Lubrication Fundamentals 2011 Part 10 ppsx

Figure 12.2 Reaction turbine with one velocity-compounded impulse stage. The first stage of this turbine is similar to the first, velocity-compounded stage of Figure 12.1. However, in the reaction blading of this turbine, both pressure and velocity decrease as the steam flows through the blades. The graph at the bottom shows the changes in pressure and velocity through the various stages. Copyright 2001 by Exxon Mobil Corporation. All Rights Reserved. Figure 12.3 Simple power plant cycle: the working fluid, here steam and water, travels a closed loop in the typical power plant cycle. steam velocity, this arrangement may be referred to as a 50% reaction turbine.) The moving blades form the walls of moving nozzles that are designed to permit further expansion of the steam and to partially reverse the direction of steam flow, which produces the reaction on the blades. The distinguishing characteristic of the reaction turbine is that a pressure drop occurs across both the moving and stationary nozzles, or blades (Figure 12.2). Nor- mally reaction turbines employ a considerable number of rows of moving and stationary nozzles through which steam flows as its initial pressure is reduced to exhaust pressure. The pressure drop across each row of nozzles is, therefore, relatively small, and steam velocities are correspondingly moderate, permitting medium rotating speeds. Reaction stages are usually preceded by an initial velocity-compounded impulse stage, as in Figure 12.2, in which a relatively large pressure drop takes place. This results in a shorter, less costly turbine. In the radial flow reaction, or Ljungstrom, turbine, the steam does not flow axially through alternating rows of fixed and moving blades; rather, it flows radially through several rows of reaction blades. Alternate rows of blades move in opposite direction. They are fastened to two independent shafts that operate in opposite directions, each shaft driving a load. After expansion in the turbine, the steam usually exhausts to a condenser, where it is condensed to provide a source of clean water for boiler feed. This simple cycle (Figure 12.3) forms the basis on which most steam power plants operate. I. STEAM TURBINE OPERATION Steam turbines are made in a number of different arrangements to suit the needs of various power plant or industrial installations. Turbines up to 40–60 MW capacity are generally Copyright 2001 by Exxon Mobil Corporation. All Rights Reserved. single-cylinder machines. (The term cylinders, chests, casings, and shells are used inter- changeably in this industry.) Larger units ranging in size up to 1250 MW are usually of compound type; that is, the steam is partially expanded in one cylinder then passed to one or more additional cylinders where expansion is completed. The simple cycle shown in Figure 12.3 is water to steam to power generation, and steam to water. This forms the basis on which most steam power plants operate. A. Single-Cylinder Turbines Single-cylinder turbines are of either the condensing or backpressure (noncondensing) type. These basic types and some of their subclassifications are shown in Figure 12.4. When the steam from a turbine exhausts to a condenser, the condenser serves two purposes: 1. By maintaining a vacuum at the turbine exhaust, it increases the pressure range through which the steam expands. In this way, it materially increases the effi- ciency of power generation. 2. It causes the steam to condense, thus providing clean water for the boilers to reconvert into steam. Figure 12.4 Typical single-cylinder turbine types: in comparison to backpressure turbines, con- densing turbines must increase more in size toward the exhaust end to handle the larger volume of low pressure steam. Copyright 2001 by Exxon Mobil Corporation. All Rights Reserved. Industrial plants frequently require steam at low to moderate pressures for process use. One of the more economical ways of generating this steam is with a combined-cycle plant, where high pressure steam is used to power equipment such as generators and the exhaust steam from this equipment is used for heating or other services. The steam is generated at high pressure and, after expansion through the turbine to the pressure desired for process use, it is delivered to the process application. This permits power to be generated by the turbine without appreciably affecting the value of the steam for process use. It may be done with a backpressure turbine designed to exhaust all the steam against the pressure required for process use, or it may be done with an automatic extraction turbine in which part of the steam is withdrawn for process use at an intermediate stage (or stages) of the turbine and the remainder of the steam exhausted