Lubrication Fundamentals 2011 Part 10 ppsx

The distinguishing characteristic of the reaction turbine is that a pressuredrop occurs across both the moving and stationary nozzles, or blades Figure 12.2.Nor-mally reaction turbines e

Figure 12.2 Reaction turbine with one velocity-compounded impulse stage The first stage ofthis turbine is similar to the first, velocity-compounded stage ofFigure 12.1.However, in the reactionblading of this turbine, both pressure and velocity decrease as the steam flows through the blades.The graph at the bottom shows the changes in pressure and velocity through the various stages.

Figure 12.3 Simple power plant cycle: the working fluid, here steam and water, travels a closedloop in the typical power plant cycle.

steam velocity, this arrangement may be referred to as a 50% reaction turbine.) The movingblades form the walls of moving nozzles that are designed to permit further expansion ofthe steam and to partially reverse the direction of steam flow, which produces the reaction

on the blades The distinguishing characteristic of the reaction turbine is that a pressuredrop occurs across both the moving and stationary nozzles, or blades (Figure 12.2).Nor-mally reaction turbines employ a considerable number of rows of moving and stationarynozzles through which steam flows as its initial pressure is reduced to exhaust pressure.The pressure drop across each row of nozzles is, therefore, relatively small, and steamvelocities are correspondingly moderate, permitting medium rotating speeds

Reaction stages are usually preceded by an initial velocity-compounded impulsestage, as in Figure 12.2, in which a relatively large pressure drop takes place This results

in a shorter, less costly turbine

In the radial flow reaction, or Ljungstrom, turbine, the steam does not flow axiallythrough alternating rows of fixed and moving blades; rather, it flows radially throughseveral rows of reaction blades Alternate rows of blades move in opposite direction Theyare fastened to two independent shafts that operate in opposite directions, each shaft driving

a load

After expansion in the turbine, the steam usually exhausts to a condenser, where it

is condensed to provide a source of clean water for boiler feed This simple cycle (Figure12.3) forms the basis on which most steam power plants operate

I STEAM TURBINE OPERATION

Steam turbines are made in a number of different arrangements to suit the needs of variouspower plant or industrial installations Turbines up to 40–60 MW capacity are generally

single-cylinder machines (The term cylinders, chests, casings, and shells are used

inter-changeably in this industry.) Larger units ranging in size up to 1250 MW are usually ofcompound type; that is, the steam is partially expanded in one cylinder then passed to one

or more additional cylinders where expansion is completed The simple cycle shown in

Figure 12.3is water to steam to power generation, and steam to water This forms thebasis on which most steam power plants operate

A Single-Cylinder Turbines

Single-cylinder turbines are of either the condensing or backpressure (noncondensing)

type These basic types and some of their subclassifications are shown in Figure 12.4.When the steam from a turbine exhausts to a condenser, the condenser serves twopurposes:

1 By maintaining a vacuum at the turbine exhaust, it increases the pressure rangethrough which the steam expands In this way, it materially increases the effi-ciency of power generation

2 It causes the steam to condense, thus providing clean water for the boilers toreconvert into steam

Figure 12.4 Typical single-cylinder turbine types: in comparison to backpressure turbines, densing turbines must increase more in size toward the exhaust end to handle the larger volume oflow pressure steam

con-Industrial plants frequently require steam at low to moderate pressures for process use.

One of the more economical ways of generating this steam is with a combined-cycle plant,

where high pressure steam is used to power equipment such as generators and the exhauststeam from this equipment is used for heating or other services The steam is generated

at high pressure and, after expansion through the turbine to the pressure desired for processuse, it is delivered to the process application This permits power to be generated by theturbine without appreciably affecting the value of the steam for process use It may bedone with a backpressure turbine designed to exhaust all the steam against the pressure

required for process use, or it may be done with an automatic extraction turbine in which

part of the steam is withdrawn for process use at an intermediate stage (or stages) of theturbine and the remainder of the steam exhausted to a condenser Such a turbine requiresspecial governors and valves to maintain constant pressure of the exhausted steam andconstant turbine speed under varying turbine load and extraction demands

