved for use in pressure vessels and boilers according ASME code case 2752, 2753 and VdTUV blatt 555. It shows higher creep rupture strength than any other austenitic stainless steels available today. It is a material for superheater and reheaters, enabling higher steam parameters of up to about 650 °C steam (ie about max 700 °C metal) without the need for expensive nickel based alloys. The aim of the present study is the investigation of the steam oxidation resistance of the Sanicro 25. The long term test was conducted in the temperature range 600 -750 °C up to 20 000 hours. The morphology of the oxide scale and the microstructure of the bulk material were investigated. In addition, the effect of surface finish and pressure on the steam oxidation were also studied.

For raising thermal efficiency and decreasing CO 2 emission, China had constructed the first 600°C ultra-supercritical(USC) fossil power plant in 2006. Now more than a hundred 600°C, 1000MW USC electric power units have been put in service. Recently, China has also developed 620°C USC power units and some of them have been put in service already. Meanwhile, more than fifty 620°C USC boilers will be produced by various China boiler companies. The austenitic steels TP347H, Super304H and HR3C are routinely used for 600°C USC boilers. Among these steels, a big amount of Super304H has been used for boiler superheater/reheater components application. However, Super304H is characterized by good stress-rupture strength but poor corrosion/oxidation resistance. On the other side, HR3C is characterized by very good corrosion/oxidation resistance but lower stress-rupture strength than Super304H. Now, the China 620°C USC project needs a new austenitic heat resisting steel with high stress-rupture strength and good corrosion/oxidation resistance to fulfill the superheater/reheater tube components application requirement. A new austenitic heat resisting steel SP2215 is based on 22Cr-15Ni with certain amount of Cu and also Nb and N for multiphase precipitation (MX, Cu-rich phase, NbCrN) strengthening in Fe-Cr-Ni austenitic matrix and M 23 C 6 carbide precipitation at grain boundaries. This SP2215 new austenitic steel is characterized by high stress-rupture strength (650°C, 105h>130MPa) and good corrosion/oxidation resistance. SP2215 austenitic steel has been commercially produced in tube product form. This SP2215 new austenitic heat-resisting steel is recommended to be used as superheater/reheater components for 620°C USC boiler application.

This paper reports the performance of HR6W iron-nickel based alloy and 617B nickel based alloy which are the candidate material for high temperature reheater outlet header of advanced secondary reheat ultra-supercritical unit boiler with reheat steam 653 °C, and analysis the applicable temperature range of the material. As a result, HR6W is the appropriate material to manufacture high temperature reheater outlet header of A-USC boiler with parameters 620°C /653°C/653°C.

This paper reports the results of a collaborative investigation of an ex-service grade 91 bend carried out by the UK generating companies Centrica, SSE, Engie and RWE. As part of the handover exercise for Centrica’s Langage power station in 2009 a number of routine checks were carried out on the main steam and hot reheat grade 91 steam pipework. In some cases low hardness readings were found with subsequent metallurgical replication showing the presence of an aberrant non martensitic microstructure. This led to a more extensive inspection programme on the steam lines and the discovery of other areas of suspect material. A review of the operating capability of the plant, including detailed pipework stress analysis and a pipework peaking assessment, along with the assumption that lower strength grade 91 material was present, led to the steam lines being down rated and returning to service under these revised conditions. At the first C inspection in December 2012, after the HRSG and associated pipework had operated for 18720 hours, a bend with a soft weld, along with a section of the straight pipe on either side, was removed from service. An investigation was undertaken to establish how long this component would have survived, had it been left in service, and to consider the implications for the future operation of the plant.

This paper reports the results of a collaborative small scale creep testing exercise carried out by the UK generating companies Centrica, SSE, Engie and RWE as part of an investigation of an ex-service grade 91 bend.

Texas Genco requested Stress Engineering Services to assist in reviewing and assessing a failure that occurred in the cold reheat (CRH) steam line at the W.A. Parish Unit around 12:10 PM on July 15, 2003, resulting in a catastrophic failure scattering components within a 1,200-foot radius. Reliant Resources and Texas Genco conducted their own investigation involving metallographic examinations, fracture surface inspection, review of operating conditions at failure time, and studies related to the CRH line weld profile. Stress Engineering Services' efforts included computational fluid dynamics studies to address how attemperator droplet sizes might impact downstream piping system behavior, followed by mock-up testing and field monitoring using high-temperature strain gauges, accelerometers, and thermocouples. The field monitoring data, along with process data from Texas Genco, were used for finite element analyses calculating static stresses and transient stresses from attemperator cycling (thermal stresses) and line vibration (mechanical stresses). A consulting firm contracted by the Electric Power Research Institute (EPRI) performed a fracture mechanics evaluation of the line, though detailed results are not included. The work by Texas Genco, Stress Engineering Services, and EPRI points to the stress concentration factor associated with the internal weld profile near the failure as the primary cause, with the cyclic thermal shocks from frequent intermittent attemperator use being sufficient to initiate the crack.

