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Chapter 16: Steam Turbines

IntroductionThe steam turbine can be said to resemble a steam windmill.

Pressurised steam is accelerated through a nozzle and then directed

(almost) tangentially onto blades attached to a rotating wheel. Torque

is generated by reaction forces as steam is redirected by the blades.

There must always be some axial velocity so the redirected flow of 

steam can make way for the incoming flow.

Axial-flow turbines have a rotor, which may be a cylindrical drum or a

number of disc wheels (of gradually varying diameters) shrunk on to a

shaft, and sets of circumferential rows of blades mounted on it. A cylindrical casing encases the rotor and has rows of 

fixed blades mounted on it. The casing blades remain stationary, and are also known as stator or guide blades. Rotor

blades are set in alternate rows with the stator blades.

Steam is admitted at one end of the turbine, and travels in the space between rotor and casing through sets of fixed and

moving blades to the exhaust where it passes to a condenser. Steam flow is directed by nozzles or fixed blades onto the

moving blades, providing a turning moment. As steam passes through the turbine, its pressure decreases and its specific

volume increases. The flow area between rotor and casing therefore needs to increase progressively, and the blades

become longer.

Clearances between moving blade tips and the casing, and between the fixed

blade tips and the rotor, are kept small to reduce leakage through the gaps,

so as much steam as possible impinges on the blades.

Mechanically, the turbine is superior to the reciprocating engine, as it hasonly one major moving part, the rotor, and this can (in theory) be balanced

perfectly. As a result, there is less friction and vibration in a turbine.

There are in fact two main types of turbines: axial-flow and radial-flow.

Virtually all modern steam turbines are axial-flow, and we will only be

looking at this type.

Features of Steam TurbinesAdvantages Disadvantages

Can give very high powers Is non-reversible

Has good steam economy Has poor starting torquesIs very reliable

Has long life

Has few working parts - no vibration

Is small in size for its power

Impulse and Reaction TurbinesThere are three principle types of axial-flow turbines: impulse turbines, reaction turbines, and a combination of the

impulse and reaction turbine contained in the same casing.

The main difference between impulse and reaction turbines is in the action of the steam and the manner in which the heat 

energy is converted into mechanical energy.

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Impulse TurbinesIn an impulse turbine, steam passes through stationary converging (sub-

sonic) or converging-diverging (super-sonic) nozzles, which reduce its

pressure (and its temperature) and increase its velocity, thus converting its

"heat energy" (enthalpy) into kinetic energy. These nozzles direct steam at

high velocity onto curved blades which are attached to the rotor. Steam

leaves the blades at an angle similar to that of its entry, but in the opposite

direction with respect to the axis of wheel rotation. The change in

momentum due to deflection of the steam produces a corresponding force on

the blades, using the steam's kinetic energy to drive the turbine shaft.

In an impulse turbine, the duct formed between adjacent blades in a row of 

blades has constant cross-sectional area, therefore the   pressure remains

constant  while steam passes through the row of blades. Since there is no

change in pressure, there is no change in steam velocity relative to the blades

inside the passages.

In a simple impulse turbine with one set of nozzles and one row of blades,

the whole pressure drop (and acceleration) occurs in the nozzles, with the

pressure constant through the blades. The absolute velocity of the steam

increases in the nozzle and then decreases in the blade passage because its

speed relative to the moving blade does not change.

Compounding for Pressure in Impulse TurbineThe lower the turbine exit pressure, the greater the increase in steam

velocity, and therefore the greater the transfer of heat energy (enthalpy) into

kinetic energy (in the nozzles). For example, the simplified SFEE is:

  W = h1 - h2

Thus, the lower the exit pressure, the lower the final enthalpy, and the

greater the amount of work done.

However, if the steam were allowed to expand from boiler pressure to

condenser (exhaust) pressure through just one set of nozzles, the excessively

high velocity developed could not be used efficiently without running the

turbine at excessive speeds. (We generally have the turbine rotating at a

constant speed, e.g. 3000rpm for 50Hz at Kwinana Power Station).

For this reason, impulse turbines are divided into a number of stages, in

each of which the pressure drops. The whole turbine is then a series of 

simple turbines and is said to be compounded for pressure. As a result, the velocity developed in each set of nozzles can

be kept within manageable limits, and can be mostly absorbed by the moving blades of each stage. A pressure stage in an

impulse turbine occurs between two successive nozzles.

Compounding for Velocity in Impulse TurbineVelocity compounding divides a  pressure stage into several velocity stages,

and is another way to transfer more kinetic energy into mechanical energy.

Velocity compounded stages consist of a series of fixed and moving blades

after a single set of nozzles.

In simple velocity compounding (one pressure stage), the entire pressure

drop of the steam takes place in one set of nozzles. The resulting high

velocity of the steam is partly absorbed by, and work done in, each of the

several rows of moving and fixed blades.

Fixed blades are placed between each row of moving blades in order to

deflect the steam back on to the next moving blade, so that the shaft rotates

in the same direction.

Fixed blades do not absorb any absolute velocity of the steam because they

are stationary. The steam strikes and leaves the fixed blades at similar

angles, and the fixed blades are generally identical in shape and size to the moving blades.

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As there is no change in cross-sectional area within the blade passages (either fixed or moving), the steam pressure

remains constant while passing through the rows of blades in an impulse turbine.

Pressure-Velocity Compounding in ImpulseTurbine

The efficiency of large impulse turbines is increased by using a

combination of pressure and velocity compounding.

Firstly, the steam velocity leaving the nozzles is too high to be absorbed

efficiently in one row of moving blades. The turbine is fitted with rows of 

blades (fixed and moving) and the steam is velocity compounded.

