Turbines Flashcards
A steam turbine is a type of external combustion engine, which is a heat engine that obtains it’s heat from where?
Fuel consumed outside the turbine cylinder
Several laws apply to the operation of steam turbines but what are the two most important?
Newton’s third law
Bernoullis principle
What is Newton’s third law?
For every action, there is an equal and opposite reaction.
Bernoulli’s principle states?
If a fluid (liquid or gas) flowing through a tube reaches a construction or narrowing of the the tube, the velocity of the fluid flowing through the constriction increases and the pressure decreases.
However when fluid leaves the constriction the velocity decreases and pressure increases
What is the difference between impulse and reaction turbines?
Small turbines tend to use impulse principle and larger turbines use the reaction principle.
Reaction blade vs impulse blade
Impulse nozzles can have one or more stationary nozzles arranged around the rotor periphery to do what?
Direct steam at the moving blades.
Is there a pressure drop across impulse blades?
No because there is no change in space between the entry and exit through the blading
How come impulse blades have minimal axial thrust?
Because of the symmetrical design
What causes a pressure drop in reaction blades?
Reaction blades are not symmetrical and their tips are not parallel to the shaft. Because the blade is smaller at the exit side causes steam to accelerate as it passes through the reaction blading creating a pressure drop.
More rows of blading is required in reaction blade systems because?
Pressure drop at each row of blades
To reduce number of blade rows in reaction turbines _____________ section at the inlet end of the turbine .
Velocity compounded impulse section
This serves two purposes:
- turbine machine is much shorter
- no pressure drop across impulse blading; therefore, at this very high pressure end of the turbine, sealing between rows of impulse wheels is not necessary
Most common methods to attach blades to rotor?
Dove tail of fir tree method
Why is it critical to place blades in their required positions?
Rotor and blades are statically and dynamically balanced from the manufacturer
When steam goes through a reaction type nozzle it :
- steam pressure decreases
- steam velocity increases
- steam volume increases
In reaction turbines, how come the blades get longer in each successive stage?
To accommodate for the steam expansion throughout the turbine
What are the two ways manufactures use to determine turbine stages?
- number of rotating rows of blades in the turbine
Or
- number of pressure drops across the turbine
Reaction turbine stage is a ring of nozzles and the associated rotor wheel with moving blades. There is often more then one casing cylinder or section. In large turbines these sections are :
HP - high pressure
IP - intermediate pressure
LP - Low pressure
Why is reaction turning less efficient higher pressures?
Leakage past the blade tips
What is the purpose of a extraction turbine?
To provide steam for other plant purposes ( feed water heating, other plant process heating)
What makes condensing steam turbines more efficient then job condensing turbines?
Latent heat of the steam transfers to the cooling water in the condenser. This creates low pressure (vacuum) in the condenser
Lack of back pressure and turbulence in the last set of turbine blades increases turbine efficiency
What is the purpose of the boiler?
Fuel is burned and water is changed to high pressure steam. steam is now piped to the steam chest
Purpose of the casing (cylinder)
Main structure or housing that contains most of the steam passages, the rotor and nozzles.
Governor and steam admission valve mount on one end of the casing in order to control the amount of steam that enters the steam turbine
Purpose of the sentinel valve?
Mounted on the turbine casing internal steam pressure. It is a warning device indicating if turning casing has excess pressure. Only found on small turbines
Function of the turbine rotor?
Blades or buckets are mounted to rotor.
Rotor assembly harnesses the energy of the steam.
What are the nozzles in a turbine?
Stationary blades or orifices that direct the steam towards the blades and change high pressure from the boiler into high velocity at the nozzle.
Clearance between nozzle ring and blade is very critical
What is the diaphragm in a turbine?
A stationary separator between the discs of the rotor. Fixed to the case and sealed around the shaft. They only allow steam to be passed through their nozzles
What is the purpose of a governor in a steam turbine?
Controls the position of the steam admission valve to control turbine speed
SAV or steam admission valve?