Steam can be also extracted without control from various stages of a turbine to heat

boiler feedwater (regenerative heating) Such turbines are called uncontrolled extraction

turbines, since the pressure at the extraction points varies with the load on the turbine

To obtain higher efficiency, large turbines (called reheat turbines) are arranged sothat after expanding partway, the steam is withdrawn, returned to the boiler, and reheated

to approximately its initial temperature It is then returned to the turbine for expansionthrough the final turbine stages to exhaust pressure

High pressure noncondensing turbines have been added to many moderate pressureinstallations to increase capacity and improve efficiency In such installations, high pres-sure boilers are installed to supply steam to the noncondensing turbines, which are designed

to exhaust at the pressure of the original boilers and supply steam to the original turbines

The high pressure turbines are called superposed or topping units.

Where low pressure steam is available from process work, it can be used to generatepower by admitting it to an intermediate stage of a turbine designed for the purpose and

expanding it to condenser pressure Such a machine is a mixed-pressure turbine, and is another form of combined-cycle operation.

Compound turbines have at least two cylinders or casings, a high pressure one and

a low pressure one To handle large volumes of low pressure steam, the low pressurecylinder is frequently of the double-flow type Very large turbines may have an intermedi-ate pressure cylinder, and two, three, or even four double-flow, low pressure cylinders

The cylinders may be in line using a single shaft, which is called tandem compound, or

in parallel groups with two or more shafts, which is called cross compound Reheat between

the high and intermediate pressure stages may be employed in large turbines Steam may

be returned to the boiler twice for reheating Some of these arrangements shown matically inFigure 12.5

diagram-II TURBINE CONTROL SYSTEMS

Although the trend is toward electronic speed sensing and control, all steam turbines areprovided with at least two independent governors that operate to control the flow of steam.One of these operates to shut off the steam supply if the turbine speed should exceed apredetermined maximum It is often referred to as an emergency trip On overspeed, itcloses the main steam valve, cutting off steam from the boiler to the turbine The other,

or main, governor may operate to maintain practically constant speed, or it may be designedfor variable speed operation under the control of some outside influence Extraction, mixed

Although most new turbine installations use electronic speed sensing and control,

a commonly used speed-sensitive element is the centrifugal, or flyball, governor (Figure12.6) Weights that are pivoted on opposite sides of a spindle and are revolving with itare moved outward by centrifugal force against a spring when the turbine speed increases,and inward by spring action as the turbine slows down This action may operate the steamadmission valve directly through a mechanical linkage, as shown, or it may operate thepilot valve of a hydraulic system, which admits and releases oil to opposite sides of apower piston, or on one side of a spring-loaded piston Movement of the power pistonopens or closes steam valves to control turbine speed

Moderate-sized and large high speed turbines are provided with a double relayhydraulic system to further boost the force of the centrifugal governor and to increase thespeed with which the system responds to speed changes

A second type of speed-sensitive element is the oil impeller Oil from a shaft-drivenpump flows through a control valve to the space surrounding the governor impeller The

Figure 12.6 Mechanical speed governor A simple arrangement such as this using a flyballgovernor is suitable for many small turbines

impeller, mounted on the turbine shaft, consists of a hollow cylindrical body with a series

of tubes extended radially inward As the oil flows inward through the tubes it is opposed

by centrifugal force and a pressure is built up that varies with the square of the turbinespeed This pressure is applied to spring-loaded bellows, which positions a pilot valve.The pilot valve, in turn, controls the flow to a hydraulic circuit that operates the steamcontrol valves

Newer turbines are equipped with electrical or electronic speed-sensitive devices.Signals from these devices, along with signals derived from load, initial steam pressure,and other variables, are fed to computer, which compares them and sends the appropriatesignals to hydraulic servovalves to adjust the steam control valves

