Steam turbine: device, operating principle, main elements

A steam turbine is a type of engine that uses steam or hot air to rotate the shaft and does not require the introduction of such parts as a crankshaft, connecting rod, pistons into the design.

Many people are familiar with the general structure of the above-described design from school. In scientific literature, the structure of a steam turbine is described as follows.

General structure of the engine

The main part of the engine is the shaft on which the disks and working blades are installed, and such elements as nozzle pipes are located nearby. The latter provide a constant supply of hot steam from the boiler. When the steam enters the nozzle, mechanical pressure is created on the working blades, and, consequently, on the entire structure of the disk. This pressure creates a torque, which makes the disks and the blades located on it move.

Today, steam turbines more commonly use a large number of disks strung on one rotating shaft. In this case, the engine operates somewhat differently. Hot steam moving through the blades of the disks loses some of its energy, giving it to the structural elements. Such a device increases the efficiency of energy use, but, in turn, requires the installation of a boiler for additional reheating of steam. Steam turbines are most popular at thermal and nuclear power plants, where their operation determines the production of alternating electric current. Here, the shaft rotation frequency can be close to 3000 rpm. This value allows for the profitable production of electrical energy generated by generators.

It should be noted that steam turbines are also currently used on sea and river vessels. Turbines cannot be used on aircraft or in ground transport due to the high water consumption required for normal operation of the generators.

Internal and external structure of the nozzle, its functions

The nozzle is one of the most important parts of a steam turbine, it is through it that the constant supply of steam occurs.

At the time when designers did not yet have sufficiently complete information about the steam expansion process, it was impossible to design a device with a high efficiency. First of all, this was determined by the structure of the nozzles, which had the same diameter throughout their entire length. At the same time, the steam passing through them moved into an area of lower pressure. Under such conditions, the flow pressure naturally decreased, transforming into speed. For normal saturation of dry steam, its pressure level at the end of the nozzle should be more than 0.58 of its initial level. This value is called the critical pressure. On its basis, the maximum flow velocity is calculated, the critical velocity, which for superheated steam is set at 0. 546 of the initial steam pressure.

But these conditions were also insufficient for the rational operation of the engine. Here, when overcoming the nozzle pipe, the steam began to rotate due to the expansion of the flow. The solution to this problem was to transform the shape of the engine nozzle. Now the nozzle had a narrower diameter, which increased as it approached the turbine disks. An additional feature of this shape was that at the outlet of the flow it was possible to bring its pressure closer to the pressure values in the external environment at the end of the nozzle. This solved the problem of steam rotation, which negatively affected the flow speed, and made it possible to achieve supercritical pressure values levels.

Steam turbine structure and operating principle

It should be noted that two operating principles are implemented in a steam turbine, determined by its design.

The first principle is the principle of active turbines. This refers to those designs where the increase in the volume of hot flow occurs in stationary pipes and before it passes to the moving disk.

The second principle is reactive. Such engines include all those in which the increase in the volume of hot flow occurs both before it enters the rotating disk and in the time interval between them. Also, devices with a similar design are designated as operating on reaction. Provided that the heat loss in the pipes is about half of all losses, the steam turbine is also called reactive.

When examining the design of the engine and its main parts, other processes must also be noted. Thus, the flow of liquid directed to the rotating disk will produce pressure on it. The pressure level here will be directly dependent on the conditions: the volume of incoming liquid, the speed of the jet at the entry and exit to the working blades, the profile of the blades and the angle of incidence of the liquid on the surface of the blades. It is not at all necessary for the water to hit the blades, rather the opposite, such an effect is often avoided and a smooth contact of the jet with the blade is sought.

Operation of a steam turbine

What is the design of a turbine operating on a similar principle. The main attention is drawn to the law that the body has greater kinetic energy if it moves at high speed. But it is necessary to understand that energy is lost when speed losses appear. Then there are the following possible scenarios for the development of events when a hot flow collides with a blade of a working blade located perpendicular to its direction.

The first option is possible: the jet collides with a static surface. Then the energy of motion is partially converted into heat, and the remaining energy will be spent on the movement of flow particles in the opposite direction from the blade, backwards. Obviously, the useful work performed in this case will be minimal.

Another option: the turbine blades will be in motion. Then a certain part of the internal energy will be spent on moving the disk with blades, and the remainder will also disappear without performing any useful work.

The design of the steam turbine and the process of its functioning - active - implements the last option. Of course, the goal should be taken into account - to minimize irrational energy costs. In addition, it is necessary to protect the blades from damage when they collide with the steam flow. A safe process can be achieved by installing a blade with the most advantageous blade shape for this.

Through surveys and relevant calculations, it was found that the most suitable for collision with the flow will be such a blade shape that will be able to produce a smooth revolution, after which the direction of the jet will be shifted to the opposite side. That is, a semicircle shape should be selected for the blades. Then, when hitting the surface of the blade. The steam will transfer the maximum of its internal energy to the turbine disk, thus implementing its rotation. The heat losses detected in this case will be close to insignificant.

The principle of operation of an active steam turbine

The structure and general principle of operation of the engine in operation is as follows.

A hot stream with a set pressure and speed is directed into the nozzle, where its volume increases to the second pressure value. Accordingly, the speed of the flow increases with this value. Gaining increasing speed as it moves along the nozzle, the flow reaches the working blades. By exerting pressure on the blades, the steam moves the disk and also the turbine shaft connected to it.

