Engineering Encyclopedia

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Classifying Steam Turbines

Note: The source of the technical material in this volume is the Professional
Engineering Development Program (PEDP) of Engineering Services.
Warning: The material contained in this document was developed for Saudi
Aramco and is intended for the exclusive use of Saudi Aramco’s employees.
Any material contained in this document which is not already in the public
domain may not be copied, reproduced, sold, given, or disclosed to third
parties, or otherwise used in whole, or in part, without the written permission
of the Vice President, Engineering Services, Saudi Aramco.

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Engineering Encyclopedia Rotating Equipment
Classifying Steam Turbines

Content Page

INTRODUCTION................................................................................................................ 1

STEAM TURBINE STAGE DESIGNS................................................................................ 2

Fixed Nozzle ............................................................................................................. 2
Rotating Blades ............................................................................................. 5
STEAM TURBINE STAGING ARRANGEMENTS............................................................ 9
Impulse (Rateau Stage) ............................................................................................. 9
Impulse (Curtis Stage)..............................................................................................12
Reaction...................................................................................................................14
Multi-Staging ...........................................................................................................16
Stage Efficiencies .....................................................................................................21
Impulse Stages..............................................................................................22
Reaction Stages ............................................................................................25
STEAM TURBINE TYPES, ARRANGEMENTS, AND APPLICATIONS ........................28
General Purpose (API 611) ......................................................................................28
Special Purpose (API 612) .......................................................................................29
Arrangements...........................................................................................................34
Condensing...................................................................................................35
Backpressure ................................................................................................37
Extraction.....................................................................................................39
Induction ......................................................................................................42
Applications .............................................................................................................42
GLOSSARY........................................................................................................................44

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Table of Figures Page

Figure 1. Convergent Nozzle .................................................................................... 4

Figure 2. Convergent-Divergent Nozzle.................................................................... 4
Figure 3. Nozzle Diaphragm (Impulse Type) ............................................................ 5
Figure 4. Elementary Impulse Turbine ...................................................................... 6
Figure 5. Curved Impulse Blade................................................................................ 6
Figure 6. Impulse Turbine Nozzle Position ............................................................... 7
Figure 7. Impulse Turbine Wheel Section ................................................................. 8
Figure 8. Reaction Turbine Stationary and Rotating Blade Arrangement ................... 8
Figure 9. Rateau Stage Impulse Turbine ..................................................................11
Figure 10. Curtis Stage Impulse Turbine..................................................................13
Figure 11. Reaction Turbine ....................................................................................14
Figure 12. Pressure-Compounded Impulse Turbine..................................................17
Figure 13. Pressure-Velocity Compounded Impulse Turbine....................................19
Figure 14. Combination Turbine Reaction Turbine with One Impulse Stage .............21
Figure 15. Vector Diagram for a Single Stage Impulse Stage (Rateau).....................22
Figure 16. Vector Diagrams Illustrating Optimum Velocity Ratio ............................23
Figure 18. Vector Diagram for a Reaction Stage......................................................26
Figure 19. Efficiency Versus Stage Type .................................................................27
Figure 20. Comparison of General-Purpose to Special-Purpose Steam Turbines ......31
Figure 20. Comparison of General-Purpose to Special-Purpose Steam Turbines ......32
Figure 20. Comparison of General-Purpose to Special-Purpose Steam Turbines ......33
Figure 20. Comparison of General-Purpose to Special-Purpose Steam Turbines ......34
Figure 21. Multi-Stage Condensing Turbine.............................................................36
Figure 22. Multi-Stage Backpressure Turbine..........................................................38
Figure 23. Single, Automatic-Extraction, Condensing Turbine.................................41

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INTRODUCTION

A steam turbine is a relatively simple type of prime mover. A steam turbine has only one major
moving part: the rotor. Turbine blades are attached to the rotor. When these rotating turbine
blades are combined with stationary nozzles or blades, they form the steam path through a
turbine. The rotor is supported on journal bearings and is axially positioned by a thrust bearing.
A housing or casing with steam inlet and outlet connections surrounds the rotating parts and
serves as a frame for the turbine.

Steam turbines are utilized by Saudi Aramco to drive electric generators, boiler fans, gas
compressors, and boiler feedwater pumps. Although a steam turbine is a relatively simple type of
prime mover, many factors enter into the design of a modern steam turbine. Modern steam
turbines are the result of many years of research and development. A steam turbine converts the
heat energy of steam into mechanical work. The heat energy is first converted to velocity energy,
or kinetic energy, and then the velocity energy is converted into mechanical work. Because steam
is a gas, all of the principles that are described in this module apply equally to the expansion
turbine section of a gas turbine.

The Mechanical Engineer must understand the principles of steam turbines because these
principles apply to Saudi Aramco. The Mechanical Engineer must understand how turbine stage
designs, turbine staging arrangements, and turbine types and arrangements affect the operation of
steam turbines and their related components. This Module provides information for the following
topics:
• Steam Turbine Stage Designs
• Steam Turbine Staging Arrangements
• Steam Turbine Types, Arrangements, and Applications

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STEAM TURBINE STAGE DESIGNS

A stage of a steam turbine is defined as the rows of fixed nozzles and rotating blades in a steam
turbine in which a single pressure decrease occurs. Steam turbines use two main types of blading
to convert the heat energy of the steam into mechanical work: impulse blading and reaction
blading. An impulse-type-bladed turbine stage consists of one row of fixed nozzles in which the
steam expands to transform heat energy into velocity energy, or kinetic energy, and one or more
rows of rotating blades that transform the kinetic energy of the steam into the power that is
delivered by the shaft. Impulse stages that contain more than one row of rotating blades have a
row of stationary blades that are placed between each row of rotating blades. In a true impulse
stage, all of the expansion of the steam takes place in the fixed nozzles. Hence, no pressure
decrease occurs while the steam passes through the rotating and/or stationary blades.

A reaction-type-bladed turbine consists of one row of stationary blades in which part of the
expansion of the steam takes place and one row of rotating blades in which the expansion of the
steam is completed. Different steam turbine characteristics are achieved by the combination of
different steam turbine stages. The combination of different stages is discussed in detail in the
multi-stage arrangement section of this module.

Fixed Nozzle

Both impulse and reaction turbines require a device that converts the stored thermal energy of the
steam into kinetic energy, or velocity energy. This device is called a nozzle. In a reaction turbine,
both the fixed blades and the rotating blades serve as nozzles. In an impulse turbine, the energy
conversion takes place when the steam passes through fixed nozzles. Nozzles are available in
many different shapes that are engineered and designed for various applications. A nozzle serves
two main functions: (1) energy conversion (thermal to kinetic) as the steam expands from a high
pressure area to a low pressure area through the nozzle and (2) the directing of the high-speed jet
of steam tangentially onto the rotating blades, where the final conversion of energy takes place
(kinetic to mechanical).

Because a nozzle is basically a smooth-shaped orifice that separates a high-pressure region from a
low-pressure region, the high-pressure steam passes through the nozzle and emerges at the low-
pressure side as a high-speed jet of steam. Nozzles may have many forms, but all are similar in
principle of operation. The two basic types of nozzles, the convergent nozzle and the convergent-
divergent nozzle, consist of an inlet section, a throat, and a mouth, or outlet section. The type of
nozzle that is used in a steam turbine depends upon the required pressure at the outlet of the
nozzle.

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The velocity of steam flow through any restricted channel, such as a nozzle, depends upon the
pressure difference between the inlet of the nozzle and the region around the outlet of the nozzle.
If the inlet and the outlet of a nozzle are at equal pressure, a static condition, in which steam does
not flow, exists. If the pressure at the inlet is maintained while the pressure at the outlet is
gradually decreased, the steam begins to flow from the high-pressure side (inlet) to the low-
pressure side (outlet). The velocity of the steam increases as the outlet pressure and temperature
decrease. When the thermal energy of the steam expands through a fixed nozzle, both pressure
and temperature decrease. A further decrease in the outlet pressure and temperature eventually
results in a point being reached where the velocity of the steam is equal to the velocity of sound in
steam. This point is called the nozzle’s critical flow. Once critical flow is reached, further
reduction of the pressure and temperature does not result in an increase in the velocity.