The fall-off of oxide scale with poor adhesion inside superheater/reheater tubes in boilers for (ultra) supercritical power unit is the main cause of accidents such as superheater/reheater blockage, tube explosion and solid particle erosion in the steam turbine which cause serious economic losses. However, there is still no method for testing and assessing the adhesion of oxide scale inside the tube. A method for testing the adhesion of corrosion products in tubes by spiral lines is proposed in this paper, and the accuracy of adhesion evaluation is improved by adopting the image recognition method.

The combustion of coal and biomass fuels in power plants generates deposits on the surfaces of superheater / reheater tubes that can lead to fireside corrosion. This type of materials degradation can limit the lives of such tubes in the long term, and better methods are needed to produce predictive models for such damage. This paper reports on four different approaches that are being investigated to tackle the challenge of modelling fireside corrosion damage on superheaters / reheaters: (a) CFD models to predict deposition onto tube surfaces; (b) generation of a database of available fireside corrosion data; (c) development of mechanistic and statistically based models of fireside corrosion from laboratory exposures and dimensional metrology; (d) statistical analysis of plant derived fireside corrosion datasets using multi-variable statistical techniques, such as Partial Least Squares Regression (PLSR). An improved understanding of the factors that influence fireside corrosion is resulting from the use of a combination of these different approaches to develop a suite of models for fireside corrosion damage.

According to ASME Case N-888-3, Similar and Dissimilar Metal Welding Using Ambient Temperature SMAW or Machine GTAW Temper Bead Technique, a 48 hr waiting period before conducting the final nondestructive examination (NDE) is required when ferritic filler weld metal is used. The purpose of the 48 hr hold is to confirm the absence of hydrogen-induced cracking in the temper bead heat-affected zone. In previous research, the effect of post-weld heat treatment (PWHT) and temper bead welding (TBW) on the hydrogen-induced cracking (HIC) susceptibility in the coarse-grained heat-affected zone (CGHAZ) in welds of SA-508, P-No. 3 Group 3, pressure vessel steel was investigated using the Delayed Hydrogen Cracking Test (DHCT). In that previous study, the Gleeble thermomechanical simulator was used to generate six CGHAZ microstructural conditions: as-welded (AW), PWHT, and AW with single a TBW reheat at 675, 700, 725, and 735°C. Hydrogen was introduced to the specimen through cathodic charging under in situ constant tensile stress. The HIC susceptibility for these microstructures was ranked by the DHCT at a diffusible hydrogen level significantly exceeding typical GTAW and SMAW processes. The work described in this paper investigates the susceptibility to HIC of these same CGHAZ microstructures with DHCT at variable current densities, further ranking each condition. Test results were analyzed by fracture surface examination of failed tests, and cross-section microstructural analysis under a scanning electron microscope (SEM). Future steps include evaluating critical hydrogen content levels using gas chromatography for each condition. The results from this study will be used to consider potential elimination of the NDE hold time requirement in Case N-888-3 when ferritic weld metal is used.

The use of the bainitic creep strength enhanced ferritic steel T/P23 has increased over the last decade in a wide range of applications including headers, superheater and reheater tubing and in waterwall tubing. Many issues have been reported in weldments of this material, such as hydrogen induced cracking, reheat cracking and stress corrosion cracking. In order to help characterize high temperature cracking phenomena, including reheat cracking, a limited number of laboratory creep crack growth tests are being conducted as part of an ongoing project. Tests were run on as-welded sections with the test specimen crack-tip located in select zones of the weldment. Test temperatures are intended to bookend the range of applications from a waterwall condition of ~482°C (900°F) to the superheat/reheat condition of 565°C (1050°F). This paper describes the results of some early testing at 482°C (900°F). The tests provided useful insight into the cracking susceptibility of the material at this temperature with respect to not only time-dependent cracking, but also fatigue crack growth and fracture toughness. The paper includes details of the test method and results, as well as findings from post-test metallographic examinations of the tested specimens.

