Steam Turbines

Front 1

Why have steam turbines almost completely replaced steam engines as prime movers in a nutshell?

Back 1

Their greater thermal efficiency and more compact size

Front 2

What is an advantage of the steam turbine's rotary motion?

Back 2

It is easily coupled with an electrical generator and thus most of the world's electricity is produced using steam turbines.

Front 3

What are some general advantages of steam turbines over steam engines?

Back 3

1. Maintenance costs and reliability are improved.

2. Lubrication is simpler.

3. Turbine requires no cylinder lubrication other than for bearings and therefore it delivers oil-free condensate or exhast steam which can be reused.

4. Bearings are located outside of the high temperature steam zones and therefore high pressure and temperature steam can be used

Front 4

What by-product do steam turbines allow plants to produce?

Back 4

Many manufacturing industries generate their own electrical energy cheaply as a by-product to the production of process steam.

Front 5

What are steam turbines used for in industry?

Back 5

Steam turbines may be used as prime movers for large alternators in central stations or as reliable drives for fans, pumps, and other auxiliaries located in the power plant itself, as well as industry in general.

Front 6

Describe in a nutshell how steam turbines work.

Back 6

The steam turbine operates through the action of a flow of steam directed by stationary nozzles or blades on to rings of rotating blades. There are basically two types of steam turbine, namely impulse and reaction, the difference being in the manner in which the steam is expanded through the turbine stages.

Front 7

What is the distinguishing characteristic of Impulse Turbines?

Back 7

If all of the steam pressure drop takes place in the stationary nozzles then the turbine is known as an impulse turbine.

Front 8

What is the distinguishing characteristic of Reaction Turbines?

Back 8

If the steam pressure drops in passing through moving blades as well as the stationary nozzles or blades, the machine is called a reaction turbine.

Front 9

What are two of the more important elements of steam turbines?

Back 9

1. Stationary Steam Passages

2. Rotor Flow Passages

Front 10

Describe Stationary Passages in Impulse Turbines.

Back 10

The stationary passage of an impulse turbine consists of one or more stationary nozzles. The nozzle allows steam at high pressure to expand and convert some of its thermal energy into kinetic energy so that the issuing flow of steam is at a lower pressure but travelling at high speed.

The nozzle is so positioned as to direct the flow of steam onto the rotor blades, imparting an impulse force to the rotating blade as the steam jet's direction of flow is changed by the shape of the rotating blades.

Front 11

Describe Stationary Passages in Reaction Turbines.

Back 11

Reaction turbines use stationary blades to redirect the steam into the rotating nozzle shaped blades of the rotor, where further expansion takes place, imparting a reaction force to the blades, causing them to rotate.

Front 12

What are Rotor Passages?

Back 12

Blades or buckets form the rotor flow passage and serve to change the direction of the steam received from the stationary nozzles. Steam emerges from the nozzle at a high velocity and strikes the blades which change its direction of flow.

Front 13

How do the shape of Impulse blades affect the steam passage?

Back 13

The impulse blades are so shaped that, when fitted into a blade wheel, the space between blades does not allow any steam pressure drop in its passage through. Thus the whole of the thrust imparted by the steam on the blades derives from the change in momentum, or impulse, of the steam as it changes direction to follow the shape of the blades.

Front 14

How do the shape of Reaction blades affect the stream passage?

Back 14

The reaction blades on the other hand are specially sized and shaped to allow a steam pressure drop. This in turn produces an increase in steam speed and causes a reaction or back thrust on the blading.

Front 15

How are impulse and reaction blades used in combination in large steam turbines?

Back 15

Most large steam turbines use a combination of impulse rotors and reaction rotors with the impulse rotors at the front end where steam velocity is the highest. As steam moves through the impulse rotors, its velocity decreases while it's pressure remains constant. Then as the steam moves into the reaction rotors, the velocity increases while the pressure reduces in each moving rotor wheel. The reaction rotor gets get larger in diameter, taking advantage of the reducing pressure.

Front 16

Describe the bearings used in steam turbines.

Back 16

The bearings have a split sleeve and oil ring with a precision bored high grade babbitt. Lubrication is by oil rings that are rotated by the shaft to bring oil up from the bearing sump. Either bearing may be removed without disturbing the wheel case cover. The governor end bearing serves as a combined thrust and journal bearing.

Front 17

Describe the Glands used in steam turbines.

Back 17

The glands consist of several adjacently mounted segmental carbon rings, each maintained in close contact with the shaft by a garter spring and each located in its own independent groove. This placement results in a negligible leakage of steam from the wheel case along the shaft.

