STEAM TURBINE

A steam turbine is a prime mover that derives its energy of rotation due to conversion of the heat energy of steam into kinetic energy as it expands through a series of nozzles mounted on the casing or produced by the fixed blades.

Important aspects for Steam Turbine functioning

* Steam at high temperature and pressure contains the potential energy.

* Potential energy of the steam is converted into a mechanical work through expansion in a nozzle and impact and/or reaction with a blade.

* Mechanical Work of many sets of blades attached to a shaft produces rotational power.

The use of Steam Turbines for :* Power Generation in Power Plants, * Drives for critical Centrifugal/Axial Compressors in

Refineries, Fertilizers & Petrochemicals * Drive units for pumps, fans

Advantages commonly attributed to Steam Turbines are:* Flexibility to use multiple inlet and exhaust pressure, * Inherent variable speed operation* Operation independent of electric power supply

Turbine Inventions

Turbine Operation:Water is converted to steam by application of heat in the boiler, which makes the steam at specified pressure and temperature.

To convert the steam’s energy into work, it must go through a thermodynamic cycle that combines expansion compression, heat input, and heat rejection.

The most efficient thermodynamic cycle for an ideal fluid is Carnot cycle. It consists of an isothermal heat input, isentropic expansion, isothermal heat rejection, and an isentropic compression.

CARNOT CYCLE

1 to 2: Isentropic expansion

2 to 3: Isothermal heat rejection 3 to 4: Isentropic compression

4 to 1: Isothermal heat supply

1 to 2: Isentropic expansion (Steam turbine)

2 to 3: Isobaric heat rejection (Condenser)

3 to 4: Isentropic compression (Pump)

4 to 1: Isobaric heat supply (Boiler)

1 to 2: Isentropic expansion (Steam turbine) 2 to 3: Isobaric heat rejection (Condenser) 3 to 4: Isentropic compression (Pump) 4 to 1: Isobaric heat supply (Boiler)

Turbine Classification

A. By the action of steam:ImpulseReactionImpulse and reaction combined

B. By the number of step reductions involved:Single stageMulti-stageWhether there is one or more revolving vanes separated by stationary reversing vanes.

C. By the direction of steam flow:AxialRadialMixedTangentialHelicalReentry

D. By the inlet steam pressure:High PressureMedium pressureLow pressure

E. By the final pressureCondensingNon-condensing

F. By the source of steam:Extraction

Impulse Stage:

Impulse nozzles organise the steam so it flows in well-formed high-speed jets. Moving blades, also called buckets, absorb the jet’s kinetic energy and convert it to shaft rotation.

Reaction Type: steam enters the fixed blade passages and leaves as steam jet that fills the entire rotor periphery. Steam flows between moving blades that in-turn, form moving nozzles. There it drops in pressure, and its speed rises relative to that of blades.

Back-pressure Type: The turbine normally operates

against a constant back-pressure. The turbine exhaust steam is supplied to the process and the electric output is dependent on the demand for the process steam.

Back pressure turbines with uncontrolled extraction: This type with uncontrolled extractions is particularly suitable when two steam systems operating at different pressures are to be supplied and the extraction flow is less than the exhaust flow.

Condensing Turbine: Straight-condensing turbines are advantageous, especially when large quantities of a reliable power source are required or an inexpensive fuel, such as process by-product gas, is

readily available.

Extraction -Condensing Turbine:Extraction-condensing turbines generate both process steam and stable electric power. Process steam, at one or more fixed pressures, can be automatically extracted as needed.

AUXILIARY STEAM

TURBINE

LARGE TURBINE INTERNALS

GOVERNOR ACTUATOR & CONTROL VALVE

Overspeed Trip:The OST is a mechanical device, which shuts down the turbine in the vent of overspeed. It initiates trip if speed exceeds an admissible limit more than 10%.

Emergency Stop Valve:

The ESV is the main shut of provision between the steam inlet and the turbine. The ESV can cut off the steam supply in a minimum time

Turbine Journal Bearing:

The function of the journal bearings is * to support the turbine rotor centrally in the outer

casing and its guide blade carriers. * to carry a load perpendicular to the axis of rotation made up

of the weight of the rotor, constant or fluctuating steam forces and kinetic forces caused by the unstable running or any residual unbalance.

Turbine Thrust Bearing:The function of the Thrust bearing is:* To keep the rotor in exact position in the casing* To absorb any axial thrust on the rotor

Governor:To control turbine load automatically speed governor takes care to measure shaft speed and adjust governor valve openings to pass the required steam flow, in turns, maintain speed within the governors regulating range.

Turbine speed will remain constant as long as shaft load, governing valve openings are unchanged and steam conditions remain steady. But when shaft load decreases with constant governing valve opening, the turbine speed increases because it is getting too much steam. To bring shaft speed to normal, governing valve must throttle steam flow to match the new load.

Types of Governor:

Construction wise: * Mechanical Governor: Simple flyweight arrangement* Mechanical + Hydraulic GovernorUsing the principle of flyweight & hydraulic amplification* Electronic GovernorMicroprocessor based controlsFunction wise:

* Constant Speed Governor* Variable Speed Governor

Governing System

Turbine Data Sheet

Operation Guidelines:

To ensure the trouble free operation, the turbine must be:* Operate as per specified operating parameters* Maintain Proper lubrication* Protective devices in line& functioning* Operate as per SOP* Regularly inspect/maintain as per Vendor guidelines* Turbine steam quality to be ensured for trouble-free

operation. The build up of deposits in turbine due to impurities in the steam can cause both thermodynamic and mechanical problem which can lead to blade failure.

Lube oil system

Turbine

Start-up

Curve

Turbine Monitoring:The principal criteria of monitoring, which can indicate a dangerous condition during normal operation are as follows:

* Oil and Bearing Temperatures* Bearing housing and Shaft vibrations* Shaft axial displacement * Steam Parameters like* Oil Quality

Steam Parameters MonitoringA.TemperatureThe satisfactory functioning and life of the various components of the turbine are to a large extent dependant on the absolute main steam temperatures and temperatures fluctuations. Consequences of exceeding the permitted tolerances are:* A shortening of the life the affected components.* A build up scale in guiding and sealing devices.* Permanent distortion of housing and casings.* Temporary or permanent changes in radial

clearances.

B. Inlet Steam Pressure Monitoring:

The objective of Inlet Steam Pressure monitoring:* Maintain the inlet steam pressure within the permitted

tolerance.* Keep the initial steam pressure as constant as possible

during steady state operation.* Match the initial steam pressure to the requirement of

power output.* Maintain the pressure and changes during start-up as

far as possible in accordance with the actual casing temperatures.

C. Wheel Chamber pressure Monitoring:

Wheel Chamber pressure monitoring is a good indication of rotor condition. As these pressure is direct function of steam mass flow. If the measurement indicates higher values at constant mass flow then it is an indication of deterioration of turbine operation. All operating conditions need to be checked.