bucket

A bucket is a curved blade mounted on the rotor of a steam or gas turbine. Hot, high-pressure fluid flows across the buckets, pushing each one forward and causing the rotor to spin at speeds reaching 3,600 rpm or higher. The bucket converts the kinetic energy of the moving fluid into mechanical rotation, which drives a generator, compressor, or pump.

Buckets are not uniform across a turbine. Early-stage buckets, closest to the steam or gas inlet, experience higher velocities and temperatures, often exceeding 1,000 degrees Fahrenheit in steam turbines. Later-stage buckets are longer and wider because the fluid expands and slows as it passes through the machine. A large steam turbine may contain 50 or more rows of buckets, each row holding dozens of individual buckets bolted or brazed to the rotor disc.

The shape of a bucket is critical to performance. Most modern designs use an airfoil profile, similar to an aircraft wing, which maximizes the force generated as fluid flows across the curved surfaces. The bucket throat, the narrowest passage between adjacent buckets, must balance accelerating the flow against creating excessive turbulence. Poor bucket aerodynamics waste energy as heat and vibration.

Common failure modes and materials

Buckets fail from erosion, corrosion, thermal fatigue, and mechanical vibration. In steam turbines running on wet steam, water droplets impact and gradually remove material from bucket surfaces. Corrosion occurs when condensate or combustion products attack the blade material. Buckets are typically cast from nickel-based superalloys such as Inconel or Mar-M for gas turbines, which tolerate extreme temperatures, while steam turbine buckets are often stainless steel or titanium for lower-temperature applications.

The term bucket originates from the visual resemblance to a water wheel bucket, though turbine buckets are far more precisely engineered. In some regions and older texts, buckets are called blades or vanes; vane more often refers to stationary buckets that direct flow, while blade or bucket describes the moving, rotor-mounted components. The distinction matters in technical documentation and equipment manuals.