CHAPTER 4

STEAM TURBINES AND AUXILIARIES

2024 _ CPUT

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 CONTENT

• Operating principles of gas and steam nozzle.

• Computation of performance parameters for design and off-design nozzles.

• Calculation of dimensions of nozzles, operating principles of impulse and

reaction turbines.

• Reasons and methods of turbine compounding.

• Calculation of diagram power, diagram efficiency, stage efficiency of turbines

using trigonometric principles and scale diagrams.

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 INTRODUCTION

• Steam turbines are heat engines and prime movers. Their primary function is to
convert thermal energy into mechanical energy. The steam’s potential energy is
transformed into kinetic and mechanical energy by rotating the turbine shaft.

• Three (3) main types of industrial heat engines cycles:

Rankine cycle Steam turbine

Bryton cycle Gas turbine
Otto cycle Internal combustion engines

• Steam turbines can be used as direct drives for rotating machinery (e.g., pumps,
compressors) or electric power generation.

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 STEAM ENGINE VS STEAM TURBINE

❖ Merits and demerits of steam turbines over steam engines are:

• Steam turbine rotational speed ranges are much higher than the steam engine.
• Steam engine thermal efficiency is much lower than that of the turbine.
• The power generated by steam turbines is almost uniform, eradicating the need for
a flywheel. Nearly vibration-free.
• Steam turbines are suitable for larger power-producing plants as they can be made
in larger sizes, up to 1200 MW.
• The rotary action of the steam turbine minimises the balancing challenge prevalent
in reciprocating engines.
• The absence of rubbing parts reduces lubrication requirements in steam turbines.

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STEAM ENGINE VS STEAM TURBINE

❖ Merits and demerits of steam turbines over steam engines are:

• Steam turbines can be overloaded with only a slight compromise in thermal

efficiency.
• Oil-free exhaust.
• High component efficiency.
• For low-speed applications, reduction gears are required in the turbine.
• Steam turbines are not reversible.
• Poor thermal efficiency for small, simple steam turbines.
• High capacity-to-weight ratio.
• Steam turbines can utilise the high pressure and temperature of steam.

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 STEAM TURBINES

• The steam energy is converted to mechanical work by expansion through the

turbine blades.
• Expansion occurs through fixed blades (nozzles) and moving blades (rotors).
• The pair of fixed and moving blades in each row is called a stage.
• The energy conversion in the blades takes place by impulse, reaction or by the
combination of impulse-reaction principle.

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 STEAM TURBINE CLASSIFICATION

❖ Steam turbines can be classified in various ways, with the most common being with
respect to the steam action in the turbine.
• Impulse turbine,
• Reaction turbine, and
• Combination of impulse and reaction turbine.
❖ According to the number of stages:
• Single stage – for small power capacity used to drive compressors, blower etc.
• Multistage – power capacity range varies and can be classified as large.

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STEAM TURBINE CLASSIFICATION(continued…)

❖ According to steam supply and exhaust conditions

• Condensing or Non-condensing (back pressure),
• Automatic or controlled extraction,
• Mixed pressure, and
• Reheat

❖ According to the direction of the steam flow

•Axial – Steam flow direction parallels the turbine’s major axis.
•Radial – steam flow direction is radial (perpendicular to the turbine’s major axis).

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STEAM TURBINE CLASSIFICATION(continued…)

❖ According to the number of exhaust stages in parallel.

• Two-flow
• Four-flow
• Six-flow

❖ According to the number of cylinders.

• Single cylinder
• Double cylinder
• Three-cylinder turbine etc.
Single-shaft turbines are called turbines with rotors mounted on the same shaft and
coupled to a generator. Those with separate rotor shafts for each cylinder are known
as multiaxial turbines.

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IMPULSE AND REACTION TURBINES

Simple Impulse or De Laval Turbine:

• Steam expansion occurs in a single set of nozzles (stators).
• The function of a steam nozzle alters the steam direction and allows pressure drop
while increasing the steam velocity.
• The steam pressure drops as it flows through the nozzle to the condenser or
atmospheric pressure in the case of a non-condensing turbine.
• High steam velocity is accompanied by energy losses, “leaving losses” due to high
velocity amounting to about 3.3% of the nozzle outlet velocity.

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11
 IMPULSE AND REACTION TURBINES(continued…)

Reaction turbine:
• There is a gradual change in pressure and velocity as the steam flows through
the fixed and moving blades.
• Fixed blades (stators) alter steam direction, expand, and increase steam
velocity.
• The steam transfers its kinetic energy to moving blades, thus driving the shaft.
• Reaction turbine requires many stages as the pressure drop per stage is
smaller than the simple impulse turbine stages.