Secondly, the pressure is expanded in two or more stages from boiler

pressure to condenser pressure, and therefore another set of nozzles and

blades compounds the pressure of the steam.

Reaction TurbinesReaction turbines use the principle of the reaction force. The reaction

principle states that the force required to accelerate a body has an equal andopposite reaction

In a reaction turbine the fixed and moving blades are shaped such that the

passages between the blades act as nozzles.

As steam passes through the shaped passages between the curved blades, its

pressure decreases. Also, the steam velocity relative to the blades increases

(as pressure is reduced).

Since the moving blades have a velocity in roughly the same direction as

the incoming steam, and steam leaves the moving blades in roughly the

opposite direction, the absolute velocity of steam leaving the moving blades

decreases in the moving blades.

In a simple reaction turbine, the “conventional” nozzles are replaced by

fixed blades which are curved such that the passages between the blades act

as nozzles.

Therefore there is a pressure drop across both the fixed and moving blades

(Note: in impulse turbines there is no pressure drop across the blades - either

fixed or moving). In reaction turbines, the fixed blades are identical in

shape to the moving blades, but their curvature is reversed so that they direct

the steam on to the next row of moving blades. The identical shape means

that there will be a pressure drop through the moving blades as well as the

fixed.

As steam moves through the moving blades, it gives them an impulse. The

velocity increases in the converging (fixed) passages between the fixed

blades, and the steam imparts an impulse force to the fixed blades. Since the

blades are fixed, there is an equal and opposite reaction force imparted to the

 previous moving blades. Therefore there are TWO forces on the moving

blades - an impulse force (from the previous fixed blade) and a reaction force

(from the next fixed blade) - both of these forces driving the moving blades.

In the fixed blade passages, the velocity of the steam is increased, and the

steam will enter the next row of moving blades at approximately its original

high velocity.

Although the total force driving the moving blades is a combination of an impulse force and a reaction force, this design

of turbine is always referred to as reaction turbines. Therefore we have the pressure falling steadily throughout the fixed

and moving blades. The drop in pressure is equal across the fixed and moving blades.

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The steam velocity increases in each row of fixed and moving blades in a reaction turbine, but this increase is absorbed

each time by the motion of the moving blades, and the steam maintains a fairly constant speed throughout the stages. To

obtain a good efficiency, a large number of steps is required to drop from boiler pressure to condenser pressure.

While an impulse turbine has no change in pressure across the moving blades (pressure drops only occur in fixed blades

or nozzles), the pressure is higher on the upstream side of a row of reaction blades than on the downstream side.

Therefore steam tends to leak around the blade tips. This is a particular problem in high pressure stages where the

pressure differential is greatest.

Key Differences between Impulse and Reaction TurbinesIn impulse turbines, the entire pressure drop takes place in fixed nozzles, and the pressure across the moving blades

remains constant. The velocity decreases through the moving blades, and remains fairly constant in the fixed blades.

In reaction turbines, specially shaped fixed and moving blades replace the nozzles, and the drop in pressure takes place

equally across both fixed and moving blades, falling progressively throughout the turbine. Absolute velocity decreases in

the moving blades, but increases in the fixed blades.

Degree of Reaction ΛThe term “drop in pressure” is normally expressed in terms of “drop in enthalpy”. This can be justified because a greater

pressure drop results in a greater temperature drop in the expanding gas or vapour, and this corresponds to a greater dropin enthalpy.

The Degree Of Reaction Λ, is defined as the fraction of the enthalpy drop that occurs in the moving (rotor) blades in a

turbine stage. That is:

Λ = enthalpy drop in moving blades / enthalpy drop in stage

In an impulse turbine, all the pressure drop in a stage occurs in the nozzles, that is, there is no drop in pressure across the

moving blades, so there is no enthalpy drop across the moving blades.

Therefore, for an impulse turbine: Λ = 0

In a reaction turbine, the pressure drop occurs in both fixed and moving blades. In a typical reaction turbine, this pressure

drop is distributed evenly, so half the enthalpy drop in a stage occurs across the fixed blades, and the other half occurs

across the moving blades.Hence, for a reaction turbine: Λ = 0.5

Comparing Impulse and Reaction Turbines

Impulse Turbines

Advantages Disadvantages

i. Suitable for efficiently absorbing Efficiency is not maintained in the lower

the high velocity of high pressure pressure stages (high velocity cannot be achieved in

steam. the lower pressure stages).

ii. Steam pressure is constant across the

blades and therefore fine tip clearances

are not necessary.

Reaction Turbines

Advantages Disadvantages

i. Efficient at the lower pressure stages Inefficient in high pressure stages due to

(high velocity maintained right down pressure leakages around the blade tips.

to condenser pressure).

ii. Fine blade tip clearances necessary to minimise

leakage. Fine clearances can result in damage to

blade tips.

iii. End thrust is generated wherever there is a pressuredrop across the moving blades. This is also worst

in high pressure stages.

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Turbine Arrangement in Large Power Stations

In modern turbines the impulse and reaction principles are combined. Typically, impulse stages are used in the initial

high pressure stages where leakage problems and end thrust on the rotor would be greatest.

In large power stations, the drop in pressure from boiler pressure to condenser pressure is very large, and generally three

turbines may be employed - a high pressure turbine (HP), an intermediate pressure turbine (IP), and a low pressure turbine

(LP). The low pressure turbines, where dimensions start to get very large, may be single-flow or double-flow. In thediagram below, the low pressure turbine is double-flow, configured to provide natural balance for end thrust.

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Review and Reinforce - Chapter 16

16.1. Why is there no axial load (theoretically) generated in an impulse turbine?

16.2. In reality there is an axial load. Why?

16.3. Impulse turbines are usually used at high pressures, reaction turbines at low pressures. Explain why.