Turbine Presentation

OVER VIEW OF STEAM TURBINE

STEAM TURBINES COMES UNDER API 611

STEAM TURBINE

A steam turbine is a prime mover that derives its energy of rotation due to conversion of the heat energy of steam into kinetic energy as it expands through a series of nozzles mounted on the casing.

Important aspects for Steam Turbine functioning

·      Steam at high temperature and pressure contains the potential energy.

·      Potential energy of the steam is converted into a mechanical work through expansion in a nozzle and impact or reaction with a blade.

·      Mechanical Work of many sets of blades attached to a shaft produces rotational power.

APPLICATIONS OF STEAM TURBINE

The applications of steam turbines are commonly in

·      Power generation in power plants

·      Drives for critical equipments such as Centrifugal Compressor and Pumps in Refineries, Petrochemical, Fertilizer and chemical plants 

Advantages

 Inherent variable speed and load conditions operation independent of electric power supply.

Turbine Operation
Water is converted into steam by application of heat in the boiler, which produces  the steam at specified pressure and temperature. To convert the steam energy into useful work, it must go through a thermodynamic cycle (eg. Carnot cycle) This steam is passed through a series of nozzles, where its pressure is converted in to high velocity. This high velocity steam impinges on the rotor bucket, causing the rotation of the shaft.

A SIMPLE CYCLE  OF STEAM TURBINE

CLASSIFICATION OF STEAM TURBINE 

  
A. By the action of steam:

·       Impulse

·      Reaction

·      Combined Impulse and reaction

B. By the number of stages

·      Single stage

·      Multi-stage

C. By the direction of steam flow:

·      Axial

·      Radial

·      Mixed            
                         

D. By the inlet steam pressure:

·      High Pressure

·      Medium pressure

·      Low pressure

E. By the final pressure

·      Condensing

·      Back pressure

F. By the source of steam

·   Extraction

IMPULSE STAGE:

Impulse nozzles organize the steam, so it flows in well-formed high-speed jets. Moving blades, also called buckets, absorb the jet’s kinetic energy and convert it to shaft rotation.

REACTION TYPE:

Steam enters the fixed blade passages and leaves as steam jet that fills the entire rotor periphery. Steam flows between moving blades that in-turn, forms as moving nozzles. There it drops in pressure, and its speed rises relative to that of blades. 

 PARTS OF STEAM TURBINE

    





















1 Sentinel warning system

     2 High-efficiency blades

     3 Shaft and wheel

     4 Carbon-ring packing

     5 Journal bearings

     6 Thrust bearing

     7 Trip collar

     8 Governors

     9 Oil rings

   10 Trip lever

   11 Governor Valve stem

   12 labyrinths & bearing-case oil seals

   13 Internal oil reservoirs

   14 Nozzle assemblies

ROTOR ASSEMBLY

The rotor assembly consists of turbine wheels shrink fitted to the shaft as shown in the figure. Over speed trip are incorporated in the shaft as a integral or separate part.

Thrust collar are fitted if hydrodynamic bearings are used (some turbine features ball bearing arrangement for thrust control)

 OVERSPEED TRIP ASSEMBLY

This is used, when the turbine speed reaches more than the setting speed. It consists of a pin which is held in position by an adjustable nut against the spring force. When the speed reaches more than the setting speed, the pin will come out of its position against spring force by the action of centrifugal force. This pin will activate the trip lever to stop the steam flow. Normally this speed is set at 110% of the operating speed. Eg. Operation speed: 3000 rpm

Trip speed: 3000 + 10% (300) = 3300 rpm

1. Body

2. Pin

3. Spring

4. Lock U-staple

5. Adjustable nut

6. Washer

7. Auxiliary weight

TURBINE JOURNAL BEARING:

The functions of the journal bearings are to support the turbine rotor centrally in the casing.

To carry a load perpendicular to the axis of rotation  made up of the weight of the rotor, constant or fluctuating steam forces and kinetic forces caused by the unstable running or any residual unbalance.

TURBINE THRUST BEARING:

The function of the Thrust bearing is:
To keep the rotor in exact position in the casing to absorb any axial thrust on the rotor.

Some versions use ball bearings and some versions use hydrodynamic bearings

SENTINEL VALVE

This assembly is used as safety device in turbine. When the turbine exhaust pressure rises above the design pressure a whistling sound will be produced giving an indication of abnormality

GOVERNOR

The function of Governor is to maintain the speed of the turbine irrespective of the load condition. If the turbines speed increases due to reduction of load, the governor will reduce the steam flow in chest valve thereby reducing the Speed.

Similarly if the speed decreases due to increases in load, the governor will increase the steam flow in the chest valve, thereby increasing the speed

TYPES OF GOVERNOR 

·      Mechanical Governor
Simple flyweight arrangement

·      Mechanical + Hydraulic Governor
Using the principle of flyweight & hydraulic amplification

·      Electronic Governor
Microprocessor based controls

 CHEST VALVE

CHEST VALVE SEAT AND DISC

HYDRAULIC TRIP & THROTTLE VALVE

 OVER SPEED TRIP CIRCUIT

The trip oil is supplied to the Trip valve through the trip mechanism. During normal operation of the steam turbine, the trip oil will be admitted to the piston in the Trip valve via trip mechanism opening port. By this action, the stem will raise against spring force to open the steam supply to the turbine.

During over speed of the turbine the over speed lever is activated. By this action the trip valve mechanism will be shifted to drain port. Causing the oil, to flow to the sump from trip valve. By this action oil in the piston of trip valve will get drained. So the valve is closed by the spring force.

Lube oil from the lube oil tank is supplied to the bearings through pumps, lube oil cooler and lube oil filters

END PACKING (CARBON RINGS)

Carbon rings are provided at each end of the turbine for sealing the steam.

They are individually separated in compartment formed by corrosion resistant steel spacers.

The partition rings are located in angular grooves in the packing case as shown. The rings are made in 3 sections and are held together by garter springs and prevented from rotation by a stop on each ring.

 

IMPORTANT POINTS TO BE CONSIDERED WHILE OVERHAULING STEAM TURBINES

·         Bearing clearances

·         Axial float

·         Nozzle clearance

·         Shaft run out

·         Rotor balancing

·         Carbon ring clearances

·         Oil seal  clearances

·         DP test for Turbine blades for crack checking

·         Split surfaces of casing and bearing housings

·         Calibration of governor

·         Sentinel valve function testing

Jobs to be done for six months preventive maintenance

Ø  Alignment checking

Ø  Cooling water jacket lines checking

Ø  Governor linkage checking and lubrication

Ø  Lube oil changing

Ø  Governor oil changing

Jobs to be done for one year preventive maintenance

Ø  6 months PM

Ø  Bearing clearance checking

Ø  Oil seal clearance checking

Ø  Trip test

Ø  Sentinel valve function testing

Ø  steam strainer cleaning

Guide lines while working in steam turbines

·         Be careful of hot surfaces

·         Check for blinding of steam inlet and exhaust lines

·         Precaution to be taken while handling governor and its linkages

·         Cleanliness to be maintained throughout  the job

·         Careful about material handling