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13
 IMPULSE AND REACTION TURBINES(continued…)

Notable differences
Impulse Reaction
Pressure drop In stator not in the rotor Both in stator and rotor
Power Limited power can be Much higher power can be generated
generated
Space requirements Less space More space
Efficiency Low High
Blade area Constant Converging
Nozzle Diaphragm contains the Attached on the casing and act as a moving
nozzle blade.
Steam admission Not all round or complete All round or complete

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 COMPOUNDING OF STEAM TURBINES

Purpose: It is used to combat “leaving losses” in a turbine. The rotor speed becomes
tremendously high when steam is expanded in a single stage (simple impulse turbine).
The solution is to compound the turbine stages.

Compounding steam turbines: it is the strategy in which energy from the steam is
extracted in several stages rather than a single stage in a turbine. A
compounded steam turbine has multiple stages. E.g., it has more than one set of
nozzles and rotors in series, keyed to the same shaft or fixed to the casing so that
the turbine absorbs either the steam pressure or the jet velocity in several stages.

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 TYPES OF COMPOUNDING

In an Impulse steam turbine, compounding can be achieved in the following three

ways:
• Velocity compounding,
• Pressure compounding, and
• Combined velocity-pressure compounding.

In a reaction turbine, compounding can be achieved only by pressure compounding.

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 VELOCITY COMPOUNDING

Steam expands to condenser pressure

from boiler pressure, flowing through the
stationary nozzle (stator) while its kinetic
energy increases. This kinetic energy is
partially transferred to the first row of
moving blades, decreasing the initial
velocity of steam. The steam then enters
the second row (fixed blades), which alters
its direction without changing its velocity.
It then enters the second row of moving
blades and leaves the turbine at low
velocity.

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 PRESSURE COMPOUNDING

• The pressure compounding in stages

corresponds to putting a number
of simple impulse stages in series.
• The total enthalpy drop is divided equally
among the stages.
• The pressure drops only in the nozzles.
• There is no pressure drop (theoretically)
while steam flows through the rotors.
• The kinetic energy of steam increases in
the nozzles at the expense of the pressure
drop and it is absorbed (partially) by the
blades in each stage, in producing torque.

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VELOCITY-PRESSURE COMPOUNDING

• This is the combination of velocity and

pressure compounding.
• Total pressure drop is divided equally in
each stage.
• Velocity of each stage is compounded.
• There are fixed blades at the beginning of
each stage, and the pressure remains
constant in each stage.

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 STEAM TURBINES

Steam turbines are axial flow machines (radial steam turbines are rarely used),
whereas gas turbines and hydraulic turbines of axial and radial flow types are used
based on applications.

The most common types of steam turbine systems are;

a) Back Pressure Steam turbine, and
b) Extraction condensing steam turbines

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 BACK-PRESSURE STEAM TURBINE

The steam turbine is used in connection with industrial processes where there is
a need for low or medium-pressure steam. The high-pressure steam enters the
back-pressure steam turbine, and while the steam expands – part of its thermal
energy is converted into mechanical energy. The mechanical energy is used to
run an electric generator or mechanical equipment, such as pumps, fans,
compressors,

The outlet steam leaves the back pressure steam turbine at “overpressure” and then
the steam returns to the plant for process steam application such as heating or drying
purposes.

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 BACK-PRESSURE STEAM TURBINE

Some of the advantages are:

- Simple configuration
- Low capital cost
- Low need of cooling water
- High overall efficiency

Disadvantages:
- Large size

Back Pressure steam turbine

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EXTRACTION CONDENSING STEAM TURBINE

• Steam is obtained by extraction

from an intermediate stage.
• Remaining steam is exhausted.
• These turbines have a relatively
high capital cost and lower total
efficiency.

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 TURBINE SELECTION

In all fields of application, the competitiveness of a turbine is a combination of several

factors:

• Efficiency,
• Life,
• Power density (power to weight ratio),
• Direct operation cost, and
• Manufacturing and maintenance costs.

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 .

Calculation of diagram power, diagram efficiency, stage efficiency of

turbines using trigonometric principles and scale diagrams.

A velocity diagram is a triangle representing the various components of velocities of the

steam on the turbine blade.

Velocity triangles may be drawn for a turbine blade's inlet and outlet sections. The
vector nature of velocity is utilised in the triangles, and the most basic form of a
velocity triangle consists of
• The tangential velocity (Vw)
• The absolute velocity (V)
• The relative velocity of the fluid (Vr)

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Velocity Diagrams, Diagram Work and Diagram Efficiency

Steam coming out from the nozzle at absolute velocity of 𝑉1 strikes the blade with
relative velocity 𝑉𝑟1 while the blades rotate with mean peripheral velocity of U. Steam
leaves the blades with relative velocity 𝑉𝑟2 while its absolute velocity is 𝑉2 . The angle 𝛼
is the nozzle angle subtended by the nozzle axis with the direction of rotation of the
wheel, 𝜃 is the inlet blade, 𝜙 is the outlet blade angle. The inlet and outlet velocity
triangles can be superimposed on a common mean peripheral velocity. With all angle
measured anticlockwise, 𝛽 is the absolute exit velocity of steam leaving the blades with
the plane of rotation of the wheel.