Front 18

Describe the Rotor and Shaft Assembly used in steam turbines.

Back 18

The rotor consists of a carefully machined and balanced forged steel disc pressed over a key on a completely ground shaft. At the gland areas, the shaft is protected by a generous coating of stainless steel applied by the metal spray process.

Front 19

Describe the Blades used in steam turbines.

Back 19

All blades are durable rolled and drawn stainless steel and are securely held in machined slots in the wheel rim by drive screws.

Front 20

Describe the Shrouding used in steam turbines.

Back 20

Inner and outer ends of blades are stainless steel shrouded to confine steam to the blade passage and stiffen the blades against vibration.

Front 21

Describe the Casing used in steam turbines.

Back 21

The top half may be removed without disturbing any other part or piping connections. The casing has true center-line support. There are no internal high pressure joints; the casing is subject to exhaust pressure only. The casing has two exhaust openings so that the exhaust line may be connected at either side. The unused exhaust opening is provided with a blind flange which may be removed for wheel, blade and nozzle inspection.

Front 22

Describe the Overspeed Trip used in steam turbines.

Back 22

When the steam turbine speed reaches the tripping speed (normally 10 - 15% over the rated speed) the trip plunger is flung out against spring force by centrifugal force or shaft rotation, far enough to strike the spring loaded trip lever. The trip valve is spring-loaded to shut and is built such that it closes in the direction of steam flow so steam flow slams shut. As the turbine rotor speed reduces, the centrifugal force is no longer sufficient to hold the plunger out against it's spring so the spring pulls the plunger back. The overspeed trip linkage must be reset, which is done externally to the machine.

Front 23

Describe the Governor used in steam turbines.

Back 23

The governor is standard equipment and is hydraulically operated, directly driven from the turbine shaft, and mechanically sensing. High sensitivity keeps the linkage constantly in motion; this helps prevent the governor valve stem from sticking. There are no expensive knife edges to replace. The governor may be replaced as a unit in the field since the governor mounting is the same for all turbine frame sizes. At rest, the governor valve is closed preventing accidental startup.

Front 24

Describe the Governor Valve used in steam turbines.

Back 24

A Monel (or stainless steel) double ported, balanced floating type with renewable cage, no-resist governor valve stem bushing and stainless steel stem is used to prevent binding and friction.

Front 25

Describe the Hand Speed Control used in steam turbines.

Back 25

The hand speed control is located on the governor with a normal range of 10% above and 50% below the rated speed.

Front 26

Describe how the Governing System works in steam turbines.

Back 26

The governor oil pump, gear-driven by the turbine shaft, furnishes pressure oil through two passages to the pressure relay.

One passage leads directly to this pressure relay and provides a signal oil pressure varying with speed; the other leads through a fixed orifice to the operating bellows of the steam control valve and to the drain valve of this pressure relay.

Thus, the signal pressure and power pressure oppose each other across the drain valve, with diaphragm spring pressure normally holding the system in equilibrium. This spring compression is adjusted by the hand speed changer, and its setting determines the speed which is to be held by the turbine.

When turbine speed changes, the two opposing forces on the drain valve become unbalanced. If speed decreases, this valve opens further, more oil drains to the tank, and the pressure in the power pressure line is reduced. When speed increases, the valve opening is reduced, and pressure in the power pressure line is increased.

The operating bellows responds to these changes in pressure by opening and closing the steam admission valve through the operating valve stem. The entire system, working as a relay, is powerful, yet sensitive.

The governing valve and the trip-throttle valve are combined in the same assembly. The trip throttle valve controls steam admission to the turbine on starting and also shuts off all steam in case of overspeeding.

After an emergency shut down it is possible to reset the combined top-throttle valve and run up the turbine again without the necessity of shutting off steam valves ahead of the turbine.

Front 27

Why is excess lubricating oil supplied to turbine bearings?

Back 27

The purpose of this excess supply is to carry away the heat which is conducted along the shaft from the steam space and so maintain the bearings at a safe working temperature.

Front 28

Describe how the Oil Lubrication system works in steam turbines.

Back 28

The oil pump, gear-driven from the main turbine shaft, furnishes oil under pressure for positive bearing lubrication. From the pump, oil passes first through an oil cooler and reducing valve on the way to the bearings. This pressure oil is then fed through an opening in the top-half of the bearing shell to distribution grooves which feed the entire surface of the bearing. Grooves cut in the surface of the thrust bearings are shaped so that oil is picked up readily by the thrust runners and fed to the thrust bearing surfaces.

