Steam Turbine - Part II (Compounding of steam turbine)
Compounding of steam Turbine
Compounding method in which energy from the steam is extracted in several stages rather than a single stage in a turbine. In all turbines, the rotating blade velocity is proportional to the steam velocity passing over the blade. If the steam is expanded only in a single stage from the boiler pressure to the exhaust pressure, its velocity must be extremely high.
A compounded
steam turbine has multiple stages, i.e., it has more
than one set of nozzles and rotors, in series, keyed to the shaft or fixed to
the casing, so that either the steam pressure or the jet velocity is absorbed
by the turbine in several stages
Compounding will be done
mostly on Impulse type Turbine and for Reaction turbine the same is not required (except
few cases of severe application, only pressure compounding can be done but it
is different from the compounding arrangement discussed below)
Velocity Compounding
This method of
velocity compounding is used to solve the problem of single-stage impulse
turbines for the use of high-pressure steam (i.e., the required velocity of the
turbine), but they are less efficient due to high friction losses
In the above diagram, the pressure drop happens in
the nozzle itself and pressure will remain same in the next row of blades, whereas
velocity increased on the nozzle (Bernoulli’s Principle) and further decreased
on the subsequent moving blades and fixed blade guides the steam to next moving
blade.
Pressure Compounding
- Rateau Turbine - Zoelly Turbine
A pressure-compounded
impulse stage is a row of fixed
nozzles followed by a row of moving blades, with multiple stages for
compounding. In this type, the total pressure drop of the steam does not take
place in the first nozzle ring but is divided up between all the nozzle rings.
The effect of absorbing the pressure drop in stages is to reduce the velocity
of the steam entering the moving blades. The steam from the boiler is passed
through the first nozzle ring in which is only partially expanded. It then passes over the first
moving blade ring, where nearly all of its velocity (momentum) is absorbed.
From this ring, it exhausts into the next nozzle ring and is again partially expanded. This method of pressure compounding is used in Rateau and
Zoelly turbines, but such turbines are bigger and bulkier in size
In the above diagram, the arrangement of blade is different from the velocity compounding turbine. The fixed blade is replaced with nozzles and the pressure drop happens in the nozzle series of nozzle fixed on the diaphragm and pressure also reducing on the nozzles and remains constant on the moving blades, whereas velocity increased on the each nozzles (Bernoulli’s Principle) and further decreased on the subsequent moving blades.
Pressure
Velocity compounding – Curtis Turbine
Impulse stages may be either pressure-compounded, velocity-compounded, or pressure-velocity-compounded. Pressure-velocity compounding is a combination of the above two types of compounding. In fact, a series of velocity-compounded impulse stages is called a pressure-velocity compounded turbine. Each stage consists of rings of fixed and moving blades. Each set of rings of moving blades is separated by a single ring of fixed nozzles. In each stage, there is one ring of fixed nozzles and 3-4 rings of moving blades (with fixed blades between them). Each stage acts as a velocity compounded impulse turbine. (refer below animation for clear understanding)
The steam coming from the steam generator is
passed to the first ring of fixed nozzles, which gets partially expanded.
The pressure partially decreases, and the velocity rises correspondingly. It
then passes over the 3-4 rings of moving blades (with fixed blades between
them), where nearly all of its velocity is absorbed. From the last ring of the
stage, it exhausts into the next nozzle ring and is again partially expanded.
This has the advantage of allowing a bigger
pressure drop in each stage and, consequently, fewer stages are necessary,
resulting in a shorter turbine for a given pressure drop. It may be seen that
the pressure is constant during each stage; the turbine is, therefore, an
impulse turbine. The method of pressure-velocity compounding is used in
the Curtis turbine.
Condensing, Non condensing and other type of turbines will be discussed on next topic. Each turbine parts, its accessories (lube oil, gland sealing, etc..), basics instrumentation and control, basic electrical related to steam turbine, calculations will be discussed on subsequent topics.
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