As indicated, the linkage between the speed-sensitive element and the steam controlvalves may be anything from a simple lever to an extensive hydraulic system controlled

by a computer

In small turbines, the flow of steam is controlled by a simple valve, usually of thebalanced type, to reduce the operating force required In large units, a valve for each ofseveral groups of nozzles controls steam flow The opening or closing of a valve cuts in

or out a group of nozzles The number of open valves, and thus the number of nozzlegroups in use, is varied according to the load The valves may be operated by a barliftarrangement, by cams, or by individual hydraulic cylinders

Additional control valves, called intercept valves, are required on reheat turbines.These are placed close to the intermediate, or reheat, cylinder and are closed by a governorsystem if the turbine starts to speed up as a result of a sudden large load reduction Thisdesign is intended to prevent large volumes of high energy steam found in the piping ofthe high pressure turbine exhaust, the reheat boiler, and the intermediate pressure turbinefrom continuing to flow and possibly cause overspeeding and emergency tripping of theturbine In older turbines, the intercept valves were controlled by a separate governorsystem, but the newer machines have the intercept valves operated by the main hydrauliccontrol system As an additional safety measure, intercept valves are preceded by stopvalves, which are actuated by the main emergency overspeed governor (or other speed-sensing control system)

Automatic extraction and backpressure turbines are provided with governors ranged to maintain constant extraction or exhaust pressure irrespective of load (within thecapacity of the turbine) The pressure-sensitive element consists of a pressure transducer,and its response to pressure changes is communicated through the control system to thevalves that control steam extraction, and to the speed governor that controls admission ofsteam to the turbine On automatic extraction turbines, the action of the pressure- andspeed-responsive elements is coordinated so that turbine speed is maintained This maynot be the case for backpressure turbines

ar-III LUBRICATED COMPONENTS

The lubrication requirements of steam turbines can be considered in terms of the partsthat must be lubricated, the type of application system, the factors affecting lubrication,and the lubricant characteristics required to satisfy these requirements

A Lubricated Parts

The main lubricated parts of steam turbines are the bearings, both journal and thrust.Depending on the type of installation, a hydraulic control system, oil shaft seals, gears,flexible couplings, and turning gear may also require lubrication

1 Journal Bearings

The rotor of a single-cylinder steam turbine, or of each casing of a compound turbine, issupported by two hydrodynamic journal bearings These journal bearings are located atthe ends of the rotor, outside the cylinder In some designs, there may be one large journalbearing between the casings that supports both turbine rotors or a turbine and generatorrotor (rigid coupling) instead of a separate bearing at the ends of each casing Clearancesbetween the shaft and shaft seals and between the blading and the cylinder, are extremelysmall, so to maintain the shaft in its original position and avoid damage to shaft seals orblading, the bearings must be accurately aligned and must run without any appreciablewear

Primarily, the loads imposed on the bearings are due to the weight of the rotorassembly The bearings are conservatively proportioned and thus pressures on them aremoderate Horizontally split shells lined with tin-based babbitt metal are usually used.The bearings are enclosed in housings and supported on spherical seats or flexible plates

to reduce any angular misalignment

The passages and grooves in turbine bearings are sized to permit the flow of ably more oil than is required for lubrication alone The additional oil flow is required toremove frictional heat and the heat conducted to the bearing along the shaft from the hotparts of the turbine The flow of oil must be sufficient to cool the bearing enough tomaintain it at a proper operating temperature In most turbines, the temperature of the oilleaving the bearings is on the order of 140–160⬚F (60–71⬚C), but in special cases it mayexceed 180⬚F (82⬚C)

consider-When a turbine is used to drive a generator, the generator bearings are similar indesign to the turbine bearings and are normally supplied from the same system

Large turbines are now frequently provided with ‘‘oil lifts’’ (jacking oil) in thejournal bearings to reduce the possibility of damage to the bearings during starting andstopping, and to reduce metal-to-metal contact during turning gear operation Oil underhigh pressure from a positive displacement pump is delivered to recesses in the bottoms

of the bearings The high pressure oil lifts the shaft and floats it on a film of oil until theshaft speed is high enough to create a normal hydrodynamic film For a shaft that is rotatedfor several hours or days to prevent rotor-sag, the oil lift is also required after turbineshutdown