The ratio of the outlet pressure to the inlet pressure at which the critical flow is reached is called
the critical pressure ratio. The critical pressure ratio is approximately 0.55 for superheated steam.
In other words, the velocity of the flow through a nozzle is a function of the pressure-differential
across the nozzle. The steam velocity increases as the outlet pressure decreases in relation to the
inlet pressure until the critical pressure ratio is reached. No further increase in steam velocity will
occur when the outlet pressure is reduced below 55 percent of the inlet pressure.

When the pressure at the outlet of a nozzle is designed to be higher than the critical pressure, a
simple parallel-wall or convergent nozzle may be used. In a convergent nozzle, which is shown in
Figure 1, the cross-sectional area at the outlet of the nozzle is the same as the cross-sectional area
at the throat of the nozzle. Because the steam will not expand beyond the throat of the nozzle, a
convergent nozzle is often referred to as a nonexpanding nozzle. High-pressure steam enters the
inlet section of the nozzle, and it expands as it passes through the throat to the low pressure area
of the nozzle.

The operation of a convergent nozzle works well in principle, but it is not very practical in most
high-pressure turbine applications. The steam will expand in all directions, and it will become
very turbulent as it exits the nozzle into the low-pressure area. The turbulent steam is difficult to
direct efficiently toward the rotating blades. Some of the steam will strike the rotating blades at
inefficient angles, and it will thereby cause the friction losses to increase as the steam flows
through the rotating blades.

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Figure 1. Convergent Nozzle

To allow the steam to expand without the turbulence that occurs in the convergent nozzle, a
section is added after the throat. The cross-sectional area of this additional section gradually
increases from the throat to the mouth of the nozzle. The increase in the cross-sectional area
causes the steam to emerge from the nozzle in a uniform steady flow. This type of nozzle, as
shown in Figure 2, is a convergent-divergent nozzle. High-pressure steam enters the inlet section
of the nozzle, and it expands as it passes through the throat to the low pressure area.

Convergent-divergent nozzles are used when the pressure at the outlet of the nozzle is required to
be lower than the critical pressure ratio. The size of the throat and the length of the divergent
section of every nozzle must be specifically designed for the pressure ratio for which the nozzle
will be used. Operation at any pressure ratio other than the design pressure ratio causes a
decrease in nozzle efficiency. Because expansion takes place from the throat of the nozzle to the
mouth of the nozzle, this type of nozzle is often called an expanding nozzle.

Figure 2. Convergent-Divergent Nozzle

Nozzles can also be formed by locating blades adjacently to one another. Figure 3 shows a nozzle
diaphragm for an impulse turbine that uses blades to form the nozzle passages.

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Figure 3. Nozzle Diaphragm (Impulse Type)

Rotating Blades

Once the thermal energy of the steam has been converted into kinetic energy by the steam turbine
nozzles, some device must be available to convert the kinetic energy into work. The conversion
of kinetic energy into work occurs in the rotating blades. Steam turbine blades are attached
around the circumference of the rotor assembly. The basic distinction between types of turbine
blades is the manner in which the steam causes the turbine rotor to move. When the rotor is
moved by a direct push, or an impulse, from the steam that is impinging on the blades, the turbine
is called an impulse turbine. When the rotor is moved by the force of reaction, the turbine is
called a reaction turbine.

To understand the manner in which kinetic energy is converted to work on the turbine blades, it is
necessary to consider both the absolute velocity of the steam and the relative velocity of the steam
in relationship to the rotating blades. In a theoretical elementary impulse turbine, such as the one
that is shown in Figure 4, the blades are merely flat vanes or plates. As the steam jet flows from
the nozzle, it impinges upon the blades and moves the rotor. If it is assumed that there is no
friction as the steam flows across the blade, the relative velocity of the steam at the blade entrance
(R1) must be equal to the vector difference between the absolute velocity of the steam at the blade
entrance minus the peripheral velocity of the blade (V1 - Vb), and the relative velocity of the steam
at the blade exit (R2) must also be equal to the vector difference between the absolute velocity of
the steam at the blade discharge minus the peripheral velocity of the blade (V2 - V1), because
theoretically, there is no change in the velocity as the steam flows across the blade. V2 is the
absolute velocity of the steam at the blade exit.

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Figure 4. Elementary Impulse Turbine

To be able to convert all of the kinetic energy of the steam into work, it would be necessary to
design a blade from which the steam would exit with zero absolute velocity. This blade would be
curved in the manner that is shown in Figure 5, and the jet of steam from the nozzle would enter
the blade tangentially rather than at an angle. The shape of the blade that is shown in Figure 5
closely approximates the shape of the blades that are used in actual impulse turbines. If the
curved blade that is shown in Figure 5 is used, the direction of the steam flow is exactly reversed.
The relative velocity of the steam at the blade entrance (R1) is again equal to the absolute velocity
of the steam at the blade entrance minus the peripheral velocity of the blade (V1 - Vb), and the
relative velocity of the steam at the blade exit (R2) is also be equal to the absolute velocity of the
steam at the blade discharge minus the peripheral velocity of the blade (V2 - Vb). Because the
direction of flow is reversed, however, the absolute velocity of the steam at the blade exit (V2) is
now equal to the absolute velocity of the steam at the blade entrance minus twice the peripheral
velocity of the blade (V1 - 2Vb2). If the absolute velocity of the steam at the blade exit (V2) is
zero, the absolute velocity of the steam at the blade entrance must be equal to twice the peripheral
velocity of the blade (V1 = 2Vb).

Figure 5. Curved Impulse Blade

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The maximum amount of work is obtained from a reversing blade when the velocity of the blade
is exactly one-half of the absolute velocity of the steam at the blade entrance. This statement
assumes that the nozzle is tangential to the blades. In an actual turbine, however, the nozzle is
positioned at an angle to the rotating blades, which causes the steam to enter the blade at an
angle, as shown in Figure 6, rather than tangentially, as was previously shown in Figures 4 and 5.
In actual turbines, it is not feasible for the steam to enter the blade tangentially and to utilize the
complete reversal of steam in the blades; to do so would require that the nozzle be placed in a
position that would place it in the path of the rotating blades. In actual impulse turbines, the
maximum amount of work is done when the blade speed is one-half times the cosine of the nozzle
angle times the absolute velocity of the steam at the blade entrance. Because the nozzle angle is
only the tangential component of the steam velocity that produces work on the turbine blades, the
nozzle angle is made as small as possible.

Figure 6. Impulse Turbine Nozzle Position

Figure 7 shows a section of an impulse turbine wheel and with the blades in place. Modern, high-
speed turbines use these types of turbine wheels (blades, shroud ring, and blade disc). Turbine
blading is designed to match the steam, PT, and volume flow conditions in the section of the
turbine in which the blading is located. The turbine wheel is contoured to approximate the
expansion characteristics of the steam. In the first stages (high-pressure or control stages) in
which the blades are subjected to shocks from steam pressures that vary as the blade passes the
inlet nozzle groups, the blades are short and sturdy. The blade length is increased from the high-
pressure end of the turbine to the exhaust end of the turbine in order to accommodate the
increased specific volume of the steam as the steam approaches the exhaust end of the turbine.
The blades at the low-pressure end of the turbine are tapered from the base of the blade to the tip
of the blade in order to meet radial loading requirements that are caused by the increased
centrifugal force in the longer blades. The blades at the low-pressure end are also normally
twisted from the base of the blade to the tip of the blade in order to accommodate the increase in
peripheral velocities.

