Introduction

In 1629, the Italian engineer Giovanni Branca designed the first impulse steam turbine. Impulse movement involves a type of turbine blading design that causes rotation of the blades and shaft when high-velocity steam from an external source pushes on the blades. Branca’s innovative design directed high-velocity steam against the blades. The impulse design is used in present-day steam turbines. Technical improvements to the steam turbine design have enhanced efficiency and power.

The primary operating principle of a turbine is to convert steam energy into mechanical energy that can be used to drive rotating equipment. A steam turbine is a device (driver) that converts kinetic energy (steam energy of movement) to mechanical energy. Steam turbines have a specially designed rotor that rotates as steam strikes it. This rotation is used to operate a variety of shaft-driven equipment. Turbines are used primarily as drivers for pumps, compressors, ocean vessels, turbo-electric locomotives, naval vessels, and electric power generation.

As high-pressure steam enters a turbine, it passes through a device called a nozzle. Nozzles restrict the flow and increase the velocity of the steam. The nozzle directs this high-velocity steam against the blades of a paddlewheel, causing it to rotate. As the steam passes through alternate sets of fixed and revolving blades, it constantly expands as it moves along. The rotating paddlewheel is attached to a shaft, and the blading and shaft together make up the rotor. Impulse or reaction movement occurs as the steam strikes the rotor, converting the steam energy into mechanical energy. The amount of steam energy needed to perform useful work depends on the pressure range through which the steam expands.

The steam used to operate a steam turbine is produced in a boiler. Boilers produce steam that can enter a turbine at temperatures as high as 538°C (average 1,000 to 1,050°F) and pressures as high as 3,500 psi inlet and 200 psi outlet. (Steam turbines can also run under a vacuum.) High pressure steam is admitted slowly into a turbine to warm it up and remove condensate (moisture produced by condensation).

Steam turbines are used to drive the electric generators in modern power plants. A multistage steam turbine is considered to be one of the world’s most powerful engines. Modern turbine technology includes 50 or more stages linked along a horizontal shaft. Each stage consists of a set of moving and stationary blades. The curved blades of each stage are designed so that the spaces between the blades act as nozzles and increase steam velocity. As the steam zigzags between the stationary and moving blades, it begins to expand as much as 1,000 times its original volume. Modern turbine design increases the size of each stage, giving the turbine a conical shape.

Basic Principles

The steam turbine operates on basic principles of thermodynamics using the the Rankine cycle. Superheated steam (or dry saturated steam, depending on application) leaves the boiler at high temperature and high pressure. At entry to the turbine, the steam gains kinetic energy by passing through a nozzle (a fixed nozzle in an impulse type turbine or the fixed blades in a reaction type turbine). When the steam leaves the nozzle it is moving at high velocity towards the blades of the turbine rotor. A force is created on the blades due to the pressure of the vapor on the blades causing them to move. A generator or other such device can be placed on the shaft, and the energy that was in the steam can now be stored and used. The steam leaves the turbine as a saturated vapor (or liquid-vapor mix depending on application) at a lower temperature and pressure than it entered with and is sent to the condenser to be cooled.

Classification

Mechanical drive steam turbines are categorized as:
• Single-stage or multi-stage
• Condensing or non-condensing exhausts
• Extraction or admission
• Impulse or reaction

1. Single-stage or multi-stage

In a single-stage turbine, steam is accelerated through one cascade of stationary nozzles and guided into the rotating blades or buckets on the turbine wheel to produce power.

A multi-stage turbine utilizes either a Curtis or Rateau first stage followed by one or more Rateau stages.

2. Condensing or non-condensing exhausts

Condensing turbines are those whose exhaust pressure is below atmospheric. They offer the highest overall turbine pressure ratio for a given set of inlet conditions and therefore require the lowest steam flow to produce a given horsepower. A cooling medium is required to totally condense the steam.

Non-condensing or back-pressure turbines exhaust steam at pressures above atmospheric and are usually applied when the exhaust steam can be utilized elsewhere.

3. Extraction or admission

Steam is extracted from, or admitted to, the turbine at some point between the inlet and exhaust.

4. Impulse or reaction

In an impulse turbine the pressure drop for the entire stage takes place across the stationary nozzle. In reaction designs, the pressure drop per stage is divided equally between the stationary nozzles and the rotating blades.

Components of a Steam Turbine

The parts of a steam turbine may be thought of as being in four groupings: rotor, fixed parts, governing mechanism, and lubrication system

ٌRotor: Steam turbines have a set of rotating blades and a row of fixed “half moon” blades. The wheel-shaped rotating blades sandwich the fixed blades. Operators commonly refer to the assembly consisting of the shaft and the rotating blades as the rotor

Fixed Parts: The principal stationary parts in a steam turbine are the fixed blades; throttle valve; steamtight casing; steam chest; nozzle; and bearings, rings, and seals.

Fixed Blades : The fixed blades are made of durable stainless steel that has been rolled and drawn. The fixed blades are a half-moon–shaped ring located in the lower section of the turbine, sandwiched between the moving blades. When fixed and rotating blades are aligned in the correct position, steam passages are formed across the wheel of the turbine.

Casing: The casing is composed of a base and covering made of carbon steel or turbine iron. The base and the covering are designed to form steamtight joints. Gaskets typically are not needed when reinforced flanges are used.

Steam Chest: The steam chest houses the governor valve and steam strainer (a mechanical device that removes impurities from steam). It is composed of carbon steel or iron and is bolted to the lower casing.

Nozzle: The nozzles and nozzle block constitute a precision instrument fabricated from a solid block of high-tensile carbon silicon steel that directs high-velocity steam against the rotor. Nozzle blocks are bolted to the steam chest. The nozzle has overlapping exits that allow the steam jets to converge before being directed against the buckets of the rotor.

Bearings: Bearings provide radial and axial support for the shaft of a steam turbine. Radial bearings (also called journal bearings) are designed to keep the rotor of a steam turbine from moving from side to side or up and down. Oil supply passages are built into the radial housing, or a slinger ring lubricates the bearings. As the shaft rotates, the lubrication forms a thin film between the shaft and the bearing that allows the system to float. This type of bearing typically is located beside the thrust bearing on one end of the turbine and by the shaft seal on the other.

Seals: Carbon-ring and labyrinth shaft seals are located at the end of each casing along the rotor. These devices are used to minimize the outward leakage of steam under pressure and the inward leakage of air. Carbon rings prevent leakage between the rotor shaft and the casing. The stainless steel spring-backed ring gland is mounted in a corrosion-resistant sleeve that is kept from rotating by a rod passing through the lower housing section. Labyrinth seals consist of a series of ridges and intricate paths designed to stop flow.

Governing Mechanism: The governor is designed to automatically regulate the speed of the turbine. A shaft-type governor is located internally and mounted to the shaft. Centrifugal force causes the weights to rotate. The weights are constrained
by a spring. As centrifugal force increases, the weights move farther from the central shaft, compressing the spring. The rotating spindle is attached to a fixed sleeve, which controls a fulcrum lever. The lever is used to position the governor valve.

Lubrication System: The lubricating oil has five functions. It lubricates bearings and gears. It cools the lubricated parts. It transfers frictional heat. It acts as the hydraulic medium for the governor. It acts as the hydraulic medium for the actuation
of the governor valves and safety devices.

Resources

• Website : Comparison between Impulse Turbine and Reaction Turbine.
• Website : Thermodynamics-Steam Turbine.
• Excel Tool : Steam Tables - Magnus Holmgren.

References

1-ENGINEERING DATA BOOK by Gas Processors Suppliers Association
2-Process Technology - Equipment and Systems by Charles E. Thomas