to a condenser. Such a turbine requires special governors and valves to maintain constant pressure of the exhausted steam and constant turbine speed under varying turbine load and extraction demands. Steam can be also extracted without control from various stages of a turbine to heat boiler feedwater (regenerative heating). Such turbines are called uncontrolled extraction turbines, since the pressure at the extraction points varies with the load on the turbine. To obtain higher efficiency, large turbines (called reheat turbines) are arranged so that after expanding partway, the steam is withdrawn, returned to the boiler, and reheated to approximately its initial temperature. It is then returned to the turbine for expansion through the final turbine stages to exhaust pressure. High pressure noncondensing turbines have been added to many moderate pressure installations to increase capacity and improve efficiency. In such installations, high pres- sure boilers are installed to supply steam to the noncondensing turbines, which are designed to exhaust at the pressure of the original boilers and supply steam to the original turbines. The high pressure turbines are called superposed or topping units. Where low pressure steam is available from process work, it can be used to generate power by admitting it to an intermediate stage of a turbine designed for the purpose and expanding it to condenser pressure. Such a machine is a mixed-pressure turbine, and is another form of combined-cycle operation. Compound turbines have at least two cylinders or casings, a high pressure one and a low pressure one. To handle large volumes of low pressure steam, the low pressure cylinder is frequently of the double-flow type. Very large turbines may have an intermedi- ate pressure cylinder, and two, three, or even four double-flow, low pressure cylinders. The cylinders may be in line using a single shaft, which is called tandem compound, or in parallel groups with two or more shafts, which is called cross compound. Reheat between the high and intermediate pressure stages may be employed in large turbines. Steam may be returned to the boiler twice for reheating. Some of these arrangements shown diagram- matically in Figure 12.5. II. TURBINE CONTROL SYSTEMS Although the trend is toward electronic speed sensing and control, all steam turbines are provided with at least two independent governors that operate to control the flow of steam. One of these operates to shut off the steam supply if the turbine speed should exceed a predetermined maximum. It is often referred to as an emergency trip. On overspeed, it closes the main steam valve, cutting off steam from the boiler to the turbine. The other, or main, governor may operate to maintain practically constant speed, or it may be designed for variable speed operation under the control of some outside influence. Extraction, mixed Copyright 2001 by Exxon Mobil Corporation. All Rights Reserved. Although most new turbine installations use electronic speed sensing and control, a commonly used speed-sensitive element is the centrifugal, or flyball, governor (Figure 12.6). Weights that are pivoted on opposite sides of a spindle and are revolving with it are moved outward by centrifugal force against a spring when the turbine speed increases, and inward by spring action as the turbine slows down. This action may operate the steam admission valve directly through a mechanical linkage, as shown, or it may operate the pilot valve of a hydraulic system, which admits and releases oil to opposite sides of a power piston, or on one side of a spring-loaded piston. Movement of the power piston opens or closes steam valves to control turbine speed. Moderate-sized and large high speed turbines are provided with a double relay hydraulic system to further boost the force of the centrifugal governor and to increase the speed with which the system responds to speed changes. A second type of speed-sensitive element is the oil impeller. Oil from a shaft-driven pump flows through a control valve to the space surrounding the governor impeller. The Figure 12.6 Mechanical speed governor. A simple arrangement such as this using a flyball governor is suitable for many small turbines. Copyright 2001 by Exxon Mobil Corporation. All Rights Reserved. impeller, mounted on the turbine shaft, consists of a hollow cylindrical body with a series of tubes extended radially inward. As the oil flows inward through the tubes it is opposed by centrifugal force and a pressure is built up that varies with the square of the turbine speed. This pressure is applied to spring-loaded bellows, which positions a pilot valve. The pilot valve, in turn, controls the flow to a hydraulic circuit that operates the steam control valves. Newer turbines are