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Figure 7. Impulse Turbine Wheel Section

In a modern reaction turbine, the stationary blades that are attached to the casing are formed and
mounted so that the spaces between the blades have the shape of nozzles. The distinction
between actual nozzles and the stationary blading that serves the purpose of nozzles in reaction
turbines is mechanical rather than functional. The previous discussion of steam flow through
nozzles applies equally well to steam flow through the nozzle-shaped spaces between the
stationary blades of reaction turbines. The stationary blades guide the steam into rotating blades.
The blades that project radially from the periphery of the rotor are formed and mounted so that
the spaces between these blades also have the shape of nozzles. The general arrangement of the
reaction-type stationary blades and the rotating blades is shown in Figure 8.
The conversion of the thermal energy of the steam into mechanical work in reaction blading is
similar to the conversion of the thermal energy of the steam into mechanical work in impulse
blading. The angles and velocities are different in the two types of blading. Because the velocity
of the steam increases as the steam expands through the rotating blades, the initial velocity of the
steam that enters the blade must be lower in a reaction turbine than it would be in an impulse
turbine with the same blade speed.

Figure 8. Reaction Turbine Stationary and Rotating Blade Arrangement

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STEAM TURBINE STAGING ARRANGEMENTS

As briefly explained previously, a basic distinction between steam turbine types is the manner in
which the steam causes the turbine rotor to move: by an impulse force or by a reaction force.
Three different staging arrangements methods are utilized in turbine construction to achieve the
desired results from a turbine. Two of the three stage arrangements methods use the impulse
principle to convert the thermal energy that is stored in the steam into useful work. The third
stage arrangement method uses the reaction principle to convert the thermal energy that is stored
in the steam into useful work. This section of the Module will discuss the following stage
arrangement types:
• Impulse (Rateau Stage)
• Impulse (Curtis Stage)
• Reaction
Impulse (Rateau Stage)

In an impulse turbine, the thermal energy of the steam is converted into mechanical energy
through a row of nozzles and one or more rows of moving blades. If the conversion of thermal
energy to mechanical energy occurs through one row of nozzles and one row of moving blades,
the impulse turbine stage is referred to as a Rateau stage. The Rateau stage impulse turbine
consists of a set of nozzles that discharges against a single row of moving blades that are mounted
on the periphery of rotor, as shown in Figure 9. The steam enters the turbine through a steam
chest and expands from some initial pressure and temperature to some final pressure and
temperature as it passes through the nozzles and acquires a very high velocity. The steam exits
the nozzles and flows through the moving blades and out of the turbine exhaust.

The steam that enters the turbine has a great deal of thermal energy due to its high pressure and
temperature. The nozzles convert the thermal energy of the steam (pressure and temperature)
into kinetic energy (velocity). As the steam expands through the nozzles, the steam's pressure and
temperature decreases and its velocity increases. The decrease in pressure and temperature and
the increase in velocity create a steam jet that is directed by the nozzles into the moving blades of
the turbine wheel. The moving blades convert the kinetic energy (velocity) of the steam jet into
mechanical energy in the form of the actual movement of the turbine wheel and shaft, or rotor. In
the moving blades, the steam's velocity decreases, but the pressure remains constant. A Rateau
stage impulse turbine utilizes both the impulse of the steam jet and, to a lesser extent, the reactive
force that results as the curved moving blades cause the steam to change its direction. The
moving blades do not serve as nozzles. Because the pressure remains constant across the moving
blades, impulse turbines do not exert any significant amount of thrust force on the rotor, and they
do not require a balance drum.

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The disadvantage of a Rateau stage impulse turbine is its relatively low efficiency due to the
inability to extract all energy from the steam. The most efficient speed of a turbine is directly
related to the velocity of the steam in the turbine. The Rateau stage impulse turbine produces
very high velocity steam. To obtain the maximum work (increase the efficiency) from a single
stage Rateau turbine, an extremely high blade speed would be required. Because the centrifugal
forces that are involved in the blade speed would exceed the design strength of the material that is
used to construct the turbine, the extremely high blade speed is not feasible. The blade speed of
the Rateau stage impulse turbine is lower than the blade speed that will provide the maximum
amount of work per pound of steam. Because single-stage steam turbines are small, low-power
units that usually drive pumps and fans, they typically operate at 3600 rpm and, as a result, they
have a low ratio of blade speed to steam velocity; therefore, single-stage steam turbine efficiencies
are typically only 30 to 35%. A decrease in the blade speed will not allow the blades to absorb
the maximum amount of kinetic energy, and the steam will leave the turbine with a relatively high
exit velocity. The relatively high exit velocity represents the kinetic energy that was not absorbed
by the blades and that was lost. The loss of energy is a decrease in efficiency. Another decrease
in efficiency is due to the increased windage losses and friction losses of the Rateau stage impulse
turbine. The windage losses and friction losses that are associated with a turbine wheel that
operates in a steam atmosphere rapidly increase as the velocity of the steam increases. Because
the Rateau stage impulse turbine has a relatively high exit velocity, the windage losses and friction
losses increase.

An advantage of the Rateau stage impulse turbine is its simplicity of design and construction.
Although this type of turbine is relatively inefficient, the simplicity of design and rugged, robust
construction make the simple Rateau stage impulse turbine well-suited for mechanical drive
applications.

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Figure 9. Rateau Stage Impulse Turbine

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Impulse (Curtis Stage)

To avoid the energy losses that are associated with the operation of the Rateau stage impulse
turbine, the Curtis stage impulse turbine was developed. As shown in Figure 10, two or more
rows of moving blades are mounted on the periphery of the shaft. The conversion of thermal
energy to mechanical energy in the Curtis stage impulse turbine occurs through one row of
nozzles and more than one row of moving blades. Fixed blades are attached to the casing
between the rows of moving blades to redirect the steam flow into the next row of moving blades.
These blades are commonly known as reversing buckets. The steam enters the turbine through
the steam chest, and it expands in a single set of nozzles as in the Rateau stage impulse turbine.
The steam passes through the first row of moving blades into a row of fixed blades that directs the
flow of steam into a second row of moving blades and out of the turbine exhaust.

Figure 10 also shows the velocity and pressure relationships across the nozzles and moving blades
(flow diagram) of a Curtis stage impulse turbine. Because the reduction of velocity occurs
through the two sets of moving blades, the Curtis stage impulse turbine is called a
velocity-compounded turbine. The nozzles convert the thermal energy of the steam (pressure and
temperature) into kinetic energy (velocity). As the steam expands through the nozzles, the
steam's pressure and temperature decreases and its velocity increases. The decrease in pressure
and temperature with the increase in velocity create a steam jet that is directed by the nozzles into
the first set of moving blades. The velocity of the steam decreases through the first set of moving
blades as the blades convert some of the kinetic energy (velocity) of the steam jet into mechanical
energy. The moving blades do not serve as nozzles, and the pressure of the steam remains
constant. The steam exits the moving blades and enters the fixed blades. The fixed blades
redirect the jet of steam into the second row of moving blades, and no pressure or velocity change
occurs in the fixed blades. The velocity of the steam decreases through the second set of moving
blades as the blades convert the remainder of the kinetic energy (velocity) of the steam jet into
mechanical energy. Because the moving blades do not serve as nozzles, the pressure of the steam
remains constant.

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Figure 10. Curtis Stage Impulse Turbine

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Reaction

In a turbine with a reaction-type blade assembly, as shown in Figure 11 the thermal energy
(pressure and temperature) of steam is converted into mechanical energy through a row of
stationary blades and a row of rotating blades. The stationary blades and rotating blades are
almost identical in shape, and both sets of blades act as nozzles. Steam expansion and redirection
take place in both sets of the blades. Figure 11 also illustrates the pressure-velocity relationship
across the reaction blading. The steam pressure decreases across every row of stationary and
rotating blades. The expansion converts the thermal energy (pressure) of the steam into kinetic
energy (velocity). The rotating blades convert the kinetic energy (velocity) of the jet of steam
into mechanical energy, which takes the form of the actual movement of the turbine rotor.

