Steam Turbine

It is defined as, "A power unit which produce power from a continuous supply of steam being delivered to the turbine at high pressure and exhausted to the condenser at low pressure".

• Motive power is due to the rate of change in momentum.
• Turbine operates at very high speed up to 40,000 rpm and 40% efficiency.
• Hero's Turbine ⇔  in which exhaust causes sphere to spin.

Applications:

• Power Generation
• Petrochemical refineries
• Paper mills
• Sugar Industry

Classification of Steam Turbine:

1. According to the Direction of Steam Flow

• Axial Turbine  ⇔  in which steam flows in the direction parallel to the axis of turbine.
• Radial Turbine  ⇔  in which steam flows in a direction perpendicular to the axis of turbine.

2. According to the Action of Steam on Blades

• Impulse 
• Reaction
• Combination of Impulse and Reaction

Ques: How can we distinguish between Impulse and Reaction Turbine?

Ans: Degree of Freedom (DOR) define it is a reaction of impulse turbine.

3. According to the Number of Stages

• Single Stage Turbine 
• Multi-Stage Turbine

Turbine Components

Components of Turbine are as follows:

1. Moving Blades  ⇔  Fixed around the circumference of a rotor shaft.
2. Blade Shroud Ring  ⇔  top of blades are connected together for rigidity.
3. Steam Chest  ⇔  It is the steam supply chamber which houses steam before being supplied to nozzles.
4. Exhaust Hood  ⇔  The portion of casing which collects and delivers the exhaust pipe (or condenser).
5. Throttle (or Stop) Valve  ⇔  Located in steam supply line.
6. Governor  ⇔  for controlling mass flow to maintain constant speed with load fluctuations.
7. Diaphragms  ⇔  which contains the fixed blades (act as nozzle).
8. Steam Turbine Casing (or Shell or Cylinder)  ⇔  outer enclosure housing containing nozzles and fixed blades. It provides structural frame.
9. Turbine Shaft  ⇔  rotating member upon which moving blades are mounted.
10. Packing  ⇔  used for preventing the leakage across the annular space between the diaphragm and shaft, casing and shaft. For E.g.: Carbon rings, labyrinth glands.

Impulse Steam Turbine

An Impulse Turbine works on the principle of Impulse, means the K.E. of steam is used to exert a force on the moving blades achieved by having the symmetrical blades (means the cross-sectional area of blades is constant.

• 5-15° of angle between nozzle and plane of rotation of steam impulse turbine.
• Pressure on the both sides of moving blades is same.

Velocity Diagram of Impulse Steam Turbine

To evaluate the force on the blades and the power developed by a turbine, it is necessary to determine the rate of change of momentum of steam across the moving blades.

• Velocity Diagram  ⇔  tell amount of velocity of fluid entering and leaving the turbine.
• Labyrinth Seal  ⇔  used to stop leakage of steam.
• On 50 Hz frequency  ⇔  Blades rpm lies between 1500-3000.

• Selection of angle '⍺' should be low as possible to decrease useful tangential component.

Important Points:

• If the steam is to enter and leaves the blades without losses, then relative velocity should be tangential to the blade inlet tip that means angle 𝜷 is minimum.
• If 𝜷1 = 𝜷2  ⇔  Symmetrical Blades (but as time goes along due to erosion/corrosion material wears off).

Steam Turbine Compounding

Compounding is defined as, "Method of reduction of rotor speed of Turbine".

• Stage  ⇔  One row of nozzle followed by one row of blades.
• High speed  ⇔  results in destruction of machine, large centrifugal force, vibration increases, overheating of blades, etc.

Ques: Why do we need Compounding?

Ans: Enthalpy of steam is very high when passed into nozzle  ⇔  steam is then expanded from boiler pressure to condenser pressure in one go  ⇔  rotor speed becomes high (30,000 - 50,000 rev/min), turbine stresses nd friction losses will be high  ⇔  efficiency becomes low  ⇔  So single stage turbine is undesirable.

• In this case, Multiple system of rotor in series (keyed to common shaft) and steam jet is absorbed in stages is used.

Methods of Steam Turbine Compounding:

1. Velocity Compounding or Curtis Staging
2. Pressure Compounding or Rateau Staging
3. Pressure-Velocity Compounding

1. Velocity Compounding

It is defined as, "The expansion of steam takes place in a set of nozzles from boiler to the condenser pressure".

• It consists of two or more rows of moving blades separated by one/two rings of fixed blades.
• Since, area of blades remain same  ⇔  Constant pressure but Velocity changes.
• Moving Blades  ⇔  keyed to shaft.
• Fixed Blades  ⇔  on Casing.

Working:

Velocity of Steam increases in nozzle  ⇔  Enter in Moving Blades (energy absorbed)  ⇔  Velocity increased by Fixed Blades (by decreasing area)  ⇔  Enter n Moving Blades (energy absorbed, blade rotates).

2. Pressure Compounding (or Series of Impulse Turbine)

It is defined as, "Rings of fixed nozzle keyed to casing installed between rings of moving blades are keyed to the turbine shaft".

• No. of stages depends on the pressure required.

Working:

Steam enters in 1st nozzle  ⇔  pressure drop, velocity increase  ⇔  enter in Moving blade-1 (area remains same, pressure constant, velocity drop)  ⇔  enter in 2nd nozzle  ⇔  velocity increase, pressure decrease  ⇔  then enter in Moving blade-2 (pressure constant, velocity drop).

Efficiency and Reheat Factor (R.F.)

Stage efficiency is defined as, "the ratio of actual heat drop to the isentropic heat drop".

• Reheat Factor  ⇔  Ratio of Cumulative Heat drop to the direct isentropic heat drop (or Rankine Heat Drop). It is used to avoid excess moisture in steam at the end of expansion to protect the turbine.

• Due to the divergence of constant pressure lines, R.F. > 1 (mostly 1.04 - 1.08) that means 

ηinternal > ηstage

Reaction Turbine

There is no nozzle to convert steam energy into mechanical energy rather blades are of Airfoil shape.

• Reaction turbine works on the principle of a reaction force on the moving blades due to the steam accelerating through the fixed blades.

Working:

Steam enters  ⇔  pressure drop (velocity increase) due to decreasing area (in fixed blades/ movable blades)  ⇔  similar to various stages.

Degree of Reaction

It is ratio of Enthalpy drop in the moving blades to the sum of enthalpy drop in the fixed and moving blades (in a stage).

• If  (Δh)MB = 0  ⇔  DOR = 0
• If  (Δh)FB = 0  ⇔  DOR = 1 (called Hero's Turbine).
• If  (Δh)MB  =  (Δh)MB  =  (Δh)Stage / 2   ⇔   DOR = 0.5 (called Parson's Turbine).

Note:

• Prototype Steam Turbine Engine
• Industrial Steam Turbine
• Steam Turbine Over Speed Protection
• Reaction Turbine Assembly
• Steam Turbine Maintenance, Repair & Overhaul

References:

• Material from Class Lectures + Book named Power plant Engineering by P.K Nag (4th Edition) + My knowledge. 
• Photoshoped pics are developed. 
• Some pics and GIF from Google.  
• Videos from YouTube (Engineering Sights).