Admits steam to the nozzle
Stop valve
Shuts off all steam to the turbine in the event of an overspeed situation
What type of shaft seals are on small turbines? And Large turbines?
Small turbines have carbon seals
Large turbines have labyrinth seals
Most failure on turbines occur because of some sort of ________ issue or overspeed conditions
Lubrication
Overspeed trip system is usually set to ______ to _______ % above operating speed?
110 to 115 %
When disassembling the overspeed trip mechanism it is important to count the number of __________. This makes it easier because when you reassemble you can get close to the correct trip speed.
Turns required to remove the spring.
Over spring must be tested after and maintenance of the trip system or a major overhaul
When does the overspeed trip system need to be checked?
On a regular basis dictated by the manufacture
Overspeed will be catastrophic
To ensure proper shutdown in the event of an overspeed the stop valve _________ seal is adequate.
Seat
Most power generation turbines use solid flange couplings. For maximum power transfer and minimum shear stess the bolts must ______________?
Fit the holes without clearance
What is the function of the case drain?
Casing drains are in the lower part of the turbine casing and steam chest.
Must be opened and checked for water before start up.
Inlet piping may also have drains. Make sure all low points are drained before start up. Failure to do so will be catastrophic
Steam admission valves can either operate on a cam lift system or a _________ lift system?
Bar
Condensers are usually ____________ type heat exchangers?
Shell and tube
Axial thrust in large turbines can mostly be eliminated using dummy (balance pistons) or rotor configuration to reduce the burden on the thrust bearing. What is a dummy piston?
A balance piston is part of the rotor with several stepped diameters that the steam pressure in the turbine can push against to counteract the thrust.
Axial thrust in large turbines can mostly be eliminated using dummy (balance pistons) or rotor configuration to reduce the burden on the thrust bearing. What is a dummy piston?
A balance piston is part of the rotor with several stepped diameters that the steam pressure in the turbine can push against to counteract the thrust.
Using _______ flow rotors also can control axial thrust. These rotors are mounted so that the steam enters the the centre of the cylinder and takes two paths in opposite directions.
Dual flow rotors
Fuel supply systems for boilers can be simple or complex. Natural gas is simple not requiring a lot of equipment to prepare the gas for burning. Other complex systems would be ________
Coal fired plants
Need machinery to bring the coal to the furnace, pulverize it , meter the coal blow it into the furnace and remove the ashes
Non condensing turbines must have a ________ ? where is it located?
Relief valve between the turbine and turbine discharge isolation valve
Condensing turbines have a atmospheric relief valve or a __________ located on the low pressure section.
Rupture disc
To keep the turbine operating efficiently ________ are used to reject heat from the steam continually. Power plants can use water from lakes, rivers, supply ponds or ________?
Condenser
Cooling towers are used if other means of water are not available
Different ways to cool the generator ?
Small generators are air cooled
Large generators are hydrogen cooled (very efficient)
What is the purpose of the turning gear(barring gear)
Used to rotate rotor slowly long before start up so that any bow in the rotor due to sag can be removed before the turbine is started. Also used when turbine is initially heated to allow even heating
Proper lubrication of the rotor bearing is critical. System mud lubricate all the high speed rotor bearing and _________(transferred from the very hot steam)
Carry the heat away
Large turbines use ________ lubrication systems
Small turbines use _________ lubrication system
Pressurized - large volume of oil at 40 psi
Oil ring lubrication
Must use oil that readily separates from water and good oxidation resistance
Large turbines have jacking oil. Jacking oil is applied when?
Before the start up of the turning gear and at very slow rotor speeds. Jacking oil is left on until about 50 to 60 rpm.
Jacking oil is also used during overhauls allowing you to turn the rotor during maintenance operations
Common issues with steam turbines could be? And how is it fixed?
Progressive deposits on the blades and nozzles. Plugging causes rubbing of parts, thrust bearing failure and other problems. Deposits can be insoluble or water-soluble.
- turbine can be shut down, dismantled and deposits be removed manually by buffing, scraping, or blasting them with walnut shells, rice, or high pressure dry ice.
