ESET 223 Lecture 3 Other types of Turbines and Maintenance.pdf

ESET-223 Power Engg. & HVAC School of Engineering Technology & Applied Science (SETAS) Lecture 3 Other Steam Turbines & Maintenance Centennial College - SDRE: ESET 223 Week 4 1 Difference between Impulse and Reaction Steam Turbines Basis of Difference Impulse Turbine Reaction Turbine The type o...

ESET-223 Power Engg. & HVAC School of Engineering Technology & Applied Science (SETAS) Lecture 3 Other Steam Turbines & Maintenance Centennial College - SDRE: ESET 223 Week 4 1 Difference between Impulse and Reaction Steam Turbines Basis of Difference Impulse Turbine Reaction Turbine The type of turbine in which only The type of turbine in which both kinetic kinetic energy of steam (impulse energy and pressure energy of steam are Definition force) is used to rotate the turbine is used to turn the turbine is called the known as impulse turbine. reaction turbine. In an impulse turbine, the steam flows In reaction turbine, the steam is guided through a nozzle, then strikes to the by the guide blades (fixed blades) to Steam flow blades of the turbine. flow over the turbine. In impulse turbine, an impulsive force In reaction turbine, a reaction force on Force on blades rotates the turbine. the blades is rotation the turbine. In impulse turbine, the pressure of In the reaction turbine, the pressure of Pressure of steam while steam remains unchanged. steam continuously decreases when it flowing over moving blades flows over the blades. In impulse turbine, the pressure of In reaction turbine, the pressure of Decrease in pressure of steam is reduced in the nozzle before steam is decreasing while flowing over steam entering the turbine. the blades. In impulse turbine, all the pressure of In reaction turbine, there is no change in Change in pressure of steam steam is converted into kinetic energy the pressure of steam before entering before striking the turbine blades. the turbine blades. Since the velocity of steam is high, But reaction turbine speed is much speed is high in impulse turbine. lower than impulse turbine because steam flow steam velocity is lower in reaction turbine. The blades of an impulse turbine are The blades of a reaction turbine are Blades profile of symmetrical profile. asymmetrical, having aero foil profile. For the same power output, the size The reaction turbine have relatively large Turbine size of impulse turbine is smaller. size for the same power output. Centennial College - SDRE: ESET 223 2 Difference between Impulse and Reaction Steam Turbines Impulse Steam Turbines Reaction Steam Turbines 3 Centennial College - SDRE: ESET 223 Impulse-Reaction Steam Turbines • As the name implies this type of turbine utilizes the principle of the combination of impulse and reaction both. • There are a number of rows of moving blades attached to the rotor and an equal number of fixed blades attached to the casing. • Steam velocity determine the amount of work performed. 4 Centennial College - SDRE: ESET 223 Impulse-Reaction Steam Turbines • The fixed blades corresponds to nozzles mentioned in connection with the impulse turbine. • Due to the row of fixed blades at the entrance, instead of the nozzles, steam is admitted for the whole circumference, and hence there is all-round or complete admission. • Passing through the first row of fixed blades, the steam undergoes a small drop in pressure and hence its velocity somewhat increases. Centennial College - SDRE: ESET 223 5 Impulse-Reaction Steam Turbines • After this, it enters the first row of moving blades and just as in the impulse turbine, it suffers a change in direction and therefore in momentum. • This momentum gives rise to an impulse on the blades. • The passage of the moving blades is so designed (converging) that there is a small drop in pressure of steam in the moving blades which results in an increase in kinetic energy of steam. Centennial College - SDRE: ESET 223 6 Impulse-Reaction Steam Turbines • This kinetic energy gives rise to reaction in the direction opposite to that of added velocity. • Thus, the gross propelling force or driving force is the vector sum of impulse and reaction forces. • Commonly, this type of turbine is called Reaction Turbine. • there is a gradual drop in pressure in both moving blades and fixed blades. 7 Centennial College - SDRE: ESET 223 Special-Purpose Steam Turbines • Special-purpose steam turbines may be required in plants with special generating or process needs. mixed-pressure steam turbines double-flow steam turbines Extraction and Bleed Steam Turbines • Steam turbines designed for special needs are used where standard steam turbines would not work. Centennial College - SDRE: ESET 223 8 Special-Purpose Steam Turbines Mixed - Pressure Steam Turbine • A mixed-pressure steam turbine is used in plants that have an abundance of low-pressure steam. • The steam turbine is designed to operate by using lowpressure steam. • High-pressure steam is used only when plant demands require it. • For example. a plant that has three boilers, two boilers at 200 psi MAWP and one boiler at 400 psi MAWP, is a candidate for a mixed-pressure steam turbine. 