Figure 11. Reaction Turbine

All reaction turbines that have more than one stage are classified as pressure-compounded
turbines. A pressure-compounded turbine is a turbine that is arranged so that the pressure drop
from the inlet to the exhaust is divided into many steps through use of alternate rows of stationary
and rotating blades. Because the entire pressure drop occurs over several stages, the pressure
drop in each set of stationary and rotating blades (each stage) is reduced. The reduced pressure
drop across each stage causes a small increase in velocity across each stage.

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The change in direction of the steam flow through the rotating blades causes the steam to
counteract or to kick back onto the rotating blades. This kickback gives more energy to the
rotating blades and the wheel to which the rotating blades are attached. The following actions of
the steam in the reaction turbine cause the turbine to move:
• The reactive force that is produced on the rotating blades when the steam increases
in velocity.
• The reactive force that is produced on the rotating blades when the steam changes
direction.
• The impact of the steam on the rotating blades as the high-velocity steam from the
stationary blades strikes the rotating blades; therefore, the reaction turbine
operates on the impulse principle as well.
A disadvantage of a reaction-bladed turbine is the reduced overall efficiency of the turbine when
used in high-pressure application. As the pressure drops in each blade row, there is a pronounced
tendency toward leakage of the steam around the blade tips. This leakage necessitates extremely
small radial clearances between the rotating blade tips and the casing, and between the stationary
blade tip and the rotor. Because the specific volume of the steam at the high-pressure end of the
turbine is small, the blades at the high-pressure end of the turbine are short, and the amount of tip
clearance is an appreciable percentage of the total blade length. The short blades and the amount
of tip clearance increase the amount of tip leakage, and they decrease the overall turbine
efficiency. Another disadvantage of a reaction-bladed turbine is the cost of the materials and
construction that would be required to manufacture the reaction-bladed turbine for use as a high-
pressure turbine. The heavy construction and more expensive materials that would be required to
manufacture a reaction-bladed turbine for use in high-pressure applications makes the reaction-
bladed turbine cost prohibitive. Because of these disadvantages, reaction-bladed turbines are
normally used for low-velocity steam applications, such as low-pressure turbines. The advantage
of reaction-bladed turbines is that because of the lower pressure and temperature, the turbines can
be constructed of lighter and less expensive materials. Another advantage of reaction-bladed
turbines is that for low-pressure applications, reaction turbine efficiency exceeds impulse turbine
efficiency by two to three percent.

Because of the pressure drop that occurs across the rotating blades, the rotor thrust that is
produced in a reaction turbine is significantly higher than the rotor thrust that is produced in an
impulse turbine. Because of this increased amount of rotor thrust, a reaction turbine usually
requires a balance drum to reduce the thrust bearing load.

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Multi-Staging

Steam turbines are classified by the arrangement of the stages of the turbine. The combination of
several stages of the various types of blading is called multi-staging. The multi-stage
arrangements use the advantages of each type of blading to increase the overall efficiency of the
steam turbine. Compounding (or the arrangement of the various stages) refers to the reduction of
the pressure and/or velocity over a series of steps. Steam turbines can be velocity-compounded,
pressure-compounded, or both pressure- and velocity-compounded. A single Curtis Stage was
referred to as a velocity-compounded turbine because the velocity reduction across the stage
occurred in two steps. A multiple-stage reaction turbine was referred to as a pressure-
compounded turbine because the velocity reduction occurred in several steps.

A reduction in the blade speed of a turbine will result in an increase in the efficiency of the
turbine. The reduced blade speed allows the turbine to produce more work by the increased
absorption of energy from the steam. One method that is used to reduce the blade speed is to
allow the steam pressure reduction to occur in steps rather than to have the entire pressure drop
occur over one set of nozzles. The combination of a number of Rateau stages results in the
reduction of the steam pressure in steps. Because the entire arrangement consists of a compound
series of pressure stages, this type of turbine arrangement is called a pressure-compounded
turbine. Figure 12 shows the four stages of a pressure-compounded impulse turbine and the
pressure velocity relationship of the pressure-compounded turbine.

The pressure-compounded impulse turbine consists of a series of Rateau stages with the nozzles
located between rows of moving blades. The steam enters the turbine through the steam chest
into the first set of nozzles. As the steam passes through the first set of nozzles, the steam
expands. The expansion of the steam causes pressure and temperature to decrease while velocity
increases. As the steam passes through the row of moving blades, the pressure remains the same,
but the velocity of the steam decreases as the blades absorb the energy of the steam to produce
work. The discharge from the moving blades is directed either into the next row of nozzles (inlet
of the next stage) or out the turbine exhaust.

As the steam passes through each nozzle, the pressure and temperature decreases and the velocity
increases. As the steam passes through each row of moving blades, the pressure remains constant
and the velocity decreases. The total pressure drop across the turbine from the steam chest to the
exhaust is divided into as many steps as there are stages. The division of the total pressure drop
into many steps results in a relatively low pressure drop across each nozzle and a relatively low
steam entrance velocity for each moving blade. An increase in the number of stages decreases the
velocity of each stage to allow the blade speed to be reduced. The advantage of a pressure-
compounded turbine arrangement is that relatively low-steam velocities can be used to achieve the
desired steam turbine blade speed.

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Figure 12. Pressure-Compounded Impulse Turbine

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The efficiency of the turbine increases as the actual blade speed approaches the desired blade
speed. The combination of enough pressure-compounded stages to result in an efficient blade
speed would require a large turbine. The combination of a pressure- compounded turbine with a
velocity-compounded turbine results in an efficient blade speed that is attained in a relatively short
turbine. Modern, high-pressure steam turbines usually use velocity-compounded stages and
pressure-compounded stages combined in one casing, as shown in Figure 13. This type of multi-
stage arrangement is called a pressure-velocity compounded turbine. The pressure-velocity
compounded turbine consists of a velocity-compounded stage (a Curtis stage) that is followed by
several pressure-compounded stages (Rateau stages). The velocity-compounded Curtis stage is
always placed at the high-pressure end of the turbine to absorb the largest portion of the total
pressure and temperature drop of the steam in a single stage. The energy that remains in the
steam is then absorbed in the pressure-compounded stages. In addition to the reduction of the
overall length of the turbine, the addition of the velocity-compounded stage as the first stage
allows the use of lighter construction materials throughout the remainder of the turbine.

Figure 13 illustrates one velocity-compounded Curtis stage followed by four, pressure-

compounded Rateau stages, and the pressure-velocity relationship of the pressure-velocity
compounded turbine. The steam enters the turbine through the steam chest into the first set of
nozzles. As the steam passes through the first set of nozzles, the steam expands with a decrease
in pressure and temperature and an increase in velocity. The pressure remains the same through
the two rows of moving blades, but the velocity decreases as the blades absorb the energy of the
steam to produce work. The fixed blades redirect the exhaust from the first row of moving blades
into the second row of moving blades. The discharge from the second row of moving blades is
directed into the nozzles of the first pressure-compounded Rateau stage.

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Figure 13. Pressure-Velocity Compounded Impulse Turbine

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It is advantageous to combine an impulse-type stage with reaction stages, as shown in Figure 14.
This multi-stage arrangement is called a combination turbine. The addition of an impulse stage at
the high-pressure end with its large temperature and pressure decrease results in a comparatively
low-pressure and low-temperature steam that enters the reaction stages. The lower-pressure and
lower-temperature steam allows for the use of light and inexpensive reaction blading.

