Steam Turbines Classification
- 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 drops.
- 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 non-condensing turbine.
- 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 blading.
- 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 securing.
- 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 turbine.
- 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 used.
- 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 condensing turbine.
- 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: Indicates available 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 engine.
- 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 pressure.
- 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 the turbines.
- 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.
- Safety 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 components.
- 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 the turbines.