Introduction to Turbines & Steam Turbines

8/31/2015 Introduction to Turbines & Steam Turbines

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Introduction to Turbines & Steam TurbinesWritten by Norrie

Wednesday, 13 January 2010 10:06

FUNDAMENTAL PRINCIPLES AND OPERATION

A. THE WINDMILL

Turbines have been in use for many years. Early turbines consisted of 'Sails' or blades mounted at an angle on acentral hub (as in the child's pinwheel in Figure: 1).

The hub was then connected to a shaft and, as the wind blew, due to the angle of the sails, the sails rotated causing thehub to rotate and so turned the shaft. The shaft was then coupled to a ' MillWheel ' used for grinding corn to makeflour and other uses.

(Picture 1).

Windmills were also used to drive water pumps which drew water from wells for farming irrigation and domestic use.In order to keep the sails facing into the wind, the windmill was fitted with a 'Rudder' which swung the sails aroundas the wind direction changed. (Picture 2).

When the generation of electricity was introduced, windmills were adapted to drive generators. One problem with a



windmill is that it depends on there being sufficient wind to drive it. Today, modern 'Windfarms' have beendeveloped with highly technical machines for producing large quantities of electricity, even when the wind velocity islow. (PICTURE: 3)

Picture: 3 Modern Windfarm

B. THE WATERWHEEL ( Figure. 2 )

The waterwheel has also been in use for many years. In this type of turbine, a wheel fitted with ' Paddles ' is placed ina stream of flowing water. As the water pushed against the paddles, the wheel rotated which, in turn, through a shaft,rotated another machine grinding mill, pump or generator. Again, the efficiency of the waterwheel depended on thevelocity and volume of water striking the paddles. [(* POTENTIAL ENERGY * Stored Energy) Energy waiting to be used.

(* KINETIC ENERGY * Energy due to motion). This high energy water is piped to large ' Water Turbines ' whichdrive the power generating plants. (See Figure. 3).

It can be seen that a 'TURBINE' is a machine which is used as a driver for other machines Generators, pumps,compressors...Etc. A turbine operation depends on the Kinetic energy contained in flowing fluids which is thenconverted into Mechanical energy. This Mechanical energy is then converted into Electrical, Heat (Thermal) orPressure energy as required. A turbine therefore, is used in the same way as a Diesel engine, Petrol engine or Electricmotor, to drive other machines. Modern turbines can produce thousands of Horsepower of energy.



STEAM TURBINES

INTRODUCTION

As already stated, a turbine is driven by the flow of a high energy fluid liquid, gas (or air). The kinetic energy ofthe fluid is converted into mechanical energy. In steam turbines, the thermal and pressure energy contained insuperheated, high pressure steam is used to drive the shaft of the turbine. Steam turbines are generally used wherethere is a plentiful supply of water. The water must first be treated to remove impurities which will cause problems inthe turbine. Chlorides, other salts, Oxygen and solid particles. This is done to prevent corrosion, erosion and scaledeposits in the system. When the water has been purified, it is then passed into a Steam Generation Plant where it isheated to produce steam. Steam at normal atmospheric conditions is Saturated (Wet) steam i.e. 212 °F (100 °C) andis of no use for driving turbines. In the type of boiler used for steam generation, the system is maintained under highpressure In this discussion we will use a steam system operated at 600 Psi. At this pressure, the water boils at 486°F. However, at this pressure and temperature the steam is still saturated (wet steam). The use of this steam in aturbine will cause erosion of the turbine internals due to droplets of water contained in the steam. The boilers aretherefore constructed with a 'Superheater' section which takes the 600 Psi wet steam and adds more heat energy to it,to a temperature of 775 °F or higher depending on requirements. At this temperature, the steam cannot contain anywater. When steam is superheated, it contains much more heat energy than wet steam and can be piped long distanceswith little loss of energy or condensation taking place.To recap, the steam used for driving steam turbines is produced from purified water to prevent corrosion and isproduced at high pressure and superheated to high temperature in order to prevent water erosion of the turbine parts.There are many types of steam turbine in use today which can produce many thousands of horsepower of energy forindustrial uses.

