Understanding the Design and Engineering of Gas Turbine Nozzle Assemblies
Gas turbine nozzle components are some of the most highly engineered parts manufactured anywhere. They need to survive high temperatures, corrosive atmospheres, vibration and centrifugal forces, and the challenges are growing as gas turbine manufacturers push for higher temperatures and speeds.
Impro Aerotek produces precision engineered components for gas turbines and our engineering teams understand what the turbine manufacturers are looking for. This blog explores the biggest challenges in gas turbine nozzle assembly engineering and explains the capabilities we have to offer.
Turning Combustion into Rotary Motion
The gas turbine is an axial flow engine. This refers to a design where airfoil-shaped blades draw air in and progressively compress it until it reaches the combustion chambers. Here the now-hot air is mixed with fuel and burnt in a continuous stream. The resulting combustion products – hot, corrosive gases – pass through a ring of nozzle guide vanes that direct the flow onto turbine blades. These have an airfoil cross-section so when fast-moving gas flows over the surface it generates lift and motion, driving the shaft or shafts used for power generation and air induction.
Engineering Gas Turbine Nozzle Assemblies
Combustion gas temperatures can reach 2,300°F (1,260°C) and their corrosive nature will oxidize and erode most metals. The main engineering challenge is therefore to find ways of enabling nozzle components to survive in these conditions. This is done through a combination of material selection and part design.
Materials for Gas Turbine Nozzle Assemblies
Some steels have melting points above the temperatures inside a gas turbine. However, they suffer from creep, (where strength declines rapidly as temperature rises), and are susceptible to corrosion.
Nickel-based superalloys like Inconel 718 overcome this problem. While Inconel and alloys like it melt at around 2,300 – 2,473°F, (1,260 – 1,356°C) they don’t start losing strength until temperatures exceed 1,400°F (760°C). This helps satisfy the requirements, but clearly isn’t sufficient. The other part of the solution is to provide cooling.
Gas Turbine Nozzle Component Design
Cooling is achieved by making the blades and vanes hollow and supplying their interiors with a stream of air. Cooling channels let this air escape through the components and flow over the external surfaces, which lowers temperatures to a point where the material can survive.
Manufacturing Processes for Gas Turbine Nozzle Components
The only practical method of forming blades and vanes is investment casting. This starts with production of wax patterns incorporating cores that will form the internal voids. Patterns are assembled into a tree around a central sprue for metal delivery, which is then given a ceramic coating. After drying, the wax is melted out, leaving cavities for metal to fill.
Superalloys are usually melted and poured under vacuum to ensure material quality and consistency. After solidification, the ceramic shell is broken away and the blades and vanes cut from the sprue. Cores may be removed at this point, or left inside until machining.
An advantage of investment casting is that, as a near-net shape process, cast parts need little metal removal. However, vanes and blades undergo precision machining to produce the mounting points and in some cases to add the cooling channels.
Machining is often CNC grinding rather than turning or milling, owing to the hardness of the superalloy. Electro-discharge machining (EDM) is sometimes used to create very precise features such as cooling channels. Heat treatment is performed to raise surface hardness, modify toughness, or remove stresses added by the machining processes.
Assembly and Inspection
Nozzle guide vane components are joined by either brazing or electron beam welding. Inspection is performed after assembly, often using ultrasound or X-ray methods. Only once parts are verified as being fit for purpose are they shipped.
Manufacturing Capabilities for Gas Turbine Nozzle Assemblies at Impro Aerotek
As a vertically-integrated manufacturer, Impro can handle the complete production process, from investment casting and precision machining to assembly and inspection. Our plants maintain rigorous QA systems certified to the leading standards, use modern equipment, and employ highly experienced personnel. If you need a source for gas turbine nozzle assemblies or other highly engineered components, we can help. Contact us to schedule a discussion.