Businesses powering industrial gas turbines with natural gas would love to run them at the stoichmetric ratio because that yields the most efficient combustion. Unfortunately, it also yields exhaust gas temperatures over 3,500°F (1,940°C). That’s a problem because components downstream of the combustors can’t survive those conditions. Consequently, maximizing efficiency means running as hot as possible while minimizing production of corrosive gases.

For manufacturers of gas turbine nozzle components, this places a high priority on raising the peak temperature their components can endure. Here’s how Impro goes about it.

The Downstream Environment

A gas turbine is a continuous combustion engine. Air is drawn in and compressed, fuel, (usually natural gas), is added, and the mixture ignited. The combustion gases then pass through three rings of turbine blades, (sometimes called buckets), with gas velocity and pressure creating rotation that powers a generator (or other rotating machinery).

The nozzle assemblies are rings of static vanes that direct the combustion gases onto the blades. Their geometry is key to maximizing the amount of kinetic energy extracted from the gas.

However, the engineering challenge is that temperatures in the latest gas turbines can go as high as 2912°F (1,600°C) and the fast-moving gas carries corrosive compounds like nitrogen oxides (NOx) and sulfur oxides (SOx). Furthermore, when demand exists, turbine operators sometimes raise the combustion temperature in a process called “peak firing” to increase output.

Materials for Nozzle Assemblies

Among metals that can be cast and machined by conventional methods, stainless steels have the highest melting points. However, they start to lose strength at relatively low temperatures and suffer creep under high loads.

As a result, the preferred materials are superalloys, particularly those that are nickel-based. These melt at around 2,300°F (1,260°C), which is lower than stainless, but don’t start to lose strength until temperatures above 1,112°F (600°C).

Engineered Cooling

Efficiency and emissions reduction requires gas turbines run at temperatures above the melting point of superalloys. To overcome this problem, downstream components like nozzle assemblies and turbine blades are manufactured with very intricate and specific internal cooling channels.  These superalloy components are then coated with high temperature ceramic based thermal barrier coatings.

Within the gas turbine a proportion of compressed air is diverted around the combustion section and into the roots of the nozzles (stationary vanes) and rotating blades and up into the casting’s hollow cooling passages. This air flows through their interior, exiting through small holes that bleed it out over the surfaces. The spacing and diameters of these holes is engineered precisely to obtain the cooling needed.

Manufacturing Processes

Superalloys are hard and tough, and machinability is poor. In addition, machining internal cooling cavities would be prohibitively expensive. As a result, nozzle assemblies, as well as turbine blades, are produced by investment casting.

Investment casting is a process where the mold cavity is formed by coating a wax pattern with a ceramic shell. Voids in the finished part are created by inserting soluble cores and sometimes ceramic cores into the pattern. Before the assembly was formed, the mold is dipped into a light acid to dissolve the soluble cores, leaving the designed hollow features inside the structure. After the pattern has been dipped and the ceramic slurry dried, the wax is melted out, leaving the ceramic cores in place. These are subsequently leached out of the cast part.

A further benefit of investment casting is the ability to manage solidification through ceramic shell thickness. Heat losses are lower where the shell is thicker, which results in slower solidification and alters grain size and shape.

Leading foundries like Impro use latest generation CAE tools to model fluid flows and solidification. This lets us optimize casting qualities and thus the mechanical properties of the part.

Investment Casting Expertise

Impro is a global leader in the investment casting industry and has the skills and capabilities to produce high quality parts like nozzle assemblies. If we can make parts for gas turbine applications we can probably produce the parts you need too. Contact us to find out.