Learning by doing

According to Dr. Markus Ess, Director of Technology of the Starrag Business Unit HPS, this order has a special background: “We’re not actually a parts supplier, even though we already have experience in casing manufacturing. However, since the scope of the contract covers the entire process chain – from unmachined parts to housing components ready for assembly – we saw this as an opportunity to expand and improve our process expertise. After all, our customers are increasingly seeking this extensive expertise.”

The customer also recognizes that Starrag is not a traditional supplier. The request was made out of necessity. This is because small-batch production was no longer economically viable for the previous supplier, and no other service provider with the necessary expertise and machinery could be found. Starrag, on the other hand, can use its ATCC facility in Rorschacherberg for the majority of the work, as it offers the ideal conditions for machining housings. Spanning approximately 2,000 m2, it is equipped with the latest four- and five-axis Starrag machining centers. In addition, experienced application specialists and developers are on site. “From an economic perspective, it’s an advantage that we don’t have to invest in new machines for this order,” says Markus Ess. “For the casings, we use the STC 1250MT and STC 800MT machines available at the ATCC, which are suitable for cutting and turning, depending on the casing size and requirements.”

Casing manufacturing – what needs to be done?

Gas turbines typically consist of several stages: the air inlet (with a booster, if necessary), the compressor (usually a multi-stage unit), the combustion chamber, the turbine itself – which converts the thermal energy of the hot gases into mechanical or electrical energy – and the exhaust. This is also the case with the smaller casings, which require six different casing components for its housing. The newer generation includes an additional casing.

Each of the rotationally symmetric casings must meet different requirements and varies in size and geometry. The materials also vary depending on the prevailing temperatures. Titanium is used in the cold section, while Inconel is used in the hot section (up to 2,000° C).

Under the contract, Starrag will manufacture the housing components for gas turbines and ship them to the United States as shipsets. A “shipset” refers to a complete set of casings for a turbine.

Technological development was also subject to economic constraints. After all, Starrag didn't just want to gain expertise. Thus, one-time costs were a major factor, and the clamping device was one of them. Stanislav Stankevich, Senior Application Engineer, explains: “We had originally planned to use four different devices, but that would have made the project uneconomical. That’s why we came up with an innovative, modular clamping concept – a combination fixture that allows us to clamp all seven housing types using a single device.” For this purpose, a matrix is marked on the pallet that specifies the clamping positions for each casing. In this process, the same grooves are always used, but different standard vertical clamping elements are employed. This allows an experienced operator to reconfigure the device relatively quickly.

Further in-house developments were necessary to ensure an efficient machining process. Stanislav Stankevich, who has extensive expertise in the field of casings, was responsible for running in the process and the tools, as well as optimizing the equipment. He explains: “To enable drilling and cutting even in hard-toreach areas, our experts developed extraslim angle heads that feature a special interface providing exceptional rigidity and are capable of handling heavy-duty cutting.” Carbide milling cutters developed and ground in-house contribute to optimized machining performance, and other tools such as deep-hole drills and reverse countersinks also originate from Starrag’s own R&D department.

At the end of the manufacturing process, each part is marked with a serial number in accordance with regulations. Bolts, pins, T-nuts, and supplied components are installed. The shipset is then placed in the reusable packaging in which the blanks were originally delivered. The casing set is then shipped to the United States, where the turbine is fully assembled and tested.