Front 29

What is the benefit of pressure lubrication?

Back 29

Pressure lubrication contributes to bearing life and helps assure continuous, reliable service of the turbine, with minimum maintenance.

Front 30

How is the oil level checked in steam turbines?

Back 30

Oil level in the reservoir is checked by means of a bayonet-type gage, or an oil sight glass.

Front 31

What is the function of thrust faces?

Back 31

Thrust faces operate to locate the turbine shaft in its correct longitudinal position by maintaining an oil wedge between the faces and the thrust runners on the shaft. This oil wedge acts to counteract thrust or axial movement of the shaft in the turbine casing and bearings, thus maintaining the relative position of the rotor wheel buckets to the steam nozzle(s).

Front 32

Describe the function of carbon packing rings for the turbine shaft steam glands.

Back 32

Steam-tight glands are necessary on steam turbine shafts to restrict the flow of steam from the steam space along the shaft to the atmosphere. It should also be pointed out that while tight radial clearance prevents steam leakage along the shaft, past the inner circumference of the carbon rings, the steam pressure forces the outboard face of the ring against the side of the packing box ring groove to prevent steam leakage "around" the outside of the ring.

Front 33

Describe Packing Boxes in steam turbines

Back 33

Packing boxes are integral units inserted and bolted in the casing, and split on the horizontal center-line. The packings contain graphite, and are self-lubricating. Shafts are Monel -metal sprayed at the carbon packing fit to minimize corrosive action.

Front 34

Describe the arrangement of Carbon Packing Rings.

Back 34

To obtain an efficient shaft steam seal, carbon packing rings are assembled around the shaft in each section of the packing boxes. The carbon packing rings in the packing boxes consist of three segments with the outer surface of each ring grooved to receive a spring. Spring pressure, combined with the steam pressure, holds the packing firmly in place axially, but permits slight radial movement for automatic concentric alignment with the shaft.

Front 35

Where are Labyrinth Glands usually employed?

Back 35

High output machines, operating with high temperature, high pressure steam usually employ labyrinth type glands to prevent the escape of steam along the shaft.

Front 36

What are Labyrinth Glands?

Back 36

A labyrinth gland, as the name implies, offers a passage to the steam which is very narrow and winding.

Front 37

What is the benefit of Labyrinth Glands?

Back 37

The clearance between turbine shafts and casing is usually only a very few microns. Consequently, any steam escaping through this fine gap is immediately reduced in pressure. A number of such pressure breakdowns, as that provided by Labyrinth Glands, is sufficient to restrict the steam leakage to atmosphere of even high pressure steam to a very small amount.

Front 38

What is the importance of having an Overspeed Trip?

Back 38

Turbines must be protected against rotating at excessive speeds. They are designed to withstand the centrifugal forces which are present when running at all speeds up to their specified maximum. A sudden loss of load, however, can cause an unprotected machine to increase in speed to such a great extent that the centrifugal forces produced will tear the blades from the blade wheels. The final result is a complete wreck of the turbine with considerable risk of injury to operating personnel.

The overspeed trip is designed to protect the machine against these dangers. Most designs rely on centrifugal force to release some catch which, in turn, closes the steam supply valve.

Front 39

Describe how the Overspeed Trip works.

Back 39

Positive overspeed protection is claimed for this emergency governor. The governor consists of a spring-loaded unbalanced weight mounted in the turbine shaft. If for any reason the speed increases to trip speed (usually 15 per cent above normal) centrifugal force causes the weight to move outward and trip a latch. This closes the trip-throttle valve through a direct mechanical linkage, instantly shutting off all steam to the turbine. To eliminate hazards in explosive atmospheres, the bolt is made of non-sparking material. The governor itself is totally enclosed in the main governor casing. Operating in an oil atmosphere, the probability of rusting or sticking is minimized and reliable operation is assured to provide greater safety for personnel and equipment.

Front 40

What is the purpose of diaphragms in steam turbines?

Back 40

These are bedded into the turbine casing and carry the fixed blades or nozzles. They form the stationary steam passages.

Front 41

What is the function of the Governer in steam turbines?

Back 41

A turbine governor is installed in order to regulate automatically the speed and the power output and to enable changes in these to be made when required.

Front 42

Describe Governors in steam turbines.

Back 42

The great majority of turbine governors are of mechanical centrifugal type. The movement of a pair of revolving weights with changes of speed is applied to the turbine steam supply valve and acts either directly, or via a relay mechanism, to vary the steam supply to the turbine.