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Velocity Diagrams, Diagram Work and Diagram Efficiency

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Velocity Diagrams, Diagram Work and Diagram Efficiency

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 CALCULATIONS

From Newton’s second law of motion

a) Force (tangential on the wheel) = mass of steam X acceleration

= mass of steam/second X change in velocity
= 𝑚ሶ 𝑠 𝑉𝑤1 + 𝑉𝑤2 (𝑁)

b) Work done on blades/sec = Force x distance travelled/sec

= 𝑚ሶ 𝑠 𝑉𝑤1 + 𝑉𝑤2 × 𝑈

c) Power per wheel = 𝑚ሶ 𝑠 𝑉𝑤1 + 𝑉𝑤2 × 𝑈 = 𝑚ሶ 𝑠 𝑉𝑤 × 𝑈 (𝑘𝑊)

1000 1000

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 𝑊𝑜𝑟𝑘 𝑑𝑜𝑛𝑒 𝑜𝑛 𝑡ℎ𝑒 𝑏𝑙𝑎𝑑𝑒
d) Blade or diagram efficiency =
𝐸𝑛𝑒𝑟𝑔𝑦 𝑠𝑢𝑝𝑝𝑙𝑖𝑒𝑑 𝑡𝑜 𝑡ℎ𝑒 𝑏𝑙𝑎𝑑𝑒

𝑚ሶ 𝑠 𝑉𝑤1 + 𝑉𝑤2 × 𝑈 2 𝑉𝑤1 + 𝑉𝑤2 × 𝑈

= 2 =
𝑉 𝑉1 2
𝑚ሶ 𝑠 × 1
2

𝑊𝑜𝑟𝑘 𝑑𝑜𝑛𝑒 𝑜𝑛 𝑡ℎ𝑒 𝑏𝑙𝑎𝑑𝑒 𝑝𝑒𝑟 𝑘𝑔 𝑜𝑓 𝑠𝑡𝑒𝑎𝑚

e) Stage efficiency =
𝑇𝑜𝑡𝑎𝑙 𝐸𝑛𝑒𝑟𝑔𝑦 𝑠𝑢𝑝𝑝𝑙𝑖𝑒𝑑 𝑝𝑒𝑟 𝑘𝑔 𝑜𝑓 𝑠𝑡𝑒𝑎𝑚 (ℎ𝑒𝑎𝑡 𝑒𝑛𝑒𝑟𝑔𝑦)

𝑉𝑤1 +𝑉𝑤2 ×𝑈 Where, ℎ1 is the enthalpy of steam

= before expansion through the nozzle
ℎ1 −ℎ2 and ℎ2 is the enthalpy after.

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 𝑉1 2
f) Nozzle efficiency =
2 ℎ1 −ℎ2

g) Stage efficiency = Blade efficiency x Nozzle efficiency

2 𝑉𝑤1 +𝑉𝑤2 ×𝑈 𝑉1 2 𝑉𝑤1 +𝑉𝑤2 ×𝑈

= × =
𝑉1 2 2 ℎ1 −ℎ2 ℎ1 −ℎ2

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h) Axial thrust on the wheel = mass of steam x Axial acceleration

𝑚ሶ 𝑠 𝑉𝑓1 − 𝑉𝑓2

i) Energy Converted to heat by blade friction = loss of kinetic energy

flow over blades.
= 𝑚ሶ 𝑠 𝑉𝑟1 2 − 𝑉𝑟2 2

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 j) Blade velocity coefficient (K). For the impulse turbine
𝑉𝑟2 = 𝐾𝑉𝑟1
In general, the steam flow over the blade is resisted by friction. The
friction reduces the relative velocity. Normally, the relative velocity is
lost between 10 and 15%. Therefore, 𝑉𝑟2 < 𝑉𝑟1K signifies how much
relative velocity is reduced.

k) Gain in kinetic energy = adiabatic heat loss

𝑉2
= ℎ1 − ℎ2 → 𝑉 = 2ℎ𝑑 = 2 × 1000ℎ𝑑 = 44.72 ℎ𝑑
2

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For the optimum value of a single-stage impulse turbine based on the
rotation of the blade speed to steam speed.
Optimum ratio of blade speed to steam speed For optimum values, the
blade speed must be
𝑈 cos 𝛼 approximately half that of
𝜌𝑜𝑝𝑡 = =
𝑉1 2 the absolute velocity to
perform the maximum work
Maximum blade efficiency and efficiency. Meaning the
absolute velocity at outlet
ⴄ𝑏𝑙𝑎𝑑𝑒,𝑚𝑎𝑥 = cos2 𝛼 will be minimal.

Maximum workdone per kg of steam

𝑊𝑚𝑎𝑥 = 2𝑈 2
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EQUIANGULAR BLADES (NO FRICTIONAL LOSSES)

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TWO ROWS VELOCITY DIAGRAM

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