Steam Turbine Principles and Design Flashcards

Overview of Steam Turbine Principles and Design

Steam turbines are essential components in power generation and industrial applications, ranging from small mechanical drives to massive turbo-generators producing in excess of $600\,MW$. This chapter details the operational mechanics, thermodynamic principles, and structural configurations of both impulse and reaction turbines, alongside their auxiliary systems.

Impulse Turbine Operating Principles and Nozzle Design

An impulse turbine operates on the principle of expanding high-pressure steam through stationary nozzles to convert thermal energy into kinetic energy. The resulting high-velocity steam jet strikes moving blades (buckets), transferring momentum and creating an impulse force that rotates the rotor.

Steam Nozzle Construction

Nozzles are typically manufactured from Monel metal, a nickel-copper alloy chosen for its high tensile strength. There are two primary configurations:

• Convergent Nozzles: These are designed for small pressure drops. The steam pressure decreases to a specific exit pressure known as the critical pressure, which is defined as $0.577$ times the inlet pressure ($P_{exit} = 0.577 \times P_{inlet}$). If the exit pressure falls below this threshold in a convergent nozzle, energy is lost to eddy currents (turbulence) rather than increasing velocity.

• Convergent-Divergent Nozzles: These are utilized for larger pressure drops. The nozzle has a throat (narrowest point) where the pressure equals the critical pressure. The divergent section allows for the increasing volume of steam as pressure continues to drop, allowing velocity to increase across the entire nozzle length without forming eddy currents.

Pressure-Velocity Profile

In a simple impulse turbine:

• Steam Pressure: Decreases significantly within the nozzles and remains constant as it passes through the moving blades.

• Steam Velocity: Increases sharply in the nozzles and decreases as it passes through the moving blades (transferring kinetic energy to the rotor).

Reaction Turbine Operating Principles

In a reaction turbine, the steam expands as it passes through both stationary and moving blades. The moving blades are shaped to create a reaction force, similar to the principle of a jet nozzle where unbalanced pressure opposite an opening creates motion.

Blading Characteristics

Reaction blading features row of stationary blades alternating with rows of moving blades. Stationary blades act as nozzles, increasing steam velocity. The moving blades have the following characteristics:

• Shape: Rounded leading edges and a long, tapered trailing edge.

• Angles: Large entrance angle (approximately $90^\circ$) and a small discharge angle.

• Pressure Drop: The distinguishing feature is that pressure drops across the moving blades as well as the stationary ones.

Pressure-Velocity Profile

In a reaction turbine:

• Steam Pressure: Decreases across both stationary and moving blades.

• Steam Velocity: Increases in the stationary blades (absolute velocity) and decreases in the moving blades.

Compounding in Impulse Turbines

Compounding is used to reduce high steam and blade velocities to manageable levels, preventing excessive centrifugal forces and reducing friction losses. To be efficient, blade velocity should be half of the inlet steam velocity. Compounding methods include:

• Pressure Compounding (Rateau Turbines): Uses two or more impulse stages in series. Each stage consists of a set of nozzles and moving blades separated by diaphragms. Pressure drops in the nozzles and stays constant in the blades.

• Velocity Compounding (Curtis Turbines): One set of stationary nozzles causes the total pressure drop. The steam then passes through multiple rows of rotating blades with stationary redirecting blades in between. The first row of rotating blades reduces velocity by approximately half.

• Pressure-Velocity Compounding: A combination where the total pressure drop is divided between two or more velocity-compounded stages. This allows for very high steam pressures while maintaining low rotational speeds.

Turbine Classifications and Configurations

Turbines are classified by steam flow, exhaust conditions, and application:

• Flow Direction: Single flow (steam travels in one direction) or Double flow (steam enters the center and flows toward both ends, eliminating axial thrust).

• Non-Condensing (Back-Pressure): Exhausts steam at pressures above atmospheric for use in heating or process headers.

• Condensing: Exhausts steam into a condenser at pressures below atmospheric (vacuum) to maximize energy extraction; primarily used for electricity generation.

• Bleeder vs. Extraction: Bleeder turbines draw off uncontrolled steam (up to $20\%$) for feedwater heating. Extraction turbines draw off a controlled amount of steam at specific pressures for industrial processes.

• Topping Turbines: Back-pressure units that take high-pressure steam and exhaust it into existing lower-pressure headers or turbines.

• Mixed-Pressure: Accepts steam from multiple sources at different pressures (e.g., HP boiler steam and LP process steam).

• Compounded Arrangements: Cross-compounded (separate shafts and loads) and Tandem-compounded (coupled shafts driving a single load).

• Reheat: Steam is extracted after the HP section, reheated in the boiler to prevent condensation in the LP stages, and returned to the IP section.

Turbine Casings and Fittings

Casing Materials and Temperature Limits

Casings (shells) are typically horizontally split for ease of maintenance. Materials are selected based on operating temperatures:

• Cast Iron: Up to $230\,^{\circ}C$.

• Cast Carbon Steel: Up to $425\,^{\circ}C$.

• Cast Alloy Steel (e.g., 3% Cr, 1% Mo): Above $550\,^{\circ}C$.

