- Turbine Classification
In order to better understand turbine operation, five basic
classifications are discussed. Type of compounding refers to
the use of blading which causes a series of pressure drops, a series
of velocity drops, or a combination of the two. Division of steam
flow indicates whether the steam flows in just one direction or
if it flows in more than one direction. Type of steam flow
describes the flow of steam in relation to the axis of the rotor.
Exhausting condition is determined by whether the turbine
exhausts into its own condenser or whether it exhausts into another
piping system. Type of blading identifies the blading as
either impulse blading or reaction blading.
- High pressure turbine: The high pressure (HP) turbine (see
Figure 1) is the first main engine turbine to receive steam from the
main steam system. It is designed to efficiently extract work out of
high pressure steam. The HP turbine is a pressure-velocity
compounded, single axial flow, non-condensing impulse turbine.
- Type of compounding: Pressure-velocity describes the
type of compounding. This refers to the use of blading which
causes a series of pressure drops and a series of velocity
- Type and division of steam flow: Single axial flow
simply means the steam flows in only one direction parallel to
the axis of the turbine rotor. Steam enters the forward end of
the turbine and exhausts through the after end of the turbine.
On a dual flow turbine the steam enters in the center of the
turbine rotor and flows both forward and aft simultaneously.
- Exhausting condition: The HP turbine exhausts into the
crossover pipe which directs the steam into the low pressure
turbine. This exhausting condition causes the HP turbine to be a
- Type of blading: The type of blading used on the HP
turbine is impulse blading because it extracts more work from
the high pressure steam than reaction blading. Impulse blading
is in the shape of a half moon. As steam impacts the moving
blade, it pushes the blade forward. This impact causes the steam
to lose velocity without losing pressure. In order to
efficiently extract the maximum amount of work out of the steam,
two different types of impulse stages are used. The Curtis stage
is the first stage of the HP turbine. The Curtis stage is
designed to initially extract a large amount of work out of the
steam as it enters the turbine. The remaining stages of the HP
turbine are Rateau stages.
- The Curtis stage
(see Figure 2) is designed to be
a power rotor, extracting a large amount of energy out of
the steam. As main steam enters the HP turbine, it first
passes through the nozzle block. The nozzle block contains
the nozzles. The velocity of steam is increased and the
pressure is decreased as the steam passes through these
nozzles. On an impulse turbine, the only time a pressure
drop occurs is when steam passes through a nozzle. After
steam passes through the nozzles, it passes through the
first set of moving blades. In the first set of moving
blades, work is extracted from the steam causing the
velocity to drop. After passing through the moving blades,
the steam then passes through the non-moving blades. The
only purpose the non-moving blades serve is to redirect
steam from the first set of moving blades to the second set
of moving blades. On an impulse turbine, non-moving blades
do not have any effect on the pressure or the velocity of
the steam passing through them. After leaving the non-moving
blades the steam passes through another set of moving
blades. This setup of a nozzle followed by a set of moving
blades, non-moving blades, and moving blades makes up a
single Curtis stage. After steam exits the nozzle there are
no further pressure drops. However, across both sets of
moving blades there is a velocity drop. This causes the
Curtis stage to be classified as velocity compounded
- The remaining stages of the HP turbine are a series of
Rateau stages . A single Rateau stage consists of a
nozzle diaphragm followed by a row of moving blades. The
nozzle diaphragm separates the stages of an impulse turbine
and provides support for the nozzles. The nozzles within the
nozzle diaphragm serve the same purpose as the nozzles
within the nozzle block. As steam passes through the nozzle,
velocity is increased and pressure is decreased. After
leaving the nozzle, steam then enters the moving blades
where once again work is extracted from the steam. As work
is extracted from the steam, its velocity will once again
decrease even though its pressure will not be effected. Even
though there is a velocity increase and a velocity decrease
in each Rateau stage, the overall velocity from the inlet of
the first Rateau stage to the exhaust of the final Rateau
stage is not changed. In contrast, there is a pressure drop
in each Rateau stage, resulting in an overall pressure drop
from the inlet of the first Rateau stage to the exhaust of
the final Rateau stage. This overall pressure drop causes
the Rateau staging to be considered pressure compounded.
