Applications

A steam turbine is a mechanical device that extracts thermal energy from pressurized steam, and converts it into rotary motion. It has almost completely replaced the reciprocating piston steam engine primarily because of its greater thermal efficiency and higher power-to-weight ratio. Because the turbine generates rotary motion, it is particularly suited to be used to drive an electrical generator – about 80% of all electricity generation in the world is by use of steam turbines. The steam turbine is a form of heat engine that derives much of its improvement in thermodynamic efficiency through the use of multiple stages in the expansion of the steam, which results in a closer approach to the ideal reversible process.

An ideal steam turbine is considered to be an isentropic process, or constant entropy process, in which the entropy of the steam entering the turbine is equal to the entropy of the steam leaving the turbine. No steam turbine is truly “isentropic”, however, with typical isentropic efficiencies ranging from 20%-90% based on the application of the turbine. The interior of a turbine comprises several sets of blades, or “buckets” as they are more commonly referred to. One set of stationary blades is connected to the casing and one set of rotating blades is connected to the shaft. The sets intermesh with certain minimum clearances, with the size and configuration of sets varying to efficiently exploit the expansion of steam at each stage.

Applications of Rotating Equipment:

Direct drive

Electrical power stations use large steam turbines driving electric generators to produce most (about 80%) of the world's electricity. Most of these centralised stations are of two types, fossil fuel power plants and nuclear power plants, but some countries are using concentrating solar power (CSP) to create the steam. Steam turbines can also be used directly to drive large centrifugal pumps, such as feedwater pumps at a thermal power plant.

It has been proposed that, given sufficient solar energy, silicon might be refined for use as a coal replacement for this type of engine.

The turbines used for electric power generation are most often directly coupled to their generators. As the generators must rotate at constant synchronous speeds according to the frequency of the electric power system, the most common speeds are 3000 r/min for 50 Hz systems, and 3600 r/min for 60 Hz systems. In installations with high steam output, as may be found in nuclear power stations, the generator sets may be arranged to operate at half these speeds, but with four-pole generators.

Marine propulsion

Another use of steam turbines is in ships; their small size, low maintenance, light weight, and low vibration are compelling advantages. A steam turbine is only efficient when operating in the thousands of RPM, while the most effective propeller designs are for speeds less than 100 RPM. The purchase cost is offset by much lower fuel and maintenance requirements and the small size of a turbine when compared to a reciprocating engine having an equivalent power. However, diesel engines are capable of higher efficiencies: steam turbine cycle efficiencies have yet to break 50%, yet diesel engines routinely exceed 50%, especially in marine applications.

Nuclear-powered ships and submarines use a nuclear reactor to create steam and either use a steam turbine directly for main propulsion, with generators providing auxiliary power, or else employ turbo-electric propulsion, where the steam drives a turbine-generator set with propulsion provided by electric motors. Nuclear power is often chosen where diesel power would be impractical (as in submarine applications) or the logistics of refuelling pose significant problems (for example, icebreakers). It has been estimated that the reactor fuel for the Royal Navy's Vanguard class submarine is sufficient to last 40 circumnavigations of the globe – potentially sufficient for the vessel's entire service life.

A heat recovery steam generator (HRSG) is an energy recovery heat exchanger that recovers heat from a hot gas stream. It produces steam that can be used in a process or used to drive a steam turbine. A common application for an HRSG is in a combined-cycle power station, where hot exhaust from a gas turbine is fed to an HRSG to generate steam which in turn drives a steam turbine. This combination produces electricity more efficiently than either the gas turbine or steam turbine alone. Another application for an HRSG is in diesel engine combined cycle power plants, where hot exhaust from a diesel engine is fed to an HRSG to generate steam which in turn drives a steam turbine. The HRSG is also an important component in cogeneration plants. Cogeneration plants typically have a higher overall efficiency in comparison to a combined cycle plant. This is due to the loss of energy associated with the steam turbine

HRSGs consist of three major components. They are the Evaporator, Superheater, and Economizer. The different components are put together to meet the operating requirements of the unit.

Modular HRSGs can be categorized by a number of ways such as direction of exhaust gases flow or number of pressure levels. Based on the flow of exhaust gases, HRSGs are categorized into vertical and horizontal types. In horizontal type HRSGs, exhaust gas flows horizontally over vertical tubes whereas in vertical type HRSGs, exhaust gas flow vertically over horizontal tubes. Based on pressure levels, HRSGs can be categorized into single pressure and multi pressure. Single pressure HRSGs have only one steam drum and steam is generated at single pressure level whereas multi pressure HRSGs employ two (double pressure) or three (triple pressure) steam drums. As such triple pressure HRSGs consist of three sections: an LP (low pressure) section, a reheat/IP (intermediate pressure) section, and an HP (high pressure) section. Each section has a steam drum and an evaporator section where water is converted to steam. This steam then passes through superheaters to raise the temperature and pressure past the saturation point.

Packaged HRSGs are designed to be shipped as a fully assembled unit from the factory. They can be used in waste heat or turbine (usually under 20 MW) applications. The packaged HRSG can have a water cooled furnace which allows for higher supplemental firing and better overall efficiency.

Some HRSGs include supplemental, or duct firing. These additional burners provide additional energy to the HRSG, which produces more steam and hence increases the output of the steam turbine. Generally, duct firing provides electrical output at lower capital cost. It is therefore often utilized for peaking operations.

HRSGs can also have diverter valves to regulate in the inlet flow into the HRSG. This allows the gas turbine to continue to operate when there is no steam demand or if the HRSG needs to be taken offline.

Emissions controls may also be located in the HRSG. Some may contain a Selective Catalytic Reduction system to reduce nitrogen oxides (a large contributor to the formation of smog and acid rain) and/or a catalyst to remove carbon monoxide. The inclusion of an SCR dramatically effects the layout of the HRSG. NOx catalyst performs best in temperatures between 650 °F (340 °C) and 750 °F (400 °C). This usually means that the evaporator section of the HRSG will have to be split and the SCR placed in between the two sections. Some low temperature NOx catalysts have recently come to market that allows for the SCR to be placed between the Evaporator and Economizer sections (350 °F - 500 °F (175 °C - 260 °C)).

Equipment

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