PRINCIPLES & OPERATION

In the pinwheel, the windmill and the water wheel, the action of the flowing fluid causes the wheel to rotate. Thispart of the machine is called the 'ROTOR'. In any turbine, the rotor is mounted on a shaft and consists of the 'Sails' or'Paddles' which we will now refer to as 'Blades'. The blades are fitted into a wheel at an angle and are called 'RotorBlades'. The wheel is then mounted on to the shaft. This arrangement of a single wheel is called 'one stage' or a'Single Stage Rotor' and does not produce high power. (See Figure. 4)

Figure. 4 Single Stage TurbineFor large processing and generation plants, very powerful turbines are needed for driving machines like compressors,large pumps or generators. In this case, 'MultiStage Turbines' are used. As stated earlier, the energy needed to drive



these turbines, comes from high pressure, superheated steam. In order to get the steam to pass to the rotor blades, weneed a means of directing the steam on to the blades. The piece of equipment used for this is called a ‘NOZZLE’. Asthe steam leaves a nozzle, its pressure decreases and its 'VELOCITY' increases. This high velocity steam jet is directedat the rotor blades and, as in the pinwheel, the rotor and shaft begins to rotate. As more and more steam is releasedon to the blades, the speed of rotation increases. (As with a windmill, stronger wind, faster rotation).MULTISTAGE STEAM TURBINESA Multistage turbine is one which has two or more wheels. The steam is directed on to the blades of the first stagewheel and, as it strikes the angled blades, they move away in an opposite direction to the flow of steam, causingrotation of the wheel and shaft. As the steam gives up energy to move the rotor blades, its pressure is decreased, itsvolume increases and it leaves the blades in an opposite direction to that taken by the wheel.(See Figure. 5)Some method of redirecting the steam on to a second wheel is now needed. To achieve this, a row of fixed,unmoving, angled blades is fitted into a ‘DIAPHRAGM’ which is mounted in the 'CASING' of the turbine. Thesestationary blades are called ' STATOR BLADES '.

Figure: 5 Steam Flow Through the Rotor & StatorThe stator blades act like further nozzles and redirect the steam on to the rotor blades of the second stage wheel.Because the steam pressure has dropped and its volume is greater, to get the same amount of energy out of it, theblades of the second stage are larger (greater area) than those of the first stage. This arrangement of alternating rotorand stator blades and increasing blade size, is continued through the turbine in order to obtain the required amount ofpower from the steam for operational needs. When the steam leaves a turbine, it may still contain a lot of energy pressure and heat. This steam may be directed for use in another process system. (See Figure. 6)



(Figure. 7) Shows a typical lube oil system for a steam turbine.The lube oil system comprises a reservoir or oil tank in which three pumps are immersed. The main oil pump isdriven by a shaft from the Steam Turbine accessory gear. The Auxiliary pump is driven by an A/C motor and is usedfor startup, shutdown and other operating conditions necessitating its use. The 3rd pump is operated by a D/C motor(battery supplied) for use on main power failure shut down of the complete system which will require lube oil whilethe units shut down. From the pumps the oil at the required pressure (controlled by PCV 1 that spills excess back tothe reservoir), passes through 1 of 2 water cooled exchangers (1 operating & 1 standby) and temperature controlled bya TCV. After cooling the main oil flow passes through 1 of 2 filters (1 operating & 1 standby). The filters are fittedwith a Differential Pressure (DP) gauge and alarm which, should the filter begin to get too dirty, at a preset DP willwarn that the filters need changing over and the dirty elements changed out. From the filters the oil passes via acontrol valve (PCV 2) which maintains the desired lube oil pressure to the bearings of the turbine and possibly also toits driven machine Compressor, Generator, pump .. etc. After lubricating and cooling the bearings, the oil returns tothe reservoir. Any oil losses are made up via the oil make up line to the reservoir. In the oil systems, a number ofalarms and shutdown devices are installed to ensure the safe operating conditions for the machine. Hydraulic Oil isprovided from the lube oil system from between the coolers and the filters. This oil may be boosted in pressure,filtered and pressure controlled by PCV/A and is used for the control and shutdown systems of high power steamturbines.

STEAM CONTROL TO A TURBINE

In order to control a turbine speed, a method of controlling the steam supply is needed. To do this, the turbine steaminlet first enters a 'STEAM CHEST'. (Figure. 8) The steam chest contains a series of steam valves which can beopened gradually as required. As each valve opens the flow of steam to the nozzle(s) is increased thus increasing theturbine speed.

Figure 8



CONTROL SYSTEM DESCRIPTION

(See Figure 9).The hydraulic/control oil enters the unit and is piped to the following parts of the system. (The oil pressure dependsupon the maker's specifications).

