Steam Turbine Principles and Design Flashcards

Flashcards covering the principles, designs, and components of steam turbines based on Chapter 1 of the Third Class Power Engineering curriculum.

What is the maximum power capacity mentioned for large turbo-generators?

$$\text{>600 MW}$$

What defines the operating principle of an impulse turbine regarding steam expansion?

Steam expands through a stationary nozzle, causing a pressure drop and a velocity increase.

In a simple impulse turbine, what happens to the steam pressure as it passes through the blades?

The steam pressure remains constant.

What specific force is produced when a high-velocity steam jet changes direction upon hitting a turbine blade?

Impulse force.

What are the two primary disadvantages noted for a simple impulse turbine?

Extreme centrifugal forces and large friction losses.

What nickel-copper alloy is typically used to manufacture turbine nozzles due to its high tensile strength?

Monel metal.

What is the collective name for the passages located outside the casing that direct steam to the nozzles?

Steam chest.

How is the critical pressure of a convergent nozzle calculated in relation to the inlet pressure?

$$0.577 \times \text{inlet pressure}$$

What occurs at the exit of a convergent nozzle if the exit pressure falls below the critical pressure?

The formation of eddy currents or turbulence.

Where is the throat located in a convergent-divergent nozzle?

The narrowest part of the nozzle at the outlet of the convergent section.

What is the purpose of the increasing area in the divergent section of a convergent-divergent nozzle?

To accommodate the increase in steam volume as pressure decreases and velocity increases.

In an impulse turbine pressure-velocity profile, where does the steam velocity reach its maximum?

At the exit of the nozzles.

In an impulse turbine profile, what happens to the steam velocity as it passes through the blades?

The velocity drops as kinetic energy is transferred to the blades.

According to the reaction principle, what happens if steam escapes from an opening in a high-pressure container?

An unbalanced pressure occurs on the opposite wall, creating a reaction force.

How does the leading edge of a reaction blade differ from an impulse blade?

A reaction blade has a rounded leading edge, while an impulse blade has a sharp leading edge.

What is the distinguishing feature of a reaction turbine regarding pressure behavior?

There is a pressure drop across the moving blades.

What is the characteristic entrance angle for a reaction turbine blade?

Large, approximately $90^{\circ}$.

What are the two main design problems caused by the pressure drop across moving reaction blades?

Steam leakage around blade tips and unbalanced axial thrust on the rotor.

Why must blade clearances be kept as small as possible in a reaction turbine?

To minimize steam leakage caused by the pressure difference between the blade inlet and outlet.

At what blade velocity is the transfer of kinetic energy most efficient in an impulse turbine?

When the blade velocity is equal to half of the inlet steam velocity.

What is the purpose of pressure compounding in impulse turbines?

To reduce steam and blade velocity by dropping pressure across multiple stages.

What are the components of a single pressure compounding stage?

A set of stationary nozzles followed by a rotor disc with moving blades.

What physical components separate the rows of moving blades in a pressure-compounded turbine?

Diaphragms.

In pressure compounding, what is the state of steam velocity after passing through the moving blades?

The velocity drops to near zero.

What are the two main operational advantages of pressure-compounded turbines?

Increased efficiency from lower friction and reduced centrifugal forces.

What identifies velocity compounding in a turbine?

One set of stationary inlet nozzles followed by a rotor with two sets of rotating blades and one set of stationary blades.

Where does the total pressure drop occur in a velocity-compounded impulse turbine?

Entirely within the stationary inlet nozzles.

What is the function of the stationary blades in a velocity-compounding arrangement?

To redirect the steam to the second set of rotating blades with minimal velocity loss.

Compared to a single-stage turbine, what is the shaft speed of a two-stage velocity-compounded unit?

Approximately half the speed.

Why are the second set of rotating blades in velocity compounding approximately twice the size of the first set?

To handle the same volume of steam at a lower velocity while maintaining the same impulse force.

What defines a pressure-velocity-compounded turbine?

A system with two or more velocity-compounding sections in series on the same shaft.

