Steinmüller Africa’s Senior Welding Engineer, Friedrich Schwim, talks to African Fusion about a current flagship welding development, the HP Heaters for Tutuka and Duvha, which have been locally designed in-house by the engineering team and are currently being manufactured out of the company’s Pretoria West fabrication facilities.
Click to download and read pdf
A view of the cramped internal space inside an HP heater.
For over five decades, Steinmüller Africa has been providing comprehensive solutions for steam generating and processing plants in every phase of their life cycles. “Design, maintenance and repair of steam generating plants are core business for us, and this includes manufacturing replacement pressure components at our local facilities in Pretoria West, before installing them on site,” begins Friedrich Schwim, Steinmüller Africa’s Senior Welding Engineer.
“While we continue to be very active on the maintenance side of steam generation, one of our current flagship projects is at our Pretoria West Workshop on the fabrication side, where we are busy manufacturing replacement HP Heaters for Eskom’s Tutuka and Duvha Power Stations,” he says.
These heaters are a crucial part of power generation boilers. They take the bled steam from the turbine – which is still at a relatively high temperature and pressure (above 250 °C and 100 bar) – and use it to preheat the boiler feed water. This relieves the pressure on the boiler, reduces the energy and the amount of fuel required to evaporate the feedwater, and therefore increases the efficiency and reliability of energy generation plant.
“We are currently in the fabrication stage of this project, which involves manufacturing a total of 14 HP heaters, which each of which contains some 54 t of mostly imported steel,” Schwim continues. Several different variations have been custom designed by Steinmüller’s South Africa’s engineering team to meet the requirements of the client’s specifications.
“This is flagship work because it is designed and manufactured in South Africa, by South African engineers. Only the raw materials are being imported: 16Mo3, which is a specified pressure vessel grade chrome-molybdenum steel alloy for use at high pressures and temperatures and 15NiCuMo (15NiCuMoNb5-4-6) for use on the steam headers, for example,” he says.
Describing the complicated structure of these heaters, Schwim says they are effectively heat exchangers with shells 12 to 14 m long and 2.0 m in diameter. Inside, the vessels are packed with tube bundles that carry steam from the low-pressure turbines back to the condenser. Headers on either side of the vessel transfer this steam into the tubes and out on the other side. In the opposite direction, boiler feed water is being pumped through the heater shell, heating up as it passes through.
The headers themselves, he says, are manufactured from forged 15NiCuMoNb5-4-6 (WB36) material. These are critical components that are manufactured locally In Steinmüller’s Pretoria West workshop. “For each connecting steam tube, we first must weld a machined solid round bar, called a nipple, onto the header pipe. Each nipple must then be drilled to the right inside diameter so connecting tubes can be welded on, in order to distribute the steam into the heater,” Schwim explains.
Most of the header work has already been completed, with the majority of the nipple welding being done using an Oerlikon submerged-arc nipple welding machine that was originally installed for manufacturing headers for the Medupi and Kusile power stations.
“With an OD of just 30 mm, though, submerged arc welding is not always ideal. So, in collaboration with eNtsa at the Nelson Mandela University (NMU), we used friction welding to do the nipple welding for four of the 28 headers – with great success. This is a world-first and has the potential to become a preferred technique for us in the future,” Schwim informs African Fusion.
eNtsa, along with Eskom, pioneered friction welding as part of an integrity testing technique – called WeldCore – for high-pressure boiler systems subject to cyclic creep. The process involves removing a core test sample from the shell of a pressure component for cyclic testing and remaining component life assessment. The shell is then repaired using a friction welding technique called friction taper hydro-pillar processing (FTHPP), which fully restores the high-pressure integrity of the system.
Weldcore is not only approved by Eskom, it also has unconditional global acceptance for application on high-integrity plant and equipment designed in accordance with ASME’s Boiler and Pressure Vessel Code (BPVC). “This SA-developed expertise in friction welding and FTHPP was used to develop a new friction-welding technique for welding nipples onto headers, which really does look promising in terms of weld integrity,” says Schwim, adding that Steinmüller has also done some successful trials using its explosive welding capability, which may also play a role in future header work.
Once the headers are complete, the ends of the steam-tube heater bundles need to be connected to the header nipples. “Here, where possible, we use orbital welding. We have two orbital systems, the first is an AMI system, but it is quite big so we could not use it for the small-bore heater tubes welded to the nipples on this project. It is used, however, for normal tube to tube welding, which is just 25 mm OD with a wall thickness of 2.9 mm. So, we bought a new TIG-based orbital welding system with feed wire and the smallest heads from Polysoude for welding tube-bundle ends to header nipples.
“We had to recruit and train a team of local welders to complete the joints where the orbital heads would not fit and to speed up production. At one point there were six tube bundles on the floor that needed to be welded and each heater has around 900 of these welds. There are not many people who can weld in the cramped space around these tubes so, as with all new projects, we started with a higher-than-expected repair rate. But we are now down to below 2%, which is excellent, particularly on difficult welds like these,” Schwim points out.
On the quality side, he says that Eskom requires 100% radiographic testing on every weld on each system, which is factored into the daily routine of the fabrication process. “We have also implemented a new cloud-based production system for this project, a tracking and traceability system called WeldEx. It is a local solution that helps us to record and track progress on a project in real time, with welding supervisors updating the system as specified by the workshop. This system is in the process of being implemented across the entire company to track progress daily on all sites. Steinmüller welds an excess of 100 000 units per year, so tracking each weld is critical to us” he says.
“We have also added fitting, cutting and prepping operations to the WeldEx system. These can now be tracked to monitor productivity on the production side and not just the welding side of things. This helps to give a complete and up-to-date picture of how production is going, and it assists in compiling the records we need for our ISO 3834 and ISO 9001 quality management systems,” he adds.
Retaining skills, according to Schwim, is a key challenge for Steinmüller as for the whole of the South African welding industry. “It has been 30-years or so since we last built these particular HP heaters, so the whole team is new. The design was done by new and relatively young engineers. Then we have had to develop people from scratch to have the expertise and experience to deliver the flawless welding results that are required,” he points out.
“We all went through a massive learning curve on this project, but after a few difficulties, things have come together and we now have a good system running. We would like to keep that going, though, with ongoing work to help us to retain the skills we have developed,” Schwim says.
“We have now manufactured and tested all 28 of the headers required for this project, with 24 of them being welded using our Oerlikon system and a further four being done using friction welding. To date, two fully completed HP heaters have been delivered to Duvha and are waiting to be installed, while a further two for Tutuka are complete and in storage at the workshop, ready to be delivered and installed by Steinmüller.
“We continue to fabricate and assemble the remaining units, all of which will be completed in the new year,” says Friedrich Schwim. “We believe we are well placed to do more of this work, which, if not done perfectly, can prove to be very costly for inexperienced fabricators,” he concludes.
