AES Lal Pir Thermal Power Plant Lal Pir Distt Muzaffargarh Punjab Pakistan
Training Module:-TO-1 Turbine General Revision-0 (June 2009)
Prepared By
Reviewed By
Maqsood Ahmad Shad
Approved By Haroon Rashid
AES Lal Pir Learning Centre 1
Contents S.no. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 21 22 23
Description Contents Steam turbine Basic turbine theory Impulse and reaction balding Turbine classification Condensing VS non condensing Single pressure VS Multi pressure Rehaet VS non reheat Single casing VS compound casing Exhaust flows Inter stage sealing Steam admission components Turbine expansion Turbine bearings Lubrication Turing gear Rupture Diaphragm Turbine Auxiliary systems Lube oil system components Gland Steam system Turbine Electro-Hydraulic Control oil system Turbine Supervisory Instruments LP/PG Turbines Turbine Protections
Page no. 2 3 4 5 6 6 7 7 8 8 8 9 11 12 13 14 15 16 16 17 18 18 19 22
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Steam Turbine Steam turbine is a mechanical device that extracts thermal energy from pressurized steam, and converts it into rotary motion. Because the turbine generates rotary motion, it is a common source to drive an electrical generator for electricity generation. Steam turbines are made in a variety of sizes ranging from small 1 HP (0.75 kW) units used as mechanical drives for pumps, compressors and other shaft driven equipment, to 2,000,000 HP (1,500 MW) turbines used to generate electricity. There are several classifications for modern steam turbines.
Power Plant Simple Cycle
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Basic Turbine Theory The steam entering a turbine has a great deal of thermal energy because of its pressure and temperature. The conversion of the thermal energy in the steam to mechanical energy in the turbine actually takes place in two steps. First, the thermal energy is converted into kinetic energy of a steam jet by nozzles. Second, the steam jets are directed against buckets or blades mounted on a rotor. The force of the jets striking the buckets or blades produces mechanical energy by turning the rotor. The nozzle converts the thermal energy of a fluid, in this case steam, into kinetic energy by expanding the fluid. The higher the pressure and temperature, the more thermal energy available. The pressure is of particular importance for expansion. As shown in this illustration, steam at temperature T 1 and pressure P 1 enters a convergent nozzle at velocity V 1. The steam leaving the nozzle is now at a lower pressure and temperature, P 2 and T2 but at a higher velocity V 2. This is because some of the energy in the steam has been converted into kinetic energy, energy of motion. Since, kinetic energy is a function of the square of the velocity, as the velocity increases, so does kinetic energy. The kinetic energy in the jet of steam is not useful as it is and the nozzle by itself cannot convert the energy in the steam to useful mechanical energy without a means of transferring this energy to the rotating blades. There are two basic types of turbine blade designs: impulse and reaction. Both use nozzles and rotors, but in different ways. Knowing the difference between these two types of turbines is helpful in understanding how a turbine changes thermal energy from the steam into mechanical energy to drive the turbine. The mechanical energy from the turbine drives the generator, making electrical energy. The operating principle of the impulse turbine is illustrated here. Steam enters the impulse turbine through a stationary nozzle. The nozzle expands the steam to create an accelerated steam jet and directs it at the rotor buckets. The accelerated steam jet then strikes the rotor buckets causing the rotor to turn. It should be noted here that the terms buckets and blades are interchangeable. Some turbine manufacturers call them buckets and others call them blades. The operating principle of a reaction-type turbine uses Newton's Third Law of Motion, which states that for every action, there is an equal, opposite reaction. In a pure reaction turbine, all expansion of the steam would occur in the rotating blades (think of the blades as rotating nozzles). As the steam expands in the rotating nozzles, the accelerated steam exerts a force on the blades, much like a rocket engine. The turbine
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rotor is forced to turn by the force of the steam jet leaving the nozzle much like that of a rocket engine. Most turbines (after the first stage) use a combination of impulse and reaction blading in their design. In other words, the steam is expanded in stationary nozzles, where it is accelerated and impacts the rotating blades (impulse design). As the steam enters the blades, it continues to expand and as a result, exerts an additional force on the rotating blades (reaction design).
Impulse and Reaction Balding The illustration here is of a simple impulse turbine stage. Each set of stationary nozzles and rotor blades (buckets) is called a stage. As the steam expands through the stationary nozzles of an impulse stage, steam velocity increases while the pressure decreases. When the steam passes through the rotating buckets, the steam velocity decreases as it gives up its energy to the blades, and the pressure remains the same. In modern power plants there is considerable thermal energy in each pound of steam. It is impractical and inefficient to build a single nozzle and rotor large enough to convert all of the steam's thermal energy into useful work. Large modern turbines are multistage turbines with each stage converting part of the steam's thermal energy to mechanical energy. Impulse turbines can be multistage in two ways. The first is with what is commonly called a wheel. The Curtis stage is what is known as a velocity compounded stage. It consists of one set of nozzles and two or more rows of moving buckets with a row of stationary guide vanes between the two rows of moving buckets. As the steam passes through the nozzle, its pressure is reduced and velocity is increased. After passing through the first row of moving buckets the steam's velocity is reduced because of the work it did on the buckets. The steam then passes through the stationary guide vanes, which change the direction of the steam without changing its pressure or velocity. The new steam direction is approximately parallel to the original steam direction leaving the nozzles. The steam then strikes a second row of buckets that are attached to the same wheel as the first row. Most Curtis stages are limited to two rows of moving buckets. The second method of multistage an impulse turbine is by using what is commonly known as a Rateau or pressure compound stage. A Rateau stage consists of a set of nozzles and a set of rotating buckets. It should be noted that there is a pressure drop through each set of nozzles (pressure-volume energy of steam is converted to kinetic energy), and a velocity drop through each set of moving buckets (kinetic energy is converted to mechanical energy). Practically all large, multistage impulse turbines contain pressure and velocity compound stages. It is not unusual to have as many as 20 stages in a turbine.