This type of multi-stage arrangement combines one impulse stage followed by a series of reaction
stages. As the steam enters the turbine through the steam inlet, the steam pressure and
temperature decrease while the velocity increases in the set of nozzles of the impulse stage. As
the steam passes through the row of moving blades, the velocity decreases as the kinetic energy is
converted to work. The steam is directed from the row of moving blades into the fixed blades or
nozzles of the first reaction stage. As the steam expands across every row of fixed and moving
blades of the reaction stages, the thermal energy (pressure) of the steam is converted into kinetic
energy (velocity). The moving blades convert the kinetic energy (velocity) of the jet of steam into
work. As the steam passes through each row of moving blades, it is directed either into the next
row of fixed blades or out the turbine exhaust.

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Figure 14. Combination Turbine Reaction Turbine with One Impulse Stage

Stage Efficiencies

A comparison of stage efficiencies based on velocity ratios and applications will improve explain
why and when Rateau, Curtis, and reaction stages are used.

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Impulse Stages

In an actual turbine, the impulse stage nozzle is positioned at an angle (α) to the rotating blades
which causes the steam to enter the blade at an angle, as shown in Figure 15. Therefore, in actual
impulse turbines, the maximum amount of work is done when the blade speed is one-half the
cosine of the nozzle angle times the absolute velocity of the steam at the blade entrance. Because
it is only the tangential component of the steam velocity that produces work on the turbine blades,
the nozzle angle is made as small as possible.

Figure 15. Vector Diagram for a Single Stage Impulse Stage (Rateau)

Figure 15 also shows the vector diagram for a single stage impulse stage (Rateau). The blade
efficiency (ηb) is defined as the ratio of the actual work per pound mass of steam flowing to the
kinetic energy of the steam entering the blade passage.
w
ηb = 2
V1
2
A blade efficiency of 100% would indicate that the work is exactly equal to the kinetic energy of
the steam entering the blade, and the kinetic energy of the steam leaving the blade is zero.
However, the steam must have some axial velocity to flow out of the blade passage. Stage
efficiency is frequently shown graphically compared to the velocity ratio. The velocity ratio is the
ratio of the blade speed (Vb) to the velocity of the steam leaving the nozzle (V1). Typical
designations for the velocity ratio are Vb/V1 or V/Co.

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Figure 16 shows three vector diagrams for different velocity ratios. Figure 16a shows a reversible
impulse stage vector diagram that has a very small entrance and blade exit angle that result in a
velocity ratio of 0.5. As the angles α and γ (inlet and blade exit angles) approach zero, the exit
velocity V2 will also approach zero, resulting in a stage that approaches 100% efficiency. Figures
16b and 16c show the vector diagrams for a reversible impulse stage with essentially zero angles,
but with the velocity ratios less than and greater than 0.5. In both cases, the exit velocity (V2)is
large, and the blade efficiency is considerably less than 100%. It is generally considered that an
impulse blade reaches the optimum efficiency when the velocity ratio is 0.5.

Figure 16. Vector Diagrams Illustrating Optimum Velocity Ratio

In practical application, a single impulse stage turbine that would receive steam at 100 psi, 482°F,
and an exhaust pressure of 2 psi, would have the steam velocity leaving the nozzle at
approximately 3609 feet per second. To have a velocity ratio of 0.5 (100% efficient), would
require a blade speed of approximately 1804 feet per second. Blade speeds of this magnitude
result in high stresses due to the centrifugal force, and irreversibility’s associated with steam flow
increase as the steam velocity increases.

Using a velocity compounded (Curtis) stage will reduce the blade speed for the same steam
velocity and entrance angle. For a reversible, zero angle turbine using a Curtis stage, the velocity
ratio for optimum efficiency is 0.25. Figure 17 shows the vector diagram for a two-row Curtis
stage.

Because the most efficient blade speed for a Curtis stage is one-half that of a Rateau stage for the
same steam velocity, the Curtis stage is placed ahead of the Rateau stage in pressure-velocity
compounded turbines. By placing the Curtis stage before the Rateau stage, the steam velocity at
the Rateau stage would be less than the steam velocity entering the Curtis Stage. All turbine
stages could operate in series and closely approach the most efficient blade speed for each stage.

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Figure 17. Vector Diagram for a Curtis Stage

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Reaction Stages

In the pure reaction stage, the entire pressure drop occurs as the steam flows through the moving
blades. The moving blades act as a nozzle, and the blade passage must have the proper contour
for a nozzle, converging if the exit pressure is greater than the critical pressure and converging-
diverging if the exit pressure is less than the critical pressure. The only purpose of the stationary
blade is to direct the steam into the moving blade at the proper angle and velocity.

In application, most turbines that are classified as reaction turbines have a pressure and enthalpy
drop in both the fixed and moving blades. The degree of reaction is defined as the fraction of the
enthalpy drop that occurs in the moving blades. The most commonly used fraction is 50 percent
reaction, where half of the enthalpy drop across the stage occurs in the fixed blade and the other
half of the enthalpy drop occurs in the moving blade.

Reaction stage performance may be shown by a velocity diagram. Figure 18 shows the velocity
diagram for a reaction stage. The component of absolute steam velocity V1 in the direction of
blade motion is shown by the vector FA=V1 cos α = VR1 cos β + Vb. For a pure reaction blade,
R1 cos β, which is the component of relative steam entrance velocity in the direction of blade
motion, must be equal to zero (angle β must be 90° so no impulse force is acting on the moving
blade). Due to the expansion of the steam as it passes through the blades, the relative exit
velocity, VR2, is greater than the relative entrance velocity, R1. If the blades are considered
frictionless, and if the drops in heat energy across the fixed and moving blades are equal, and
angle α = angle γ, then V1 = VR2 and VR1 = V2. To obtain the maximum work from the blades,
vector V2, the absolute steam exit velocity, must be minimized because it performs no work.
Vector V2 is minimized when V2 is perpendicular to Vb. Since VR2 = V1, the condition of
maximum work is obtained when Vb = V1 cos α. For high steam velocities, a reaction turbine
would have too high of a blade speed to operate at the most efficient point, therefore, reaction
turbines are not normally used in high pressure steam applications. Reaction turbines are typically
used in low velocity steam applications, such as low pressure turbines, because the turbine can
operate closer to the most efficient blade speed. Because of the low pressure and temperature
steam used for reaction turbines, the turbine can be constructed of lighter and less expensive
materials.

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Figure 18. Vector Diagram for a Reaction Stage

Reaction stages has a maximum efficiency when the velocity ratio is approximately equal to .707.
Vb 1
= =.707
V1 2

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For a given enthalpy drop per stage, the maximum efficiency for a reaction stage requires a higher
blade speed than for an impulse blade. Because the most efficient blade speed for a Curtis stage is
lower than the efficient speed of a reaction stage for the same steam velocity, a Curtis stage can
be placed ahead of the reaction stages in a combination, velocity-compounded impulse and
pressure-compounded reaction turbine. By placing the Curtis stage before the reaction stages, a
large temperature and pressure drop can be effected in the first stage nozzles so that the pressure
and temperature of the steam striking the reaction stages are lower. The Curtis stage converts a
large part of the available kinetic energy in the velocity-compounded wheel, requiring fewer
remaining reaction rows to complete the extraction of energy, and resulting in a shorter turbine.
All turbine stages could operate in series and closely approach the most efficient blade speed for
each stage.

Figure 19 shows a comparison of stage efficiencies to velocity ratios for the different stage
arrangements. The effects of the stage arrangements to the relative work per stage and the
number of stage required can also be seen on Figure 19. By adding a two-row Curtis stage,
efficiency curve 2, to reaction stages, efficiency curve 5, results in the efficiency curve 3.
Efficiency curve 3 reaches approximately 80% when the velocity ratio is 0.3. The efficiency for
the combination Curtis stage/reaction stage turbine is greater than the efficiency of just a two-row
Curtis stage turbine. However, the efficiency of the combination Curtis stage/reaction stage
turbine is less than the efficiency of a reaction turbine. The decrease in reaction stage efficiency is
offset by the number of stages required to obtain maximum efficiency.