- There are various other components of the HP turbine which
must be considered.
- Foundation: The aft end of the HP turbine is rigidly
mounted to the frame of the ship. The forward end is mounted
using either sliding feet (similar to what is used on the
boiler) or using a flexible I-beam (see Figure 5). The
mounting is designed to support the weight of the forward
end of the HP turbine as well as compensate for the
expansion and contraction encountered during start up and
- Steam chest: The steam chest, located on the
forward, upper half of the HP turbine casing, houses the
throttle valve assembly. This is the area of the turbine
where main steam first enters the main engine. The throttle
valve assembly regulates the amount of steam entering the
turbine. After passing through the throttle valve, steam
enters the nozzle block.
- Turbine casing drains
remove the condensate from the
turbine casing during warm-up, securing, maneuvering and other
low flow conditions.
- Low pressure (LP) turbine: The LP turbine (see Figure 6) is
located next to the HP turbine. The LP turbine is a pressure
compounded, either single or dual axial flow, condensing reaction
- Division of steam flow: On ships where space is a
consideration, the LP turbine is designed to be a dual flow
turbine. Steam enters the center of the turbine from the
crossover pipe and flows across the reaction blading in two
opposite directions. This configuration reduces axial thrust on
the turbine and allows for a smaller turbine installation. On
ships where space is not a concern, a single flow turbine is
- Direction of steam flow: Just like on the HP turbine,
steam flows parallel to the turbine rotor.
- Exhausting condition: Unlike the HP turbine, the LP
turbine exhausts into the main condenser. Because the LP turbine
exhausts into its own dedicated condenser, it is considered a
- Type of blading: The major difference between the HP
turbine and the LP turbine is the type of blading used. Because
the steam entering the HP turbine is at a high pressure it is
more efficient to use impulse blading. The steam entering the LP
turbine is at a significantly lower pressure than the steam
entering the HP turbine. In order to efficiently extract work
out of this lower pressure steam, reaction blading is used on
the LP turbine. Reaction blading works on the same concept as a
jet engine. A jet engine is designed to take in air, compress
it, heat it up and discharge it through the back. As the air
exits the jet engine, it expands, pushing the jet engine
forward. As the jet engine is pushed forward, it propels the jet
through the air. Similarly, each moving reaction blade, is
designed to act as a nozzle (miniature jet engine). As the steam
passes through a reaction blade it causes the reaction blade to
be propelled forward, resulting in rotation of the LP turbine
rotor. Both the moving blades and the non-moving blades of a
reaction turbine are designed to act like nozzles. As steam
passes through the non-moving blades, no work is extracted.
Pressure will decrease and velocity will increase as steam
passes through these non-moving blades. In the moving blades
work is extracted. Even though the moving blades are designed to
act like nozzles, velocity and pressure will decrease due to
work being extracted from the steam.
- Type of compounding: Due to the overall effect being a
loss of pressure across the LP turbine blading, the LP turbine
is a pressure compounded turbine.
Monitoring of parameters: In order to operate the main
engines safely, various parameters must be constantly monitored.
These main engine indicators will give the operator an idea of the
operating condition of the main engine.
- Astern turbine: The astern turbine is designed to propel the
ship in the astern direction. The concept is the same as in a car. A
car is designed to go both forward and reverse. The designers could
have designed the car with two totally separate transmissions, one
for forward and one for reverse. Instead, they designed cars with
one transmission capable of going both forward and reverse. The same
concept exists on the ship's main engines. The ship is not designed
with two engines per shaft. Rather, it is designed with one engine
per shaft. In a car, the ability to go in reverse is contained
within the transmission. On the ship's main engine, the ability to
go astern is contained within the LP turbine. The astern turbine is
designed as an integral part of the LP turbine rotor. On a double
flow LP turbine, the ahead elements of the LP turbine are located
towards the center of the LP turbine. The astern elements are
located on the forward and after end of the LP turbine rotor. On a
single flow LP turbine, the astern elements are located on the
forward end of the LP turbine. The astern turbine is a single axial
flow, velocity compounded, condensing impulse turbine consisting of
one or two Curtis stages located on the forward and/or after end of
the LP turbine.