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Newton's Third Law of Motion says, "For every action there is an equal and opposite reaction." This is the principle on which reaction turbine blading works. The turbine rotor is forced to turn by the active force of the steam jet leaving the nozzle. Therefore, in an ideal reaction turbine, the moving buckets would be the only nozzles and all steam expansion would occur in the moving buckets. This is impractical because it is difficult to admit steam to moving nozzles. Large turbines use fixed nozzles to admit steam to moving nozzles and therefore use a combination of impulse and reaction principles. Each stage of a reaction turbine consists of a set of fixed nozzles and a set of moving buckets. The pure reaction turbine stationary blades would be "guide vanes" and the nozzles would be rotating. Being the same shape as nozzles with the steam expanding, gaining velocity, and losing pressure while passing through. The reaction of this velocity upon the rotating blades powers the rotor. The pressure drop would occur over the fixed and rotating nozzles. In reality, reaction turbines are typically 50% reaction turbines, and use a combination of impulse and reaction staging. Each pair of fixed and moving nozzles makes up one stage.
Turbine Classification Impulse and reaction turbines can further be classified using other important characteristics. These characteristics are: •
Condensing versus non-condensing
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Extraction versus non-extraction
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Single-pressure versus multiple-pressure
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Reheat versus non-reheat
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Single casing versus compound
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Exhaust flows
Condensing vs. Non-Condensing One characteristic for classifying steam turbines is whether they are condensing or noncondensing turbines. In a condensing type turbine, the steam is exhausted to a condenser. By condensing the steam, the turbine exhaust pressure and temperature can be very low. Low exhaust pressure allows the turbine to make maximum use of the thermal energy in the steam and makes the power plant more efficient. Nearly all large utility steam turbines are of this type.
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In non-condensing turbines, the exhaust is not condensed. The steam is typically used for some useful purpose such as heating a building. If a non-condensing turbine exhausts to a pressure greater than atmospheric pressure, it is called a backpressure unit. This type of turbine is most often seen in process plants such as steel mills, refineries, and paper mills.
Extraction vs. Non-Extraction Turbines may also be classified by whether or not they are extraction type turbines. Extraction turbines are sometimes called "bleeder" turbines because steam is removed or bled from between stages. An extraction type turbine is a multistage turbine where some of the steam is extracted or bled from between the stages at what is commonly referred to as extraction points. Extraction steam may be used for regenerative feedwater heating or other purposes. In addition, there are two common types of extraction used with steam turbines. On utility turbines, the extractions are referred to as non-controlled because the extraction pressure and flow is strictly a function of the operating load of the unit. In cogeneration facilities, it is common to have controlled extraction turbines, where the extraction pressure is controlled by an additional set of control valves located in the turbine's steam path. The control system modulates the extraction control valves in addition to the admission control valves to obtain the desired combination of electric generation and extraction steam flow to the steam host.
Single-Pressure vs. Multiple-Pressure Most turbines have steam admitted to the first stage from a single source. Some turbines also have steam admitted at a lower pressure in the steam path at some point after the first stage. This arrangement is common in steam turbines in combined cycle plants because it is common to have a heat recovery steam generator (HRSG) that operates at more than one pressure.
Reheat vs. Non-Reheat A reheat turbine is a multistage turbine in which the steam is directed from some intermediate point of the turbine back to the boiler. In the boiler, the steam is reheated and piped back to the turbine. Some large turbines return the steam to the boiler a second time to be reheated and are called double reheat turbines. There are two advantages to reheating steam: 1. It makes the power plant more efficient thermodynamically. 2. It delays the onset of steam condensation in the turbine so that wet steam is only "handled" by the turbine in the last LP stage.
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Nearly all modern, large steam turbines use reheat. Most have only one stage of reheat; however, some larger units use two stages of reheat.
Single Casing vs. Compound A single casing turbine has all the turbine stages in one casing. As turbines became larger, it was not practical to have all the stages in one casing. Turbines with two or more casings are classified as compound turbines. There are two different types of compound turbines: tandem compound and cross compound. Tandem-compound turbines have the turbine sections in line with one another and are on the same shaft. The tandem-compound unit shown here only has two sections but some larger, modern units may have as many as five sections Cross-compound turbines have different turbine sections on different shafts. For power plants, this means that two separate generators are used. Some large cross-compound units have two or more turbine sections on each shaft, and would be a combination of cross-compound and tandem-compound. Nearly all large steam turbines are multiple casing units. The tandem-compound arrangement is most common.
