Compounding of Steam Turbines

A velocity-compounded impulse stage consists 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 takes 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 onto 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 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.

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.

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.

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.

A compounded steam turbine has multiple stages, i.e., it has more than one set of nozzles and blades, 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. For example, a large HP Turbine used in nuclear power plants can be a double-flow reaction turbine with about ten 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 the last three stages).

In a reaction steam turbine compounding can be achieved only in the pressure compounding. In fact, it is not exactly the same as what it was discussed in impulse turbines. Note that, there is steam expansion in both the fixed and moving blades.

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