What Are Turbine Buckets?
Turbine buckets are an important part of gas turbine stage design. Each stage unit contains a nozzle with a wheel and accompanying bucket. Further turbine sections include the turbine rotor, turbine shell, nozzle, shroud, exhaust diffuser, spacers, and exhaust frame.
A turbine bucket consists of a leading edge, trailing edge, root portion, and tip portion. Turbine buckets also include cooling holes for cooling liquid running through the bucket. First stage buckets are the first surface that the gases from the first nozzle encounter. Given this, they require high heat resistance to function at standard operating temperatures without failure.
As the air passes through the first stage bucket, it flows through cooling holes and exits out of the recessed bucket tips. Cooling holes spaced evenly around the turbine bucket's interior allow air through the plenum and function to cool the airfoil without requiring compressor air.
Due to the pressure drop between each stage of a gas turbine, progressive turbine buckets need to be larger than the previous. The second stage buckets do not require shank cooling, so they use cooling holes to reduce airfoil temperature without losing energy during the cycle. Enclosing the third stage bucket is a turbine shroud that connects to each bucket to dampen vibrations from rotor motion.
Each bucket connects to its turbine wheel with a set of tang dovetails that fit into cutouts placed around the wheel rim. The dovetails then connect to bucket vanes that line the top and bottom of the turbine rotor. The lengthened design of the wheel attachment and spacers ensure that the dovetails do not overheat.
Turbine Nozzle and Vane Design
Each turbine section contains a stationary nozzle that directs the compressed combustion gas into the buckets. The force of the gas stream emitted from the nozzles pushes against the turbine buckets and turbine blades, causing the entire rotor to spin. This setup creates high pressure and heat flow. That's why each gas turbine has seals on the interior to prevent thermodynamic leakage. Turbine nozzles or vanes are an essential part of a gas turbine and must be capable of withstanding thermal stresses and intense pressure from heated gas.
Like buckets, there are three stages of nozzles. The first stage nozzle directly receives gas from the main combustion system through transition pieces. These transition pieces minimize compressor leakage and attach to a retaining ring, which also works to seal the gap between turbine vane and the outer shell. The nozzle support ring supports the first stage vane via bolts that anchor it to the outer casing.
By design, each stage vane reuses combustion gases from the previous stage. To that end, the second stage nozzle redirects pressure against the second stage turbine bucket. Likewise, the third stage nozzle receives gas exiting the second stage turbine bucket and directs it to the third stage turbine bucket. The tiered stage design of the modern gas turbine ensures maximum efficiency and minimal heat loss from each part during the process.
Materials capable of higher temperatures reduce the need for cooling holes, so there is less heat flow and less energy lost during the gas transfer process. Thanks to these advancements, modern blade and turbine buckets often have a 30-year operational life.