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STEAM TURBINES BASIC INFORMATION

Various types of turbines are used for Powergeneration.

A particular type of turbine is used in each areadepending upon available options. For example, ifstrong winds are available at some location, WindTurbine can be the best solution for ElectricityGeneration at that area.

Diesel, Natural Gas and Furnace Oil are mostly usedto drive Combustion turbines for Electricity generationat larger ratings, provided that these sources areavailable easily and at right price.

Many Combustion Turbine IPPs are functioning inPakistan.

However, these fuel sources are not only expensive,but also a major source of pollution. Efforts are madethrough out the World to reduce dependence onPetroleum products.

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The major source of Electricity Generation in Pakistanare Dams. Water Turbines are rotated through waterflow to produce electricity, providing free and vastenergy.

This natural gift is not abundant anymore due to lack ofwater flow, Rain timings, Political issues, absence offunds and skill for machinery refurbishing, etc.

Hence the load-shedding.

Steam Turbines Introduction

Power generated by steam turbines have lit the firstlight bulbs for over one and a half century. In fact, the

first power plant (run by Thomas Edison using hisdynamos and located at Pearl Street in New York City)was a CHP plant that generated power using a steamturbine. The excess steam was used to heat homes.

Because a steam turbine runs off of steam producedby a boiler, it can support many different types of fuels.

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Natural gas, coal, nuclear, wood, biomass, municipalsolid waste and more can all be used to run a steamturbine. As a result, facilities that have an excess ofwaste products such as oil or wood tend to implementsteam turbines. As the figure below indicates, currentsteam turbine CHP systems run off of a variety of fuels.

Another attractive feature of steam turbines is that theycan be modified to fit any CHP system. Therefore, thesteam turbine can be fitted to match a facilitiespressure and temperature requirements.

Furthermore, steam turbines can be retrofitted into anexisting steam system. Furthermore, water and steamare very well understood. Using the steam tables, wecan know exactly the properties of our working fluids atcertain temperatures and pressures. Therefore, steamis very predictable.

Brief Technological Description

Steam turbines are slightly different than other CHPprime movers in that they require a separate boiler or“HRSG” (Heat Recovery Steam Generator) to create itsworking fluid (steam). Sometimes, plants will alreadyhave a boiler producing steam for a process or tosatisfy a heating/cooling load and rather than using a“pressure reducing valve” to “isenthalpically” reducethe pressure of the steam, they will run the steamthrough a “back pressure steam turbine” and generateelectricity. In CHP applications, a boiler or HRSG willgenerate steam which will be put through a steamturbine. The steam turbine will produce electricity andthe remaining exhaust steam can be used for hotwater or heating/cooling. The diagram belowsummarizes a boiler/steam turbine cycle.

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The basic process behind steam power generation isthe “Rankine Cycle”. Water is heated until it is asaturated liquid. From there, it is compressed intosteam. The steam is transferred to a turbine where thepressure of the steam is reduced (usually to subatmospheric pressures) by expansion over the turbineblades. This process produces electricity. The lowpressure steam is condensed back to a liquid. Thewater, now referred to as return water, is mixed withnew water, referred to as “feedwater”, and pumpedback to the boiler. The figure below shows a commondiagram used to describe the Rankine Cycle.

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Types of Steam Turbines:

There are basically two types of steam turbines:

- Condensing,

- Non-condensing,

Condensing steam turbines are very economic andfuel efficient. These turbines are mostly used for largeamount of power generation. Small industires whereelectricity unit rate is a basic issue, can also applycondensing steam turbine system.

Non-condensing steam turbines are also referred toas “back pressure” steam turbines. Here, steam isexpanded over a turbine and the exhaust steam isused for to meet a facilities steam needs. The steamis expanded until it reaches a pressure that the facilitycan use. The figure below shows the process of aback pressure steam turbine.

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The other type of steam turbine used in CHPapplications is called an extraction turbine. In theseturbines, steam in extracted from the turbine at someintermediate pressure. This steam can be used tomeet the facilities steam need. The remaining steamis expanded further and condensed. Extractionturbines can also act as admission turbines. Inadmission turbines, additional steam is added to theturbine at some intermediate point. The figure belowschematically shows the process of an extractionsteam turbine.

Waste heat from a steam turbine (either collectedthrough the exhaust or from extraction), can be used forspace heating or cooling, for process, or it can be usedto create chilled or hot water. Steam turbines can alsobe part of a “combined cycle” process. In theseprocesses, waste steam from an electricity producingprocess (i.e. waste steam produced by a gas turbine)is run through a steam turbine to produce moreelectricity. While this is very energy efficient, it is notconsidering CHP because no heating or cooling loadsare satisfied in any part.