- Other Components
- Deflector plate: During astern operations the steam will
naturally want to flow into the ahead elements of the LP
turbine. Similarly, during ahead operations the steam will
naturally want to flow into the astern elements of the LP
turbine. If this were permitted, the rotation created by the
ahead elements would be hindered by the steam acting on the
astern elements. To prevent this, a deflector plate is
installed. This deflector plate provides a physical barrier to
prevent steam from the ahead elements from impinging on the
astern elements and vice versa.
- Sentinel valves: There are two sentinel valves installed
on the LP turbine. One sentinel valve is located on the
crossover pipe leading to the LP turbine and the second is
located on the forward end of the LP turbine casing. Both of
these sentinel valves warn the operator of over-pressurization
of the LP turbine. A sentinel valve does not relieve system
pressure. It only acts to provide an audible alarm in the event
of overpressurization of the LP turbine. Some crossover pipes
also have relief valves installed.
- Bearings: In order to support the weight of the turbine
and to maintain radial and axial alignment, two different types
of bearings are used.
- Turbine journal bearings maintain the radial alignment
of the turbine and supports the weight of the rotor.
Bearings are spherically seated allowing for slight radial
misalignment during installation only. They are
located on the forward and after end of both turbine rotors.
- Turbine thrust bearings absorb any axial thrust created
in the turbine and also maintain the axial position of the
rotor in the casing. The thrust bearings are double acting,
segmented shoe, Kingsbury type thrust bearings. They are
usually located on the forward end of each turbine rotor.
- Flexible coupling:
Transmits the torque from the
turbines to the reduction gears. The flexible couplings are
designed so that any thrust created in the turbines will not be
transmitted to the reduction gears. They also allow for slight
radial misalignment and provide a means of disconnecting the
turbines from the main reduction gears.
- The rotor position indicator (RPI), Located on the
forward end of the turbine rotor, this device indicates a safe
distance between fixed and moving blades inside the turbine and
gives indication of thrust bearing wear. As the thrust bearing
naturally wears, this reading will gradually increase.
- HP turbine steam chest pressure:
steam pressure to the main engine from the boiler. This pressure
gage senses the pressure of the steam entering the steam chest.
The gage is located on the throttle board.
- First stage pressure:
Measured at the point where steam
is exhausting the first stage nozzle block. As the throttle
valve is opened, admitting more steam to the turbine, this
pressure will increase. As the throttle valve is shut this
pressure will decrease. The gage is located on the throttle
board. During speed changes, the throttle man controls the amount
of steam admitted to the turbine. This gage allows the
throttle man to monitor the amount of steam admitted to the
- First stage temperature:
Indicates the temperature of
the first stage of the HP turbine. During astern operations,
there is no flow of steam through the HP turbine to cool the
turbine blading. However, due to the HP turbine being connected
to the reduction gear, it will still rotate in the reverse
direction. As the HP turbine blading passes through the air
inside the turbine casing, friction is created. This is known as
windage. During extended astern operations, windage will create
large amounts of heat. If the turbine rotor overheats, damage
will occur to the turbine blading and rotors. The watch standers
monitor this temperature in order to be aware of any overheating
which may be occurring. Windage also occurs on multi-shaft ships
when a shaft is trailing.
- HP turbine exhaust pressure:
Indicates the pressure of
steam exhausting from the HP turbine before it enters the LP
turbine. At low speeds, the vacuum of the main condenser may be
pulling the steam through the turbines and this gage may
indicate a vacuum.
- Main condenser vacuum:
Indicates the vacuum (pressure
below atmospheric pressure) in the main condenser. The main
condenser is designed to operate under a vacuum. If a decrease
in vacuum occurs, the main engine will no longer operator
efficiently. If vacuum is totally lost, this could result in
damage to the main engine. This low pressure area is the most
efficient place for turbine exhaust steam.