Figure 19. Efficiency Versus Stage Type

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STEAM TURBINE TYPES, ARRANGEMENTS, AND APPLICATIONS

Although a steam turbine is a relatively simple type of prime mover, the type of steam turbine and
the arrangement of the steam turbine that is used in a process depends upon the needs of the
process. Modern steam turbines are the result of many years of research, development,
accumulated knowledge, and experience. Steam turbine users are continuously interested in
achieving improved energy effectiveness and reduced total life costs without sacrifice of safety or
reliability. The years of research, development, accumulated knowledge, and experience have led
the petroleum industry to classify all steam turbines into two types of turbines based upon the
needs of the process.

The object of this classification is to provide a purchase specification to facilitate the manufacture
and procurement of different classes of turbines for refinery service. Steam turbines that are used
for refinery service are classified by the American Petroleum Institute (API) as either special-
purpose steam turbines (API 612) or general-purpose steam turbines (API 611). These
classifications cover the minimum requirements for steam turbines for general refinery systems,
which include basic design, materials, related lube-oil systems, controls, and auxiliary equipment.

Both special-purpose and general-purpose steam turbines may be further classified by the type of
arrangement of the steam turbine and the application of the steam turbine. This section of the
module presents the following turbine classifications:
• General Purpose (API 611)
• Special Purpose (API 612)
• Arrangements
• Applications

General Purpose (API 611)

General-purpose steam turbines for use in the petroleum industry are defined in accordance with
API standard number 611. General-purpose steam turbines are defined as horizontal or vertical
turbines that are used to drive equipment that is usually spared, that is relatively small in size
(power), or that is in noncritical service. General-purpose steam turbines are generally used
where steam conditions will not exceed a pressure of 600 psig (41 bar) and a temperature of
750°F (400°C), or where speed will not exceed 6000 rpm.

General-purpose steam turbines are generally classified as low horsepower turbines. General-
purpose steam turbines use standard designs that use off-the-shelf components. Although
general-purpose steam turbines are durable, they generally have a low efficiency (35 to 40%). A
steam turbine vendor may offer alternative designs other than the specified designs in the API
standard; however, any substitutions must be mutually agreed upon by the purchaser and the
vendor.

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The equipment and auxiliaries that are used to construct a general-purpose steam turbine must be
designed and constructed for a minimum service life of 20 years with at least 3 years of
uninterrupted operation between overhauls. General-purpose steam turbines must be designed to
satisfy the following requirements:
• Capable of operation at normal power and speed under normal steam conditions.
• Capable of delivery of rated power at the rated power speed with simultaneous
minimum inlet and maximum exhaust conditions.
• Capable of continuous operation at maximum continuous speed for the turbine and
at any speed within the specified range for the turbine.
• Capable of continuous operation at rated power and speed under maximum inlet
steam conditions and maximum or minimum exhaust steam conditions.
• Capable of operation with variations from rated steam conditions.
• Capable of operation without damage up to the trip speed of the turbine and relief
valve settings.
• Single-stage turbines must be capable of immediate startup and operation at full
load without a warmup period.
• The turbine wheels must be located between the bearings.
• Oil reservoirs and housings that enclose moving lubricated parts (such as bearings,
shaft seals, highly polished parts, instruments, and control elements) must be
designed to minimize contamination by moisture, dust, and other foreign matter.
• Designed to permit rapid and economical maintenance. Major parts such as casing
components and bearing housings must be designed and manufactured to ensure
accurate alignment on reassembly.

The Saudi Aramco requirements that are associated with general-purpose steam turbines are
documented in Saudi Aramco Materials System Specifications 32-SAMSS-009, General Purpose
Steam Turbines. This standard establishes the requirements of API 611, Second Edition, dated
January 1982 as part of the specification. The requirements that are contained in 32-SAMSS-009
are either additional requirements or exceptions to the requirements that are set forth in API 611.

Special Purpose (API 612)

Special-purpose steam turbines for use in the petroleum industry are defined in accordance with
the API standard number 612. Special-purpose steam turbines are horizontal turbines that are
used to drive equipment that is usually not spared, that is relatively large in size (power), or that is
in critical service. Special-purpose steam turbines are not limited by steam conditions or turbine
speed.

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Special-purpose steam turbines are highly engineered, high-efficiency (65 - 83%) steam turbines
that have an application-specific design. A steam turbine vendor may offer alternative designs
other than the designs that are specified in the API standard; however, any substitutions must be
mutually agreed upon by the purchaser and the vendor.

The equipment and auxiliaries that are used to construct a special-purpose steam turbine must be
designed and constructed for a minimum service life of 20 years with at least 3 years of
uninterrupted operation between overhauls. Special-purpose steam turbines must be designed to
satisfy the following requirements:
• Capable of operation at normal power and speed under normal steam conditions.
• Capable of delivery of rated power at the rated power speed with simultaneous
minimum inlet and maximum exhaust conditions.
• Capable of continuous operation at maximum continuous speed for the turbine and
at any speed within the specified range for the turbine.
• Capable of continuous operation at rated power and speed under maximum inlet
steam conditions and maximum or minimum exhaust steam conditions.
• Capable of continuous operation at the lowest speed at which maximum torque is
required with minimum inlet and maximum exhaust conditions.
• Extraction and induction steam turbines must be capable of continuous operation
at conditions agreed upon between the purchaser and the vendor.
• Capable of operation with variations from rated steam conditions.
• Capable of operation while uncoupled with maximum inlet steam conditions.
(Operation while uncoupled may result in governing instability that may require
action such as throttling of inlet pressure.)
• Capable of operation without damage up to the trip speed of the turbine and relief
valve settings.
• Oil reservoirs and housings that enclose moving lubricated parts (such as bearings,
shaft seals, highly polished parts, instruments, and control elements) must be
designed to minimize contamination by moisture, dust, and other foreign matter.
• Designed to permit rapid and economical maintenance. Major parts such as casing
components and bearing housings must be designed and manufactured to ensure
accurate alignment on reassembly.
The Saudi Aramco requirements that are associated with special-purpose steam turbines are
documented in Saudi Aramco Materials System Specifications 32-SAMSS-010, Special Purpose
Steam Turbines. This standard establishes the requirements of API 612, Second Edition, dated
June 1979 as part of the specification. The requirements that are contained in 32-SAMSS-010 are
either additional requirements or exceptions to the requirements that are set forth in API 612.

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Figure 20 lists some of the major differences between general-purpose steam turbines and special-
purpose steam turbines as noted in 32-SAMSS-009 and API 611 for general-purpose steam
turbines and in 32-SAMSS-010 and API 612 for special-purpose steam turbines.

API 611 and 32-SAMSS-009 API 612 and 32-SAMSS-010

Scope General-purpose turbines are horizontal or Special-purpose turbines are those horizontal
vertical turbines that are used to drive turbines that are used to drive equipment that is
equipment that is usually spared, that is usually not spared, that is relatively large in size
relatively small in size (power), or that is in (power), or that is in critical service. This
noncritical service. General-purpose category is not limited by steam conditions or
turbines are generally used where steam turbine speed.
conditions will not exceed a pressure of 600
psig (41 bar gauge) and a temperature of
750°F (400°C) or where speed will not
exceed 6000 rpm.
General-purpose turbines must not be
applied for requirements that exceed 1000
kW (1340 HP).
General Provision must be made for hot alignment
during operation, using the Acculign System.
Provisions for borescope inspection must be
installed if specified.
Configuration Vertical or horizontal. Horizontal only.
Flow Backpressure or condensing only. Backpressure, induction, extraction,
Arrangement condensing.
Pressure The turbine’s casing may be either axially The turbine’s casing must be axially split.
Casings or radially split.
Turbine casings may also be split radially
between high-pressure and low-pressure
portions.

