What are the losses in steam turbines?
Answers:
- Residual Velocity Loss
- This is equal to the absolute velocity of the steam at the blade exit. - Loss due to Friction -
Friction loss occurs in the nozzles, turbine blades and between the steam and rotating discs. This loss is about 10%. - Leakage Loss.
- Loss due to Mechanical Friction
- Accounts for the loss due to friction between the shaft and bearing. - Radiation Loss
- Though this loss is negligible, as turbine casings are insulated, it occurs due to heat leakage from turbine to ambient air which is at a much lower temperature than the turbine. - Loss due to Moisture -
In the lower stages of the turbine, the steam may become wet as the velocity of water particles is lower than that of steam. So a part of the kinetic energy of steam is lost to drag the water particles along with it.
What are the main causes of turbine vibration?
Answer:
- unbalanced parts
- poor alignment of parts
- loose parts
- rubbing parts
- lubrication troubles
- steam troubles
- foundation troubles
- cracked or excessively worn parts
What are the points of SCC attack?
Answers:
- SCC attack predominates where corrodents deposit and build up i.e. in those blading areas where flowing steam cannot provide a washing effect.
- What are these points in particular?
- Tie wires.
- Tie wire holes.
- Brazings.
- Blade covers.
- Tenon holes.
- At what points does corrosion fatigue does show up?
It attacks trailing edges, near the base of the foil and also the blade-root serration’s.
What are the possible causes for the turbine not running at rated speed?
Answers:
- The possible causes are:
- too many hand valves closed,
- oil relay governor set too low,
- inlet steam pressure too low or exhaust pressure too high,
- load higher than turbine rating,
- throttle valve not opening fully,
- safety trip valve not opening properly,
- nozzles plugged,
- steam strainer choked.
What are the possible causes of a governor not operating?
Answers:
- Restriction of throttle valve reflex.
- Failure of governor control on start-up.
If it is found that after start-up, the speed increases continuously and the governor is not closing the throttle valve, it may be that the governor pump has been installed in the wrong direction.
What are the possible causes of excessive vibration or noise in a steam turbine?
Answers:
- Misalignment.
- Worn bearings.
- Worn coupling to driven machine.
- Unbalanced coupling to driven machine.
- Unbalanced wheel.
- Piping strain.
- Bent shaft.
What are the possible causes of the speed of the turbine rotor increasing excessively as the load is decreased?
Answers:
- Throttle valve not closing fully.
- Wearing of throttle valve seats.
What are the stresses to which a steam turbine rotor is subjected during its service life?
Answers:
- Mechanical stress
- The factors that contribute to mechanical stress in the shaft are the centrifugal forces and torque’s generated due to revolving motion of the shaft as well as bending arising during steady-state operation. - Thermal stress
- Transient operating phases i.e. startup and shutdown the genesis of thermal stress induced to the turbine shaft. - Electrically induced stress
- They originate due to short circuits and faulty synchronization.
What are these points in particular?
Answers:
- Tie wires.
- Tie wire holes.
- Brazings.
- Blade covers.
- Tenon holes.
What are three types of condensers?
Answer:
- surface (shell-and-tube)
- jet
- barometric.
What are topping and superposed turbines?
Answer:
Topping and superposed turbines are high-pressure, non-condensing units that can be added to an older, moderate-pressure plant. Topping turbines receive high-pressure steam from new high-pressure boilers. The exhaust steam of the new turbine is at the same pressure as the old boilers and is used to supply the old turbines.
What are two types of clearance in a turbine?
Answers:
- radial - the clearance at the tips of the rotor and casing
- axial - the fore-and-aft clearance, at the sides of the rotor and the casing
What design modification is adopted to reduce susceptibility of last low-pressure stages to fatigue failure?
Answer:
One modification is to join the blade segments together at the shroud band.
What does "upgrading" generally means in the context of steam turbines?
Answer:
Upgrading is a most widely used tern. It encompasses a variety of meanings verses life extension, modernization and up-rating of steam turbines.
What does the term "ramp rat" mean?
Answer:
Ramp rate is used in bringing a turbine up to operating temperature and is the degrees Fahrenheit rise per hour that metal surfaces are exposed to when bringing a machine to rated conditions. Manufactures specify ramp rates for their machines in order to avoid thermal stresses. Thermocouples are used in measuring metal temperatures.
What factors are responsible for turbine-blade failures?
Answers:
- In the high pressure cylinder, the turbine blades are mostly affected by:
- solid-particle erosion (SPE),
- high cycle fatigue,
- Whereas in the last few stages of the low-pressure cylinder, the blade damage is mainly afflicted by:
- erosion,
- corrosion,
- stress/fatigue damage mechanism.
- According to EPRI (Electric Power Research Institute, USA) data stress-corrosion cracking and fatigue are the chief exponents for turbine-blade failures in utility industries.
What factors cause excessive steam leakage under carbon rings?
Answers:
- Dirt under rings. - steam borne scale or dirt foul up the rings if steam is leaking under the carbon rings.
- Shaft scored.
- Worn or broken carbon rings.
These should be replaced with a new set of carbon rings. The complete ring is to be replaced.
What factors contribute to excessive speed variation of the turbine?
Answers:
- Improper governor droop adjustment.
- Improper governor lubrication.
- Throttle assembly friction.
- Friction in stuffing box.
- High inlet steam pressure and light load.
- Rapidly varying load.
What is a balance piston?
Answer:
Reaction turbines have axial thrust because pressure on the entering side is greater than pressure on the leaving side of each stage. To counteract this force, steam is admitted to a dummy (balance) piston chamber at the low-pressure end of the rotor. Some designers also use a balance piston on impulse turbines that have a high thrust. Instead of pistons, seal strips are also used to duplicate a piston's counter force.
What is a combination thrust and radial bearing?
Answer:
This unit has the ends of the babbitt bearing extended radially over the end of the shell. Collars on the rotor face these thrust pads, and the journal is supported in the bearing between the thrust collars.
What is a diaphragm (turbine)?
Answer:
Partitions between pressure stages in a turbine's casing are called diaphragms. They hold the vane-shaped nozzles and seals between the stages. Usually labyrinth-type seals are used. One-half of the diaphragms are fitted into the top of the casing, the other half into the bottom.
What is a multiport governor valve? Why is it used?
Answer:
In large turbines, a valve controls steam flow to groups of nozzles. The number of open valves controls the number of nozzles in use according to the load. A bar-lift or cam arrangement operated by the governor, opens and close the valves in sequence. Such a device is a multiport valve. Using nozzles at full steam pressure is more efficient than throttling the steam.
What is a radial-flow turbine?
Answer:
In a radial-flow turbine, steam flows outward from the shaft to the casing. The unit is usually a reaction unit, having both fixed and moving blades. They are used for special jobs and are more common to European manufacturers.
What is a shrunk-on-disc rotor?
Answer:
These are built by heat expanding the discs, so that upon cooling they shrink on the main rotor forging.
What is a stage in a steam turbine?
Answer:
In an impulse turbine, the stage is a set of moving blades behind the nozzle. In a reaction turbine, each row of blades is called a "stage." A single Curtis stage may consist of two or more rows of moving blades.