Exhaust Flows Condensing turbines can be further classified by their exhaust flows. A single-flow condensing turbine passes all its exhaust steam to the condenser through one exhaust opening. As turbines got larger, the low pressure sections had to be split up into more than one section because of design limitations related to the length of the last stage blades in the LP turbine. Turbines with as many as six exhaust flows are not uncommon.
Interstage Sealing Interstage sealing prevents steam from bypassing the turbine buckets, instead of flowing through the nozzles and buckets. The illustration on the right is the inner element of the study turbine. The green sections represent the fixed blading, or nozzles. The blue sections represent the rotating turbine blades or buckets. It is important to note that the rotating blades use seals at the blade tips to minimize leakage of steam over the blade tips. This is more critical in reaction blade design than in a pure impulse blade design. Since the pure impulse blade has no pressure drop across the blade, leakage around the blade tips is minimal. The reaction blade design, however, does have a pressure drop across the rotating blades and is therefore more prone to tip leakage.
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The blade tip clearance on turbine blading is very minimal. During start-up, the rotor assembly will experience higher than normal vibration as it passes through "critical" speeds, and will expand at a faster rate than the casing due to being "immersed" in the steam and having less mass. If vibration or the difference in thermal growth between the rotor and the casing (differential expansion) become excessive, the tip (radial) seals and the shaft packing seals will rub. This will result in increased steam seal clearances and a less efficient turbine section. Any vibration or differential expansion in excess of the manufacturer's recommended limits is to be considered serious, and immediate corrective action should be taken, including removing the turbine from service. The stationary blade rows (nozzles) are sealed between the diaphragm (blade ring) and the rotor to avoid leakage of steam past this point. This is done to maximize steam flow through the nozzle where the steam's pressure can be converted to velocity.
Main Steam Admission Components The control of high pressure, high temperature steam flow through a turbine is one of the most critical functions performed by mechanical components associated with the steam turbine. For this purpose following steam admission components are discussed: •
Main turbine stop/throttle valve
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Main turbine control/governor valves
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Steam chest
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First stage nozzle block
The main turbine stop valve and governor valve(s) work together with the turbine's control system to control the flow of steam to and through the turbine. As the name implies, the stop valve is an isolation valve and the governor valve is a flow control valve. The actual construction of these valves will differ from one turbine manufacturer to another and will be referred to by different names. Westinghouse uses the terms "throttle valve" and "governor valve," while GE uses the terms "stop valve" and "control valve." The main turbine stop valve is the first main steam isolation component. It is normally either fully open or fully closed, and is designed to provide positive shutoff of steam flow to the turbine. The valve is hydraulically opened against large springs that can close the valve very quickly, within a few tenths of a second. This is especially important in the event of an emergency trip of the turbine. Although the normal position of the stop/throttle valve is fully open or fully closed, during startup these valves can be used, in some designs, to control the flow of steam to the turbine when the turbine is in what is known as full arc admission. Full arc admission refers to allowing the steam to flow the full 360 degrees of the turbine inlet - this facilitates even heating of the turbine and minimizes thermal stress.
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In older GE machines, one of the stop valves is typically provided with an internal valve that is referred to as the full arc supplement valve. The main stop valves are designed to prevent opening if there is a large differential pressure across the valves. However, the full arc supplement valve can be opened and is used to control steam flow during the spin-up, synchronization, and initial loading of the turbine. After the turbine is on-line, the control valves are modulated toward the closed position until steam pressure builds up adequately between the stop valves and control valves. At that point, the stop valves will open fully and turbine loading will be controlled by the control valves. On newer GE designs, the control valves are used in full arc mode from the initial roll-off to synchronization and loading of the turbine. With this type of design, the stop valves still play an important steam isolation role, but there is no internal bypass (full arc supplement) provided. On Westinghouse designs, the throttle valves are used to roll-off and accelerate the turbine to a speed near synchronous speed. The control system then closes the governor valves until they take over speed control. From that point on, the governor valves are used to accelerate, synchronize, and load the turbine. The steam chest is the area that receives steam from the turbine main stop valves and houses the control (governor) valve(s). Steam leads direct the steam from the control/governor valves to the nozzle block. These are external pipes when the steam chest is not mounted on the turbine casing, and internal passages when the steam chest is mounted on the turbine casing. Notice the different openings on the control valves. The nozzle block contains the nozzles that direct the flow of steam to the first stage blades of the turbine rotor. The nozzles convert the thermal energy in the steam to kinetic energy to rotate the turbine rotor. The governor valve controls the flow rate of steam through the turbine. The number of governor valves varies considerably with the turbine design and manufacturer. Some turbines may have only one while others could have as many as 10 or 12. These valves are also hydraulically opened, but are used to throttle steam flow to the nozzle block. To increase turbine load, these valves open sequentially. This is called partial arc admission. As with the main stop valves, these valves have large springs to quickly close them in the event of an emergency. Partial Arc Partial arc admission gets its name from the circle or arc of the nozzle block. Partial arc admission simply means only part of the arc or nozzle block (only one or two governor/control valves open) is admitting steam. Full Arc Full arc admission occurs when all the governor (control) valves are 100% open, admitting steam evenly around the turbine first stage nozzle block. Full arc admission is normally used during startup of the turbine by admitting steam to the turbine through the throttle valve or stop valve bypass.