The capacity of PC power plants in Japan rose to 35GW in 2004. The most current plants have a 600 deg-C class steam temperature and a net thermal efficiency of approximately 42% (HHV). Older plants, which were built in the ‘70s and early ‘80s, will reach the point where they will need to be rebuilt or refurbished in the near future. The steam temperatures of the older plants are 538 deg-C or 566 deg-C. We have done a case study on the refurbishment of one of these plants with the advanced USC technology that uses a 700 deg-C class steam temperature in order to increase the thermal efficiency and to reduce CO 2 emissions. The model plant studied for refurbishing has a 24.1MPa/538 deg-C /538 deg-C steam condition. We studied three possible systems for the refurbishing. The first was a double reheat system with 35MPa/700 deg-C /720 deg-C /720 deg-C steam conditions, the second one was a single reheat 25MPa/700 deg-C/720 deg-C system, the last one was a single reheat 24.1MPa/610 deg-C/720 deg-C system. In addition to these, the most current technology system with 600 deg-C main and reheat temperatures was studied for comparison. The study showed that the advanced USC Technology is suitable for refurbishing old plants. It is economical and environmentally-friendly because it can reuse many of the parts from the old plants and the thermal efficiency is much higher than the current 600 deg-C plants. Therefore, CO 2 reduction is achieved economically through refurbishment.

Alloy 33 is a weld overlay material that has generated a lot of interest in the fossil boiler industry. The high chromium content of Alloy 33 has been shown to provide excellent corrosion protection in both waterwall and superheater/reheater tube applications. For waterwall applications, the corrosion resistance has been demonstrated in both laboratory and field tests conducted over the last 5 years. In addition to corrosion resistance, the Alloy 33 has also shown that it is also resistant to cracking (although no material is 100% immune). In the superheater/reheater, the use of spiral clad weld overlay tubes is able to provide resistance to excellent coal ash corrosion. Laboratory and field tests have shown Alloy 33 to have among the best corrosion resistance of all materials studied. The application of Alloy 33 is also easier than other more highly alloyed materials (such as FM-72) and is less expensive. As a result of these favorable experiences, Alloy 33 is now being used commercially to weld overlay both waterwall and superheater/reheater tubes on fossil boilers.

Advanced austenitic stainless steels, such as Super 304H, have been used in reheater and superheater tubes in supercritical and ultra-supercritical power plants for many years now. It is important to characterize the microstructure of ex-service reheater and superheater tubes as this will help researchers understand the long-term microstructural evolution and degradation of the material, which can impact the performance and lifetime of the components that are in service. In this research, the microstructure of an ex-service Super 304H reheater tube that has been in service for 99,000 hours at an approximate metal temperature of 873K (600°C) has been characterized. The characterization techniques used were electron microscopy-based and included imaging and chemical analysis techniques. Seven phases were observed as a result of the characterization work. The phases observed were MX carbonitrides rich in niobium, copper-rich particles, M 23 C 6 , sigma phase, Z phase, a cored phase, and a BCC phase.

The use of the bainitic class of creep strength enhanced ferritic steels T/P23 and T24 has increased over the last decade in a wide range of applications including replacement headers, superheater and reheater tubing and in waterwall tubing. Many issues have been reported in one or both of these materials including hydrogen induced cracking, reheat cracking and stress corrosion cracking. To appropriately address these issues, work has been initiated that includes a literature review, development of a database of phase transformation temperatures, investigation of tempering behavior, and an analysis of the effect of phase transformation on residual stresses. Such information will be provided in the context of understanding why these two materials appear highly susceptible to these cracking mechanisms.

Recent advances in materials technology for boilers materials in the advanced USC (A-USC) power plants have been reviewed based on the experiences from the strengthening and degradation of long term creep properties and the relevant microstructural evolution in the advanced high Cr ferritic steels. P122 and P92 type steels are considered to exhibit the long term creep strength degradation over 600°C, which is mainly due to the instability of the martensitic microstructure strengthened too much by MX carbonitrides. This can be modified by reducing the precipitation of VN nitride and by optimizing the Cr content of the steels. An Fe-Ni based alloy, HR6W strengthened by the Fe2W type Laves phase is found to be a marginal strength level material with good ductility at high temperatures over 700°C and to be used for a large diameter heavy wall thick piping such as main steam pipe and hot reheat pipe in A-USC plants, while Ni-Co based alloys such as Alloys 617 and 263 strengthened by a large amount of the y’ phase are found to be the high strength candidate materials for superheater and reheater tubes, although they are prone to relaxation cracking after welding and to grain boundary embrittlement during long term creep exposure. A new Ni based alloy, HR35 strengthened by a-Cr phase and other intermetallic phases has been proposed for piping application, which is specially designed for a good resistance to relaxation cracking as well as high strength and a good resistance to steam oxidation and fire-side corrosion at high temperatures over 700°C.

