What is a steam turbine? Types and practical classification

One of the most important machines in the energy conversion field is the steam turbine. The cumulative electrical and mechanical power installed by these devices exceeds the amount of power generation of other similar units. The operational procedures and design of the steam turbines are highly advanced. These devices offer the highest power output (up to 300 MW), highest stage efficiency (from 0.80 to 0.95), and also cover an extremely varied range of applications. To know more about these mature technological components, Visit – mdaturbines.com/steam-turbines/

Steam Turbine: Types 

The main functional principle of steam turbines depends on the dynamic nature of the steam. Steams that come out from the nozzles immediately strike the blades of the rotator that are fitted on a shaft disc. The high-velocity steam produces a dynamic pressure on the blade and as a result the blades and shaft both begin to rotate in a similar direction. 

The governing system of the steam turbine is another important aspect to consider. Irrespective of the varied types of loads with which the turbines work, the governing system is a process utilized to maintain the fixed steady velocity of the turbine. The main purpose of this is to supply the steam into the turbine blades in such a way that it offers a fixed velocity under different types of loads. 

Based on their construction, size, pressure, and numerous other parameters, there are mainly two types of the steam turbine – Reaction steam turbine and Impulse steam turbine. The basic difference between these two types of steam turbines is the way the steam experiences expanding while passing through the turbine. 

1. Reaction steam turbine – During the passing over of the steam over the blades, it gets expanded in both moving and fixed blades due to the mechanism of the turbine. This results in a continuous pressure drop in both fixed and moving blades. The difference between this reaction turbine and the impulse turbine is that reaction turbines are composed of moving blades that are alternatively placed with the fixed nozzles. Also, the per-stage pressure drop amount in the reaction turbine is much lower than the impulse turbine. Moreover, these steam turbines usually have more efficiency. 

Generally, the requirement for the number of blade rows is almost half in the impulse turbine than that of the reaction turbine for the same amount of thermal power conversion. Therefore, these reaction turbines are much heavier and longer. 

The main features of the reaction steam turbine are: 

• In reaction turbines, the pressure cannot be entirely expanded. Only it can be partially expanded when it rests on the blades of the rotor or passes through the nozzle. 

• The speed of the reaction turbine is much lower than the impulse turbine because of the lower steam velocity. 

• The blade efficiency curve of the reaction turbines is much lower than the impulse turbines. 

• The main mechanism lies in how the rotor blades are got stricken by the vector sum amount of the steam’s impulsive and reactive force. 

• Here, the shapes of the blades are asymmetrical. 

• The reaction turbine requires a higher number of stages to produce the same amount of power. 

2. Impulse steam turbine – In the impulse turbine, the steam comes out at a fairly high speed through a fixed turbine nozzle and strikes the rotor blades. The blades change the direction of flow without altering their pressure. Here, the rotation of the shaft basically happens because of the variation in momentum. The impulse steam turbines can even function by utilizing only a single-blade wheel. 

The main features of the impulse steam turbine are:

• During the passage through the fixed nozzles, the steam expands entirely and the pressure produced by the steam remains constant. 

• Since the speed of the steam is fairly high, so impulse turbine offers a high velocity. 

• The curve that defines the efficiency of the blades is also high. 

• The main mechanism lies in how the rotor blades in this steam turbine are stricken by impulsive force. 

• The rotor blades of reaction steam turbines are shape-wise symmetrical. 

• The same amount of power can be produced by utilizing only a few stages. 

Also, there are different types of steam turbines depending on the number of pressure stages they involve to produce power: 

• Multistage reaction and impulse turbines – These turbines are designed in a huge range of power capacities. There are also wide variations of sizes. 

• Single-stage turbines – The single staged turbines are mostly utilized for driving blowers, centrifugal compressors, and other similar machinery. 

In addition, steam turbines can be also classified on the basis of steam flow: 

• Radial turbines – In these radial turbines, the steam flow occurs in a direction that’s perpendicular to the turbine’s axis. Some of the low-pressure stages are even more axial than this. 

• Axial turbines – In axial turbines, the steam flow occurs in a parallel direction to the axis of the turbine’s rotor. 

The basic advantage of all these categories of steam turbines is that the thermal efficiency is fairly high and a heavy foundation is not required for perfect balancing. Moreover, the lubrication procedure is simple, and achieving a great range of steam velocity is also possible.