Front 43

Describe how Governors work.

Back 43

A pair of governor weights are made to revolve by the speed of the turbine shaft. An increase of speed causes them to fly outwards and to compress a spring until the increased pressure in the spring just balances the centrifugal force and the governor sleeve takes up the new position dictated by the increased speed. Movement of the governor sleeve can then be utilized to operate the turbine steam supply valve either directly or through a relay mechanism.

Front 44

What is the mechanism by which steam is throttled in steam turbines?

Back 44

Oil pressure, exerted above or below a piston in the relay cylinder, is used to move the turbine throttle valve. The pilot valve controls the flow of this oil under pressure by covering or uncovering ports leading to the relay cylinder.

Front 45

How does the Governer Sleeve motion affect Turbine speed?

Back 45

A change of turbine speed causes movement of the governor sleeve. This in turn moves the pilot valve and causes oil to flow to the relay piston. The resulting movement of the throttle valve spindle (and relay piston rod) resets the pilot valve and the throttle valve movement ceases. The whole system is then set in a new position to suit the new speed.

Front 46

How can steam turbine speed be adjusted?

Back 46

A hand control is superimposed on this equipment, named the “Speeder” handwheel, to allow adjustment of the turbine speed when running on load.

Front 47

What is the general procedure for starting larger turbines of the condensing type?

Back 47

1. Start the auxiliary oil pump. Note whether it is delivering oil to the bearings at the correct pressure.

2. Start the circulating water pump and see that it is properly primed.

3. Start the condenser air pump and keep it running slowly.

4. Start the condensate extraction pump.

5. Open all the drains in preparation for warming up and removing condensate.

6. Open the throttle valve or throttle by-pass valve quickly to start the rotor moving. As soon as the rotor begins to turn, close the throttle valve to a point which enables the machine to run light at a speed suitable for warming up the unit. The sealing steam should now be turned on to the glands, and the speed of the air pump (or steam supply to the steam jet air ejectors where these are fitted) should be adjusted so that the vacuum is suitable for warming up purposes. This will vary from 380 to 635mm (15 to 25 in.) of mercury.

7. If it is the practice to test the hand-trip gear every time the machine is started, then this should now be done. The turbine is re-started in accordance with (6) above.

8. Turn on the water supply to the oil cooler.

9. While the machine is being warmed up, attention should be given to the bearings to ensure that they are receiving an ample supply of oil.

10. When the turbine is properly heated, the throttle valve may be opened further, so as to cause a gradual increase in speed. As the machine approaches its full speed, the governor will take charge and maintain the correct speed. As the machine is gaining speed, the operator should listen for internal noises or signs of vibration. If there are obvious signs of a vibration which is not understood, the turbine should be at once shut down, and an attempt made to re-start after further warming through. If, on a second attempt to start, the vibration recurs, then, of course, it is advisable to shut down and seek the cause of the trouble.

11. As the turbine speed is increasing, the main oil pump should take charge of the oil supply to the bearings and the governor gear, and the auxiliary pump should either stop or run at a very slow speed. This should be verified.

12. If the glands are sealed hydraulically, the water may be turned on, and the steam supply shut off.

13. Close the drains and increase the vacuum to its maximum value.

14. If the turbine is running satisfactorily, the throttle valve may be opened full, and then closed half a turn to prevent it from locking. The turbine is then ready to take a load, which should be added gradually, particularly with large machines.

Front 48

What is the general procedure for stopping larger turbines of the condensing type?

Back 48

1. If the turbine is driving an alternating current generator, then, in general, the generator is first unloaded and electrically disconnected and the operator reduces the speed of the turbine in question.

2. As the load is being taken off, regulate the supply of steam to the glands to conform to the new conditions.

3. Start the auxiliary oil pump. Where the control is automatic, check its operation later as the turbine comes to rest.

4. When the order to stop the machine is received, the stop valve may be closed gradually or tripped by hand. Once a week (or at other specified intervals) the operation of the emergency governor should be checked.

5. See that the bearings are receiving a proper supply of oil while the machine is slowing down.

6. Shut off the supply of steam and water to the glands.

7. When the machine has stopped, shut off the water supply to the oil coolers and stop the auxiliary oil pump.

8. Close the steam isolating valve and open the drain between the isolating valve and stop valve. This will prevent steam from escaping past the stop valve, while the machine is standing, in the event of the isolating valve and stop valve being not quite tight.

9. Stop the condenser air and circulating water pumps.