- turbine can be shut down causing the deposits to crack off due to temperature change
Where would you look to find specific details about rigging and components weights when dismantling a turbine?
Service manual
Before casing is lifted with rigging it needs to first be lifted evenly with the ?
Jacking bolts
Acceptable range of oil viscosity entering the bearings occurs when the oil temperature is between 105 F to 140 F and exits the bearing at _____________
130 to 160 F
How is the rotor axial positions set?
With shims at the thrust bearing location in the turbine housing
All diaphragm positioning is referenced to this turbine centre line using bearing centrelines. What else is based of that reference?
All manufacturers specifications
Why must you check manufacturers specifications when aligning turbine couplings?
Thermal growth limits are different for each machine. Projected values are set by the manufacturer
What type of thrust bearings are typically found in large turbines?
Tilting pad Kingsbury thrust bearings
Gas turbines are?
Internal combustion, constant flow engine designed to convert the energy of fuel into useful horsepower.
Efficiency of a gas turbine depends primarily on?
The temperature to which the air can be raised
The more energy added to the gases, the greater the expansion of the gases, which translates into more power output and efficiency
Gas turbines are used where?
Aircraft propulsion
High speed marine propulsion
Stationary power generation both as standby and main power generation
Gas pipeline compressor drives
Regeneration is when some of the exhaust heat is used to heat the air of combustion. Cogeneration is what?
When exhaust heat is used to generate steam for power generation of other plant processes
Types of shaft configuration
Single shaft - very common good for single speed operation
Split shaft - suitable for variable speed operating conditions
Twin spool shaft - one shaft running Inside the other. Variable speed
Gas generation sections job?
Brings air in from the compressor to mix with the fuel in the combustion chamber to create hot expanding gases that can be harnessed
Power sections job?
This section delivers rotary motion that is useable to drive the machinery
Harness energy from the hot gases as efficiently as possible without adversely affecting the operation of the gas generation section.
Many industrial turbines are _______ cycle because they are simple, low cost, and light weight.
Open
Typically smaller gas turbines
Semi closed cycle turbines differ from open cycle turbines how?
Semi closed have a heat exchanger or regenerator attached to it in order to use the waste exhaust heat.
The idea of using regeneration is to improve what?
The thermal efficiency of the gas turbine by using the exhaust heat to assist in the heating of the gases before the turbine
Greatest advantage of gas turbine is?
High power to weight ratio
Horse power per dollar installed cost
Low maintenance cost
Minimum water cooling system
Quick start to full load operation
Why do gas turbines work well for standby and emergency electricity generators?
Can be started very quickly with no harmful affects and can be shut down in a shorter amount of time
Disadvantages of gas turbines
Low thermal efficiency
Experienced technicians required
High air intake
Complex control systems
The diffuser lowers _______ and increases the _______
Velocity (kinetic)
Pressure (potential)
Turbine engine and piston engine are similar in a few aspects. Both are internal combustion engines and have the same cycles of operation. That have :
Intake compression
Ignition
Power
And exhaust
As a continuum of their cycles
In a piston engine each event happens at the same place but at different times. In a gas turbine the events happen when?
At the same time but different places
Restriction in air flow will cause?
Surging, catastrophic failure to the turbine engine when this happens
What is the purpose of the compressor in an gas turbine?
Provide the turbine with with pressurized air for combustion (primary air) and cooling (secondary air)
Two types of gas turbine compressors?
Radial flow
Axial flow
Three parts of the radial turbine compressor?
Impeller
Diffuser
Compressor manifold
Axial flow compressors are more efficient in larger turbines than the radial flow types. Why are they not used in small turbines?
To difficult to build small
Two main parts of the axial flow turbine compressor?
Rotor with blades
Stator with blades
For every row of blades on the rotor there is a set of blades on the stator. Each consecutive set of rotor and stator blades forms a pressure ______?
Stage
Gas turbine blades are made of?
Stainless or semi stainless materials