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Starting a GE turbine in full arc admission can vary depending on the design of the machine. On older machines, starting is accomplished by opening the control valves 100% and then opening a small bypass (full arc supplement) valve that is built into the main stop valve. Once the bypass valve begins opening, the steam flows evenly to all of the sections of the turbine nozzle block. This provides for even heating of the turbine during startup. It is essential that turbine starting and loading guidelines be followed. Failure to do so will impose excessive thermal stresses on the turbine. Because of tremendous centrifugal forces, turbine over speed conditions must also be avoided. One of the worst situations that can occur is a loss of generator load. When a generator is on-line, it is electrically tied to the power grid and will hold turbine speed at 3600 or 1800 RPM depending on whether the generator is a two pole or four pole machines. Increasing steam flow to the turbine under these conditions will not increase turbine speed. Increasing the steam flow increases the horsepower output of the turbine, which in turn increases the generator power output. If the generator output breaker opens, the generator is no longer tied to the power grid. Once this happens, there is nothing holding the turbine at a constant speed. All the horsepower being supplied by the turbine will cause the turbine and generator to accelerate rapidly. This rapid acceleration will exert tremendous centrifugal forces on the turbine and generator rotating elements. If allowed to continue accelerating, the forces will cause damage to the rotating elements. To better visualize this, compare what happens when the generator breaker opens to a car climbing a steep hill. As the car climbs the hill, the accelerator is depressed further and further to maintain the car's speed until the engine is at its maximum horsepower output. Now, if the transmission were to jump out of gear into neutral, the engine RPM would red line and possibly destroy itself if the accelerator was not released. Easing up on the accelerator is comparable to stopping steam flow to the turbine. In an over speed condition, the turbine control system will take rapid action to isolate all sources of steam to the turbine to prevent the turbine from accelerating to a destructive speed. This includes the stop/throttle valves, the control governor valves, intercept, and reheat stop valves. It should be noted that all of these valves are designed to fail in the closed position by using large springs as well as steam pressure to help drive them closed.
Turbine Expansion Recall that a large utility boiler grows over six inches when brought up to normal operating temperature. This phenomenon also affects the turbine. The larger the turbine, the more it grows. Allowances for growth must be made due to this thermal expansion experienced by the turbine. These allowances are monitored by the operator using the Turbine Supervisory Instrumentation (TSI), and are called differential expansion and casing expansion.
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If the rotor of a turbine were heated up to 650°F (343°C) but the casing were left at 100°F (38°C), the rotor would become longer than the casing and clearances between the rotating blades and the stationary components would cease to exist. Similarly, if the casing were heated to 650°F (343°C) and the rotor left at 100°F (38°C), the rotor would be far too short for the casing. In either of these cases, if the turbine rotor were turning at its rated speed, substantial damage to the turbine components would occur. These two scenarios, though exaggerated, are called differential expansion. Differential expansion is the difference in growth between the rotor and the turbine casing. This growth differential is caused by uneven heating as a result of the difference in exposure to the hot steam as well as the mass between the rotor and the casing. The difference in types of materials used in the construction of the casing and rotor will also affect the differential expansion Rapidly changing load is a major factor in differential expansion. Raising or lowering load too fast causes the rotor to heat or cool more quickly than the casing. This increases differential expansion. Virtually all modern turbines have trips (emergency shutdowns) associated with excessive differential expansion. Turbine load or main steam temperature should not be raised too quickly because the rotor will go long (grow faster than the casing). Likewise, reducing load or steam temperature too quickly will make the rotor go short (shrink faster than the casing). Casing expansion is an external measurement of casing thermal growth. Every power turbine is keyed or locked at one point and allowed to expand out from that point. By locking, or keying one end of the turbine, either at a bearing or a casing, and placing the remaining bearings and casings on keyed sliding feet, the turbine is allowed to freely move axially due to thermal expansion. If the sliding feet under a bearing pedestal or casing should bind in one position, the turbine case would not be able to expand or contract properly as it heats or cools. This would cause differential expansion problems as well as excessive stress on the casing and thrust bearing assembly, usually on the inactive face. This is one of the reasons attention must be paid to abnormal or unusual thrust indications. Sometimes turbine casing binding is easily fixed by the addition of grease to the sliding feet. Abnormal turbine casing thermal growth indications or abnormal thrust bearing readings should always be treated seriously.
Bearings Steam turbines have bearings to support and align the turbine rotor. Journal bearings are used to support the weight of the rotor and maintain rotor radial alignment, while a thrust bearing is provided to maintain the rotor's axial position. A journal bearing is also called a sleeve bearing. It is simply a cylinder of Babbitt cast into a cylindrical steel liner. Babbitt is a soft alloy made of lead, tin, and antimony, and is softer than the steel journals. The journal bearing is split at the horizontal joint for assembly and fits into a circumferential slot or fits into a bearing standard or pedestal.