equipped with electrical or electronic speed-sensitive devices. Signals from these devices, along with signals derived from load, initial steam pressure, and other variables, are fed to computer, which compares them and sends the appropriate signals to hydraulic servovalves to adjust the steam control valves. As indicated, the linkage between the speed-sensitive element and the steam control valves may be anything from a simple lever to an extensive hydraulic system controlled by a computer. In small turbines, the flow of steam is controlled by a simple valve, usually of the balanced type, to reduce the operating force required. In large units, a valve for each of several groups of nozzles controls steam flow. The opening or closing of a valve cuts in or out a group of nozzles. The number of open valves, and thus the number of nozzle groups in use, is varied according to the load. The valves may be operated by a barlift arrangement, by cams, or by individual hydraulic cylinders. Additional control valves, called intercept valves, are required on reheat turbines. These are placed close to the intermediate, or reheat, cylinder and are closed by a governor system if the turbine starts to speed up as a result of a sudden large load reduction. This design is intended to prevent large volumes of high energy steam found in the piping of the high pressure turbine exhaust, the reheat boiler, and the intermediate pressure turbine from continuing to flow and possibly cause overspeeding and emergency tripping of the turbine. In older turbines, the intercept valves were controlled by a separate governor system, but the newer machines have the intercept valves operated by the main hydraulic control system. As an additional safety measure, intercept valves are preceded by stop valves, which are actuated by the main emergency overspeed governor (or other speed- sensing control system). Automatic extraction and backpressure turbines are provided with governors ar- ranged to maintain constant extraction or exhaust pressure irrespective of load (within the capacity of the turbine). The pressure-sensitive element consists of a pressure transducer, and its response to pressure changes is communicated through the control system to the valves that control steam extraction, and to the speed governor that controls admission of steam to the turbine. On automatic extraction turbines, the action of the pressure- and speed-responsive elements is coordinated so that turbine speed is maintained. This may not be the case for backpressure turbines. III. LUBRICATED COMPONENTS The lubrication requirements of steam turbines can be considered in terms of the parts that must be lubricated, the type of application system, the factors affecting lubrication, and the lubricant characteristics required to satisfy these requirements. A. Lubricated Parts The main lubricated parts of steam turbines are the bearings, both journal and thrust. Depending on the type of installation, a hydraulic control system, oil shaft seals, gears, flexible couplings, and turning gear may also require lubrication. Copyright 2001 by Exxon Mobil Corporation. All Rights Reserved. 1. Journal Bearings The rotor of a single-cylinder steam turbine, or of each casing of a compound turbine, is supported by two hydrodynamic journal bearings. These journal bearings are located at the ends of the rotor, outside the cylinder. In some designs, there may be one large journal bearing between the casings that supports both turbine rotors or a turbine and generator rotor (rigid coupling) instead of a separate bearing at the ends of each casing. Clearances between the shaft and shaft seals and between the blading and the cylinder, are extremely small, so to maintain the shaft in its original position and avoid damage to shaft seals or blading, the bearings must be accurately aligned and must run without any appreciable wear. Primarily, the loads imposed on the bearings are due to the weight of the rotor assembly. The bearings are conservatively proportioned and thus pressures on them are moderate. Horizontally split shells lined with tin-based babbitt metal are usually used. The bearings are enclosed in housings and supported on spherical seats or flexible plates to reduce any angular misalignment. The passages and grooves in turbine bearings are sized to permit the flow of consider- ably more oil than is required for lubrication alone. The additional oil flow is required to remove frictional heat and the heat conducted to the bearing along the shaft from the hot parts of the turbine. The flow of oil must be sufficient to cool the bearing enough to maintain it at a proper operating temperature. In most turbines, the temperature of the oil leaving the bearings is on the order of 140–160ЊF (60–71ЊC), but