1. Power Piston2. Trip and Throttle Valve3. Overspeed Trip Mechanisms & Slide Valves 'A' and 'B'4. Trip Solenoid Valve

1. The Power Piston The oil passes to the power piston via Port 'Y' of Slide Valve 'A'. The Governor, on thesignals from the control system will, through a ServoMechanism', adjust the hydraulic oil to the power piston. Thisin turn, controls the steam flow via the steam chest valves to the nozzles, thus controlling the turbine speed.

2. The Trip and Throttle Valve This valve passes the H.P. steam to the steam chest. On startup of the turbine, thesteam chest valves are fully open. The T/T valve is opened slowly by a handwheel until the machine comes toMinimum Governor Control setting the power piston falls and the steam supply to the nozzles comes underGovernor control. When the turbine is under the control of the governor, the T/T valve handwheel is swung to thefully open position. (This will not increase steam flow to the turbine due to the governor control of the steam chestvalves). The high pressure hydraulic oil passes to the T/T valve cylinder via a restriction orifice and the ' Y ' Port ofSlide Valve ' B '. This oil pressure acting on the piston keeps the main steam valve fully open during normaloperation. In order to periodically check the operation of the T/T valve, a 25% stroke solenoid operated valve isfitted. On operation of the stroke check button, the valve is energised and bleeds off an amount of oil from the T/Tvalve cylinder. The T/T valve closes down by 25% without affecting the steam flow to the turbine. When the checkbutton is released, the T/T valve goes to the fully open position again.

3. The Overspeed Trip Mechanism & Slide Valves ' A ' and ' B 'The O/S trip, as its name implies, is a Mechanical shutdown device in the event of turbine excessive speed.(Overspeed trips are discussed later). The slide valves are kept in the ' RUN ' position by applying oil pressure to thevalve piston against a return spring. The oil feed to these mechanisms also passes through a restriction orifice. Fromthis feed line, oil is also piped to the 'Trip Solenoid valve’.

4. The Trip Solenoid Valve This is an Electrical shutdown device which receives a signal from the electrical tripcircuit which includes High vibration, Low lube oil pressure, High bearing temperature, Low hydraulic oil pressure... etc. The electrical signal energises the solenoid which opens the valve and dumps the hydraulic oil back to thereservoir. The oil pressure is dumped to zero Psi due to the oil flow rate through the restriction orifices being LESSthan the flow of oil returning to the reservoir. When the oil dumps, Slide valves ' A ' & ' B ' are pushed across bytheir springs. This CLOSES the oil supply to the T/T valve and to the Power piston via the ' Y ' Ports and OPENSthe ' X ' Ports to dump the oil. The T/T valve closes and the steam flow is stopped. At the same time the Power pistonrises to fully open the steam chest valves. (No steam can flow as the T/T valve has closed). Before resetting the tripcondition electrical or mechanical, the T/T valve handwheel must be spun to the closed position and made ready forstartup and the governor control system set to minimum governor. When the trip system is reset, the hydraulic oilpressure is restored and the two slide valves move across to the ' GO ' position again. The machine can now be restarted.



TURBINE CONTROL SYSTEM

Figure. 9

TYPES OF STEAM TURBINE

(Figure. 10)For any steam turbine to operate, a pressure difference must exist between the steam supply and the exhaust. Wherethe exhaust steam is above atmospheric pressure, the turbine is classed as a ‘Back Pressure Turbine' or 'NonCondensing Turbine'.I. BACKPRESSURE STEAM TURBINESAs an example, taking a 600 Psi steam supply to a turbine, the turbine speed is controlled by the steam input. If wehave an exhaust pressure of say 125 Psi (a D.P. of 475 Psi), this exhaust steam will still contain a lot of heat andpressure energy and may be used to drive other smaller turbines and for heating purposes in reboilers, heaters,vaporisers...etc. In this type of turbine, the exhaust must be maintained at a constant pressure by a PCV control systemdownstream of the turbine exhaust to prevent changes in the exhaust pressure that would affect the turbine speed bychanging the pressure drop across it. The governor would be fighting against these pressure fluctuations and speedcontrol would be erratic. II. CONDENSING STEAM TURBINESIn a condensing steam turbine, the maximum amount of energy is extracted from the steam. This is achieved bypassing the exhaust steam into a condenser (called a Surface Condenser). The steam is condensed by surface contactwith bundles of tubes through which cooling water is passing. As the steam condenses, its volume, on changing towater, decreases by about 1800 times. This great decrease in volume causes a vacuum to form in the condenser. Dueto this, the pressure drop across the turbine and therefore the turbine power is maximised. The steam condensate(water) is level controlled in the condenser and pumped back to the steam generation plant. However, although thewater for the steam generation is purified and treated, the steam will still contain some Noncondensibles. These willbuild up in the surface condenser and gradually destroy the vacuum, thereby decreasing the P.D. across the turbineand thus decreasing its efficiency and power. In order to maintain the vacuum, the noncondensibles must be removedfrom the surface condenser. This is carried out by a system of ‘STEAM EJECTORS' and ‘Ejector Condensers' whichpull the gases from the surface condenser and eject them to the atmosphere.

