Multiple Choice: For each of the following sentences circle the most appropriate letter.

1. The turbine blades are

A. straight      

B. circular

C. curved

 

2. Efficiency of the turbine depends on the following parameters:

A. Inlet and outlet angle of the blade

B. Surface finishing of the blade

C. Profile of the blade

D. All of above

 

3. A steam nozzle converts

A. heat energy of steam into kinetic energy

B. kinetic energy into heat energy of steam

C. heat energy of steam into potential energy

D. potential energy into heat energy of steam

 

4. A nozzle is said to be a convergent nozzle

A. when the cross-section of the nozzle increases continuously from entrance to exit

B. when the cross-section of the nozzle decreases continuously from entrance to exit

C. when the cross-section of the nozzle first decreases from entrance to throat and then increases from its throat to exit

D. none of the above

 

5. The variation of steam pressure in the nozzle depends upon

A. velocity of steam

B. specific volume of steam

C. dryness fraction of steam

D. all of these

 

6. A nozzle is said to be a divergent nozzle

A. when the cross-section of the nozzle increases continuously from entrance to exit

B. when the cross-section of the nozzle decreases continuously from entrance to exit

C. when the cross-section of the nozzle first decreases from entrance to throat and then increases from its throat to exit

D. none of the above

 

7. The nozzle efficiency is the ratio of

A. workdone on the blades to the energy supplied to the blades

B. workdone on the blades per kg of steam to the total energy supplied per stage per kg of steam

C. energy supplied to the blades per kg of steam to the total energy supplied per stage per kg of steam

D. noneoftheabove

 

 

UNIT 7

CASINGS AND SEALS

CASING

The turbine casing completely surrounds the rotor and provides the inlet and exhaust passages for the steam. The casings of turbine cylinders are of simple construction to minimize any distortion due to temperature changes. Casings are usually split horizontally to permit easier access for inspection and repair. 

The flanges are accurately machined to ensure a steam-tight metal-to-metal fit, and the flanges are strongly bolted together. Flange seals, except for corrective maintenance measures, are not normally used. In some (high temperature) units, both top and bottom casing halves are made of two vertical casings bolted together and seal-welded. The inlet end is made of alloy steel (Cr-Ni), while the exhaust end is made of carbon steel. The flanges in the casing are bolted together.

One method of joining the top and bottom halves of the cylinder casing is by using flanges with machined holes. Bolts or studs are insertion into these machined holes to hold the top and bottom halves together. To prevent leakage from the joint between the top flange and the bottom flange the joint faces are accurately machined.

Another method of joining the top and bottom cylinder flanges is by clamps bolted radially around the outer of the cylinder. The outer faces of the flanges are made wedge-shaped so that the tighter the clamps are pulled the greater the pressure on the joint faces.

Each casing has a steam chest to receive the incoming steam and to deliver it to the first stage nozzles. In marine turbines the steam chest is mounted directly on the casing.

The steam chest, located on the forward, upper half of the HP turbine casing, houses the throttle valve assembly. This is the area of the turbine where main steam first enters the main engine. The throttle valve assembly regulates the amount of steam entering the turbine. After passing through the throttle valve, steam enters the nozzle block.

Since a reaction force acts on stationary nozzles and blading, the turbine casing must be securely fixed to a foundation to resist these forces and to prevent the turbine from moving. 

Access openings are sometimes provided in the casing to allow the checking of blading clearances. Other opening in the casing include drain connections, steam bypass connections, openings for rotor ends, pressure gages, thermometers, relief valves and holes for balancing the rotor.

GLANDS AND GLAND SEALING

Steam is prevented from leaking out of the rotor high-pressure end and air is prevented from entering the low-pressure end by the use of glands. A combination of mechanical glands and a gland sealing system is usual.

Mechanical glands are usually of the labyrinth type.

Figure 7.1

(a) Water-sealed glands and labyrinth seals as used on the high-pressureend of condensing

turbines.

(b) Labyrinth-type gland as used on noncondensing turbines.

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 that is grooved on the inside. 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 gland sealing system operates in conjunction with the labyrinth gland where a number of pockets are provided. The system operates in one of two ways.

When the turbine is running at full speed steam will leak into the first pocket and a positive pressure will be maintained there. Any steam which further leaks along the shaft to the second pocket will be extracted by an air pump or air ejector to the gland steam condenser. Any air which leaks in from the machinery space will also pass to the gland steam condenser.

At very low speeds or when starting up, steam is provided from a low-pressure supply to the inner pocket. The outer pocket operates as before.

The gland steam sealing system provides the various low-pressure steam supplies and extraction arrangements for all the glands in the turbine unit.

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. 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 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.

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.

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.

EXERCISES

 

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