in special cases it may exceed 180ЊF (82ЊC). When a turbine is used to drive a generator, the generator bearings are similar in design to the turbine bearings and are normally supplied from the same system. Large turbines are now frequently provided with ‘‘oil lifts’’ (jacking oil) in the journal bearings to reduce the possibility of damage to the bearings during starting and stopping, and to reduce metal-to-metal contact during turning gear operation. Oil under high pressure from a positive displacement pump is delivered to recesses in the bottoms of the bearings. The high pressure oil lifts the shaft and floats it on a film of oil until the shaft speed is high enough to create a normal hydrodynamic film. For a shaft that is rotated for several hours or days to prevent rotor-sag, the oil lift is also required after turbine shutdown. A phenomenon that occurs in relatively lightly loaded, high speed journal bearings, such as turbine bearings, is known as ‘‘oil whip’’ or ‘‘oil film whirl.’’ The center of the journal of a hydrodynamic bearing ordinarily assumes a stable, eccentric position in the bearing that is determined by load, speed, and oil viscosity. Under light load and high speed, the stable position closely approaches the center of the bearing. There is a tendency, however, for the journal center to move in a more or less circular path about the stable position in a self-excited vibratory motion having a frequency of something less than half the shaft speed. In certain cases, such as some of the relatively lightweight, high pressure rotors of compound turbines that require large-diameter journals to transmit the torque, this whirling has been troublesome and has required the use of bearings designed especially to suppress oil whip. Bearings designed to suppress oil whip are available in several types. Among the common types are the pressure or pressure pad bearings (Figure 12.7), the three-lobed bearing (Figure 12.8), and the tilting pad antiwhip bearing (Figure 12.9). The pressure pad bearing suppresses oil whip because oil carried into the wide groove increases in Copyright 2001 by Exxon Mobil Corporation. All Rights Reserved. pressure when it reaches the dam at the end. This increase in pressure forces the journal downward into a more eccentric position that is more resistant to oil whip. The other types illustrated depend on the multiple oil films formed to preload the journal and minimize the tendency to whip. 2. Thrust Bearings Theoretically, in impulse turbines, the drop in steam pressure occurs almost entirely in the stationary nozzles. The steam pressures on opposite sides of the moving blades are, therefore, approximately equal, and there is little tendency for the steam to exert a thrust in the axial direction. In actual turbines, this ideal is not fully realized, and there is always a thrust tending to displace the rotor. In reaction turbines, a considerable drop in steam pressure occurs across each row of moving blades. Since the pressure at the entering side of each of the many rows of moving blades is higher than the pressure at the leaving side, the steam exerts a considerable axial thrust toward the exhaust end. Also, when rotors are stepped up in diameter, the unbalanced steam pressure acting on annular areas thus created adds to the thrust. Usually the total thrust is balanced by means of dummy, or balancing, pistons on which the steam exerts a pressure in the opposite direction to the thrust. In double-flow elements of com- pound turbines, steam flows from the center to both ends, ensuring that thrust is well balanced. Regardless of the type of turbine, thrust bearings are always provided on each shaft to take axial thrust and, thus, hold the rotor in correct axial position with respect to the stationary parts. Although thrust caused by the flow of steam is usually toward the low pressure end, means are always provided to prevent axial movement of the rotor in either direction. The thrust bearings of small turbines may be babbitt-faced ends on the journal bearings, or rolling element bearings of a type designed to carry thrust loads. Medium- Figure 12.10 Combined journal and tilting pad thrust bearing. A rigid collar on the shaft is held centered between the stationary thrust ring and a second stationary thrust ring (not shown) by two rows of titling pads. Copyright 2001 by Exxon Mobil Corporation. All Rights Reserved. Figure 12.11 Tapered land thrust bearing and plain journal bearing. The thrust bearing consists of a collar on the shaft and two stationary bearing rings, one on each side of the collar. The babbitted thrust faces of the bearing rings are cut into sectors by radial grooves. About 80% of each sector is beveled to the leading radial groove, to permit the formation of wedge