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A turbine can develop a tremendous amount of thrust. The pressure drop across the reaction blades or the steam impinging on the impulse blades produces an unbalanced axial force on the rotor. On Westinghouse turbines, this force is toward the governor; on GE turbines, the force pushes the rotor towards the generator end. This force, or thrust, requires some method of counteracting it, or the rotor would be pushed into the stationary diaphragms. One method of counteracting the thrust in larger turbines is to oppose the steam flow in the different sections of the turbine. However, smaller turbines with only one section cannot use this method. They must rely on a "dummy piston" or thrust bearing assembly to control the thrust. The thrust bearing is a simple device that holds the rotor axially stationary. It consists of an oversized ring, or collar machined into the turbine rotor. On each side of the collar are bearing surfaces that prevent the collar (which is part of the rotor) from moving. The Kingsbury-type thrust bearing illustrated here is commonly used in turbines to contain, or balance thrust. Thrust bearings have an active face, or side, and an inactive face, or side. The active face is the side that, under normal on-line conditions, is pushing against (or holding) the rotor in position. The inactive face is the side that is normally not in contact with the thrust collar on the rotor. It is important to note that thrust direction can actually change while rolling the machine up to speed. Most turbines built in the last 20 years have a means of indicating thrust. These are typically thermocouples mounted into the thrust bearing that measure the amount of thrust on each face of the thrust bearing. An operator should monitor this for unusual indication. If the thrust-loading forces change from the normal, a serious problem could be developing in the turbine. Changes in the thrust bearing loading could be indicative of: 1. Possible turbine water induction. This is a serious condition where water enters the turbine through the main steam line or an extraction line. The impact of water on turbine blades can be damaging. Water is 1600 times more dense than steam, and the water is obviously much cooler than steam. This added density and thermal shock can damage steam path components or permanently distort the casing. It can also damage the thrust bearing due to the added thrust. 2.
Possible hang-up of the turbine casing sliding feet. The larger the turbine, the more it grows as it is heated, and as previously discussed, this expansion must be accommodated by allowing the turbine to move on slides. The turbine supervisory system provides the operator with indications to verify that the turbine is expanding properly.
Lubrication The bearings used in large steam turbines are Babbitt-type, forced oil lubricated bearings. They must be supplied with clean, relatively cool oil for proper operation.
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The bearings are fed oil under pressure from the lubricating oil pump; as the rotor turns, it produces a pumping action that builds additional pressure to create a wedge of oil between the journal surface and the Babbitt. In normal operation, the surfaces are separated by the film of oil and never touch. This animation shows an exaggerated view of what the rotor of a turbine (or even a pump or fan shaft) looks like in its bearing. The inner circle represents the rotor, and the outer circle is the clearance in the bearing. Although there is not this much clearance in actual bearings, this exaggerated view provides an understanding of how lubrication works in the turbine bearings (or any other sleeve-type bearing). With the rotor in its stopped position, it is resting in the bottom of the bearing. As the rotor begins turning, it climbs the bearing wall and the void under the rotor fills with oil. The rotor then slides down the bearing where comes in contact with the oil. The oil underneath forms a film between the bearing and the rotor. As the speed is increased the film grows thicker because of the hydraulic action of the rotor. This sequence is basic, but serves to illustrate oil wedge lubrication. Oil temperature is a major factor in turbine operation. The reason oil temperature is critical to turbine operation is viscosity. Viscosity is the measure of the thickness or pour-ability of oil. Oil that is so thick it can barely run out of a can when turned over is said to have a high viscosity. Likewise, oil that flows like water is said to have a low viscosity. As oil temperature changes, so does the viscosity. Most turbines use a high grade, synthetic-type oil that resists viscosity changes. However, even the most premium oils thin out with an increase in temperature. If the turbine lube oil is allowed to get too cool (less than approximately 105°F, or 41°C) when the turbine is rolling, the oil will build too much of a film, or wedge under the rotor. This wedge will not remain because of the spinning action of the rotor hydraulically pushing it out. Since the oil is thicker and slower moving at lower temperatures, the void under the rotor will not be replaced with oil quickly enough. The rotor falls due to no oil wedge, and then re-climbs the journal, creating another thick wedge. This cycle repeats itself rather rapidly, violently shaking the turbine. This is called an oil whip and is indicated by high radial vibration. Oil whip can be avoided by keeping the oil temperature within the manufacturer's recommended limits. Just as too low of an oil temperature is a problem, too high of a temperature is a problem as well. Too high of an oil temperature allows the oil to thin out. When this happens, the oil wedge will not adequately support the weight of the turbine rotor. The rotor won't bounce as it did with oil whip, but will be too close to the bearing. The result is friction and an additional build-up of heat. The original high temperature will be compounded, and significantly worsened by bearing friction. As long as turbine lube oil temperatures are maintained within the turbine manufacturer's limits, these problems should not occur.
