Steam turbines may be classified into different categories depending on their construction, working pressures, size and many other parameters. But there are two basic types of steam turbines:
- impulse turbines
- reaction turbines.
The main distinction is the manner in which the steam is expanded as it passes through the turbine.
Impulse Turbine and Reaction Turbine
Steam turbine types based on blade geometry and energy conversion process are impulse turbine and reaction turbine.
The impulse turbine
is composed of moving blades
alternating with fixed nozzles
. In the impulse turbine, the steam is expanded in fixed nozzles and remains at constant pressure when passing over the blades. Curtis turbine
, Rateau turbine
, or Brown-Curtis turbine
are impulse type turbines. The original steam turbine, the De Laval, was an impulse turbine having a single-blade wheel.
The entire pressure drop of steam take place in stationary nozzles only. Though the theoretical impulse blades have zero pressure drop in the moving blades, practically, for the flow to take place across the moving blades, there must be a small pressure drop across the moving blades also.
Impulse vs Reaction Turbine – comparison
In impulse turbines, the steam expands through the nozzle, where most of the pressure potential energy is converted to kinetic energy. The high-velocity steam from fixed nozzles impacts the blades, changes its direction, which in turn applies a force. The resulting impulse drives the blades forward, causing the rotor to turn. The main feature of these turbines is that the pressure drop per single stage can be quite large, allowing for large blades and a smaller number of stages. Except for low-power applications, turbine blades are arranged in multiple stages in series, called compounding, which greatly improves efficiency at low speeds.
Modern steam turbines frequently employ both reaction and impulse in the same unit, typically varying the degree of reaction and impulse from the blade root to its periphery. The rotor blades are usually designed like an impulse blade at the rot and like a reaction blade at the tip.
Since the Curtis stages reduce significantly the pressure and temperature of the fluid to a moderate level with a high proportion of work per stage. An usual arrangement is to provide on the high pressure side one or more Curtis stages, followed by Rateau or reaction staging. In general, when friction is taken into account reaction stages the reaction stage is found to be the most efficient, followed by Rateau and Curtis in that order. Frictional losses are significant for Curtis stages, since these are proportional to steam velocity squared. The reason that frictional losses are less significant in the reaction stage lies in the fact that the steam expands continuously and therefore flow velocities are lower.
Compounding of steam turbines
is the method in which energy from the steam is extracted in a number of 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 number of stages. For example, large HP Turbine used in nuclear power plants can be double-flow reaction turbine with about 10 stages with shrouded blades. Large LP turbines used in nuclear power plants are usually double-flow reaction turbines with about 5-8 stages (with shrouded blades and with free-standing blades of last 3 stages).
In an impulse steam turbine compounding can be achieved in the following three ways:
- pressure compounding
- velocity compounding
- pressure-velocity compounding
Impulse Turbine – velocity compounding
A velocity-compounded impulse stage consist of a row of fixed nozzles followed by two or more rows of moving blades and fixed blades (without expansion). This divides the velocity drop across the stage into several smaller drops. In this type, the total pressure drop (expansion) of the steam take place only in the first nozzle ring. This produces very high velocity steam, which flows through multiple stages of fixed and moving blades. At each stage, only a portion of the high velocity is absorbed, the remainder is exhausted on to the next ring of fixed blades. The function of the fixed blades is to redirect the steam (without appreciably altering the velocity) leaving from the first ring of moving blades to the second ring of moving blades. The jet then passes on to the next ring of moving blades, the process repeating itself until practically all the velocity of the jet has been absorbed.
This method of velocity compounding is used to solve the problem of single stage impulse turbine for use of high pressure steam (i.e. required velocity of the turbine), but they are less efficient due to high friction losses.
Rateau Turbine – pressure compounding
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 it 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.
Curtis Turbine – pressure-velocity compounding
Impulse stages may be either pressure-compounded, velocity-compounded, or pressure-velocity compounded. The 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.
The steam coming from the steam generator is passed to the first ring of fixed nozzles, where it 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, less 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.