TRIAL ON STEAM TURBINE

INTRODUCTION

A steam turbine is a mechanical device that converts thermal energy in pressurized steam into useful mechanical work.  The original steam engine which largely powered the industrial revolution in the UK was based on reciprocating pistons.   This has now been almost totally replaced by the steam turbine because the steam turbine has a higher thermodynamic efficiency and a lower power-to-weight ratio and the steam turbine is ideal for the very large power configurations used in power stations.   The steam turbine derives much of its better thermodynamic efficiency because of the use of multiple stages in the expansion of the steam.   This results in a closer approach to the ideal reversible process.

Steam turbines are made in a variety of sizes ranging from small 0.75 kW units used as mechanical drives for pumps, compressors and other shaft driven equipment, to 1,500,000kW turbines used to generate electricity.  Steam turbines are widely used for marine applications for vessel propulsion systems.  In recent times gas turbines , as developed for aerospace applications, are being used more and more in the field of power generation once dominated by steam turbines.

Power plants generate electrical power by using fuels like coal, oil or natural gas. A simple power plant consists of a boiler, turbine, condenser and a pump. Fuel, burned in the boiler and superheater, heats the water to generate steam. The steam is then heated to a superheated state in the superheater. This steam is used to rotate the turbine which powers the generator. Electrical energy is generated when the generator windings rotate in a strong magnetic field. After the steam leaves the turbine it is cooled to its liquid state in the condenser. The liquid is pressurized by the pump prior to going back to the boiler A simple power plant is described by a Rankine Cycle.

Saturated or superheated steam enters the turbine at state 1, where it expands isentropically to the exit pressure. The steam is then condensed at constant pressure and temperature to a saturated liquid, The heat removed from the steam in the condenser is typically transferred to the cooling water. The saturated liquid then flows through the pump which increases the pressure to the boiler pressure where the water is first heated to the saturation temperature, boiled and typically superheated. Then the whole cycle is repeated.

When steam leaves the turbine, it is typically wet. The presense of water causes erosion of the turbine blades. To prevent this, steam is extracted from high pressure turbine and then it is reheated in the boiler  and sent back to the low pressure turbine.

Regeneration helps improve the Rankine cycle efficiency by preheating the feedwater into the boiler. Regeneration can be achieved by open feedwater heaters or closed feedwater heaters. In open feedwater heaters, a fraction of the steam exiting a high pressure turbine is mixed with the feedwater at the same pressure. In closed system, the steam bled from the turbine is not directly mixed with the feedwater, and therefore, the two streams can be at different pressures.

DISCUSSION

The steam energy is converted mechanical work by expansion through the turbine.   Th expansion takes place through a series of fixed blades (nozzles) and moving blades each row of fixed blades and moving blades is called a stage.   The moving blades rotate on the central turbine rotor and the fixed blades are concentrically arranged within the circular turbine casing which is substantially designed to withstand the steam pressure.

On large output turbines the duty too large for one turbine and a number of turbine casing/rotor units are combined to achieve the duty.  These are generally arranged on a common centre line (tandem mounted) but parallel systems can be used called cross compound systems

Turbine types

When people began to use water power to win mechanical work, they looked first for the best forms of impellers. Three types were established thereby and variations of them are used today in various applications, among other in steam turbines in power stations, as marine propellers, as compressors in gas turbines etc.

Steam power cycle(practical)

RANKINE CYCLE

Saturated or superheated steam enters the turbine at state 1, where it expands isentropically to the exit pressure at state 2. The steam is then condensed at constant pressure and temperature to a saturated liquid, state 3. The heat removed from the steam in the condenser is typically transferred to the cooling water. The saturated liquid then flows through the pump which increases the pressure to the boiler pressure (state 4), where the water is first heated to the saturation temperature, boiled and typically superheated to state 1. Then the whole cycle is repeated.

Typical Modifications

REHEAT

When steam leaves the turbine, it is typically wet. The presense of water causes erosion of the turbine blades. To prevent this, steam is extracted from high pressure turbine (state 2), and then it is reheated in the boiler (state 2') and sent back to the low pressure turbine.

REGENERATION

Regeneration helps improve the Rankine cycle efficiency by preheating the feed water into the boiler. Regeneration can be achieved by open feed water heaters or closed feed water heaters. In open feed water heaters, a fraction of the steam exiting a high pressure turbine is mixed with the feed water at the same pressure. In closed system, the steam bled from the turbine is not directly mixed with the feed water, and therefore, the two streams can be at different pressures.

Applications of steam turbine

Electrical power stations use large steam turbines driving electric generators to produce most (about 80%) of the world's electricity. Most of these centralised stations are of two types, fossil fuel power plants and nuclear power plants, but some countries are using concentrating solar power (CSP) to create the steam. Steam turbines can also be used directly to drive large centrifugal pumps, such as feedwater pumps at a thermal power plant.

Another use of steam turbines is in ships; their small size, low maintenance, light weight, and low vibration are compelling advantages. A steam turbine is only efficient when operating in the thousands of RPM, while the most effective propeller designs are for speeds less than 100 RPM. Therefore precise (thus expensive) reduction gears are generally used, although several ships, such as Turbine and RMS Titanic, had direct drive from the steam turbine to the propeller shafts. The purchase cost is offset by much lower fuel and maintenance requirements and the small size of a turbine when compared to a reciprocating engine having an equivalent power. However, diesel engines are capable of higher efficiencies: steam turbine cycle efficiencies have yet to break 50%, yet diesel engines routinely exceed 50%, especially in marine applications

A steam turbine locomotive engine is a steam locomotive driven by a steam turbine.The main advantage of a steam turbine locomotive is better balance and reduced hammer blow on the track. However, a disadvantage is that its output power is less flexible and so turbine locomotives were best suited for long