PRACTICAL NOTES ON STEAM TURBINES

A steam turbine (ST) generates mechanical power from steam at high temperature and pressure. It can deliver constant or variable speed and is capable of close speed control. Drive applications include pumps, compressors, electric generators and many more. Steam supplies power to about 20%–30% of all pumps and compressors

that move fluids in plants and other facilities worldwide.

STs have been classified by mechanical arrangement as single-casing, cross-compound (more than one shaft

side-by-side) or multi-casing (tandem-compound, which is two or more casings in a single train). They have also been identified by steam flow. This tends to be axial direction for most, and radial for a few. In addition, STs can be categorized by steam cycle, whether condensing, non-condensing, automatic extraction or reheat.

In a steam turbine, the steam flows through directing devices and impinges on curved blades mounted along the periphery of the rotor. By exerting a force on the blades, the steam flow causes the turbine rotor to rotate. Unlike a reciprocating steam engine, an ST makes use of kinetic rather than the potential energy.

Steam turbines have evolved in the direction of multistage axial designs, in which the expansion of steam is performed in a row of sequentially arranged stages. Staging permits a considerable increase in power output, while preserving the speed required for direct coupling of driven equipment.

Steam cycle

The Rankine cycle closely describes the process by which a steam turbine generates power. The cycle consists of a heat source (boiler, heat recovery unit, and so on) that converts water to high-pressure steam. Water is pumped to elevated pressure using boiler-feed water pumps (BFW pumps), which is medium- to high-pressure depending on the size of the unit and the temperature to which the steam is heated. Steam is then heated to the boiling point corresponding to the pressure, boiled (heated from liquid to vapor). Most frequently, it is superheated above the temperature of boiling.

The pressurized steam is expanded to lower pressure in a multistage ST, and exhausted either to a condenser at vacuum conditions (condensing) or into an intermediate-temperature steam distribution system (non-condensing) that delivers the steam to other applications. The condensate is returned to the BFW pumps for continuation of the cycle.

An ST usually consists of a stationary set of blades (called nozzles) and a moving set of adjacent blades (called buckets or rotor blades) installed within a casing. The two sets of blades work together, such that the steam turns the shaft of the turbine and the connected load. The stationary nozzles accelerate the steam to high velocity by expanding it to lower pressure. A rotating bladed disc changes the direction of the steam flow, thereby creating a force on the blades that manifests as torque on the shaft on which the bladed wheel is mounted. The combination

of torque and speed is the output power of the ST.

Material selection

The evolution of steam systems has been coupled to advances in high-strength steel alloys. Steam admission temperatures have continuously been improved. Traditional alloy steels were used up to 550°C. For 580°C and beyond, modern alloy steels are used. Super alloys are needed to keep creep deformation within acceptable limits.

In addition to temperatures, centrifugal forces put high stress on rotor and blade materials. High-yield strength combined with fracture toughness are important requirements. However, these are metallurgical contradicting properties. A challenge in recent decades has been stress corrosion cracks (SCC) in discs exposed to wet steam.

Many steam turbine rotor blades have shrouding at the top, which interlocks with that of adjacent blades to increase

damping and reduce blade flutter. In large STs, the shrouding is often complemented (especially in the long blades of a low-pressure turbine) with lacing wires. These wires pass through holes drilled in the blades at suitable distances from the blade root. They are usually brazed to the blades at the point where they pass through. Lacing wires reduce blade flutter in the central part of the blades. The introduction of lacing wires substantially reduces blade failure in low-pressure turbines.