Figure 20. Comparison of General-Purpose to Special-Purpose Steam Turbines

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API 611 and 32-SAMSS-009 API 612 and 32-SAMSS-010

Casing All nozzles or nozzle blocks must be All bladed nozzle rings must be replaceable.
Appurtenances replaceable. All other stationary blading
must be mounted in replaceable
diaphragms or segments.
Diaphragm horizontal joints must be rabbet fit.
Rotors Each rotor must be clearly marked with a
unique identification number.
Rotor Dynamics Stiff shaft, Ncr ≥ 1.2N Separations margin defined.
Seals Outer glands must be sealed at the shaft by Outer glands must be sealed with replaceable
carbon-ring packing. labyrinth packing.
A separate vacuum device must be furnished to
reduce external leakage from the glands and
possible contamination of the bearing oil.
Sealing System Ejector exhausts to atmosphere. Ejector exhausts to gland condenser.
Bearings and Hydrodynamic radial bearings are required Radial bearings must be of the hydrodynamic
Bearing where anti-friction-bearing dN factors are design.
Housings 300,000 or more, when standard anti-
friction bearings fail to meet an L10 rating
life, or when the turbine horsepower rating
exceeds 50 kW (67 hp). The anti-friction
bearing L-10 life must be calculated in
accordance with ISO 281.
Horizontal turbines must be equipped with Thrust bearings must be of the hydrodynamic,
thrust bearings that are designed to handle steel-backed, babbitted multiple-segment type,
axial loads in either direction. Multi-stage designed for equal thrust capacity in both
turbines must have hydrodynamic thrust directions and arranged for continuous
bearings when specified or where anti- pressurized lubrication to each side. Thrust
friction bearings fail to meet the minimum bearings must have two temperature sensors in
L10 rating life. each of the active and inactive sides.
Vertical turbines may have oil- or grease-
lubricated ball- or roller-type radial and
thrust bearings.
Figure 20. Comparison of General-Purpose to Special-Purpose Steam Turbines
(Continued)

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API 611 and 32-SAMSS-009 API 612 and 32-SAMSS-010

Bearings and Hydrodynamic radial bearings must have
Bearing forced feed lubrication when the shaft
Housings velocity at the bearing exceeds 14 m/s (45
(Continued) ft/s).
Non-pressure-fed bearings must be limited
to turbines that have shaft velocities below
11 m/s (35 ft/s) unless water is cooled via
jackets or cooling coils; then, the limit may
be increased to 12 m/s (40 ft/s). If water is
cooled via an external cooling system, shaft
velocities up to 14 m/s (45 ft/s) are
acceptable.
When specified, provisions for the mounting of
accelerometers on the bearing housings must be
made in accordance with API Standard 678.
When specified, resistance temperature When specified, thrust bearings and radial
detectors must be installed in accordance bearings must be fitted with bearing-metal
with ISS 8020-416-ENG. temperature sensors.
Lubrication and Closed circulation or pressure lubrication Lube and control oil systems must be in
Control Oil systems must be in accordance with 32- accordance with 32-SAMSS-013 and API 614.
SAMSS-013 and API 611.
Permissive starting features for lube oil pressure
must be furnished.
Vibration As required by AES-J-604. Required.
Probes and
Bearing RTDs
Nameplates and Minimum allowable turbine speed must be A rotation arrow must be located on the thrust
Rotation included. bearing housing.
Arrows
Gear Units Integral gear units must not be used for Gear units must be in accordance with 13-
driven equipment that requires more than SAMSS-001 and API 613.
75 hp (56 kW).
Gear units must be in accordance with 13-
SAMSS-001 and API 613 or API 677 for
operating speeds of 3600 rpm and above.

Figure 20. Comparison of General-Purpose to Special-Purpose Steam Turbines

(Continued)

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API 611 and 32-SAMSS-009 API 612 and 32-SAMSS-010

Coupling No specification. In accordance with API 671.
Mounting Plates The mounting surfaces for the equipment The upper and lower surfaces of bearing
feet must be machined flat and parallel pedestals and mounting plates must be
within 0.002 inch per foot. machined parallel to within 0.0005 inch per
foot.
Baseplate A baseplate must be a single, fabricated A baseplate must be a single fabricated steel
steel unit. unit unless the purchaser and the vendor
mutually agree that it may be fabricated in
multiple sections. Multiple-section baseplates
must have machined and doweled mating
surfaces to ensure accurate field reassembly.
Unless otherwise specified, a NEMA SM The speed governor for generator drive
23 Class A oil-relay governor will be applications must conform to NEMA SM 23,
supplied. Class D minimum, with an adjustable droop.
Governor Accomplished pneumatically, remote speed Details of electrical governors must be mutually
adjustment must utilize a control signal of agreed upon by the purchaser and the vendor;
3 to 15 psig. Dual speed sensors must be however, as a minimum, dual speed-sensing
provided as a minimum. systems must be provided.
The trip and throttle valve may be of either The speed of the turbine must vary linearly with
the single-seated or the double-seated the control signal. Unless otherwise specified,
balanced type, with renewable parts. an increase in control signal will increase the
turbine speed.
The trip and throttle valve must be of the
double-seated balanced-type with renewable
parts.
When a solenoid-operated valve is provided in
the trip system, it must activate closure of the
trip and throttle valve and the control valves
when deenergized.
Controls Built-in trip and throttle valve. Separate trip and throttle controls.
Piping and The minimum size of any connection must The minimum size of any connection must be
Appurtenances be 1/2 inch nominal pipe size. 3/4 inch nominal pipe size.
Testing No witness required. Witness required.

Figure 20. Comparison of General-Purpose to Special-Purpose Steam Turbines

(Continued)
Arrangements

The steam turbine arrangement that is used in a process depends on the needs of the process. In
this section of the Module, the Mechanical Engineer will examine the following turbine
arrangements:

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• Condensing
• Backpressure
• Extraction
• Induction
Condensing

A condensing steam turbine is a turbine that exhausts to a condenser. Condensing turbines can be
either single-stage or multi-stage design. A multi-stage condensing turbine is a turbine that
contains more than one stage (reaction and/or impulse type) and exhausts to a condenser. The
exhaust from a multi-stage condensing turbine is at a pressure that is less than atmospheric
pressure. The exhaust steam is condensed by cooling water condensers or air-fin condensers.
Figure 21 is an illustration of a multi-stage condensing turbine. The figure includes a blowup of
the first four stages and a diagram of the steam supply, exhaust, and condensing system. Steam,
which is at a pressure of 125 psig or higher, is supplied to the turbine. The turbine extracts the
energy from the steam and produces work. The turbine exhaust is directed into a condenser. The
exhaust pressure of a condensing turbine is very low, usually between 4 and 6 in. Hg absolute (2
to 3 psia). The low exhaust pressure allows the maximum pressure energy to be extracted from
each pound of steam. The condensed water (condensate) is recovered, and for reuse, it is pumped
back to the steam generating system.

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Figure 21. Multi-Stage Condensing Turbine

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Because only a portion of the steam energy is converted to work, the condensing turbine has a
relatively low cycle efficiency even though the turbine efficiency is the highest of all of the turbine
arrangements (70 to 83%). A large part of the steam energy is lost in the condenser. In fact,
more heat is transferred to the cooling water or the air in the condenser than is converted to work
in the turbine. However, condensing turbines are necessary if mechanical power generation from
steam is required and if there is no use for the exhaust steam.

Backpressure

A backpressure turbine is a steam turbine that exhausts at a pressure that is greater than
atmospheric pressure, which is normally 15 psig or higher. A backpressure turbine is a non-
condensing steam turbine. A multi-stage backpressure turbine is a steam turbine that contains
more than one stage (reaction and/or impulse type), and it exhausts at a pressure that is greater
than atmospheric pressure. The exhaust steam from a backpressure turbine can be used for some
other process, such as heating steam, or the exhaust steam can be exhausted into the atmosphere.

