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:
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.
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:
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.
What are four types of thrust hearings?
Answer:
Babbitt-faced collar bearings.
Tilting pivotal pads.
Tapered land bearings.
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:
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.
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:
Impulse type.
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.