9 Centennial College - SDRE: ESET 223 Special-Purpose Steam Turbines Double - Flow Steam Turbine • A double-flow steam turbine is used to balance the axial thrust on the rotor. • Steam is admitted in the center of the casing and flows in two directions • to equalize the axial thrust on the rotor. 10 Centennial College - SDRE: ESET 223 Special-Purpose Steam Turbines Extraction and Bleed Steam Turbines • Extraction and bleed steam turbines are special-purpose steam turbines used to capture the heat in steam for use in process or feedwater heating rather than being sent to the condenser. • Extraction steam is steam withdrawn from a steam turbine that is under pressure control. • Bleed steam is steam withdrawn from a stage of the steam turbine without pressure control. 11 Centennial College - SDRE: ESET 223 Special-Purpose Steam Turbines Extraction Steam Turbines • Extraction steam can be taken from one or more pressure stages of the steam turbine. • The flow is controlled by a positioning control that actuates a servo-motor connected to a grid extraction valve and the governor system controlling the steam turbine speed. • Extraction steam at various pressures is used for all types of steam process loads. Centennial College - SDRE: ESET 223 12 Special-Purpose Steam Turbines Bleed Steam Turbines • Bleed steam is used for heating feedwater or constant load use, such as process work in a paper mill. • Bleed steam can also be taken from one or more stages of the steam turbine for heating feedwater. 13 Centennial College - SDRE: ESET 223 Maintenance of Turbine Lubrication Systems • The lubrication system is a very important part of steam turbine operation. The steam turbine rotor is supported on a film of lubricating oil to prevent wear from friction. • Lubricating Oil • Because of the high operating speeds of the steam turbine, the proper lubricating oil (free of foreign matter) must be used. • Lubricating oil must have the proper viscosity to flow properly and handle high temperatures without breaking down. • Steam turbine manufacturers recommend specific lubricating oils that have been tested and meet operating requirements. 14 Centennial College - SDRE: ESET 223 Lubrication Systems Ring Oilers: Ring oilers are used on small auxiliary steam turbines. Ring oilers consist of a loose-fitting ring that revolves in a small oil reservoir. This Supplies lubricating oil to bearing grooves for even distribution over bearing surfaces. A ring oiler is a simple device, consisting of a large metal ring placed around a horizontal shaft, adjacent to a bearing. An oil sump is underneath this shaft and the ring is large enough to dip into the oil. As the shaft rotates, the ring is carried round with it. The rotating ring in turn picks up some oil and deposits it onto the shaft, from where it flows sideways and lubricates the bearings. Centennial College - SDRE: ESET 223 15 Lubrication Systems Gravity-Fed Lubrication Systems • Gravity-fed lubrication systems use oil cups or reservoirs to feed lubricating oil to the bearings on steam turbines. • Oil cups can also be used on small auxiliary steam turbines. Oil cups are mounted on the top of bearings and can be adjusted for rate of feed of lubricating oil to the bearings. Oil cups can also be mounted on the side of bearings with a cap that lifts up to show the amount of lubricating oil in the cup. 16 Centennial College - SDRE: ESET 223 Lubrication Systems Force-Fed Lubrication Systems: • Force-fed lubrication systems are used on large steam turbines. • Lubricating oil is supplied to the steam turbine bearings and hydraulic governor system that controls the steam turbine speed. 17 Centennial College - SDRE: ESET 223 Lubrication Systems Force-Fed Lubrication Systems: • The lubricating oil must be cooled to prevent it from breaking down and losing its lubricating ability. • In addition, it must be clean and free of impurities that could plug up an oil line or cause a hearing to become scratched. • It must also be free of water, which emulsion or sludge to develop. Centennial College - SDRE: ESET 223 18 Lubrication Systems Force-Fed Lubrication Systems: • The auxiliary lubricating oil pump must be started so that lubricating oil is supplied to the steam turbine bearing and governor before the steam turbine is started. • The main lubricating oil pump is driven by the shaft. • As the steam turbine reaches operating speed, the auxiliary lubricating oil pump cuts out. • The main lubricating oil pump delivers oil through the duplex oil strainer to the lubricating oil cooler. Before it reaches the lubricating oil cooler, a small amount of oil is taken from the line and passes to a centrifuge. Centennial College - SDRE: ESET 223 19 Lubrication Systems Force-Fed Lubrication Systems: • The centrifuge removes dirt and water from the lubricating oil, discharging the clean oil to the oil reservoir, and the water and dirt to waste. • A portion of lubricating oil in the lubrication system is constantly being cleaned. • The duplex oil strainer should be cleaned at least once each shift with the steam turbine in operation. • When the steam turbine is taken off-line, an oil pressure signal switch automatically starts up the auxiliary lubricating oil pump to circulate lubricating oil to the steam turbine bearings until properly cooled. Centennial College - SDRE: ESET 223 20 Lubrication Systems Lubricating Oil Reconditioning • From the centrifuge, the lubricating oil is pumped to the clean lubricating oil storage tank until it is ready to be put back in service. 