Figure 22 is an illustration of a reaction-type, multi-stage backpressure turbine. The figure

includes a blowup of the first four stages and a diagram of the steam supply and exhaust system
arrangements. The steam system shows the high-pressure steam inlet line and the exhaust line.
High-pressure steam (400 to 650 psig) is supplied to the turbine. The turbine extracts energy
from the steam in order to produce work. The turbine exhaust steam leaves at medium to low
pressure (225 to 15 psig), and it is distributed to other parts of the plant that use the useful heat of
the steam for other processes.

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Figure 22. Multi-Stage Backpressure Turbine

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The backpressure turbine arrangement has a cycle efficiency that is high, with little lost energy.
The energy of the steam that is not used to produce work in the turbine is used in other plant
processes. The high-efficiency cycle assumes that there is a use for the exhaust steam and that the
exhaust steam will not be vented to the atmosphere. The typical efficiencies of backpressure
turbines range from 65 to 75%.

Extraction

Many industrial plants require various quantities of process steam at various pressure applications.
The extraction turbine is used to balance the process steam requirements of the various plant
process pressure requirements. An extraction turbine is a multi-stage turbine in which some of
the steam is exhausted, or bled, from between the turbine stages. The extraction steam is used for
various processes, such as to drive general-purpose turbines, to heat feedwater, or to heat
buildings.

Extraction turbines can be adapted to a variety of plant conditions. Many different types of
extraction turbines are built. Extraction turbines can be non-condensing or condensing turbines
that have one or more extraction points. Extraction turbines can have automatic or non-
automatic extraction. The pressure of the steam at any stage of a multi-stage turbine is
determined by the steam flow or the turbine load. In a non-automatic extraction turbine, no effort
is made to control the extraction steam pressure or the extracted steam flow. The steam pressure
or steam flow varies with the load of the turbine. In an automatic extraction turbine, valves are
used at the inlet to the next section of turbine. Both the main turbine valves and the extraction
turbine valves receive the output of the control signal in order to regulate the extraction steam
pressure and/or the extraction steam flow.

The most frequently used extraction turbine is the single, automatic-extraction, condensing
turbine that is shown in Figure 23. Figure 23 also shows a diagram of the steam supply, the
exhaust, and the condensing systems that are associated with the extraction condensing turbine.
High-pressure (HP) steam (400 to 1500 psig) is supplied to the inlet of the turbine. After one or
more stages, medium-pressure or low-pressure (MP/LP) steam (15 to 400 psig) is extracted from
the turbine and is supplied to other plant processes. The steam that is not extracted proceeds
through the low-pressure stages of the turbine, and it exhausts to a condenser at a normal
condensing pressure of 2 to 3 psia. The exhaust portion of the steam is condensed by cooling
water. The condensate is returned to the steam generator for reuse.

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For design purposes, the extraction turbine that is shown in Figure 23 may be considered as a
backpressure turbine and a condensing turbine that operate in series on a common rotor and that
are built into a single casing. Because of the emphasis that is placed on compactness and simple
construction, the number of stages of an extraction turbine is usually limited. Because of the
compactness and simple construction, the performance may not be equal to the combined
performance of a backpressure turbine and a straight condensing turbine that is built in two
separate units. The extraction-type of turbine is more complex and, therefore, more expensive
than either a backpressure turbine or a straight-condensing turbine. On the other hand, the cost of
an extraction turbine is less than the total cost of two independent units, a backpressure turbine
and a straight-condensing turbine.

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Figure 23. Single, Automatic-Extraction, Condensing Turbine

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Induction

Another type of turbine that is very similar to the extraction turbine is the induction turbine or
automatic admission turbine. The induction turbine is also used to balance the process steam
requirements of the plant with the electrical power requirements. An induction turbine is a multi-
stage turbine that has the provision to use low-pressure steam and high-pressure steam in
proportion to the available steam supply. Unlike the extraction steam (where steam is extracted
from the turbine to be used for various processes, such as feedwater heating or heat building), an
induction turbine generally uses low-pressure steam that is exhausted from other plant processes
to generate electrical power.

Low-pressure steam is admitted to the turbine to carry normal load conditions. If the available
low-pressure steam is insufficient to supply the turbine, or if the electrical load requirements
exceed the capacity of the low-pressure steam supply, high-pressure steam is admitted to the
latter stages of the turbine in order to provide sufficient energy to operate the turbine. If a
complete loss of low-pressure steam occurs, induction turbines are normally designed to operate
satisfactorily on high-pressure steam.

Induction turbines can be adapted to a great variety of plant conditions. Induction turbines are
normally condensing-type turbines that have one or more induction points. The steam pressure,
or steam flow, varies with the load of the turbine. Both the main turbine valves and the high-
pressure steam supply valves receive the output of the control signal to regulate the high-pressure
steam flow to the turbine.

Applications

Multi-stage condensing turbines are typically used in large horsepower applications and in
applications in which there is no suitable use for the exhaust steam. Saudi Aramco typically uses
multi-stage condensing turbines for generator drives, but they may also be used to drive the
following:
• Large centrifugal pumps
• Compressors
• Blowers
Multi-stage backpressure turbines are typically used in applications in which there is a suitable use
for the exhaust steam, such as process steam or plant heating. Saudi Aramco typically uses multi-
stage backpressure turbines for compressor drives. Multi-stage backpressure turbines may also be
used to drive the following:
• Compressor drives
• Generator drives
• Pump drives

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Saudi Aramco typically uses extraction turbines for generator drives. Extraction turbines can also
be used to drive the following:
• Large centrifugal pumps
• Compressors
• Blowers
Induction turbines are typically used for generator drives. Saudi Aramco currently does not have
any induction turbines in use in their facilities.

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GLOSSARY

automatic A steam turbine with the capacity to extract steam. The pressure, or flow
extraction turbine rate, of the extracted steam is controlled by a valve gear at the inlet to the
low-pressure section of the turbine and the main valve gear. (Steam
turbines can be furnished with automatic extraction and admission
capability.)
backpressure A steam turbine that exhausts at a pressure that is equal to or greater than
turbine atmospheric pressure. Also known as a non-condensing steam turbine.
blades Blades are attached around the circumference of the rotor assembly. The
blades receive the steam from the nozzles and convert the steam velocity
into useful work.
casing A casing is the housing of the turbine that contains the steam, supports
the stationary internals (nozzles and interstage diaphragms) of the turbine,
and houses the gland labyrinths, the steam admission valves (except on
large electric utility steam units), and the journal and thrust bearings.
governor A turbine control and protection device that is used to sense or measure a
single quantity, such as turbine speed, inlet pressure, extraction pressure,
induction pressure, exhaust pressure, or any combination of these
quantities, and to control the turbine to regulate the quantities that are
sensed. A governor limits turbine load, varies turbine load to maintain
constant power, and/or shuts down the turbine in an emergency.
induction turbine A steam turbine with the capacity to admit steam at two or more
pressures. Valve gear at the low-pressure opening can automatically
control the pressure in the low-pressure opening. Commonly called an
automatic admission turbine.
nozzle A device that converts the stored thermal energy of the steam into kinetic
energy, or velocity, and guides the steam to the blades at the correct
incident angle.
rotor A turbine rotor consists of the rotating elements of a steam turbine: the
shaft, the blade disks, and the blades. The rotor transmits the rotating
mechanical energy from the turbine blades to the load.
seal A device or material that prevents excessive leakage of fluids (gases or
liquids) by creating and/or maintaining a fluid-pressure differential across
the gap that exists between two relatively movable and/or separable
components of a fluid system
steam chest The section of a turbine that serves as the steam inlet to the turbine. The
steam chest houses the control valves, receives the supplied steam, and
directs the steam to the first stage nozzle assembly.

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