AN introduction to Steam Turbines
A heat engine is one that converts heat energy into mechanical energy. The steam turbine is classified as a heat engine, as are the steam engine, the internal combustion engine, and the gas turbine. Steam turbines are used in industry for several critical purposes: to generate electricity by driving an electric generator and to drive equipment such as compressors, fans, and pumps. The particular process dictates the steam conditions at which the turbine operates.
The turbine makes use of the fact that steam, when passing through a small opening, attains a high velocity. The velocity attained during expansion depends on the initial and final heat content of the
steam. This difference in heat content represents the heat energy converted into kinetic energy (energy due to velocity) during the process.
the steam turbine permits the steam to expand and attain high velocity. It then converts this velocity energy into mechanical energy. There are two general principles by which this can be accomplished
A turbine that makes use of the impulsive force of high-velocity steam is known as an impulse turbine.A turbine that makes use of the reaction force produced by the flow of steam through a nozzle is a reaction turbine.
In practically all commercial turbines, a combination of impulse and reactive forces is utilized. Both
impulse and reaction blading on the same shaft utilize the steam more efficiently than does one alone.
An example of an impulse force is illustrated in Fig. 1 a. Here water is seen striking a flat plate (P) and being scattered so that any energy remaining is splashed, lost, or dissipated. Only impulse force is used. Figure 1b shows a combination of impulse and reaction forces on the bowl (B).
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| FIG 1 |
pressure causes the steam to flow with high velocity from a small jet or nozzle. This steam is directed against the paddle wheel, and rotation results. All pressure drop takes place in the nozzle or stationary elements, and the moving paddles absorb the velocity energy in the steam issuing from the nozzles.
The required velocity of the moving parts is reduced by applying the principle of pressure staging. Pressure staging consists of allowing only a limited pressure drop in one set of nozzles. After the steam from a set of nozzles has passed the rotating element (blades), it is expanded in another set of nozzles.
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| FIG 2 |
The principle of velocity compounding is also used in the operation of impulse turbines. This means applying the velocity energy in steam coming from the nozzles to two or more sets of moving blades.
The operation makes use of a set of stationary blades, which reverse the flow of steam, between each two sets of rotating blades.
Large high-pressure turbines usually have many pressure stages and in addition use velocity compounding in the high pressure stages.
If one set of nozzles is used and all the pressure drop occurs in this group of nozzles, and if all the energy is directed against a single wheel, we have a single-stage simple velocity turbine.
The reaction turbine is one in which the pressure drop takes place in the rotating element.
The reaction turbine is in reality an expansion nozzle in which the pressure of the steam is used to increase its velocity. This velocity is converted into mechanical energy by the rotating element. During this process of expansion, the steam increases in volume. The area of the steam passage through the blading must increase, from the high pressure to the low-pressure end of the turbine, in order to accommodate this larger volume of steam.
The illustration in Fig. 1b shows a combination of impulse and reaction forces in action. Note that, with the same stream of water, the bowl (B) moves farther than the plate (P). The water from the
stream of water produces an impulse force in the direction in which the stream of water is moving, and on leaving the bowl, the water produces a reaction force against the bowl. All turbine blades are
shaped to give somewhat the same effect as the water in the bowl.
The efficiency of a turbine is optimized as the steam expands and does work in a number of steps or stages as it flows through the turbine. These stages are identified from the manner by which the energy is removed from the steam.
There are two types of turbine stages, impulse and reaction, and most turbines combine features of both
Impulse turbine. This stage design is often compared with a water wheel because nozzles direct the steam that flows through high-velocity jets. These steam jets, which contain kinetic energy, flow against the moving turbine blades or buckets. This energy is converted into mechanical energy by rotating the shaft. In a pure impulse turbine, when the steam passes through the stationary blades, it incurs a pressure drop. There is no pressure drop in the steam as it passes through the rotating blades. Therefore, in an impulse turbine, all the change of pressure energy into kinetic energy occurs in the stationary blades, while the change of kinetic energy into mechanical energy takes place in the moving blades of the turbine.
Reaction turbine. This design uses the reaction force resulting from the steam accelerating through the nozzles. The nozzles are actually created by the blades, Each stage of the turbine consists of a stationary set of blades and a row of rotating blades on a shaft. Since there is a continuous drop of pressure throughout each stage, steam is admitted around the entire circumference of the
blades and, therefore, the stationary blades extend around the entire circumference. Steam passes through a set of stationary blades that direct the steam against the rotating blades. As the steam passes
through these rotating blades, there is a pressure drop from the entrance side to the exit side that increases the velocity of the steam and produces rotation by the reaction of the steam on the blades.
Major components of a turbine
The turbine consists of a shaft, which has one or more disks to which are attached moving blades, and a casing in which the stationary blades and nozzles are mounted. The shaft is supported within the casing by means of bearings that carry the vertical and circumference loads and by axial thrust bearings that resist the axial movement caused by the flow of steam through the turbine. Seals are provided in the casing to prevent the steam from bypassing the stages of the turbine.
Blades.
Blades generally are made from lowcarbon stainless steel; however, for high-temperature applications and where high moisture is expected, alloy steels are used to provide the strength and erosion resistance needed. Special coatings on the blades are often used where high erosion is anticipated.
The turbine efficiency, as well as its reliable performance, depends on the design and construction of the blades.
Rotor shaft and bearings.
The rotor shaft is supported at each end by bearings. These are normally ball bearings on small turbines; however, on the larger turbines, a pressure-lubricated journal bearing is used. Because of the axial thrust along the shaft that results from the difference in steam pressure across the stages of the turbine, thrust bearings are used to maintain the clearances between the moving blades and the stationary portions in each stage of the turbine.
