Main components of steam turbine:

Stationary components:

Almost all stationary parts are two halves.

Diaphragm: Partitions between pressure stages in a

turbine casing are called diaphragms. They hold the vane-

shaped nozzles and seals between the stages. Usually

labyrinth-type seals are used. One-half of a diaphragm is

fitted into the top of the casing, the other half into the

bottom as in Fig. and Fig. . The interstage

diaphragms are located in grooves in the casing accurately

Fig. Top Diaphragm and bottom diaphragm from Michigan State

University power plant [9]

Steam nozzles: Steam nozzles are installed on the

peripheral of the diaphragms. There are admission Nozzles and

interstage Nozzles their function is to accelerate the steam flow to

high velocity by expanding it to low pressure. Located in the

casing are the steam-admission nozzles which are cut into a solid

block of bronze or alloy steel, depending on steam conditions.

Nozzles are so proportioned as to be contributory to efficient

operation and are made of corrosion- and erosion-resistant

materials. This nozzle block is bolted to the steam chest, which in

turn is bolted to the base of the turbine casing. The entire assembly

of nozzles for one stage is called a diaphragm. The casing

Fig. Two Diaphragm halves

assembly with the stationary blading or nozzles is referred to as the

turbine cylinder. The cylinder of an impulse turbine is frequently

referred to as the wheel casing. (See Fig. )

Rotating parts:

Rotors for small turbines consist of a machined-steel disk

shrunk and keyed onto a heavy steel shaft. The shaft is rust

protected at the gland zones by a sprayed coating of stainless

steel. The rotor is statically and dynamically balanced to

ensure smooth operation throughout its operating range.

Fig. Nozzles, buckets, diaphragm, wheel and Shrouds [ ]

Rotors for large turbines are formed from a single piece

forging, including both the journals and the coupling flange.

Thrust-bearing collar and oil impeller may be carried on a

stub shaft bolted to the end of the rotor. Forgings of this type

are carefully heat treated and must conform to specifications.

Rotors are machined, and after the blades are in place, they

are dynamically balanced and tested. (Fig. )

1.4.2.1 Wheel: A simple turbine consists of a shaft on which is

mounted one or more wheels (discs). On the circumference of the

wheels are located blades or buckets to receive the steam and

convert it into useful work. The rims of the wheels have dovetail

channels for receiving the blades. The ends of the blades are made

to fit these dovetail channels. (See Fig. )

Fig. Rotor of a turbine in Michigan State University power plant

]

Fig. Rotors for various types of turbines (a) rotor for condensing

turbine; (b) rotor for non-condensing turbine; (c) rotor for non-

condensing single-extraction turbine; (d) rotor for condensing double-

extraction turbine. (Siemens Westinghouse Power Corp.)

Turbine blades: On the outer portion, or circumference, of

each wheel located on the shaft are blades where steam is directed

and converted into work by rotation of the shaft (Fig.1.29). There

are many blades in each turbine stage, and larger turbines have

more stages. (Fig.1. ). as the steam flows through the turbine, it

expands and its volume increases. This increased volume is

handled by having longer blades and thus a larger casing for each

stage of the turbine. Figure 1.31 and Figure 1. are a schematic

showing how the blade size varies as the steam flows through the

turbine. The turbine efficiency, as well as its reliable performance,

depends on the design and construction of the blades. Blades not

only must handle the steam velocity and temperature but also must

be able to handle the centrifugal force caused by the high speed of

the turbine. Any vibration in a turbine is significant because there

is little clearance between the moving blades and the stationary

portions on the casing. A vibration of the moving blades could

cause contact with the stationary components, which would result

in severe damage to the turbine. Vibration has to be monitored

continuously and corrected immediately when required.

Fig. Turbine blades of a turbine in Michigan State University

power plant ]

Shroud ring: It is placed around the blades outer ends (Fig.1.26).

The tips of the blades pass through holes in the shroud ring. The

Fig A Turbine blade

ends are then welded so that they are held securely by the ring.

When the blades are very long, extra lacing is sometimes used.

The function of shrouds:

a. Stiffen the blades against vibration

b. Confine the steam to the blade path and prevent steam

axial flow.

Fig. Double-flow low-pressure turbine showing variation in

blade size. (Power Magazine, a McGraw-Hill publication.)

Packing: (steam Sealing)

The shaft at the high-pressure end of the turbine must be packed to

prevent leakage of steam from the turbine. The one at the low-

pressure end of a condensing turbine must be packed to prevent the

leakage of air into the condenser.

There are external steam sealing (high pressure sealing at the

boiler side and low pressure sealing at the condenser side

Fig.1.3 ) and Interstage steam sealing Fig.1.3 .

Types of packing:

Fig. Turbine components

1: Shrouds

2: Diaphragm

3: Nozzles

4: wheel

5: Blades

6: Shaft

Labyrinth packing.

Water seals.

Carbon packing.

Flexible metallic packing.

Fig. external steam sealing [9]

Labyrinth packing

Labyrinth packing is used widely in steam turbine practice. It gets

its name from the fact that it is so constructed that steam in leaking

must follow a winding path and change its direction many times.