Impulse & Reaction:

Most modern steam turbines are a combination of thereaction and impulse design. Typically, higherpressure sections are impulse type and lowerpressure stages are reaction type.

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An impulse turbine has fixed nozzles that orient thesteam flow into high speed jets. These jets containsignificant kinetic energy, which the rotor blades,shaped like buckets, convert into shaft rotation as thesteam jet changes direction. A pressure drop occursacross only the stationary blades, with a net increasein steam velocity across the stage.

As the steam flows through the nozzle its pressurefalls from steam chest pressure to condenserpressure (or atmosphere pressure). Due to thisrelatively higher ratio of expansion of steam in thenozzle the steam leaves the nozzle with a very highvelocity. The steam leaving the moving blades is alarge portion of the maximum velocity of the steamwhen leaving the nozzle. The loss of energy due to thishigher exit velocity is commonly called the "carry overvelocity" or "leaving loss".

In the reaction turbine, the rotor blades themselves arearranged to form convergent nozzles.

This type of turbine makes use of the reaction forceproduced as the steam accelerates through thenozzles formed by the rotor.

Steam is directed onto the rotor by the fixed vanes ofthe stator. It leaves the stator as a jet that fills the entirecircumference of the rotor. The steam then changesdirection and increases its speed relative to the speedof the blades.

A pressure drop occurs across both the stator and therotor, with steam accelerating through the stator anddecelerating through the rotor, with no net change in

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steam velocity across the stage but with a decrease inboth pressure and temperature.

Net result reflecting the work performed in the driving ofthe rotor.

Casing or Shaft Arrangements

These arrangements include single casing, tandemcompound and cross compound turbines.

- Single casing units are the most basic style where asingle casing and shaft are coupled to a generator.

- Tandem compound are used where two or morecasings are directly coupled together to drive a singlegenerator.

- Cross compound turbine arrangement features twoor more shafts not in line driving two or moregenerators that often operate at different speeds. Across compound turbine is typically used for manylarge applications.

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Warming & Starting

When warming up a steam turbine for use, the mainsteam stop valves (after the boiler) have a bypass lineto allow superheated steam to slowly bypass the valveand proceed to heat up the lines in the system alongwith the steam turbine.

Also a turning gear is engaged when there is no steamto the turbine to slowly rotate the turbine to ensure evenheating 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 andsteam is admitted to the turbine, first to the asternblades then to the ahead blades slowly rotating theturbine at 10 to 15 RPM to slowly warm the turbine.

Maintenance

Problems with turbines are now rare and maintenancerequirements are relatively small. Any imbalance of therotor can lead to vibration, which in extreme cases canlead to a blade letting go and punching straightthrough the casing. It is, however, essential that theturbine be turned with dry steam. If water gets into the steam and is blasted onto theblades (moisture carryover) rapid impingement anderosion of the blades can occur, possibly leading toimbalance and catastrophic failure. Also, waterentering the blades will likely result in the destructionof the thrust bearing for the turbine shaft. To preventthis, along with controls and baffles in the boilers toensure high quality steam, condensate drains areinstalled in the steam piping leading to the turbine

Speed regulation

The control of a turbine with a governor is essential, asturbines need to be run up slowly, to prevent damagewhile some applications (such as the generation ofalternating current electricity) require precise speedcontrol.

Uncontrolled acceleration of the turbine rotor can leadto an over-speed trip, which causes the nozzle valvesthat control the flow of steam to the turbine to close.

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Efficiencies

Efficiencies of steam systems are hard to calculate.40-50% is a number that is usually attached to steamturbine efficiencies. However, this number could bemisleading because it does not take into accountboiler efficiencies.

Boilers are generally 80-85% efficient. If such a boileris included in the steam system, then the efficiency ofthe steam system drops to 32-42.5%.

However, if the boilers are already in place and asteam turbine is added later, then generally the boilerefficiency need not be considered.

Another point to consider is if a back pressure steamturbine is replacing a pressure reducing valve orsteam blow off practice, then any energy you can get isenergy efficient.

The reasoning behind this is that the plant waspreviously loosing energy either by isenthalpicexpansion or by simply blowing excess steam away.Now, the plant is using the energy and therefore, whencompared to before, the plant is much more energyefficient.

Emissions

Steam turbines do not emit anything themselves.

However, the steam generator emits pollutants.Therefore, the emissions from a steam turbine systemare highly variable and depend on the type of fuel beingused to create the steam and the method by whichsteam is created.

Boilers will emit NOx, SOx, PM, CO, and CO2.

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There are a variety of emissions control technologiesfor steam systems. Some of these include:

-Flu-gas recirculation,

-low excess air firing,

-combustion control,

-using low nitrogen fuel oil,

-inserting water/steam to lower NOx,

-selective non-catalytic reduction,

-selective catalytic reduction and others.

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