Casing Fittings

• Drainage: Grooves in the casing remove moisture (allowable wetness limit is approximately $14\%$).

• Sentinel Valves: Located at the highest point of the casing. These provide an audible whistle as a high-pressure warning during start-up; they are not intended to replace full-flow relief valves.

Shaft Sealing Arrangements

To prevent steam leakage out and air leakage into (ingress) the turbine:

• Carbon Ring Seals: Used on small turbines (Shaft\,Diameter < 150\,mm). Segments are held by garter springs.

• Labyrinth Seals: Consist of thin interleaved rings (rings on casing or shaft) creating a difficult zigzag path for steam. In condensing units, gland seal steam ($10\,kPa$ to $20\,kPa$) is used to prevent air ingress.

• Water Seals: An impeller creates a water barrier; useful for LP glands to prevent air infiltration but requires high rotational speed to function.

Rotor and Blade Design

Rotor Types

• Disc Rotors: Used in impulse turbines. Discs are either shrunk/keyed onto the shaft or solid-forged. Shrink fits involve making the disc opening $0.1\%$ smaller than the shaft and heating it for installation.

• Drum Rotors: Used in reaction turbines. Hollow or welded construction reduces surface area, thereby minimizing axial thrust.\n

Blade Attachment

• Impulse: Mounted in diaphragms (stationary) or attached to disc grooves (moving). Moving blades often feature shrouding (metal strips) joined via tangs to stabilize the assembly.

• Reaction: Stationary blades fit into casing grooves; moving blades fit into rotor grooves. Knife-edge seals and radial fins are used to minimize tip leakage.

Managing Turbine Thrust

Thrust is minimal in impulse turbines but severe in reaction turbines due to the pressure drop across moving blades.

• Double Flow Design: Cancels thrust by flowing steam in opposing directions.

• Thrust Bearings: Kingsbury (or Michell) bearings utilize tilting pads on a stationary plate to create an oil wedge against a rotating collar.

• Dummy Pistons: Enlarged rotor sections at the inlet where steam pressure creates a counter-force to the blade thrust. A balance pipe connects the LP side of the piston to a point along the turbine casing.

• Thrust Adjusting Gear: Manually adjusts the axial position of the shaft relative to the casing to set clearances during operation.

Small and Large Mechanical Drive Turbines

Small Turbines

• Capacity: $8\,kW$ to $900\,kW$.

• Speed: $1000\,rpm$ to $6500\,rpm$.

• Steam Conditions: Typical units consume steam at $4200\,kPa$ and $400\,^{\circ}C$.

Large Turbines

Large mechanical drives (e.g., $3955\,kW$ at $3600\,rpm$) drive high-volume loads like feedwater pumps or gas compressors. They often incorporate multiple extraction points for process flexibility.

Speed Reduction Gears

Reduction gears match high-speed turbines to slower-driven machinery (generators, pumps, etc.).

• Ratio Calculation: The speed ratio is inverse to the gear tooth ratio. For a turbine at $5600\,rpm$ with a $30$-tooth pinion and a $120$-tooth driven gear: $\text{Driven Speed} = \frac{30}{120} \times 5600\,rpm = 1400\,rpm$

• Gear Types: Helical gears reduce noise and vibration. Double helical (Herringbone) gears are used to eliminate axial gear thrust.

1. What are the two primary types of steam turbines?

   • Impulse turbines and reaction turbines.

2. How does an impulse turbine convert thermal energy into mechanical energy?

   • It expands high-pressure steam through stationary nozzles to create a high-velocity steam jet that strikes moving blades, transferring momentum and rotating the rotor.

3. What is the critical pressure in a convergent nozzle?

   • The exit pressure defined as $P_{exit} = 0.577 \times P_{inlet}$.

4. What is the role of convergent-divergent nozzles?

   • They allow for larger pressure drops and prevent the formation of eddy currents while enabling an increase in steam velocity.

5. In an impulse turbine, how does steam pressure change?

   • Steam pressure decreases significantly within the nozzles and remains constant during its passage through the moving blades.

6. What characterizes a reaction turbine?

   • It has both stationary and moving blades where the steam expands and creates a reaction force.

7. What are the two methods of compounding in impulse turbines?

   • Pressure compounding (Rateau turbines) and velocity compounding (Curtis turbines).

8. What does a non-condensing (back-pressure) turbine do?

   • It exhausts steam at pressures above atmospheric for heating or process headers.

9. What are the materials typically used for turbine casings?

   • Cast iron, cast carbon steel, and cast alloy steel.

10. What prevents steam leakage and air ingress in turbine seals?

    • Carbon ring seals, labyrinth seals, and water seals.

11. What are the characteristics of disc rotors used in impulse turbines?

    • They can be shrunk/keyed onto the shaft or solid-forged, and involve a shrink fit for installation.

12. How is thrust managed in turbines?

    • Through designs like double flow, thrust bearings, dummy pistons, and thrust adjusting gears.

13. What are the speed capacities of small turbines?

    • Ranges from $8 kW$ to $900 kW$ and speeds from $1000 rpm$ to $6500 rpm$.

14. What is the purpose of speed reduction gears?

    • To match the high-speed turbines to slower-driven machinery such as generators and pumps.