Power Systems Components Lec. 6

Steam Turbine Overview

• Definition: A steam turbine is a mechanical device that extracts thermal energy from pressurized steam and converts it into rotary motion.

• Invention: Modern steam turbines were invented by Sir Charles Parsons in 1884.

• Efficiency and Usage:

  • It has largely replaced reciprocating piston steam engines due to greater thermal efficiency and higher power-to-weight ratio.

  • Approximately 80% of global electricity generation relies on steam turbines.

• Thermodynamic Efficiency: Achieved through multiple stages in the expansion of steam, approaching the ideal reversible process.

Historical Development

• Early Devices:

  • The Aeolipile described by Hero of Alexandria in the 1st century was one of the first steam engines.

  • In 1543, Blasco de Garay used a steam machine for marine propulsion.

  • Taqi al-Din described a practical steam turbine in 1551.

• Parsons' Contribution:

  • Sir Charles Parsons' first model generated 7.5 kW.

  • His invention revolutionized electricity generation, with turbines being easily scalable.

  • The capacity of generators has increased from 7.5 kW to units of 50,000 kW.

Types of Steam Turbines

• Classification: Steam turbines can be classified based on size and application:

  • Sizes: Vary from small (<1 hp) units to large (2,000,000 hp) turbines.

  • Types Based on Operation:

    • Condensing Turbines: Commonly found in power plants, exhaust steam in a partially condensed state.

    • Noncondensing (Backpressure) Turbines: Used in industrial applications, exhausting steam at the conditions needed.

    • Reheat Turbines: Commonly used in power plants where steam is re-superheated after leaving a high-pressure section.

    • Extracting Type Turbines: Release steam for process needs at various turbine stages.

    • Induction Turbines: Introduce low-pressure steam mid-cycle to generate additional power.

Turbine Design and Operation

• Casing Arrangements:

  • Single casing: One casing and shaft coupled to a generator.

  • Tandem compound: Two or more casings coupled to a generator.

  • Cross compound: Two or more shafts driving multiple generators at different speeds.

• Isentropic Process:

  • Ideal turbine operation resembles an isentropic process, though real turbines operate with efficiencies between 20%-90%.

• Blade Design: Interior consists of sets of stationary and rotating blades (buckets), maximizing steam expansion across various stages.

Turbine Efficiency

• Efficiency Types:

  • Impulse Turbines: Use high-speed jets of steam directed from fixed nozzles, converting kinetic energy to rotation.

  • Reaction Turbines: Rotor blades create convergent nozzles, utilizing the reaction force as steam accelerates through them.

Operation and Maintenance

• Warm-up Procedures: Involves using bypass lines and turning gears to prevent uneven expansion.

• Common Issues: Imbalances can cause vibrations, potential blade failure, or thrust bearing damage.

• Steam Quality: Essential to maintain dry steam to prevent blade erosion and operational failure.

• Speed Regulation: Governed by precision mechanisms to prevent overspeed and ensure efficient electrical generation.

Applications of Steam Turbines

• Electric Power Generation: Form the backbone of centralized power stations (fossil fuel and nuclear) producing 80% of the world’s electricity.

• Marine Propulsion: Small size and light weight lead to use in ships which might require reduction gears for effective speed management.

• Industrial Uses: Providing mechanical drives for pumps and compressors.

• Locomotives: Steam turbine technology was also applied in steam locomotion, offering benefits in balance, but limited flexibility in power output.