What is the main advantage of utilizing pressure-velocity compounding?

High steam pressures can be used while maintaining relatively low rotational speeds.

What is a common arrangement for the first stage of a very large impulse turbine?

A single velocity-compounded stage followed by several pressure-compounded stages.

In a multi-stage arrangement, which nozzle produces the highest steam velocity?

The first nozzle in the velocity-compounded stage.

What defines a single flow turbine?

The steam flows in only one direction along the axis between the inlet and exhaust.

What is the primary advantage of a double flow turbine design regarding axial forces?

The elimination of end thrust.

Why is double flow beneficial for large condensing turbines?

It allows for many stages without requiring excessively large casing diameters for high-volume exhaust steam.

What is another term used for a non-condensing turbine?

Back-pressure turbine.

Where does a back-pressure turbine typically discharge its exhaust steam?

Directly into a lower pressure header for process or heating use.

What is the typical application for a condensing turbine?

As a driver for an electric generator.

What defines a bleeder turbine?

A turbine where steam is removed at intermediate points, with the amount changing as the load changes.

What is the typical maximum percentage of total steam that is bled from a bleeder turbine?

Approximately $20\%$.

How does a mixed-pressure condensing turbine function?

Additional steam is added to the turbine partway between the inlet and exhaust points.

In what type of power plant are mixed-pressure condensing turbines commonly used?

Gas turbine combined cycle power plants.

How does an extraction turbine differ from a bleeder turbine in its operation?

The amount of steam extracted is controlled to maintain a specific required pressure.

What is the specific purpose of a topping turbine?

To take steam from a high-pressure header and reduce it to a lower pressure for use by processors or other turbines.

What does the term 'compounded turbines' refer to in large-scale systems?

Two separate turbines where the exhaust from the first becomes the inlet for the second.

Compare cross compounding and tandem compounding.

Cross compounding uses separate shafts and loads; tandem compounding couples the shafts together for a single load.

In a cross-compounded system, which turbine section usually runs at a higher speed?

The high-pressure (HP) section.

What is the purpose of a reheat turbine?

To maintain efficiency and prevent condensation in the LP stages by reheating steam in the boiler.

What is the most common turbine casing design for ease of assembly and inspection?

Horizontally split casing.

What determines the metal used in the construction of a turbine casing?

The size, operating pressure, and temperature requirements.

Up to what temperature can cast iron be used for turbine casings?

$$230^{\circ}C$$

What is the temperature limit for cast carbon steel casings?

$$425^{\circ}C$$

What alloy is used for high-pressure casings operating above $550^{\circ}C$?

Cast alloy steel containing $3\%\text{ chromium}$ and $1\%\text{ molybdenum}$.

How are casing joints made steam tight without the use of gaskets?

By carefully machining the flange faces.

Why are turbine casing flanges sometimes heated by steam?

To prevent distortion and leakage by ensuring the flanges heat at the same rate as the casing walls.

What is the layout of a double casing (double-shell) turbine?

The high-pressure steam is contained in an inner casing which is then surrounded by an outer casing.

What is the maximum allowable moisture content in turbine exhaust steam?

Approximately $14\%\text{ moisture}$.

What is the main purpose of a sentinel valve?

To provide a visual and audible warning (whistle) of abnormally high casing pressure.

Is a sentinel valve intended to act as a primary pressure relief valve for the turbine casing?

No, its purpose is to warn the operator, not to relieve full pressure.

What fitting is required on the exhaust line if the casing is not designed for full inlet pressure?

A relief valve capable of discharging the full-load steam flow.

When is the sentinel valve most useful for an operator?

During turbine start-up, to verify that the exhaust valve is fully open.

What is the function of shaft seals in a condensing turbine under vacuum?

To prevent the ingress of ambient air into the casing.

Why are carbon ring seals generally limited to shafts less than $150\,mm$ in diameter?

Because of the heat generated when the rings ride on the rotating shaft.

How are the segments of a carbon ring seal held together?

By the force of a garter spring.