A phenomenon that occurs in relatively lightly loaded, high speed journal bearings,such as turbine bearings, is known as ‘‘oil whip’’ or ‘‘oil film whirl.’’ The center of thejournal of a hydrodynamic bearing ordinarily assumes a stable, eccentric position in thebearing that is determined by load, speed, and oil viscosity Under light load and highspeed, the stable position closely approaches the center of the bearing There is a tendency,however, for the journal center to move in a more or less circular path about the stableposition in a self-excited vibratory motion having a frequency of something less than halfthe shaft speed In certain cases, such as some of the relatively lightweight, high pressurerotors of compound turbines that require large-diameter journals to transmit the torque,this whirling has been troublesome and has required the use of bearings designed especially

to suppress oil whip

Bearings designed to suppress oil whip are available in several types Among thecommon types are the pressure or pressure pad bearings(Figure 12.7), the three-lobedbearing (Figure 12.8), and the tilting pad antiwhip bearing (Figure 12.9) The pressurepad bearing suppresses oil whip because oil carried into the wide groove increases in

pressure when it reaches the dam at the end This increase in pressure forces the journaldownward into a more eccentric position that is more resistant to oil whip The other typesillustrated depend on the multiple oil films formed to preload the journal and minimizethe tendency to whip.

2 Thrust Bearings

Theoretically, in impulse turbines, the drop in steam pressure occurs almost entirely inthe stationary nozzles The steam pressures on opposite sides of the moving blades are,therefore, approximately equal, and there is little tendency for the steam to exert a thrust

in the axial direction In actual turbines, this ideal is not fully realized, and there is always

a thrust tending to displace the rotor

In reaction turbines, a considerable drop in steam pressure occurs across each row

of moving blades Since the pressure at the entering side of each of the many rows ofmoving blades is higher than the pressure at the leaving side, the steam exerts a considerableaxial thrust toward the exhaust end Also, when rotors are stepped up in diameter, theunbalanced steam pressure acting on annular areas thus created adds to the thrust Usuallythe total thrust is balanced by means of dummy, or balancing, pistons on which the steamexerts a pressure in the opposite direction to the thrust In double-flow elements of com-pound turbines, steam flows from the center to both ends, ensuring that thrust is wellbalanced

Regardless of the type of turbine, thrust bearings are always provided on each shaft

to take axial thrust and, thus, hold the rotor in correct axial position with respect to thestationary parts Although thrust caused by the flow of steam is usually toward the lowpressure end, means are always provided to prevent axial movement of the rotor in eitherdirection

The thrust bearings of small turbines may be babbitt-faced ends on the journalbearings, or rolling element bearings of a type designed to carry thrust loads Medium-

Figure 12.10 Combined journal and tilting pad thrust bearing A rigid collar on the shaft is heldcentered between the stationary thrust ring and a second stationary thrust ring (not shown) by tworows of titling pads

Figure 12.11 Tapered land thrust bearing and plain journal bearing The thrust bearing consists

of a collar on the shaft and two stationary bearing rings, one on each side of the collar The babbittedthrust faces of the bearing rings are cut into sectors by radial grooves About 80% of each sector

is beveled to the leading radial groove, to permit the formation of wedge oil films The unbeveledportions of the sectors absorb the thrust load when speed is too low to form hydrodynamic films

sized and large turbines are always equipped with thrust bearings of the tilting pad (Figure12.10),or tapered land (Figure 12.11) type

3 Hydraulic Control Systems

As discussed earlier, medium-sized and large turbines have hydraulic control systems totransmit the motion of the speed or pressure-sensitive elements to the steam control valves.Two general approaches are used for these systems

In mechanical hydraulic control systems, the operating pressure is comparativelylow (⬍150 psi), and oil from the bearing lubrication system may be used safely as thehydraulic fluid Separate pumps are provided to supply the hydraulic requirement Anemergency tripping device is provided to shut down the turbine if there is any failure inthe hydraulic system

Larger turbines now being installed are equipped with electrohydraulic control tems To provide the rapid response needed for control of these units, the hydraulic systemsoperate at relatively high pressures, typically in the range of 1500–2000 psi

sys-The systems consist of an independent reservoir and two separate and independentpumping systems The large fluid flow required for rapid response to sudden changes inload is usually provided by gas-charged accumulators.