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Turning Gear The turbine turning gear is another important component associated with large steam turbines. Turning gears are also referred to as jacking gears or turning devices. Turning gears are used to slowly rotate the turbine rotor during shutdown, prior to startup, and when the turbine rotor is hot. Normally turning gears rotate the turbine shaft at 3 to 5 RPM. A typical turning gear consists of an electric motor driving a speed reducing gear train. The gear train drives a pinion gear or "clash pinion" as it is often called, that can swing in towards and out away from the "bull gear" mounted on the turbine rotor. The "bull gear" is normally mounted between the turbine and generator couplings. The turning gear components are lubricated with oil from the turbine lube oil system. A pressure switch in the oil line to the turbine prevents starting the turning gear motor unless there is adequate oil pressure to the turbine. Depending on the age and design of the turbine, the turning gear may be operated locally, remotely, manually, and/or automatically. Turning gear operation on many older units was totally manual and performed locally. On newer turbines, turning gears are designed to be operated locally and from the control room. Some turning gear systems are automated. Turbines with automated turning gears are equipped with a device to sense when the turbine shaft has come to rest. This device, commonly called a "zero speed indicator" may be mechanicalhydraulic or electronic. On an automated turning gear, the oil supply valve to the turning gear opens, the turning gear motor starts, and the clash pinion is moved into engagement with the bull gear automatically when a zero speed signal is generated by the zero speed indicator. Regardless of the vintage of the turbine, an ammeter is usually provided to monitor turning gear motor current. A high turning gear motor current could be the result of clearance problems in the turbine that are causing rubbing. Turning gear motor current alternating between a high value and a normal value is indicative of a rub in the turbine.
Rupture Diaphragm The turbine rupture diaphragm is located on the low-pressure turbine exhaust hood. It is a rudimentary device that serves the same function as a safety valve. The rupture diaphragm protects the low-pressure turbine casing from excessive pressure should the circulating water pump fail, or anything happen to pressurize the condenser. Rupture diaphragms are used instead of safety valves because of the tremendous volume of steam they relieve. A safety valve with the relieving capacity of a rupture diaphragm would be very large and expensive. The typical rupture diaphragm has a diaphragm made of lead clamped around its circumference and supported by a grid underneath. When there is a vacuum in the low-
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pressure turbine casing, the diaphragm is pulled downward against the grid. When pressure in the exhaust hood increases above about 5 psig (1.4 bar, or 20 psia), the soft lead diaphragm lifts against a cutting edge and ruptures, thereby relieving the pressure from the condenser and low-pressure turbine. Rupture diaphragms are relatively inexpensive and are relatively easy to replace. However, a properly operated plant should never blow a rupture diaphragm.
Turbine auxiliary systems •
Turbine Lube Oil System
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Turbine Steam (Gland) Seal System
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Turbine Electro-Hydraulic Control (EHC) System
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Turbine Supervisory Instrumentation (TSI) System
Because lubrication of the bearings is critical to turbine operation, backup pumps are provided to maintain oil flow to the bearings in case the primary oil pump fails or is not available due to operating conditions (when the turbine is on turning gear, for example). Another interesting point about the turbine lube oil system is the piping. Most systems use guard piping, where the supply piping is located inside the drain piping. Guarded prevents loss of oil from leaks in the supply piping. A leak in the supply piping could quickly pump the lube oil reservoir dry, preventing the turbine from safely spinning down. Piping is also guarded to reduce fire hazards. If a leak in the supply piping were to spray on hot steam lines, a major fire could result. Placing the supply piping in the drain piping reduces this risk.
Turbine lubes oil system components: •
Lube Oil Reservoir
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Motor (AC and DC) Driven Pumps
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Vapor Extractor
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Lube Oil Coolers
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Lube Oil Filters
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Supply and Return Piping
The turbine lube oil reservoir serves as the suction source for the turbine lube oil pumps. It is sized so that it maintains an adequate suction to the pumps during normal operation. It also has adequate storage capacity to contain the oil when the system is shutdown and all the oil drains to the reservoir.
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The typical turbine lube oil system will have Ac & DC motor driven oil pumps. These pumps are usually submerged suction pumps with the motor mounted on top of the reservoir. The first pump is normally an AC motor-driven pump capable of supplying all of the system's oil requirements during startup, shutdown, and emergency conditions, such as a turbine trip or main shaft-driven oil pump failure. This pump is typically called the auxiliary oil pump. The second pump always uses a direct current (DC) electric motor and is normally called the emergency DC oil pump. This pump serves as the last line of defense against a loss of lube oil event. The emergency oil pump will only supply oil to the bearings and is strictly for coast down protection of the turbine. It is not meant for normal service. The emergency DC oil pump electrical supply originates from the plant battery system. It provides power to the emergency oil pump if plant electrical power fails. The vapor extractor is a blower that draws the oil mist and vapor off the reservoir and maintains a negative pressure at the bearing housing. The negative pressure at the bearing housing prevents oil mist from leaking out around the bearings. This mist and vapor can be explosive, especially in systems where the generator is hydrogen-cooled. The turbine lube oil system typically supplies and receives oil from the generator seal oil system. The oil returning from the seal oil system can contain some hydrogen gas and release it in the turbine lube oil reservoir. Hydrogen gas is extremely explosive. Regardless of whether the generator is hydrogen-cooled or not, the oil mist and vapors in a turbine lube oil system are very flammable, and require the vapor extractor to operate when the oil system is in service. Turbine lube oil systems normally have two 100% capacity oil coolers. These coolers are used to control the temperature of the oil being supplied to the bearings. Normally the coolers have special valve assemblies on the oil side of the cooler that allow swapping coolers without interrupting oil flow to the bearings. Turbine lube oil systems normally have some kind of oil conditioning system to maintain oil quality. The purpose of oil conditioning systems is to remove water and particulate matter from the oil that could score the shaft journal.
