Single stage steam turbine components

TEXT 4 *. Coppus Model RLHB24 Single Stage Turbine

Inlet Flange. This is the connection to the steam supply. It is part of the Combination Overspeed Trip/Throttle Valve (hereinafter termed the combo Valve). Flange type, size, and material are a function of steam conditions and customer specification.

Combo Valve. This Combination Overspeed Trip/Throttle Valve is mounted between the turbine casing and the inlet steam line. It houses both a throttle valve and an overspeed trip valve. The Overspeed Trip Valve is a mechanically actuated valve that interrupts the supply of steam to the turbine during an overspeed condition or other emergency, thereby bringing the turbine to a complete stop. In the event of overspeed, the valve is activated by the overspeed trip collar, which is attached to the turbine shaft inside the Governor Mounting Housing. In the event of other emergencies, the valve can be activated using the Overspeed Trip Lever, which protrudes from the Governor Mounting Housing.

The Throttle Valve is contained in the Combo Valve upstream of the Overspeed Trip Valve. It controls the amount of steam entering the turbine and thereby determines the speed and power produced by the turbine.

Trip Linkage (not visible). This linkage connects the Overspeed Trip Valve to the trip mechanism inside the Governor Mounting Housing. The trip Linkage is activated by either the overspeed trip collar or the Overspeed Trip Lever.

Governor. The Governor senses the speed of the turbine and opens or closes the throttle valve, as appropriate, to maintain the set speed. A variety of governors are available for different applications.

Throttle Linkage. This is the linkage between the Governor and Throttle Valve.

Governor Mounting Housing. This is the structure supporting the Governor and connecting it to the Governor End Bearing Housing. The Trip Collar, Overspeed Trip Lever, and Governor Drive Coupling are contained within the Governor Mounting Housing.

Overspeed Trip Lever. The Overspeed Trip Lever is part of the Trip Linkage, allowing manual activation of the Overspeed Trip Valve.

Overspeed Trip Reset Handle. This handle is used to reset (open) the Overspeed Trip Valve, permitting recovery from an overspeed trip condition. When recovering from a trip condition, the handle is initially opened slightly to permit pilot valve operation, and then is opened fully to reset the valve.

Full Flow Handle. This handle is used to close the throttle valve when recovering from an overspeed trip condition. The throttle valve is held against its seat with this handle to reduce incoming steam pressure, and then the overspeed trip reset handle is used to reset the overspeed trip valve.

Governor End Bearing Housing. RLHA turbines have one sleeve shaft support bearing and a thrust bearing in this housing. RLHB turbines have on ball bearing serving both purposes in this housing. The standard housing also contains an oil ring, seals, the oil reservoir and the cooling water jacket. An Oil Level Gauge and Constant Level Oiler are mounted on the bearing housing, along with the oil filler/vent, oil drain plug, and plugs for cooling water inlet and outlet openings.

Drive End Bearing Housing. This housing is similar to the Governor End Bearing Housing. The RLHA contains one sleeve bearing, while the RLHB utilizes a ball bearing.

Oil Level Gauge. The Oil Level gauge indicated the oil level in the bearing housing. This level corresponds with a mark inscribed on the bearing housing.

Constant Level Oiler. The Constant Level Oiler is an oil reservoir that is set to maintain a constant oil level in the bearing housing.

Gland Housing. Gland Housings contain Carbon Ring Seals that prevent steam from leaking along the shaft to atmosphere. Some steam will escape past the carbon rings, lubricating them. This steam is conveyed by the gland leakoff connection to a safe location.

Cover. The Cover is the turbine component that seals the turbine casing. It contains an eye bolt, used for lifting the cover during turbine service. The eye bolt must not be used for lifting the entire turbine.

Inlet Casing. The Inlet Casing is the casing section containing the high-pressure steam. Steam enters the Inlet Casing from the Overspeed Trip Valve and exits through nozzles in the Nozzle Block.

