A turbine is a machine that develops rotary power by directing a high-velocity fluid against blades mounted around the rim of a wheel. Steam turbines
How a Steam Turbine WorksSteam is admitted to the turbine wheel through a streamlined nozzle that efficiently expands the steam to very high velocity. The blades of the turbine wheel are curved in such a manner to receive the impact of the steam on the nozzle side, and to form a backwards jet on the side of the wheel opposite the nozzle. In this manner, both impulse and reaction forces are put to use. Exhaust ConfigurationsSteam turbines may be either condensing, noncondensing or extracting, depending on how the steam leaving the turbine is used. Condensing TurbinesA condensing turbine exhausts into a water-cooled condenser that turns all of the exhaust steam into water. The condensing action recovers water for re-use in the boilers, and it creates a vacuum to achieve the lowest possible exhaust pressure for the turbine. This helps force steam through the turbine and reduces the amount of steam required to operate the turbine. Noncondensing (Topping) TurbinesThe exhaust steam from noncondensing turbines is exhausted into a lower-pressure system. If the turbine exhaust is used for process steam requirements, the turbine is classified as a “back pressure” turbine. Extracting TurbinesSteam may be extracted from a mid-stage in a multi-stage turbine to drive an intermediate-pressure steam source. Extracting turbines may be either condensing or topping. For example, a multi-stage turbine operating with 600-psig steam pressure and exhausting to a condenser may have a mid-stage connection to draw steam at 100 psig. This steam may be furnished to a process for heating purposes. Multi-stage Steam TurbineAs steam passes through a multistage turbine, it expands to as much as 1000 times its original volume. Each successive stage of the turbine is, therefore, larger than the previous one in order to make efficient use of the expanding steam. This arrangement of larger and larger stages gives steam turbines their characteristic conical shape. Critical SpeedIn large multi-stage turbines there is a speed, referred to as the critical speed, that is in tune with the natural vibrating frequency of the turbine shaft. If the critical speed is below the operating speed, the turbine must not run within the critical speed range. Severe vibration and damage to the turbine will result. Speed Control - Governor SystemsGovernor systems are speed-sensitive control systems that are integral with the steam turbine. They control normal operation by varying the amount of steam to the turbine. They commonly consist of spring-opposed rotating weights, a steam valve, and an interconnecting linkage or servo motor system. Control SystemsControl systems respond to pressure changes in the process system and then reposition the turbine governor valve to maintain the preset pressures. Overspeed Trip SystemThe overspeed system usually consists of a spring-loaded pin or weight mounted in the turbine shaft or on a collar, a quick-closing valve that is separate from the governor valve, and interconnecting linkage. The centrifugal force created by rotation of the pin in the turbine shaft exceeds the spring loading at a preset speed. The resultant movement of the trip pin causes knife-edges in the linkage to separate and permit the spring-loaded trip valve to close. This mechanism stops the turbine when the speed has exceeded the control range of the operating governor. Lubrication SystemOil Ring Lubrication SystemThe oil ring lubrication system employs an oil ring that rotates on the shaft with the lower portion submerged in the oil contained in the bearing case. The rotating ring transfers oil from the oil reservoir to the bearings. The oil in the bearing-case reservoirs is cooled by water flowing in cooling water chambers or tubular heat exchangers. Pressure Lubrication SystemThe pressure lubrication system consists of an oil pump driven by the turbine shaft, an oil reservoir, a tubular oil cooler, an oil filter, and the interconnecting piping. Oil is supplied to the bearing cases under pressure. The oil rings may be retained in this system to provide oil to the bearings during start-up and shutdown, when the operating speed and bearing design permit. Gas turbines
How a Gas Turbine WorksThe air compressor and turbine are at the opposite ends of a common shaft. There is space between the two devices that houses multiple combustion chambers, arranged in a circle around the shaft. The inlets to the combustion chambers receive the preheated compressed air from the compressor and regenerator. The outlets of the combustion chambers are directed at the turbine blades through special nozzles. Compression of the Combustion AirThe compressor draws in fresh air and compresses it to a pressure of 50 to 75 psig. The air is drawn by the compressor through a heat exchanger (the regenerator) where it is preheated before entering the compressor. Combustion of the FuelIn the combustion chamber, the compressed air combines with fuel and the resulting mixture is burned. An electric igniter starts the ignition. RotationThe burning gases expand rapidly and rush into the turbine, where they cause the turbine wheels to rotate. Hot gases move through a multistage gas turbine in much the same way that steam moves through a steam turbine. RegenerationThe hot gases leaving the turbine may be circulated to a regenerator. There, the gases warm the incoming air before it enters the compression chamber. Preheating the air reduces the amount of fuel needed for combustion. Operating ConditionsGas turbines run at even hotter temperatures than steam turbines. The hotter a gas turbine runs, the more efficiently it operates. The temperature in many gas turbines is 1600°F or higher. The turbine cannot transmit its entire power output to the load, since a substantial portion is needed to drive the compressor. The turbine is started with the aid of an electric motor, which first has to set the compressor in motion. When a supply of compressed air is available to the combustion chamber, the turbine can start running. |