Turbine Gland Steam Seal System The steam seal system arrangement shown here is similar to almost all of the steam seal systems found on condensing steam turbines. It consists of two headers; the seal steam header and the steam seal leak-off header. Condensing type steam turbines have to seal against leakage anywhere the rotor penetrates the casing. This means sealing against steam outward leakage on the highpressure end of the turbine and air in-leakage on the low-pressure (condenser) end of the turbine.
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The turbine illustrated here is a single casing type turbine and therefore has only one steam seal on each end. Many of the turbines found in power plants today have multiple casings to house low-pressure and high-pressure sections. The turbine gland seal system for those turbines would be the same, only with more seal steam supplies, and leak-off lines for the additional seals. The steam seal supply system illustrated here is a typical system. The seal steam supply valve provides sealing steam from the main steam system during startup, low loads, and shutdown. Upon low header pressure (typically below 3 psig, or 1.2 bar), the main steam supply valve opens, supplying high-pressure steam to the steam seal header, which directs the steam to the individual labyrinth seals on the turbine shaft. As turbine load increases, leakage from the pressurized sections of the turbine meets or exceeds the requirements of the seal system for the LP section seals. At this time the turbine is said to be "self-sealing." The steam seal supply valve closes and the steam seal unloading valve opens to maintain the proper pressure on the steam seal header. If header pressure should increase too much due to the leakage rate from the pressurized seals exceeding the sealing needs of the LP turbine seals, the excess steam will be relieved to the condenser. The steam seal leak-off system typically consists of piping, gland steam condenser, and a vent blower for the non-condensable gases that are drawn off the steam seal system. The gland steam condenser is a shell and tube type heat exchanger.The purpose of the gland steam condenser is to maintain a slightly negative pressure at the outer portion of the shaft seals to avoid steam flowing out of the seals during normal operation. It is important to avoid steam leakage to the atmosphere around shaft seals not only for "housekeeping" purposes, but more importantly to avoid steam blowing around the bearing housings, which are usually in close proximity to the shaft seals. As previously mentioned, the bearing housings are maintained at a negative pressure by the vapor extractor on the lube oil reservoir. Steam leaking from the shaft seals will be drawn into the lube oil system and result in water contamination of the lube oil system. The air and steam from the outer seal region flows to the gland steam condenser where the steam is condensed by condensate flowing through the tube side of the heat exchanger. A blower mounted on the shell exhausts the air that is pulled in from the outer seal region to atmosphere.
Turbine Electro-Hydraulic Control Oil System EHC systems supply high-pressure oil for positioning the various valves on the turbine. The design and operation of these hydraulic control systems vary with turbine manufacturer, but all provide similar functions.
Turbine Supervisory Instruments Turbines are large, expensive pieces of equipment that operate at high temperatures and speeds. To protect this equipment, instrumentation is provided to monitor various
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operating parameters. These instruments are commonly referred to as the Turbine Supervisory Instrument (TSI) System. Some of the parameters monitored by this system include: •
Turbine rotor speed (rpm)
•
Turbine casing metal temperatures
•
Rotor eccentricity
•
Shaft (bearing) vibration
•
Casing expansion
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Differential expansion
•
Bearing metal temperatures
Turbine speed is one of the most important operating parameters that is monitored and controlled. If turbine speed were not monitored and controlled, the centrifugal forces produced by the rotating elements of the turbine could cause it to come apart during excessive overspend conditions. For this reason, the design of the speed control system is such that it will never allow the turbine to exceed 110% of rated speed following a tripped condition. Turbines casing metal temperatures are monitored so the operator can control how fast the casings are heated or cooled. Because these casings are thick pieces of metal it takes time for temperatures to equalize. If the casings are heated or cooled too rapidly, undesirable thermal stresses will result. This will eventually lead to metal fatigue and shorten the life of the equipment. Rotor eccentricity is monitored when the turbine is on turning gear prior to startup to ensure that the rotor has not bowed. Good operating practice calls for an acceptable level of eccentricity before accelerating the turbine from turning gear speed to operating speed. Failure to do so will result in high vibration and damage to the close steam path seal clearances. Once the rotor rolls off turning gear, the eccentricity monitor switches to another function, such as differential expansion or vibration monitoring. Excessive vibration will cause damage to the bearings and/or other turbine components, such as the internal steam path seals. Because the mass and the material from which the turbine casings and rotor are made are different, they heat up and cool down at different rates. This means they expand at different rates when starting and loading the turbine. This difference in expansion rate must be monitored and controlled because the axial clearances between the stationary and rotating elements of the turbine are very close. Failure to monitor and control differential expansion will result in metal-to-metal contact between the rotating and stationary parts of the turbine, and damage will result. Bearing metal temperatures are also monitored to prevent equipment damage. Rotor bearings support the rotor and maintain its radial alignment. Thrust bearings are used to maintain the axial alignment of the turbine rotor. If the bearings are damaged, the chances of rotating and stationary element damage are substantially increased.