oil films. The unbeveled portions of the sectors absorb the thrust load when speed is too low to form hydrodynamic films. sized and large turbines are always equipped with thrust bearings of the tilting pad (Figure 12.10), or tapered land (Figure 12.11) type. 3. Hydraulic Control Systems As discussed earlier, medium-sized and large turbines have hydraulic control systems to transmit the motion of the speed or pressure-sensitive elements to the steam control valves. Two general approaches are used for these systems. In mechanical hydraulic control systems, the operating pressure is comparatively low (Ͻ150 psi), and oil from the bearing lubrication system may be used safely as the hydraulic fluid. Separate pumps are provided to supply the hydraulic requirement. An emergency tripping device is provided to shut down the turbine if there is any failure in the hydraulic system. Larger turbines now being installed are equipped with electrohydraulic control sys- tems. To provide the rapid response needed for control of these units, the hydraulic systems operate at relatively high pressures, typically in the range of 1500–2000 psi. Copyright 2001 by Exxon Mobil Corporation. All Rights Reserved. The systems consist of an independent reservoir and two separate and independent pumping systems. The large fluid flow required for rapid response to sudden changes in load is usually provided by gas-charged accumulators. The critical nature of the servovalves used in these systems requires that careful attention be paid to the filtration of the fluid, and strict limits on particulate contamination usually are observed. The need for precise control also calls for one use of both heaters and coolers to maintain the temperature of the fluid and, thus, its viscosity, in a narrow range. Since a leak or a break in a hydraulic line could result in a fire if the high pressure fluid sprayed onto hot steam piping or valves, fire-resistant hydraulic fluids are widely used in these systems. 4. Oil Shaft Seals for Hydrogen-Cooled Generators Because it is a more effective coolant than air, hydrogen is commonly used to cool medium- sized and large generators. Shaft-mounted blowers circulate the gas through rotor and stator passages, then through liquid-cooled hydrogen coolers. Gas pressures up to 60 psi (413 kPa) are used. A further development, which has permitted increases of generator ratings over hydrogen cooling alone, is the direct liquid cooling of stator windings. Some liquid systems use transformer oil while others use water. Even with water-cooled stators, the interior of the generator is still filled with hydrogen. The main connection between type of cooling and turbine lubrication is that when hydrogen is used for cooling, some of the oil is exposed to the hydrogen. Oil shaft seals (Figure 12.12) are used to prevent the escape of the hydrogen. Turbine oil for these seals may be supplied from an essentially separate system having its own reservoir, pumps, and so forth, or may be supplied directly from the main turbine lubricating system. In either case, before entering the reservoir, oil returning from the seals must be passed through a special tank to remove any traces of hydrogen. Otherwise hydrogen could accumulate in the reservoir and form an explosive mixture with air. In addition, the main turbine oil reservoir of all units driving hydrogen-cooled generators must be equipped with a vapor extractor to remove any traces of hydrogen that may be carried back by the sealing oil or the oil from the generator bearings. 5. Gear Drives Efficient turbine speed is often higher than the operating speed of the machine being driven. This may be the case, for example, when a turbine drives a direct current generator, paper machine drives, centrifugal pumps, or other industrial machines. It is also the case when a turbine is used for ship propulsion. In these applications, reduction gears are used to connect the turbine to the driven unit. Reduction gears used with moderate-sized and large turbines are usually of the precision-cut, double-helical type. Double reduction gear sets are required with marine propulsion turbines, and epicyclic reduction gears are sometimes used instead of conven- tional gear sets. Usually, the gear sets are enclosed in a separate oil-tight casing and are connected to the turbine and the driven machine through flexible couplings. Small ma- chines may have the gear housing integral with the turbine housing and the pinion on the turbine shaft. Reduction gears may have a circulation system that is entirely separate from the turbine system, or circulation may be supplied from the turbine system. In the latter case, a separate pump (or pumps), is provided for the gears. Some older small-geared turbines have ring-oiled turbine bearings and splash-lubricated gears. Copyright 2001 by Exxon Mobil Corporation. All Rights Reserved. [...]