Question & Answers on Steam Turbines
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Question & Answers on Steam Turbines

What is important to remember about radial bearings?

Answer:  A turbine rotor is supported by two radial bearings, one on each end of the steam cylinder. These bearings must be accurately aligned to maintain the close clearance between the shaft and the shaft seals, and between the rotor and the casing. If excessive bearing wear lowers the he rotor, great harm can be done to the turbine.

How many governors are needed for safe turbine operation? Why?

Answer:  Two independent governors are needed for safe turbine operation. One is an over speed or emergency trip that shuts off the steam at 10 percent above running speed (maximum speed). The second, or main governor, usually controls speed at a constant rate; however, many applications have variable speed control.

How is a flyball governor used with a hydraulic control?

Answer:  As the turbine speeds up, the weights are moved outward by centrifugal force, causing linkage to open a pilot valve that admits and releases oil on either side of a piston or on one side of a spring-loaded piston. The movement of the piston controls the steam valves.

What is a multi-port 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 closes these valves in sequence. Such a device is a multi-port valve. Using nozzles at full steam pressure is more efficient than throttling the steam.

What is meant by critical speed?

Answer:  It is the speed at which the machine vibrates most violently. It is due to many causes, such as imbalance or harmonic vibrations set up by the entire machine. To minimize damage, the turbine should be hurried through the known critical speed as rapidly as possible. (Caution, be sure the vibration is caused by critical speed and not by some other trouble).

How is oil pressure maintained when starting or stopping a medium-sized turbine?

Answer:  An auxiliary pump is provided to maintain oil pressure. Some auxiliary pumps are turned by a hand crank; others are motor-driven. This pump is used when the integral pump is running too slowly to provide pressure, as when starting or securing a medium-sized turbine.

Besides lubrication, which are two functions of lubricating oil in some turbines?

Answer: In large units, lube oil cools the bearings by carrying off heat to the oil coolers. Lube oil in some turbines also acts as a hydraulic fluid to operate the governor speed-control system.

What is meant by the water rate of a turbine?

Answer:  Water rate is another term used for the steam rate.

 

What is the difference between partial and full arc admission?

Answer:  In multi-valve turbine inlets, partial arc ad mission allows the steam to enter per valve opening in a sequential manner, so as load is increased, more valves open to admit steam. This can cause uneven heating on the high-pressure annulus as the valves are individually opened with load increase. In full-arc admission, all regulating valves open but only at a percentage of their full opening. With load increase, they all open more fully. This provides more uniform heating around the high-pressure part of the turbine. Most modern controls start with full-arc and switch to partial arc to reduce throttling losses through the valves.

At what points does corrosion fatigue does show up?

Answer:  It attacks trailing edges, near the base of the foil and also the blade-root serration’s.

Besides lubrication, what are two functions of lubricating oil in some turbines?

Answer:  In larger units, lube oil cools the bearings by carrying off heat to the oil coolers. Lube oil in some turbines also acts as a hydraulic fluid to operate the governor speed-control system.

Despite preventive measures, damage due to moisture impingement has been found, in certain cases, in the shield and beyond. Why?

Answers:

  1. Shields are designed and fabricated on the basis of predicted range of steam/water quantities impacting the blades at specific angles.
  2. Now if the operating conditions deviate significantly from design parameters then the erosion damage will occur. And in some cases it may go beyond nominal erosion wear and warrant repair.
  3. Also the corrosion of casing can occur due to blockage/clogging of water drains or extraction thereby forcing the water back into the casing. If this condensate water is carried over to steam path and impacts the blade, thermal-fatigue failure can occur within a short period.

By monitoring the exhaust steam temperature, how can the blade deposition be predicted?

Answers:

  1. Immediately after the 1st commissioning, the different values of exhaust temperature for different steam flow rates are precisely determined and plotted against steam flow. This will produce the first actual graph. This is for a clean turbine.
  2. Similar graphs are to be drawn at later periods for comparing with the initial graph.
  3. A rise in exhaust steam temperature under the same conditions refers to deposit formation.
  4. An increase of exhaust steam temperature by more than 10% in the range of 70 to l00% steam flow, indicates inadmissible blade depositions. Shutdown is to be taken and blades are to be washed off deposits.

Do the radial axial-bore cracks occur in the LP rotor/shaft alone?

Answer:  These are also known to occur in the HP as well as HP rotors.

Do you stop cooling-water flow through a steam condenser as soon as the turbine is slopped?

Answer:  You should keep the cooling water circulating for about 15 mill or more so that the condenser has a chance to cool down gradually and evenly. Be sure to have cooling water flowing through the condenser before starting up in order to prevent live steam from entering the condenser unless it is cooled. Overheating can cause severe leaks and other headaches.

Do you think that turbine blade failure is the only cause of unreliability of steam turbines? Does upgrading of turbine means replacement of blades and/or improvement of blade design?

Answers:

  1. Like the blades, the steam-turbine rotors are highly stressed components. They are subject to cracking by a variety of failure mechanisms. Rotor failures do occur. And when they occur the result is catastrophic with the complete destruction of the unit and the total loss of generating capacity.
  2. Therefore, special attention should be given to rotor upgrading and repairing techniques.