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The instruments in the TSI System not only provide indication, but many of them will annunciate alarms and trip the turbine if unsafe conditions exist, such as high vibration or excessive rotor axial movement.
Lal Pir/PG Turbines Specification Type……………………………………….. .Single Reheat condensing tandem .Two cylinder double flow exhaust MCR………………………………………....362 MW Speed……………………………….………. 3000 rpm Direction of rotation………………............... Clockwise (from GV end) Inlet pressure………………………...............169kg/cm2 Inlet temperature…………….….……………538 C0 (Main steam and reheat) Exhaust pressure ………………….…………692 mmHg No of Extractions………………….………... 8 Blading: HP Turbine…………………Impulse ………1 Stage (Rateau). Reaction………11Stages. IP Turbine………………… Reaction………10 Stages. LP Turbine………… ……...Reaction………6 Stages.
LP Turbine
HP-IP turbine is of combination impulse (Rateau stage) and reaction type while LP turbine is of reaction double flow type. Turbines consist of double
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casing. Inner casing having fixed blades and outer casing serves to minimize heat losses. Labyrinth seals are fixed between outer casing and rotor to minimize steam leakage. A thrust bearing is installed at governor pedestal that absorbs axial thrust. A 2-stage dummy piston is installed at steam inlet zone of HP turbine that is designed to balance the thrust on the blades and thus produce a thrust towards the inlet end of the machine under normal operating conditions. With this arrangement, any floating of the rotor, such as is possible in case of loss of load, can occur towards the exhaust end only thus temporarily increasing the axial running clearance by the amount of clearance in the thrust bearing but maintaining at least the desired minimum clearance at all times. The balance piston labyrinth seals are of the radial clearance type.
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There are 2 no of MSVs and 4 no of GVs in HP turbine while IP turbine is equipped with 2 RSVs and 2 ICVs. The steam enters the high-pressure element through 2 MSV-steam chest assemblies, which are located on each side of the turbine. The chest outlets are connected with HP-IP casing through four inlet sleeves, each connected to its nozzle chamber by a slip joint. Two of these inlet sleeves connections are in the base and two are in the cover. The steam passes through the impulse stage and high- pressure balding forward to the reheater; through two exhaust openings in the outer casing base. The steam returns from the re-heater to the intermediate pressure element through two reheat stop-interceptor valve assemblies, installed in the mezzanine floor in front of the HP pedestal. Two outlets of these valve assemblies are connected to the LP casing bottom. The steam passes through the intermediate pressure element (Reaction balding), which is located on the generator end of the HP-IP turbine. The low pressure turbine is a reaction, double flow type element with steam entering at the center of the blade path and flowing towards an exhaust opening at each end, hence downward into a combined exhaust to the condenser. A turning gear is installed in-between LP turbine and generator. During start up and shut down, turbine is put on turning gear that rotates at very slowly to avoid eccentricity. HP and IP turbines are coupled with LP turbine and LP turbine with generator by means of rigid couplings.
Turbine Protections To assure safe operation of plant, different protection systems have been employed all over the plant where necessary. Turbine protections are designed to protect the turbine automatically during any emergency conditions. The protections are as follows:-
1. Over speed trip In case of emergency, for example when the generator suddenly is disconnected from the power system, the turbine may over speed. The first response to an over peed occurs in the turbine control system. The turbine governor senses the increase in speed and immediately begins to close the control valves to decrease the steam flow to the turbine. Ultimate result is the turbine trip. Mechanical trip mechanism is also available that trips the turbine when its speed reaches 10% above its normal speed (3300 rpm).Second defense line is the Electrical over speed, trip value is 111% (3330 rpm)
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2. Low Lube oil Pressure protection The function of the lube oil protection system is to initiate action if a low lubricating oil pressure occurs. If the pressure decreases to a minimum value (0.56 kg/ cm 2), the turbine will trip.
3. Thrust bearing protection This system protects the turbine from excessive thrust. Thrust bearing is the turbine component that maintains the axial thrust of the shaft and helps to absorb the thrust of the turbine. Following thrust bearing wear pressures produce upon rotor axial displacement of; 2.1 kg/cm2 corresponds to a rotor displacement of 0.9 mm (alarm). 5.6 kg/cm2 corresponds to a rotor displacement of 1.0 mm (trip) 4.
Low vacuum protection
Turbine trips if a low vacuum condition exists in the condenser. Following are the values for alarm and trip: 5.
At 615 mm of Hg (alarm) At 500 mm of Hg (trip).
5. High vibration Turbine also trips due to high vibration on turbine bearings.
6. Hand trip/push button trip During emergency conditions, turbine may also be tripped by means of a hand lever located on the turbine or by means of a push button located near CRTs in control room.
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