Frequently asked questions on turbines Part 1

Frequently Asked Questions
TURBINE PART 1
Questions 1 to 43
Manohar Tatwawadi
total output power solutions, Pune
Mob. No. +91 9372167165

Question-Answers
1. What is a stage in a steam turbine?
• 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.
2. What is a diaphragm?
• 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.
3. What is a radial-flow turbine?
• 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).
06-01-2022 total output power solutions 2

4. What are four types of turbine seals?
• Carbon rings fitted in segments around the shaft and held
together by garter or retainer springs.
• Labyrinth mated with shaft serration’s or shaft seal strips.
• 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.
• Stuffing box using woven or soft packing rings that are
compressed with a gland to prevent leakage along the
shaft.
5. In which turbine is tip leakage a problem?
• 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.

6. What are two types of clearance in a turbine?
• Radial - clearance at the tips of the rotor and casing &
• Axial - the fore-and-aft clearance, at the sides of the rotor and the casing.
7. What are four types of thrust bearings?
• Babbitt-faced collar bearings.
• Tilting pivotal pads.
• Tapered land bearings.
• Rolling-contact (roller or ball) bearings.
8. What is the function of a thrust bearing?
• Thrust bearings keep the rotor in its correct axial position.
9. What is a balance piston?
• 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.

10. Why should a steam or moisture separator be installed in the steam line next
to a steam turbine?
• 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.
11. What are some conditions that may prevent a turbine from developing full
power?
• The machine is overloaded.
• The initial steam pressure and temperature are not up to design conditions.
• The exhaust pressure is too high.
• The governor is set too low.
• The steam strainer is clogged.
• Turbine nozzles are clogged with deposits.
• Internal wear on nozzles and blades.
12. 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?
• 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.

13. What is steam rate as applied to turbo-generators?
• The steam rate is the pounds of steam that must be supplied per
kilowatt-hour of generator output at the steam turbine inlet.
14. What is the operating principle of an impulse turbine?
• 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.
15. What is the operating principle of a reaction turbine?
• 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.

16. What are topping and superposed turbines?
• 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.
17. What is an extraction turbine?
• 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."
18. What is a combination thrust and radial bearing?
• 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.
19. What is a tapered-land thrust bearing?
• 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.

20. What is important to remember about radial bearings?
• 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.
21. How many governors are needed for safe turbine operation? Why?
• Two independent governors are needed for safe turbine operation. One is an
overspeed 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.
22. How is a flyball governor used with a hydraulic control?
• 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.
23. What is a multi-port governor valve? Why is it used?
• 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.

24. What is meant by critical speed?
• 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).
25. How is oil pressure maintained when starting or stopping a medium-sized turbine?
• 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.
26. Besides lubrication, which are two functions of lubricating oil in some turbines?
• 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.
27. What is the difference between partial and full arc admission?
• In multi-valve turbine inlets, partial arc admission 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.

28. 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.
29. But despite these preventive measures, damage due to moisture impingement
has been found, in certain cases, in the shield and beyond. Why?
• Shields are designed and fabricated on the basis of predicted range of
steam/water quantities impacting the blades at specific angles.
• 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.
• 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.
30. Do the radial axial-bore cracks occur in the LP rotor/shaft alone?
• These are also known to occur in the HP as well as HP rotors.

31. By monitoring the exhaust steam temperature, how can the blade deposition be
predicted?
• 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.
• Similar graphs are to be drawn at later periods for comparing with the initial
graph.
• A rise in exhaust steam temperature under the same conditions refers to deposit
formation.
• 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.
32. Do you stop cooling-water flow through a steam condenser as soon as the
turbine is slopped?
• 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.

33. 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?
• 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. Therefore, special attention should be given to rotor
upgrading and repairing techniques.
34. What are the factors of Blade Failures?
• Stress-Corrosion Cracking, High-Cycle Fatigue, Corrosion-Fatigue Cracking, Temperature
Creep Rupture, Low-Cycle Fatigue, Corrosion, 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.
35. How can damaged tenons be repaired?
• By adopting modern welding techniques, tenons can be rebuilt. This in some cases
results in extended blade life.
36. How can problems of "excessive vibration or noise" due to piping strain be avoided on
steam turbines?
• The inlet as well as exhaust steam lines should be firmly supported to avoid strains from
being imposed on the turbine.
• Adequate allowance should be made for expansion of steam pipes due to heat.

37. How can steam turbines be classified?
• By the action of steam: Impulse, Reaction, Impulse and reaction combined.
• The number of step reductions involved: Single stage, Multi-stage,
• Whether there is one or more revolving vanes separated by stationary reversing
vanes.
• The direction of steam flow: Axial, Radial, Mixed, Tangential, Helical & Reentry.
• The inlet steam pressure: High pressure, Medium pressure & Low pressure.
• The final pressure: Condensing & Non-condensing.
• The source of steam: Extraction & Accumulator.
38. How can the deposits be removed?
• Water soluble deposits may be washed off with condensate or wet steam.
• Water insoluble deposits are removed mechanically after dismantling the turbine.
Experience shows that water soluble deposits are embedded in layers of water
insoluble deposits. And when the washing process is carried out, water soluble
parts of the deposit dissolve away leaving a loose, friable skeleton of water
insoluble deposits which then break loose and wash away.
39. How can the detection of deposits in a turbine be made during operation?
• Pressure monitoring.
• Internal efficiency monitoring.
• Monitoring exhaust steam temperature.
• Monitoring specific steam consumption.

40. How can the misalignment be rectified?
• The bolts holding the flanges together are to be tightened. The coupling is to be
checked for square ness between the bore and the face. At the same time axial
clearance is to be checked. Using gauge block and feeler gauges, the gap between
coupling faces 1800 apart is to be measured. After rotating the coupling-half 1800,
the gap at the same points is to be measured. After this, the other coupling is to
be rotated 1800 and the gap at the same points is to be re-measured. These
measures should come within a few thousands of an inch. Dividing the coupling
faces into four intervals, the distance between the coupling faces at this intervals
is to be measured with the aid of a gauge block and feeler gauges. These gap
measurements should come within 0.005 inch for proper angular shaft alignment.
After proper alignment at room temperature, the two halves of the coupling are to
be connected.
41. How can the disadvantages of the impulse turbine can be overcome?
• By Velocity compounding , By Pressure compounding or by Pressure-Velocity
compounding.
42. How can the fatigue damage on high-pressure blades be corrected?
• Fatigue-damage on high-pressure blades arises due to vibration induced by
partial-arc admission. This can be corrected by switching over to full arc admission
technique.

Frequently asked questions on turbines Part 1

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Frequently asked questions on turbines Part 1