Handvalves. Handvalves allow the operator to open or close the passages from the Steam chest to a portion of the nozzles-thereby turning some nozzles on and off. This permits the operator to improve turbine efficiency at partial load. The reasoning behind this is as follows: the Throttle Valve opens or closes in response to the Governor in an attempt to maintain a constant speed as the load imposed on the turbine varies. At low loads, the Throttle Valve is almost closed, resulting not only in reduced steam flow through the turbine, but in reduced steam pressure in the Steam Chest. When steam pressure in the chest is low, then according to the laws of thermodynamics, turbine efficiency is low. By closing some nozzles, power can be decreased by reducing steam flow, without throttling and reducing pressure. The number of handvalves on the turbine is determined by operating conditions and customer requirements. To avoid steam cutting damage to the handvalve seats, handvalves must be either completely open or completely closed, and never used as a throttle.

Exhaust Casing. The Exhaust Casing contains the exhaust steam and is integral with the Exhaust Flange. The Exhaust Casing supports the Drive End Bearing Housing.

Turbine Pedestal and Flex Plate. The Turbine Pedestal consists of two legs that are bolted to the Exhaust casing. The legs are drilled for mounting bolts and dowel pins which hold the turbine in position and help maintain alignment with the driven equipment. The flex plate, mounted to the Governor End Bearing Housing, supports the opposite end of the turbine.

Exhaust Flange. This flange connects the turbine to the exhaust steam line. Flange type, size, and material are a function of steam conditions and customer requirements. Refer to the certified drawing at the end of this manual fro a complete description.

Shaft Extension. This is the output shaft of the turbine, which is ground and keyed to accept a coupling.

TEXT 5. Reaction and Impulse Steam Turbines

There are two principal turbine types: reaction and impulse. In a reaction turbine, the steam expands in both the stationary and moving blades. The moving blades are designed to utilize the steam jet energy of the stationary blades and to act as nozzles themselves. Because they are moving nozzles, a reaction force—produced by the pressure drop across them—supplements the steam jet force of the stationary blades. These combined forces cause rotation. To operate efficiently the reaction turbine must be designed to minimize leakage around the moving blades. This is done by making most internal clearances relatively small. The reaction turbine also usually requires a balance piston (similar to those used in large centrifugal compressors) because of the large thrust loads generated.

The impulse turbine has little or no pressure drop across its moving blades. Steam energy is transferred to the rotor entirely by the steam jets striking the moving blades. Since there is theoretically no pressure drop across the moving blades (and thus no reaction), internal clearances are large, and no balance piston is needed. These features make the impulse turbine a rugged and durable machine that can withstand the heavy-duty service of today’s mechanical drive applications.

TEXT 6. Steam Turbine Capacity

Multistage steam turbines are economical, smooth, and quiet, and are made in very large sizes. Powers of 500 and 660 megawatts (670,000 and 885,000 horsepower) are common, and steam turbines of more than 1,000 megawatts (1,340,400 horsepower) have been built. Others of 2,000 megawatts (2,680,700 horsepower) are being designed. Small high-speed steam turbines with a single row of blades were developed in 1887 by the Swedish engineer Carl Gustaf de Laval (1845-1913). It was de Laval who invented the special reduction gearing which allows a turbine rotating at high speed to drive a propeller or machine at a comparatively low speed.

Steam turbines used in ships are always geared because high-speed propellers are inefficient. The first turbine ship, the Turbine, was designed by Parsons and built at Wallsend-on-Tyne, England. It had three turbines producing a total power of 1,492 kilowatts (2,000 horsepower).

Steam turbines may be either condensing or non-condensing. In a condensing turbine the steam goes from the turbine into a condenser and is cooled by cold water circulating in pipes. The steam becomes water and a vacuum is created because the water takes up less space than the steam does. The vacuum helps force steam through the turbine. The water is then pumped back into the boiler to be made into steam again. In a non-condensing turbine the steam which has passed through the turbine is used to provide heat for buildings or in other industrial processes.

TEXT 7. Steam Turbine Operating Characteristics

Steam turbines, especially smaller units, leak steam around blade rows and out the end seals. When an end is at a low pressure, as is the case with condensing steam turbines, air can also leak into the system. The leakages cause less power to be produced than expected, and the makeup water has to be treated to avoid boiler and turbine material problems. Air that has leaked in needs to be removed, which is usually done by a compressor removing non-condensable gases from the condenser. Because of the high pressures used in steam turbines, the casing is quite thick, and consequently steam turbines exhibit large thermal inertia.