TYPES OF STEAM TURBINES



Figure. 10

REMOVAL OF NONCONDENSIBLES

As stated, the gases are removed from the surface condenser by a system of 1st and 2nd stage ejectors and condensers.An ejector consists of a 'Venturi Tube' through which a jet of high velocity steam is passed. This high velocity steamcreates ‘suction’ (vacuum) in the Venturi tube. Vacuum is increased by condensing the steam as it leaves the ejector.The noncondensibles are piped into the low pressure area of the 1st stage ejector and are carried with the steam intothe 1st stage ejector condenser. The water produced from the steam is piped back to the surface condenser. Again,these gases, as they build up in the 1st stage ejector condenser, will tend to destroy the vacuum. To prevent this, thegases are pulled from the 1st stage system into the 2nd stage by another ejector. Again the steam is condensed andpiped back to the surface condenser. In the 2nd stage condenser, the gases are allowed to build up pressure until, atjust above atmospheric pressure, a check (nonreturn) valve will open and pass them to atmosphere. As they escape,the pressure drop causes the check valve to close again. This is a continuous process. A water level is maintained inthe ejector condensers by a ' Loop ' seal tube to prevent the gases also returning to the surface condenser.(Figure. 11)SURFACE CONDENSER

Figure. 11



TURBINE SEALSTEAM SYSTEM (Figure. 12)

When a condensing steam turbine is first started up and in a lowload condition, steam from the inlet (H.P.) end willleak from the outboard gland even though carbonring seals and labyrinth seals are installed to minimise the leakage.Superheated steam is invisible and, due to its high temperature, is very dangerous. Leakage of steam is also a wasteand is not desirable. Conversely, under the same lowload conditions, the L.P end of the turbine will be under thevacuum of the surface condenser. The vacuum will tend to pull in cold atmospheric air through the seals along theshaft. Cold air will have a detrimental effect on the hot metal of the shaft which can lead to damage. In order tominimise these problems, a manually controlled supply of low pressure SEAL steam (about 2 Psi), is piped to acommon line feeding the glands of the machine. This pressure will prevent the ingress of air at the L.P. end andensure a positive pressure at the H.P. end during startup. (The ejector units are started and vacuum pulled beforestarting the turbine). When the turbine load is increased, the leakage of steam into the Sealsteam header will causegreater pressure than the Seal steam supply and will begin to flow to the L.P. end seal. At this point, the Seal steamsupply can be shut down and the Sealsteam taken from the H.P. end and manually maintained at 2 Psi by ventingexcess into the surface condenser. Even when the seal pressures are maintained at 2 Psi, some leakage occurs fromboth seals. This is taken care of by the LEAK steam recovery system.

TURBINE LEAKSTEAM SYSTEM (Figure. 13)Leakage of steam from turbine glands to atmosphere (in both types of turbine), as already mentioned, is dangerousand a waste of steam. The turbine inboard and outboard glands are fitted with carbon ring seals and labyrinth seals tohelp prevent leakage. These seals do not completely stop the escape of steam. In order to completely prevent it, aturbine will also be fitted with a LEAKsteam system which pipes the leakage into a water cooled condenser. Thecondensate from the condenser is drained away to a 'Hotwell' from where it is returned to the steam generation plant.The leak steam condenser is fitted with an ejector system to remove noncondensibles from the steam and dischargethem to atmosphere with the ejector exhaust steam. This, in the case of the condensing turbine, is to prevent noncondensibles from reentering the surface condenser.

SEAL STEAM SYSTEM

Figure. 12

TURBINE LEAK STEAM SYSTEM



Figure. 13

About the Author

Norrie is a retired professional who has been working in Oil and Gas and LNG production in MarsaelBrega, Libyafor 30 years.

Norrie used to be in the Training Dept. and prepared Programmes for Libyan Trainees.

Last Updated on Wednesday, 24 February 2010 20:09

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Introduction to Turbines & Steam Turbines