21 Centennial College - SDRE: ESET 223 Lubrication Systems Lubricating Oil Coolers • Lubricating oil coolers are commonly shell-and-tube coolers. • Lubricating oil passes through the tubes while the cooling water discharges out the top and enters the bottom of the shell. • Lubricating oil coolers should be removed from service, dismantled, and thoroughly cleaned at regular intervals. • If necessary, the tube bundle should be removed and cleaning solution should be used to remove scale. 22 Centennial College - SDRE: ESET 223 Governors and Overspeed Trip • Governors maintain the speed of rotation of steam turbine as constant. • The steam turbine requires an automatic overspeed tripping device also. • As the steam turbine rotor (shaft) rotates, it builds up centrifugal force, which pulls the blades or buckets away from the shaft. • Centripetal force holds the buckets or blades on the shaft. 23 Centennial College - SDRE: ESET 223 Governors and Overspeed Trip • Centripetal force is exerted inward toward the shaft opposite centrifugal force. • If the centrifugal force exceeds the centripetal force, the steam turbine will destroy itself. • Governor and overspeed trip must be tested as required to ensure proper operation. Types of Governors The two types of governors are •mechanical •Hydraulic 24 Centennial College - SDRE: ESET 223 Governors Mechanical Governors: Mechanical governors commonly consist of a speed-sensing element and linkage that control the governor valve by the force acting on the speed sensing element. • Note: Over speed trips are designed to trip at 10% over rated RPM. For example, a steam turbine rated for 3600 RPM trips at 3600 + 10%, or 3960 RPM. 25 Source: Wikipedia Centennial College - SDRE: ESET 223 Governors and Overspeed Trip Operation of Mechanical Overspeed Trips • A bolt is mounted in the shaft and opposed by an adjustable spring. • It remains in one position until the shaft speed reaches the point where the centrifugal force on the bolt overcomes the tension of the spring. • The bolt, as it moves outward, trips a latch device that releases a butterfly valve. The butterfly valve shuts off the steam supply. Centennial College - SDRE: ESET 223 26 Governors Hydraulic Governors: Hydraulic governors are very similar to mechanical governors and consist of a speed-sensing element and speed changer. However, instead linkage hydraulic governors have a servomotor, which amplifies the force to actuate admission valves. The basic Westinghouse oil-relay governor has a closed hydraulic system, using a single source of oil for governing and steam turbine lubrication. The essential elements of this system are: •oil pump, •governor weights, •governing oil valve, and •servo-motor Centennial College - SDRE: ESET 223 27 Noncondensing and Condensing Steam Turbines Noncondensing Steam Turbines • A noncondensing steam turbine is a steam turbine that exhausts at atmospheric pressure or above. • Noncondensing steam turbines are normally used in steam plants that require large amounts of low-pressure process or heating steam. • A paper mill that heats a considerable amount of water for process uses a noncondensing steam turbine. • In addition, noncondensing steam turbines have no condensate returns. 28 Centennial College - SDRE: ESET 223 Noncondensing and Condensing Steam Turbines Noncondensing Steam Turbines • The exhaust steam from small, noncondensing steam turbines, is sometimes used to heat feedwater in the open feedwater heater. • Noncondensing steam turbines must use a large amount of the available exhaust steam to maintain maximum efficiency. • If the exhaust steam is not used and allowed to go to waste into the atmosphere, the steam turbine would have a very low thermal efficiency and would waste a considerable amount of energy. Centennial College - SDRE: ESET 223 29 Noncondensing and Condensing Steam Turbines Condensing Steam Turbines • Condensing turbines turn the steam at least partly to water using condensers and giant concrete cooling towers. This allows the steam to expand more and helps the turbine extract the maximum energy from it, making the electricity generating process much more efficient. • Condensing steam turbines are commonly used on medium to large units and operate with an exhaust pressure of 26" Hg to 29" Hg vacuum. • The higher the vacuum is, the greater the efficiency is. This exhaust pressure allows the steam to expand to a greater volume and release more heat to perform more work. • An increase in the vacuum on the exhaust of a steam turbine increases the efficiency of the steam turbine. • A condensing steam turbine is a steam turbine that allows condensate to be reclaimed for use in the system. 30 Centennial College - SDRE: ESET 223