Steam turbines must be warmed up and cooled down slowly to minimize the differential expansion between the rotating blades and the stationary parts. Large steam turbines can take over ten hours to warm up. While smaller units have more rapid startup times, steam turbines differ appreciably from reciprocating engines, which start up rapidly, and from gas turbines, which can start up in a moderate amount of time and load follow with reasonable rapidity. Steam turbine applications usually operate continuously for extended periods, although the steam fed to the unit and the power delivered may vary (slowly) during such periods of continuous operation.

TEXT 8. Steam Turbine Maintenance

Steam turbines are rugged units, with operational life often exceeding 50 years. Maintenance is simple, comprised mainly of making sure that all fluids (steam flowing through the turbine and the oil for the bearing) are always clean and at the proper temperature. The oil lubrication system must be clean and at the correct operating temperature and level to maintain proper performance. Other items include inspecting auxiliaries such as lubricating-oil pumps, coolers and oil strainers and checking safety devices such as the operation of overspeed trips.

In order to obtain reliable service, steam turbines require long warm-up periods so that there are minimal thermal expansion stresses and wear concerns. Steam turbine maintenance costs are quite low, typically less than $0.004 per kWh. Boilers and any associated solid fuel processing and handling equipment that is part of the boiler/steam turbine plant require their own types of maintenance.

One maintenance issue with steam turbines is solids carry over from the boiler that deposit on turbine nozzles and other internal parts and degrades turbine efficiency and power output. Some of these are water soluble but others are not. Three methods are employed to remove such deposits: 1) manual removal; 2) cracking off deposits by shutting the turbine off and allowing it to cool; and 3) for water soluble deposits, water washing while the turbine is running. Data on steam generator costs shows cost increasing with decreasing size, with a 5.25 MW, 900 psig, 850°F, 125 psig backpressure steam turbine/generator costing $285/kW (installed). In that installation the boiler alone, excluding fuel handling and pollution control equipment, cost 150% of the cost of the steam turbine.

TEXT 9. Steam Turbine Operation and Maintenance

When warming up a steam turbine for use, the main steam stop valves (after the boiler) have a bypass line to allow superheated steam to slowly bypass the valve and proceed to heat up the lines in the system along with the steam turbine. Also, a turning gear is engaged when there is no steam to the turbine to slowly rotate the turbine to ensure even heating to prevent uneven expansion. After first rotating the turbine by the turning gear, allowing time for the rotor to assume a straight plane (no bowing), then the turning gear is disengaged and steam is admitted to the turbine, first to the astern blades then to the ahead blades slowly rotating the turbine at 10 to 15 rpm to slowly warm the turbine.

Problems with turbines are now rare and maintenance requirements are relatively small. Any imbalance of the rotor can lead to vibration, which in extreme cases can lead to a blade letting go and punching straight through the casing. It is, however, essential that the turbine be turned with dry steam - that is, superheated steam with a minimal liquid water content. If water gets into the steam and is blasted onto the blades (moisture carryover) rapid impingement and erosion of the blades can occur, possibly leading to imbalance and catastrophic failure. Also, water entering the blades will likely result in the destruction of the thrust bearing for the turbine shaft. To prevent this, along with controls and baffles in the boilers to ensure high quality steam, condensate drains are installed in the steam piping leading to the turbine.

TEXT 10. Steam Turbine Speed Regulation

The control of a turbine with a governor is essential, as turbines need to be run up slowly, to prevent damage while some applications (such as the generation of alternating current electricity) require precise speed control. Uncontrolled acceleration of the turbine rotor can lead to an overspeed trip, which causes the nozzle valves that control the flow of steam to the turbine to close. If this fails then the turbine may continue accelerating until it breaks apart, often spectacularly. Turbines are expensive to make, requiring precision manufacture and special quality materials. During normal operation in synchronization with the electricity network, power plants are governed with a five percent droop speed control. This means the full load speed is 100% and the no-load speed is 105%. This is required for the stable operation of the network without hunting and drop-outs of power plants. Normally the changes in speed are minor. Adjustments in power output are made by slowly raising the droop curve by increasing the spring pressure on a centrifugal governor. Generally this is a basic system requirement for all power plants because the older and newer plants have to be compatible in response to the instantaneous changes in frequency without depending on outside communication.