The critical nature of the servovalves used in these systems requires that carefulattention be paid to the filtration of the fluid, and strict limits on particulate contaminationusually are observed The need for precise control also calls for one use of both heatersand coolers to maintain the temperature of the fluid and, thus, its viscosity, in a narrowrange Since a leak or a break in a hydraulic line could result in a fire if the high pressurefluid sprayed onto hot steam piping or valves, fire-resistant hydraulic fluids are widelyused in these systems

4 Oil Shaft Seals for Hydrogen-Cooled Generators

Because it is a more effective coolant than air, hydrogen is commonly used to cool sized and large generators Shaft-mounted blowers circulate the gas through rotor andstator passages, then through liquid-cooled hydrogen coolers Gas pressures up to 60 psi(413 kPa) are used A further development, which has permitted increases of generatorratings over hydrogen cooling alone, is the direct liquid cooling of stator windings Someliquid systems use transformer oil while others use water Even with water-cooled stators,the interior of the generator is still filled with hydrogen

medium-The main connection between type of cooling and turbine lubrication is that whenhydrogen is used for cooling, some of the oil is exposed to the hydrogen Oil shaft seals

(Figure 12.12)are used to prevent the escape of the hydrogen Turbine oil for these sealsmay be supplied from an essentially separate system having its own reservoir, pumps, and

so forth, or may be supplied directly from the main turbine lubricating system In eithercase, before entering the reservoir, oil returning from the seals must be passed through aspecial tank to remove any traces of hydrogen Otherwise hydrogen could accumulate inthe reservoir and form an explosive mixture with air In addition, the main turbine oilreservoir of all units driving hydrogen-cooled generators must be equipped with a vaporextractor to remove any traces of hydrogen that may be carried back by the sealing oil orthe oil from the generator bearings

5 Gear Drives

Efficient turbine speed is often higher than the operating speed of the machine beingdriven This may be the case, for example, when a turbine drives a direct current generator,paper machine drives, centrifugal pumps, or other industrial machines It is also the casewhen a turbine is used for ship propulsion In these applications, reduction gears are used

to connect the turbine to the driven unit

Reduction gears used with moderate-sized and large turbines are usually of theprecision-cut, double-helical type Double reduction gear sets are required with marinepropulsion turbines, and epicyclic reduction gears are sometimes used instead of conven-tional gear sets Usually, the gear sets are enclosed in a separate oil-tight casing and areconnected to the turbine and the driven machine through flexible couplings Small ma-chines may have the gear housing integral with the turbine housing and the pinion on theturbine shaft

Reduction gears may have a circulation system that is entirely separate from theturbine system, or circulation may be supplied from the turbine system In the latter case,

a separate pump (or pumps), is provided for the gears Some older small-geared turbineshave ring-oiled turbine bearings and splash-lubricated gears

Smaller couplings may be lubricated by a bath of oil carried inside the case, or withgrease.

7 Turning Gear

When one is starting and stopping large turbines, it is necessary to turn the rotor slowly

to avoid uneven heating or cooling, which could cause distortion or bowing of the shaft.This is done with a barring mechanism or turning gear The turning gear usually consists

of either an electric or hydraulic motor that is temporarily coupled to the turbine shaftthrough reduction gears Rotor speed, while the turning gear is operating, is usually below

100 rpm To provide adequate oil flow to the bearings during this low speed operation,

a separate auxiliary oil pump usually is provided The oil coolers are used at maximumcapacity to increase oil viscosity and to help maintain oil films in the bearings If oil liftsare provided in the turbine bearings, they are also operated while the turning gear isoperating