- Main engine lube oil pressure:
Indicates oil pressure at
the most remote bearing. This is the bearing farthest away from
the lube oil pump. In the event system pressure is lost, this
bearing will normally be the first one to lose lube oil
- Bearing oil outlet thermometers:
Give an indication of
the temperature of the oil leaving the bearings. If a bearing
overheats, the babbitt will begin to break down causing a
bearing failure. By monitoring bearing temperature, a
watch stander will notice any bearings which are abnormally hot
and will be able to take corrective action in order to prevent
any further bearing damage from occurring.
Sight flow indicators
(SFI) are located on the outlet of
the bearing and allow the watch stander to monitor oil flow
through a bearing. The SFI is constructed with small glass
windows which permit the watch stander to look into the
indicator. Normally a flow of oil can be seen through the SFI.
In the event lubrication is lost to the bearing, the
watch stander will not see a flow of oil through the SFI.
NOTE: All bearings have both a sight flow indicator and a
temperature gage attached to them. A watchstander monitoring the
RPI�s, SFI�s and bearing oil temperature on main engines is
conducting what is called a THREE POINT CHECK and can
quickly report the satisfactory or unsatisfactory condition of
- Steam flow:
During normal ahead operations, main steam first
enters the steam chest. The amount of steam then allowed to flow
from the steam chest into the turbine is controlled by the throttle
valve located in the bottom of the steam chest. After passing
through the throttle valve, steam then passes through the nozzle
block. The nozzle block causes the pressure of steam to drop while
increasing steam velocity. The Curtis stage extracts work from the
steam and sends the steam to the Rateau stages. After exhausting
from the final Rateau stage, steam flows through the crossover pipe
into the LP turbine. The reaction blading of the LP turbine extracts
more work from the steam. After steam exhausts from the LP turbine
blading, it flows through the exhaust trunk into the main condenser.
- During astern operations steam will not enter the
steam chest of the HP turbine. Instead, after flowing through
the main engine guarding valve, main steam is directed to the
astern elements of the LP turbine located in the forward and
after end of the LP turbine. After passing through the astern
elements, steam then flows through the exhaust trunk and into
the main condenser.
is of the utmost importance while operating the main
engine. Failure to observe safety precautions can result in damage
to the main engine, personnel injuries, or even death. Even though
many safety precautions seem to be common sense, many times
personnel fail to consider the results of their actions.
- Never place any part of the body near rotating machinery.
While it is highly unlikely anyone will ever attempt to grab the
main shaft while it is rotating at 200 rpm, there are other
things to be considered. While the main engine jacking gear is
engaged, the shaft is rotating at a very slow rate. Despite this
slow rotation, watchstanders still should not be permitted to do
any type of work to the main shaft, such as painting or cleaning
the main shaft.
- Do not wear jewelry, neckties, or loose fitting clothing
while operating equipment. This clothing can become entangled in
the machinery and cause injury or death.
- Oil leaks shall be corrected at their source. Spills of any
kind shall be wiped up immediately and the wiping rags disposed
of immediately or stored in fire safe containers. Failure to
observe safety with any petroleum product can result in a major
Class B fire.
- Promptly reinstall shaft guards, coupling guards, deck
plates, handrails, flange shields and other protective devices
removed as interferences immediately after completion of
maintenance on machinery, piping, valves or other system
- An open main engine presents special safety precautions.
While the main engine is open, an E-5 or above is required to
stand guard. A security area is established around the main
engine using ropes and signs. No tools are permitted within the
security area without first being inventoried by the guard.
Before personnel are permitted to enter the security area, they
are required to remove all jewelry, securely fasten eyeglasses
and tools to their body using lanyards, and all clothing
fasteners must be covered with tape. As an added safety
precaution, ensure all warfare and rank devices are removed
before entering the security area. This precaution prevents
inadvertent introduction of anything that could cause damage to