... large turbines are equipped with circulation systems that supply oil to all parts of the unit requiring lubrication Separate circulation systems may be provided for the seal oil for hydrogen-cooled turbogenerators and for the hydraulic control systems C Factors Affecting Lubrication Steam turbines in themselves do not represent particularly severe service for petroleumbased lubricating oils Because of... higher viscosity oils or oils formulated with antiwear additives may be required to provide satisfactory lubrication of the gears Oils of ISO viscosity grade 68 (61.2–74.8 cSt at 40ЊC) are used in many of these systems Oils of viscosity grade 100 (90– 110 cSt at 40ЊC) are also used in some machines, particularly marine propulsion turbines With some geared turbines, the oil is passed through a cooler immediately... Figure 13.12 Bulb turbine installation II LUBRICATED PARTS The main parts of hydraulic turbines requiring lubrication are the turbine and generator bearings, the guide vane bearings, the control valve, governor, and control system, and the compressors A Turbine and Generator Bearings Horizontal shaft machines require journal bearings to support the rotating parts, including the generator armature With the... noted earlier, steam turbine lubrication systems are conservatively designed Bearing loads are moderate Under these conditions, mineral oil lubricants of the correct viscosity normally provide adequate load-carrying ability However, in turbines not equipped with oil lifts, boundary lubrication conditions occur in the bearings during starting and stopping Under boundary lubrication conditions, some... optimum improvement in the particular base oils used Oxidation life of steam turbine oils is frequently specified in terms of the turbine oil stability test (TOST: ASTM D 943; see Chapter 3) Manufacturer’s specifications typically call for a test life to reach a neutralization number of 2.0 of 100 0 h minimum Commercial turbine oils of ISO VG 32 typically run from 4000 h to over 10, 000 h Higher viscosity... cause the formation of common red rust and also black rust, similar in appearance to pipe scale Rusting may occur both on parts covered by oil and on parts above the oil level In either case, in addition to damage to the metal surfaces, rusting is harmful for a number of reasons Particles of rust in the oil tend to stabilize emulsions and foam and to act as catalysts that increase the rate of oil oxidation... long life 1 Viscosity In direct-connected steam turbines, the main lubrication requirement is for the journal and thrust bearings Higher viscosity oils provide a greater margin of safety in these bearings, but at the same time, increase pumping losses and friction losses due to shearing of the lubricant films In high speed machines, particularly, the latter can become an important cause of power loss... long service life, oils must be carefully formulated for the specific conditions encountered in steam turbine lubrication systems 1 Circulation and Heating in the Presence of Air The temperature of the oil in steam turbine systems is raised both by the frictional heat generated in the lubricated parts and by heat conducted along the shaft from the rotor As the oil flows through the system, it is broken... contaminant that is most prevalent in steam turbine lubrication systems Common sources of water contamination are as follows: 1 2 3 4 Steam from leaking shaft seals or the shaft seals of turbine-driven pumps Condensation from humid air in the oil reservoir and bearing pedestals Water leaks in oil coolers Steam leaks in oil heating elements (where used) The lubrication systems of turbines that operate intermittently... the addition of the extreme pressure properties; thus they may be used throughout the turbine lubrication system E Oxidation Stability The most important characteristic of turbine oils from the standpoint of long service life is their ability to resist oxidation under the conditions encountered in the turbine lubrication system Resistance to oxidation is important from the standpoint of retention of . COMPONENTS The lubrication requirements of steam turbines can be considered in terms of the parts that must be lubricated, the type of application system, the factors affecting lubrication, and. may occur both on parts covered by oil and on parts above the oil level. In either case, in addition to damage to the metal surfaces, rusting is harmful for a number of reasons. Particles of rust. provide satisfactory lubrication of the gears. Oils of ISO viscosity grade 68 (61.2–74.8 cSt at 40ЊC) are used in many of these systems. Oils of viscosity grade 100 (90 – 110 cSt at 40ЊC) are also