Casings and seals.
Casings are steel castings whose purpose is to support the rotor bearings and to have internal surfaces that will efficiently assist in the flow of steam through the turbine. The casing also supports
the stationary blades and nozzles for all stages. In addition to being designed to support the weight of the stationary nozzles and blades, the casing also must resist the mechanical stresses that are caused by the reaction forces on these nozzles and blades as well as the thermal stresses that are caused by the steam temperature differentials that occur during operation in the various stages of the turbine.
Since the shaft penetrates through the casing. seals are necessary to minimize the leakage of steam. In small, low temperature turbines, carbon packing ring seals are used. These seals are located directly on the shaft and are held in place by a spring assembly. In larger turbines, labyrinth seals are used to control steam leakage. In many turbine designs, a combination of the two types of seals are used at the ends of the shaft.
Small turbines are housed in a single casing that admits high-pressure steam at one end, and low-pressure steam leaves at the back end of the turbine to the condenser or as steam for a process or heating. On large, high-pressure turbines, two or three separate casings are used, with the turbine having three sections:
high pressure (HP), intermediate pressure (IP), and low pressure (lp)
Steam flow control.
The part load performance and responsiveness of a steam turbine are to a large degree dependent on the method used to control the steam flow to the first stage of the turbine.
There are two admission methods used: partial arc and full arc. Partial-arc admission requires the adjusting of the active nozzle area, where the nozzles in the first stage are divided into groups and
controlled separately with throttling valves. When the load is increased, the throttling valves for each group of nozzles are opened in sequence to the full-throttle position until the desired load is
attained.
Full-arc admission requires the steam pressure entering all the first stage nozzles to be adjusted by either of two ways:
1. By operating the boiler at constant pressure and throttling the steam flow, with all valves being opened together until the required load is obtained
2. By varying the boiler operating pressure with the throttle valves basically full open, except at the lowest loads
The selected control system is based on the planned operation of the turbine, since both systems have disadvantages. A base-loaded turbine would have a different control system than would a turbine
planned for variations in load.
1. Full-arc admission that requires constant throttle pressure is a relatively simple system, but when the turbine operates at part loads, pump power is wasted.
2. When the steam flow is throttled for full-arc admission, this results in a turbine inlet steam temperature drop.
3. When a partial-arc admission system is used that has a constant throttle pressure, this reduces the energy loss caused by steam throttling, but it is generally less efficient at full load.
4. By varying the boiler pressure, a less responsive system results for partial arc admission.
Therefore, each system affects the part load efficiency differently. Differences also exist on the turbine inlet steam temperature and on the system responsiveness. As a result, systems are designed with combined features of each control strategy in order to optimize the controls for the expected operation of the turbine.
Turbine types and applications
As described previously, a steam turbine takes the thermal energy of the steam, which is provided by a boiler, and converts it into useful mechanical work by means of the steam expanding as it flows
through the turbine. Steam is introduced into the turbine through small stationary nozzles, where the steam expands and reaches a high velocity. This process converts the thermal energy in the steam
to kinetic energy as it passes through the nozzle openings and moves the turbine blades, which are attached to the rotor.
Steam supply and exhaust conditions. When classifying steam turbines
by their steam supply and exhaust conditions, they are categorized as condensing, non condensing or back pressure, reheat-condensing, and extraction and induction.
1. Condensing turbine.
This type of steam turbine is used primarily as a drive for an electric generator in a power plant. These units exhaust steam at less than atmospheric pressure to a condenser
2. Noncondensing or backpressure turbine.
This type of turbine is used primarily in process plants, where the exhaust steam pressure
is controlled by a regulating station that maintains the process steam at the required pressure
3. Reheat-condensing turbine.
This type of turbine is used primarily in electricity-producing power plants. In these units, the main steam exhausts from the high-pressure section of the turbine and is returned to the boiler, where it is reheated with the associated increase in steam temperature.
4. Extraction and induction turbine.
This type of turbine is also found primarily in process plants. On extraction turbines, steam is
taken from the turbine at various extraction points and is used as process steam.
Casing or shaft arrangement.
Steam turbines are also classified by their casing or shaft arrangement as being single, tandem-compound, or cross-compound and are described as follows:
1. Single casing .
This is the basic arrangement for smaller units, where a single casing and shaft are used.
2. Tandem-compound casing.
This arrangement has two or more casings on one shaft that drives a generator.
3. Cross-compound casing.
This arrangement has two or more shafts that are not in line, with each shaft driving a generator.
These units are found in large electric utility power plants.
PRESSURE, TEMPERATURE, AND FLOW RELATIONSHIPS
The general flow equation for all turbine stages can be expressed as follows:
Most stages, including altlh ose between the first and last stage, operate at a nearly constant pressure ratio under changing governing valve setting, throttle flow, condenser pressure, and throttle steam conditions.
For these stages, by assuming a constant p2/p1,a nd by ignoring the very small changes in y and A pre. equation becomes:
Although Cq, varies slightly with Reynolds number, practically it can also be considered a constant. Thus:
where, in the above and in Fig
w = rate of flow, lbm/hr
Cq = flow coefficient
An = nozzle area, ft2 (stationary-blade flow area)
pl = stage-inlet pressure, psia
p2 = pressure between stationary and rotating blade rows, psia
p3 = stage-outlet pressure, psia
R1= universal gas constant at stage inlet
g = acceleration due to gravity, ft/sec2
v1 = specific volume at stage inlet, ft3/lbm
TI = absolute temperature at stage inlet, O R
V = velocity, ft/sec
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