connectors between the header and the manifold,” said Fontaine. The dissimilar welds are essentially located within the connections where Incoloy transition sections are placed between the P91 connectors and 347H connectors. Explaining the arrangement, Fontaine said: “Incoloy has a thermal expansion just in between carbon steel and stainless steel. The transition pieces are 150 mm long to make the connection between stainless steel and ferritic steel. So there are two critical welds – one for stainless steel to Incoloy and one from Incoloy to carbon steel.” Notably, there will be access to these welds so they can be monitored over the lifetime of the plant to see how they hold up under the heavy cycling that is expected. “The welds will expand and shrink daily over time, so good accessibility has been provided so we can perform some specific inspection procedures that have been established. We can do non-destructive testing over time,” said Fontaine. While this is the solution for Bouchain, he noted one benefit of locating the dissimilar welds in the tubes themselves. “The tubes in the superheater/ reheater have smaller diameter, so the thermal expansion is potentially much smaller. This means the stress on the welding will be much less,” he said. Here, specific procedures are followed to weld ferritic steel alloy to austenitic steel and there will be no need to use Incoloy transitions. Extensive studies have been carried out in-house in the laboratory, as well as with tube boiler suppliers such as Sumitomo, using Finite Element Analysis to measure parameters such as accelerated stress fatigue. “Each solution has its advantages but the preferred solution of CMI is to have the dissimilar welds in the tubes,” Fontaine commented. In terms of general design, the HRSG at Bouchain is a traditional bundle-type boiler, with 18 pre-fabricated bundles arranged in modules. Each boiler is three modules wide by six rows. There is no duct firing as this has a negative impact on efficiency. There is also no selective catalytic reduction, although a 6 m cavity is provided for potential future installation if needed to meet any tightening in emission regulation. The boiler has to handle gas flowing from the turbine at a rate of 750 kg/s and a temperature of around 650°C. This led CMI to design a boiler with a heating surface of 405 428 m2, provided by 14 142 tubes, each measuring 21.7 m in length. Fontaine explained the current market trend to this long tube length. “Going to more than three modules wide in order to handle greater gas flow is a big step in price; it’s a killer. So as gas flow is increased, we have to increase the cross-section by increasing the tube length. This allows us to keep to three modules, even if the size and weight of the modules is greater.” It is notable that it is a drum-type boiler, since the trend is towards once-through design at such high pressure. Fontaine explained: “Drum type boilers have been proven in the industry up to steam outlet pressures of around 170-180 bar. But the reason for the limitation is quite simple – as the pressure increases the steam becomes denser. It becomes closer to water as you approach the supercritical point.” This means it becomes more difficult to effectively separate steam and water in the drum because one of the purposes of the drum is to have dry steam going into the superheater. Fontaine noted: “This task becomes more difficult as the pressure increases; so 180 bar at the outlet is considered as the limit.” While the pressure at the steam outlet is usually slightly lower than that inside the drum, the pressure inside the drum at Bouchain is still 160 bar, which is close to the considered limit. In order to increase the pressure and therefore efficiency, once-through technology is considered. In the once-through design, water enters at one end and exits as superheated steam. The circulation ratio is one and there is no need for circulation pumps. Since the phase change takes place in the heat exchanger in a once-through boiler, there is no need for either steam drum or blow-down tank. In this scenario, there would be a steam separator instead of a drum to separate water from steam and to accommodate the swelling effect during start-up. Yet while once-through design has many advantages, an important feature of the drum-type boiler is the flow stability in the heat exchanger. Special Project Supplement under the ASME Codes and meets Europe’s Pressure Equipment Directive (PED), has been used in supercritical boilers for some time. “Super304H is available on the market and accepted by ASME Codes. It has a long history in coal fired boilers,” said Fontaine. “But considering it for combined cycle boilers is quite a new development. We are using it in the HRSG at Hamitabat, which will use the Siemens H gas turbine and at the Norte project in Mexico, which will use the MHPS J class machine.” According to Fontaine the decision to change the material in the superheater/ reheater sections for the first time at Bouchain was a big step for this and future projects. While the use of a new material may sound straightforward, it called for a lot of developments. “Ferritic steels are basically from the same family, so there is no problem in welding them to each other, as they have almost the same thermal expansion,” said Fontaine. “But where you have to weld the stainless steel tube to carbon steel tube, like at Bouchain, it is a challenge. We are talking about welding two different families – ferritic to austenitic.” One of the biggest challenges is welding dissimilar materials when connecting austenitic steel and carbon steel components. The difference in coefficient of thermal expansion of the different materials means that the joints between the materials are subject to greater stress, which can be a particular challenge for plants in cycling operation. At Bouchain Super304H is used in the first rows of the superheater/reheater – the hottest part of the boiler. Conventional carbon steel is used in areas of the boiler where temperatures are lower. Special attention has to be paid in welding the two materials in order to reduce shearing stress, especially when cycling. “It is like mixing water and fire. Because one material expands more than the other, we have to make a deal between the two,” noted Fontaine. Dealing with the challenges posed by dissimilar welds in HRSGs of the Bouchain type was one of the reasons CMI established a welding expertise centre at its Seraing site in Belgium. There are options on where to locate these dissimilar welds – they can be in the connections between the steam manifold and the header of the superheater (6-inch diameter tubes)/reheater tubes (diameter tubes of 31/38 mm) or alternatively downstream in the superheater/reheater piping (diameter > 650 mm). “We had some discussions with GE on where they should be but finally decided to have them on the THE ENERGY INDUSTRY TIMES - JANUARY 2017 CAD drawing of the Bouchain boiler. The HRSG is a triple-pressure, drum-type design Construction of CMI’s HRSG at Hamitabat: the boiler uses stainless steel tubes for an operating steam temperature of 600°C for both high pressure superheater and reheater Stainless steel superheater tubes. The shiny surface indicates that it is not carbon steel
Energy Industry Times January 2017
To see the actual publication please follow the link above