C Factors Affecting Lubrication

Steam turbines in themselves do not represent particularly severe service for based lubricating oils Because of the costs and time involved in shutting down mostturbines to change the oil and clean the system, however, and because of the relativelylarge volume of oil contained in large turbine systems, turbine operators expect extremelylong service life from the turbine oil To achieve long service life, oils must be carefullyformulated for the specific conditions encountered in steam turbine lubrication systems

petroleum-1 Circulation and Heating in the Presence of Air

The temperature of the oil in steam turbine systems is raised both by the frictional heatgenerated in the lubricated parts and by heat conducted along the shaft from the rotor Asthe oil flows through the system, it is broken into droplets or mist, which permits greaterexposure to air System designs are usually conservative, to ensure that maximum oiltemperatures are not excessive, but in long-term service some oxidation of the oil doesoccur This oxidation may be further catalyzed by finely divided metal particles resultingfrom wear or contamination and water

Slight oxidation of the oil is harmless The small amounts of oxidation productsformed initially can be carried in solution in the oil without noticeable effect As oxidationcontinues, some of the soluble products may become insoluble, or insoluble materials may

be formed As an oil oxidizes, it should also show an increase in viscosity; but in mostturbine systems the rate of oxidation is very slow, and viscosity increases are rarely noted.Insoluble oxidation products may be carried with the oil as it circulates Some maythen settle out as gum, varnish, or sludge on governor parts, in bearing passages, oncoolers, on strainers, and in oil reservoirs Their accumulation may interfere with thesupply of oil to bearings and with governors or control of the unit Under moderate tosevere oil oxidation, there is also the potential for the oil degradation materials to plateout on bearing surfaces, reducing clearances, and increasing bearing temperatures.Some oxidation products that are soluble in warm oil become insoluble when theoil is cooled This can result in insulating deposits forming on cooling coils or othercool surfaces The resultant reduction in effectiveness of cooling may cause higher oiltemperatures and contribute to more rapid oxidation.

An increase in viscosity within a range that has proved satisfactory in service is notnecessarily harmful However, excessive viscosity increase may reduce oil flow to bear-ings, increase pumping losses, and increase fluid friction and heating in bearings All theseeffects tend to increase the operating temperature of the oil and contribute to more rapidoxidation of it

2 Contamination

Water is the contaminant that is most prevalent in steam turbine lubrication systems.Common sources of water contamination are as follows:

1 Steam from leaking shaft seals or the shaft seals of turbine-driven pumps

2 Condensation from humid air in the oil reservoir and bearing pedestals

3 Water leaks in oil coolers

4 Steam leaks in oil heating elements (where used)The lubrication systems of turbines that operate intermittently are more likely tobecome contaminated with water than are the systems of turbines that operate continuously.When a turbine oil is agitated with water, some emulsion will form If the oil isnew and clean, the emulsion will separate readily The water will then settle in the reservoir,where it can be drawn off or removed by the purification equipment Oxidation of the oil,

or contamination of it with certain types of solid material such as rust and fly ash, mayincrease the tendencies of the oil to emulsify and to stabilize emulsions after they areformed Persistent emulsions can join with insoluble oxidation products, dirt, and so forth

to form sludges The character of these sludges may vary; but, if they accumulate in oilpipes, passages, and oil coolers, they can interfere with oil circulation and cause high oiland bearing temperatures

Water in lubricating oil, combined with air that is always present, can cause theformation of common red rust and also black rust, similar in appearance to pipe scale.Rusting may occur both on parts covered by oil and on parts above the oil level In eithercase, in addition to damage to the metal surfaces, rusting is harmful for a number ofreasons Particles of rust in the oil tend to stabilize emulsions and foam and to act ascatalysts that increase the rate of oil oxidation Rust is abrasive and when carried by theoil to the bearings may scratch the journals and cause excessive wear If carried into thesmall clearances of governor or control mechanisms, it can cause sluggish operation or,

in extreme cases, sticking, overspeed, or tripping the unit off-line

Solid materials of many types can contaminate turbine oil systems Pulverized coal,fly ash, dirt, rust, pipe scale, and metal particles are typical examples These solid materialsmay enter the system in the following ways: