STEAM TURBINES AND ITS GOVERNING

MOHAMMED FARIS A K42

INTRODUCTION

The steam turbine is a prime mover in which the potential energy of steam is transformed into kinetic energy , and latter in its turn is transformed to mechanical energy of rotation of the turbine shaft.

Utilised in most diverse fields of industry, for power generation and for transport

CLASSIFICATION

Based on action of steam Impulse Reaction Combination of impulse and reaction Based on number of pressure stages Single stage Multi stage Impulse and Reaction turbines Based on Direction of steam flow Axial turbines Radial turbines Based on no of cylinders Single Double Three cylinder Four cylinder

Based on method of governing Throttle governing Nozzle governing According to heat drop process Condensing turbines with generators Back pressure turbines Topping turbines Low pressure turbines Mixed pressure turbines According to steam conditions at inlet Low pressure (1.2-2 atm) Medium pressure ( upto 40 atm ) High pressure (above 40 atm ) Very high pressure ( 170 atm and above and temp of 550 C

and above ) Super critical (above 225 atm ) Based on usage in industry Stationary ( variable and constant speed ) Non stationary

Advantages of Steam Turbine over the Steam Engines• Thermal efficiency higher for steam

turbine• Power generation at uniform rate • Higher speeds and higher outputs• No internal lubrication and minimised

balancing problem• No loss due to initial condensation of

steam• Utilise high vaccum very advantageously

COMMON TYPES OF TURBINES• Simple impulse turbine

Steam expands in the nozzles and its pressure does not alter as it moves over the blades.Eg: De laval turbine Expansion takes place in one set of nozzles.

• Reaction turbine Steam expands continuously as it passes over the blades and thus there is gradual fall in the pressure during expansion. No of stages required is more.

Difference between Impulse and Reaction Turbines

Sl no

Particulars Impulse Turbine Reaction Turbine

1.

2.

3.4.

5.

6.

7.

8.9.

10.

Pressure drop

Area of blade channelsBladesAdmission of steamNozzles

Power

Space

EfficiencySuitability

Blade manufacture

Only in nozzles and not in moving blades.Constant.

Profile type.Not all round.

Diaphragm contains the nozzles.Small power capacities.

Less space for same power.

LowSmall power requirements.

Easy

In fixed blades(nozzles) as well as in moving blades.Varying.

Aerofoil type.All round.

Fixed blades attached to the casing.Much power can be developed.More space required.

HighFor medium and high power requirements.Difficult

Methods for reducing rotor speed• Velocity compounding

• Pressure compounding

• Pressure velocity compounding

Steam turbine governing and control

Objective is to keep turbine speed fairly constant irrespective of load . Different methods are• Throttle governing

• Nozzle governing

• By pass governing

Comparison of Throttle and Nozzle control Governing

Sl no.

Particulars Throttle control Nozzle control

1.2.

3.

4.

5.

Throttling lossesPartial admission lossesHeat drop availableUse

Suitability

SevereLow

Lesser

In both Impulse and Reaction turbines.Small turbines

Little lossesHigh

Larger

Mostly in Impulse turbine.Medium and Large turbines.