SUPER304H (18Cr-9Ni-3Cu-Nb-N, ASME CC2328) and TP347HFG (18Cr-12Ni-Nb, ASME SA213) are advanced fine-grained microstructure steel tubes developed for high strength and superior steam oxidation resistance. Their exceptional performance is demonstrated by the longest creep rupture tests, with SUPER304H tested at 600°C for 85,426 hours and TP347HFG at 700°C for 55,858 hours, both maintaining stable strength and microstructure with minimal σ phase formation and absence of other brittle phases compared to conventional austenitic stainless steels. HR3C (25Cr-20Ni-Nb-N, ASME CC2115) was specifically developed for high-strength, high-corrosion-resistant steel tubes used in severe corrosion environments of ultra-supercritical (USC) boilers operating at steam temperatures around 600°C. The longest creep test for HR3C, conducted at 700°C and 69 MPa for 88,362 hours, confirmed its high and stable creep strengths and microstructural integrity across the 600-800°C temperature range. These innovative steel tubes have been successfully installed in the Eddystone No. 3 USC power plant as superheater and reheater tubes since 1991, with subsequent microstructural investigations after long-term service exposure revealing their remarkable performance. The paper provides an up-to-date analysis of the long-term creep rupture properties and microstructural changes of these steels following extended creep rupture and aging processes, highlighting their successful application as standard materials for superheater and reheater tubes in newly constructed ultra-supercritical boilers worldwide.

Development activities initiated over a decade ago within the COST 522 program and continuing through the COST 536 Action have yielded significant progress in constructing a new generation of steam power plants capable of operating under advanced steam conditions. These innovative plants promise substantially improved thermal efficiency, with steam temperatures reaching up to 620°C (1150°F). Recent successful power plant orders in Europe and the United States stem from critical advancements, including the development, testing, and qualification of 10% Cr steels with enhanced long-term creep properties for high-temperature components such as turbine rotors and valve casings. Extensive in-house development efforts have focused on fabrication, weldability, mechanical integrity, and fracture mechanics evaluations of full-sized forged and cast components. These materials will be implemented in several new coal-fired power plants, notably the Hempstead plant in the USA, which will operate with live steam temperatures of 599°C (1111°F) and reheat steam temperatures of 607°C (1125°F). The improved creep properties enable the construction of casings with reduced wall thicknesses, offering greater thermal flexibility at lower component costs and facilitating welded turbine rotors for high-temperature applications without requiring cooling in the steam inlet region. Looking forward, further efficiency improvements are anticipated through the introduction of nickel alloys in steam turbine and boiler components, with the European AD700 project targeting reheat steam temperatures of 720°C (1328°F) and the US Department of Energy project aiming even higher at 760°C (1400°F). The AD700 project has already demonstrated the technical feasibility of such advanced steam power plants, with engineering tasks progressing toward the construction of a 550 MW demonstration plant, while DOE activities continue to address boiler concerns and focus on rotor welding, mechanical integrity, and steam oxidation resistance.

The rising global energy demand has led to a surge in the construction of high-efficiency power plants with advanced steam parameters. National and international projects indicate that fossil fuels will continue to be the primary source of power generation in the coming years, despite significant efforts and progress in utilizing alternative energy sources. Economic pressures and climate protection concerns necessitate more cost-efficient and environmentally sustainable energy production. Achieving this requires reducing specific fuel and heat consumption per kilowatt-hour, making it essential to improve the efficiency of new power plants beyond those commissioned in Germany between 1992 and 2002. While new construction and process innovations contribute to efficiency gains, the primary factors driving improvement are increased steam pressure and temperature. Current design parameters include steam temperatures of 605 °C (live steam) and 625 °C (hot reheat steam), along with pressures of 300 bar (live steam) and 80 bar (hot reheat steam), which have become critical for obtaining building and operating licenses in Germany. However, the European Creep Collaborative Committee’s (ECCC) 2005 reassessment of the creep strength of steel T/P92 (X10CrWMoVNb9-2) has placed limitations on further increasing steam temperatures beyond 625 °C.

The oxide exfoliation is one of the main problems that cause the explosion of superheater or reheater, which threaten the safety of power plant units, but there is no direct test method of the particle concentration of the scales in high temperature steam. Based on the study of ferromagnetic and optical characteristics of scales, the technology and equipment were developed for on-line measurement based on magnetic sensitivity and granularity behavior. Through numerical simulation and dynamic simulation experiments of scale movement under high temperature and high pressure steam, calculating method of the particle concertation of scales in the main steam or reheated steam pipeline was retrieved by local sampling concentration.