This device consists of a drum that turns with the shaft and is

grooved on the outside. The drum turns inside a stationary cylinder

Fig. Interstage steam sealing

that is grooved on the inside (Fig. Fig1.3 ). There are many

different types of labyrinth packing, but the general principle

involved is the same for all. Steam in leaking past the packing is

subjected to a throttling action. This action produces a reduction in

pressure with each groove that the steam passes. The amount of

leakage past the packing depends on the clearance between the

stationary and the rotating elements. The amount of clearance

necessary depends on the type of equipment, steam temperatures,

and general service conditions. The steam that leaks past the

labyrinth packing is piped to some low-pressure system or to a low

stage on the turbine.

Fig. (a) Water-sealed glands and labyrinth seals as used on

the high-pressure end of condensing turbines. (b) Labyrinth-type

gland as used on no condensing turbines. (Siemens Westinghouse

Power Corp.)

Water seals: A water-packed gland consists of a centrifugal-

pump runner attached to the turbine shaft. The runner rotates in a

chamber in the gland casing. In some designs, water is supplied to

the chamber at a pressure of 3 to 8 psi and is thrown out against the

sides by the runner, forming a seal. Water seals are used in

connection with labyrinth packing to prevent the steam that passes

the packing from leaking into the turbine room. Such a seal is also

used on the low-pressure end of condensing turbines. In this case

the leakage to the condenser is water instead of air.

Figure1.3 shows water-sealed glands and labyrinth seals as used

on the high-pressure end of condensing turbines. They are used

singly or in combination, depending on the service required. Each

labyrinth consists of a multiplicity of seals to minimize steam

leakage. The seal rings are spring backed and made of material that

permits close running clearances with safety. The glands are

usually supplied with condensate water for sealing to prevent

contamination of the condensate water. Seal designs are

continuously being improved to minimize steam leakage and thus

improve turbine performance. The illustrated designs are typical of

those found on operating turbines.

Carbon packing: Carbon packing is composed of rings of

carbon held against the shaft by means of springs. Each ring

fits into a separate groove in the gland casing. When

adjustments are made while the turbine is cold, carbon

packing should have from 0.001 to 0.002 in of clearance per

inch of shaft diameter. The width of the groove in the

packing casing should exceed the axial thickness of the

packing ring by about 0.005 in. Carbon packing is sometimes

used to pack the diaphragms of impulse turbines. Steam seals

are used in connection with carbon packing. This is essential

when carbon packing is used on the low-pressure end of

condensing turbines, because if there is a slight packing leak,

steam instead of air will leak into the condenser. In operating

a turbine equipped with carbon packing, a slight leak is

desirable because a small amount of steam keeps the packing

lubricated.

Flexible metallic packing: It is used to pack small single-

stage turbines operating at low backpressure. In most cases

the pressure in the casing of these turbines is only slightly

above atmospheric pressure. The application is the same as

when this packing is used for other purposes, except that care

must be exercised in adjusting. Due to the high speed at

which the shaft operates, even a small amount of friction will

cause overheating.

Bearings: Bearings support and/or properly position the

turbine rotor with respect to the stationary turbine parts.

Types of bearings:

Journal Bearings.

Thrust Bearings.

.1 Journal Bearing: Their main function is to the journal or

radial bearings support the weight of the rotor and position it

radially.

Utility turbines use journal bearing instead of ball or roller

bearings. Journal bearings have a smooth surface of a soft material

called Babbitt. The bearings are fed with oil as the rotor turns; it

produces a pumping action that builds up pressure and a film of oil

between the journal surface and the Babbitt so that in normal

operation the surfaces never touch. Figure 1.36 shows the pressure

distribution of the oil in the bearing.

Thrust Bearing: The thrust bearing absorbs axial forces on

the rotor and positions it axially with respect to the stationary

turbine parts.

The thrust bearing (see Figs. and ) consists of a collar

rigidly attached to the turbine shaft rotating between two Babbitt-

lined shoes. The clearance between the collar and the shoes is

small. The piston is attached to the spindle, and steam pressure is

Fig. Formation of Oil Film in Journal Bearing

exerted on one side and atmospheric pressure is exerted on the

other side. The difference in pressure produces a force that

balances the thrust exerted on the rotating blades. If the shaft starts

to move in either direction, the collar comes into contact with the

shoes, and the shaft is held in proper position. Larger thrust

bearings have several collars on the shaft and a corresponding

number of stationary shoes.

The Kingsbury thrust bearing (Fig. ) is used when a large

thrust load must be carried to maintain the proper axial position in

the turbine cylinder. (The one shown in Fig. 1.37 is a combination

of the Kingsbury and collar types.) The thrust collar is the same as

that used in the common type of thrust bearing. The thrust shoes

are made up of segments that are individually pivoted. With this

arrangement, the pressure is distributed equally not only between

the different segments but also on the individual segments. The

openings between the segments permit the oil to enter the bearing

surfaces. Almost 10 times as much pressure per square inch can be

carried on the Kingsbury-type bearing as on the ordinary thrust

bearing. Axial position of the bearing and turbine rotor may be

adjusted by liners, located at the retainer rings, on each end of the

bearing. The bearing is lubricated by circulating oil to all its

moving parts. The impulse turbine does not require as large a

thrust bearing as the reaction turbine because there is little or no

pressure drop through the rotating blades. However, the thrust

bearing must be used to ensure proper clearance between the

stationary and rotating elements. Reaction turbines that do not have

some method of balancing the force caused by the drop in pressure

in the rotating blades must be equipped with large thrust bearings.

