Steam Turbines Expansion Phase References Required – Introduction to Naval Engineering (Ch. 8) Recommended – Principles of Naval Engineering (pp. 144-145)

Steam TurbinesExpansion Phase

References

• Required– Introduction to Naval Engineering (Ch. 8)

• Recommended– Principles of Naval Engineering (pp. 144-145)

Objectives

A. Comprehend the basic design of steam turbines.

B. Comprehend the energy conversions present in steam turbines and represent those conversions on a pressure velocity diagram.

C. Know the various uses of steam turbines used onboard ships.

Introduction

• Turbines are energy conversion devices• Turbines used for:– Propulsion– Electricity generation– Prime mover for various pumps• MSW Pumps• MFP• Fire pumps• Lube Oil Pumps

Turbine Components

• Casing - containment vessel• Nozzle(s)- energy conversion devices that

converts the thermal energy of the steam to kinetic energy

• Blades- energy conversion devices that convert kinetic energy to rotational mechanical energy

• Rotor- supports the blades and transfers rotational mechanical energy out of the turbine

Nozzles

• Definition: mechanical device which:– uses a reduction in surface area to

convert thermal energy to kinetic energy (enthalpy to kinetic energy)

– directs fluid flow onto turbine blading

• Components: inlet, throat (smallest diameter), and mouth (outlet)

Nozzles

• As Pressure drops velocity KE(=mv2/2)• Types:

ConvergingConverging-Diverging

Blades

• Steam exiting nozzles hits blades on rotor and pushes them through a distance

• KE Work• Two types of blades:– Impulse– Reaction

Blades

• Impulse– Steam hits blade @ high

velocity, moves blade by a “direct push” and exits at low velocity

• Reaction– “Fixed vane” nozzles increase

velocity, and “moving vane” blades move by a reaction force or “kickback”

Blades

• Reaction Turbines are moved by three main forces:– Reactive force produced on the

moving blades as the steam expands between the blades.

– Reactive force produced on the moving blades when the steam changes direction.

– Push or impulse of the steam impinging on the blading.

Classification of Turbines

• Staging (groups of blades)• Compounding (groups of stages)• Direction of Steam Flow • Division of Steam Flow

Staging

• Rateau Stage One set of nozzles and moving impulse blades– Only one pressure drop

P

V

Staging

• Curtis Stage • 1 nozzle, 2 sets of moving blades, one set

of fixed blades– Two velocity drops

P

V

Staging

• Reaction Stage (called Parsons stage)– One set of fixed vane nozzles and moving

nozzle-shaped blades– Two pressure drops

Compounding

• Building turbine with multiple stages to extract maximum energy

• Velocity Compounding– Only one pressure drop with multiple

velocity drops using multiple stages• Pressure Compounding– Multiple pressure drops using

multiple stages

Compounding - Impulse Turbines• Velocity-Compounded

Impulse Turbine– One nozzle, row of

moving blades, row of fixed blades, & second row of moving blades (called Curtis stage)

– Adv: Good if initial high pressure

Compounding - Impulse Turbines

• Pressure-Compounded Impulse Turbine– Two or more

impulse/Rateau stages– Adv: increases

efficiency by using lower pressures

Compounding - Impulse Turbines

• Velocity-Pressure Compounded Impulse Turbine– One Curtis stage followed

by single or series of Rateau stages

– Common for many propulsion turbines (allows for lower blade speed)

Compounding - Reaction Turbines• Pressure-Compounded

Reaction Turbine– Called Parsons turbine

because uses Parsons stages

– Adv: efficient at low pressure/low velocity

– Disadv: long stage and impractical

– Used in some auxiliary applications

USS Narwhal (SSN-671)

Direction of Steam Flow

• Axial Flow– Steam flow parallel to

turbine shaft axis (used a lot, particularly for propulsion)

• Radial Flow– Steam flow

perpendicular to turbine shaft axis (used some for auxiliary turbines)

Division of Steam Flow

• Single Flow– Steam enters at inlet and flows in

one direction to the exhaust

• Double Flow– Steam flow split & flows in two

directions– Allows turbine size to be reduced– Axial thrust on shaft avoided

(axial flows cancel)

Turbine Components

Questions?