What prevents carbon ring seals from rotating along with the turbine shaft?

A keyway and key located in the bearing housing.

What are labyrinth seals typically constructed from?

Brass, stainless steel, or other metal alloys.

What is the 'knife edge' in a labyrinth seal design?

The tapered outer edge of the ring that maintains a minute clearance with the shaft.

At what pressure is gland seal steam typically supplied to prevent air infiltration?

$10\,kPa$ to $20\,kPa$.

What is a stepped labyrinth seal?

A seal where some rings are longer and fit into grooves in the rotating shaft for a more difficult leak path.

How does a water seal create a leak-proof barrier?

An impeller on the shaft throws water outward against a casing to form a barrier.

Why must gland steam be used during start-up for a turbine equipped with water seals?

Because the water seal requires high speed to create sufficient pressure for sealing.

What is the typical construction of an impulse turbine rotor?

A disc rotor consisting of thin discs (wheels) mounted on a small diameter shaft.

Why is there normally no axial thrust on the discs of an impulse turbine?

Because there is no pressure drop across the moving impulse blades.

What is the hub of a turbine disc?

The wider central section of the disc that is shrunk or keyed onto the shaft.

By what percentage is a disc diameter typically smaller than the shaft diameter for a shrink fit?

Approximately $0.1\%$.

What is the primary drawback of machining a solid-forged disc rotor?

High cost, as about $50\%$ of the initial forging is wasted as metal shavings.

What rotor design is used in reaction turbines to eliminate surfaces where pressure drops could cause thrust?

Drum rotor.

How is a hollow drum rotor typically constructed?

The drum is machined in two parts, with the end cover and shaft installed via a shrink fit.

Where are the stationary blades or nozzles of an impulse turbine mounted?

In diaphragms.

How are diaphragms secured within the turbine casing?

They are held in place by keys.

What are 'tangs' on the moving blades of an impulse turbine?

Projections at the tips used to secure the shrouding.

What are the two functions of shrouding in an impulse turbine?

To strengthen the blade assembly and stabilize the blade ends.

How are reaction blades locked into the grooves of the casing or rotor?

Using serrations and a locking strip.

What is the purpose of knife-edge seals in reaction turbines?

To minimize steam leakage across the blade tips caused by pressure differentials.

In reaction blading, what component is used on rotating blades to minimize leakage to the casing?

Radial fins.

What are the two main purposes of a thrust bearing in a steam turbine?

To keep the rotor in a precise axial position and to absorb axial thrust.

Why is a thrust bearing usually located at the steam inlet end of a turbine?

Because blade clearances are most critical at the high-pressure inlet.

What is a Kingsbury (or Michell) thrust bearing?

A bearing using tilting pads that create an oil wedge to carry high thrust loads.

How is the oil wedge formed in a Kingsbury bearing?

The pads tilt as the shaft rotates, allowing oil to be pumped between the pads and the rotating collar.

What is a dummy piston in a reaction turbine?

An enlarged rotor section at the inlet end designed to produce a counter-thrust.

What is the purpose of a balance pipe on a dummy piston?

To subject the low-pressure side of the piston to intermediate pressure, maintaining thrust balance.

What is the function of thrust adjusting gear?

To manually adjust the axial position of the shaft to maintain safety clearances, particularly during start-up.

What is the power rating range for small mechanical drive turbines?

$8\,kW$ to $900\,kW$.

What is the typical speed range for small mechanical drive turbines?

$1000\,rpm$ to $6500\,rpm$.

What are small industrial turbines commonly used for?

Driving centrifugal pumps, compressors, fans, or blowers.

Why are helical gears used in turbine reduction gear sets?

To reduce noise and vibration by allowing gears to mesh more smoothly.

What gear design is used to eliminate the axial thrust produced by standard helical gears?

Double helical or herringbone pattern.

How is the speed of a driven machine calculated when using a reduction gear?

$$\frac{\text{pinion teeth}}{\text{driven gear teeth}} \times \text{turbine speed}$$