BLADE FAILURES

Unknown 26%

Stress-Corrosion Cracking 22%

High-Cycle Fatigue 20%

Corrosion-Fatigue Cracking 7%

Temperature Creep Rupture 6%

Low-Cycle Fatigue 5%

Corrosion 4%

Other causes 10%

Besides, many damage mechanisms operate in combination of, poor steam/water chemistry, certain blade design factors that vary from one turbine manufacture to other, system operating parameters,

How can damaged tenons be repaired?

Answers:  By adopting modern welding techniques, tenons can be rebuilt This in some cases results in extended blade life.

How can problems of "excessive vibration or noise" due to piping strain be avoided on steam turbines?

Answers:

  1. The inlet as well as exhaust steam lines should be firmly supported to avoid strains from being imposed on the turbine.
  2. Adequate allowance should be made for expansion of steam pipes due to heat.

How can steam turbines be classified?

Answers:

By the action of steam:

  1. Impulse.
  2. Reaction.
  3. Impulse and reaction combined.

    The number of step reductions involved:

  4. Single stage.
  5. Multi-stage.
  6. Whether there is one or more revolving vanes separated by stationary reversing vanes.

    The direction of steam flow:

  7. Axial.
  8. Radial.
  9. Mixed.
  10. Tangential.
  11. Helical.
  12. Reentry.

    The inlet steam pressure:

  13. High pressure.
  14. Medium pressure.
  15. Low pressure.

    The final pressure:

  16. Condensing.
  17. Non-condensing.

    The source of steam:

  18. Extraction.
  19. Accumulator.

 

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 blade.

What is a diaphragm?

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 diaphragm is fitted into the top of the casing, the other half into the bottom.

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, such as Sta-Laval (now ABB).

What are four types of turbine seals?

Answers:

  1. Carbon rings fitted in segments around the shaft and held together by garter or retainer springs.
  2. Labyrinth mated with shaft serration’s or shaft seal strips.
  3. Water seals where a shaft runner acts as a pump to create a ring of water around the shaft. Use only treated water to avoid shaft pitting.
  4. Stuffing box using woven or soft packing rings that are compressed with a gland to prevent leakage along the shaft.

In which turbine is tip leakage a problem?

Answer:  Tip leakage is a problem in reaction turbines. Here, each vane forms a nozzle; steam must flow through the moving nozzle to the fixed nozzle. Steam escaping across the tips of the blades represents a loss of work. Therefore, tip seals are used prevent this.

What are two types of clearance in a turbine?

Answer:

  1. Radial - clearance at the tips of the rotor and casing.
  2. Axial - the fore-and-aft clearance, at the sides of the rotor and the casing.

What are four types of thrust hearings?

Answer:

  1. Babbitt-faced collar bearings.
  2. Tilting pivotal pads.
  3. Tapered land bearings.
  4. Rolling-contact (roller or ball) bearings.

 

What is the function of a thrust bearing?

Answer:  Thrust bearings keep the rotor in its correct axial position.

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 piston, seal strips are also used to duplicate a piston's counter force

Why should a steam or moisture separator be installed in the steam line next to a steam turbine?

Answer:  All multistage turbines, low-pressure turbines, and turbines operating at high pressure with saturated steam should have a moisture separator in order to prevent rapid blade wear from water erosion.

What are some conditions that may prevent a turbine from developing full power?

Answers:

  1. The machine is overloaded.
  2. The initial steam pressure and temperature are not up to design conditions.
  3. The exhaust pressure is too high.
  4. The governor is set too low.
  5. The steam strainer is clogged.
  6. Turbine nozzles are clogged with deposits.
  7. Internal wear on nozzles and blades.

 Why is it necessary to open casing drains and drains on the steam line going to the turbine when a turbine is to be started?

Answers:

To avoid slugging nozzles and blades inside the turbine with condensate on start-up; this can break these components from impact. The blades were designed to handle steam, not water.

What is steam rate as applied to turbo-generators?

Answer:  The steam rate is the pounds of steam that must be supplied per kilowatt-hour of generator output at the steam turbine inlet.

What are the two basic types of steam turbines?

Answers:

  1. Impulse type.
  2. Reaction type.

What is the operating principle of an impulse turbine?

Answer:  The basic idea of an impulse turbine is that a jet of steam from a fixed nozzle pushes against the rotor blades and impels them forward. The velocity of the steam is about twice as fast as the velocity of the blades. Only turbines utilizing fixed nozzles are classified as impulse turbines.

What is the operating principle of a reaction turbine?

Answer:  A reaction turbine utilizes a jet of steam that flows from a nozzle on the rotor. Actually, the steam is directed into the moving blades by fixed blades designed to expand the steam. The result is a small increase in velocity over that of the moving blades. These blades form a wall of moving nozzles that further expand the steam. The steam flow is partially reversed by the moving blades, producing a reaction on the blades. Since the pressure drop is small across each row of nozzles (blades), the speed is comparatively low. Therefore, more rows of moving blades are needed than in an impulse turbine.

What are topping and superposed turbines?

Answer:  Topping and superposed turbines arc 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 has the same pressure as the old boilers and is used to supply the old turbines.

What is an extraction turbine?

Answer:  In an extraction turbine, steam is withdrawn from one or more stages, at one or more pressures, for heating, plant process, or feedwater heater needs. They are often called "bleeder turbines."

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 tapered-land thrust bearing?

Answer:  The babbitt face of a tapered-land thrust bearing has a series of fixed pads divided by radial slots. The leading edge of each sector is tapered, allowing an oil wedge to build up and carry the thrust between the collar and pad.

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