Turbine bearings are subjected to very severe service and require

careful attention on the part of the operator. Most turbines operate

at high speed (3600 rpm) and are subjected to the heat generated in

the bearing itself as well as that received from the high-

temperature steam. These conditions make necessary some method

of cooling. In some cases the bearings are cooled by water

jacketing; in others the oil is circulated through a cooler.

Fig. Main and thrust bearings: (a) main bearing; (b) section of

thrust bearing and housing; (c) thrust bearing cage in place. (Siemens

Westinghouse Power Corp.)

Casing: Casings are steel castings whose purpose is to

support the rotor bearings and to have internal surfaces that

will efficiently assist in the flow of steam through the turbine. The

casing also supports the stationary blades and nozzles for all stages

and also it keeps the steam in the turbine and the air out. The

casing is divided into two halves upper casing and lower casing

Fig.1.39.

The HP/IP turbine always has shells or castings. When steam

pressures and temperatures are high enough, there are two shells

used to split up the pressure and temperature change. The inner

shells are supported and positioned within the outer shell. The

inner shells in turn support and position the other internals,

Fig. Turbine thrust end showing balance piston and thrust bearing.

diaphragms and labyrinth seals. The shells have bolted joints at

the horizontal centerline to permit assembly of the internals. In

operation, the shells are covered with insulation to prevent heat

loss.

The low pressure turbine always has inner and outer shells or

casings. Shells are most common in smaller and older units and

casings on larger newer units. The outer shell or casing prevents

air from entering the turbine exhaust and condenser and directs the

steam from the turbine exhaust to the condenser.

The exhaust hood is connected directly to the condenser, usually at

the end of turbine, and so is under a partial vacuum in operation.

There is a safety device (rapture disc) in the exhaust hood to

prevent excessive pressure buildup if the condenser loses its

vacuum.

1.8 Oil pumps: To pump the lubricating oil to the bearings.

Main oil pump.

AC auxiliary pump.

DC auxiliary pump.

Steam driven pump.

Hydraulically driven pump.

Fig.1.39 Single-casing condensing turbine for approximately35-MW

output. (Siemens Westinghouse Power Corp.).

Front standard: It is an extension to the turbine connected

to it through a key. Also it is not insulated.

The function of the front standard :

Support all control systems (Main oil pump, speed

governor, over speed trip and thrust wear detector).

Support all measuring equipments (Pressure indicators,

Temperature indicators, Speed indicators...).

Steam chest (Nozzle box): In high-temperature turbines

these components are separate from the main turbine structure. In

smaller units, the steam chest is usually mounted directly on the

casing.

It contains the inlet control valves and the admission nozzles

Fig.1. .

The steam chest and valve assembly shown in Fig.1.4 shows

another design, and the illustration identifies the major

components, including the steam inlet, the throttle valve, governor

valve, and valve actuators.

Fig.1 Turbine steam admission section. (Siemens Westinghouse Power

Corp.) . ]

The steam chest is bolted to the base and is made of iron or steel. It

contains a governor valve, a strainer, and an operating hand valve

that is used for manual adjustment to obtain maximum efficiency.

Regardless of whether or not a hand valve is provided, the steam is

made to pass through the governor-controlled admission valve

contained in the steam chest. These are only typical illustrations of

a small turbine design, since there are numerous designs with

different features that vary between manufacturers.

The multi valve steam chest (Fig.1.4 ) is cast integrally with the

cylinder cover with a cored passage from each valve to a nozzle

group. Single-seated valves are used, arranged in parallel within

the steam chest and surrounded by steam at throttle pressure. The

Fig.1. Turbine steam chest and valve assembly. (Siemens

Westinghouse Power Corp.). [1]

governor mechanism raises and lowers the valve-lift bar in a

horizontal plane, opening the valves in sequence, with an

unbalanced force tending to close the valves.

Turning gear:

If a turbine is shut down and the rotor was allowed to rest in one position then due to unequal heating the spindle bends. If the

turbine is a large one, vibration may occur when the turbine is

started again. For these reasons most large steam turbines are provided with motor driven gear to turn the rotor slowly while the

unit is out of service Fig.1.4 . and Fig. 1.44

Fig.1. Simplified steam chest with multiple valves. (Siemens

Westinghouse Power Corp.)

The function of turning gear:

Rotates the turbine rotor after shutdown at low speed.

Rotates the turbine rotor at low speed (20-30 rpm) before

starting up.

Decreases the starting torque.

Fig.1. Turning gear of a steam turbine at Michigan State University

power plant ]

Fig.1. Simplified Turning Gear