After passing through the blades, the flow, due to collisions with obstacles, reduces the speed values - a significant part of the internal kinetic energy is converted into mechanical energy. The pressure level also decreases here. However, at the inlet and outlet from the blades, these steam values are equal, which is due to equal cross-sections of the channels along the entire length between the blades of the working blades. Also, maintaining the original state of the steam is due to the fact that inside the parts themselves, there is no additional increase in the original volume of steam. To remove the exhaust steam, there is a special branch pipe in the turbine design.

Technical device of a steam turbine

The turbine design contains three cylinders, which are stators in a fixed shell, and a powerful rotating rotor. Several separated rotors are fastened with couplings. The chain, composed of the rotors of the cylinders, the generator of electric current and the exciter, is combined into a shaft line. The dimensions of this structure of the design with the largest dimensions of its parts are about 80 meters in length.

During operation, the turbine and its work are as follows. The shaft line rotates in the support bearings of the sliding liners. The revolutions are performed on a dense lubricating layer, the shaft does not directly touch the metal surfaces of the liners during operation. Today, as a rule, the rotors of the device are installed on two support bearings.

Sometimes, among the rotors related to the HPC and MPC, only one support bearing works. The flow, increasing its volume in the turbine, forces the rotors to rotate. The energy generated by the rotors is connected in the half-coupling and here it receives its greatest value.

Also, all elements experience the effect of axial stress. The efforts are added up and their greatest indicator - the axial stress in total - is transferred to the rotor segments.

Technical structure of the turbine rotor

Individual rotors are located in cylinders. The pressure values in them in modern engines often reach 500 MPa, therefore the housings are made with two walls, which allows to reduce the pressure differences. This also makes it possible to make the process of tightening flange connections much simpler and faster. With this precaution, a sharp change in the value of the power generated by the engines is possible.

The presence of a horizontal hole is necessary, allowing for quick installation of parts inside the housing of the structure, and also creates access to the already built-in rotor when checking and repairing the device. When installing the turbine itself, all connectors and housing holes are located accordingly. In order to simplify the installation procedure of the steam turbine, it is agreed that all horizontal planes are connected into a single one.

When further installing the shaft-turning device, it is located in the prepared horizontal connector, which guarantees the centring of the parts. This is required primarily to prevent collisions between the stator and the rotor during engine operation. This problem can create a serious accident of the steam turbine. Since the steam flow inside the steam turbine has high temperatures, and the rotor turns over a lubricating layer, the oil temperature should not exceed 100 ᵒ Celsius. Such frames are optimal both in accordance with fire safety standards and in order to preserve the lubricating properties of the liquid. In order to achieve these values, the bearing shells are located outside the cylinder walls in prepared supports.

Operation of turbines at nuclear power plants

The design of a turbine at a nuclear power plant is studied using the example of saturated steam devices, which are present only at facilities that use steam as an energy source. The primary indicators of structures at nuclear power plants have low indicators. Therefore, to achieve the desired effect, a larger amount of liquid is passed through them. Due to this, the humidity deposited on the elements of the turbine structures increases. The solution here is moisture traps inside and outside the engine casings.

Increasing the humidity level also reduces the final efficiency of the steam turbine and causes erosive destruction of the nozzles. To avoid possible damage, the parts of the structure are chromed, hardened, and subjected to electric spark treatment. So, in the conditions of gas stations, the main task of designers is to protect structures from destruction by high humidity.

The most rational method of removing excess liquid from turbines is the method of steam extraction, performed on regenerative heaters. Moreover, if these extractions are placed on the turbine in stages, then they carry out complete removal of excess moisture and the need to install moisture traps inside the turbines disappears. Possible humidity values directly depend on the diameter of the blades of the working blades and on the frequency of disk rotation.

The structure of steam and gas turbines

The main advantage of a steam turbine, like AEG steam turbines, is the absence of the need to connect an electric current generator to the turbine shaft. It is resistant to overloads and can be controlled using a shaft speed control device. Their efficiency is also relatively high, which, taking into account all other qualities, puts them in first place in terms of operating efficiency.

Gas turbines, which are almost no different in design from steam turbines, have similar characteristics. They are also blade-type devices, and the rotor movement here is also carried out by converting the kinetic energy of the flow.

The main difference is in the type of working substance used. Just as in steam this is water or steam, so in gas turbines, gas is used, released by combustible materials or representing a composition of steam and air. An additional difference is in the equipment necessary for the release of these working substances. Therefore, in general, the designs are almost the same, but their additional equipment is different.

Steam turbine with built-in condensate

Condensers and steam turbines were studied in the monograph by S.M. Losev, published in 1964. The book contained a theoretical description of the design and operation of turbines and their condenser units.

The turbine unit, located in the heater, contains several environments - water, gas and condenser, which together make up a complete cycle. Under this condition, a minimum amount of steam and water is spent in the environment during the transformation process. To replenish them, natural water, previously passed through a water purifier, is poured into the unit. Here the water is exposed to chemicals that cleanse it of excess impurities.

The principle of operation of the condenser unit:

The gas flow that has passed through the turbine blades and has a comparatively lower pressure and amount of heat is discharged into the condenser.
At the same time, there are tubes along the path of the steam, with the help of which the cooling liquid is drawn out by pumps. It is often used from natural reservoirs.
When it touches the cold walls of the tubes, the steam is converted into condensate, which is due to its higher temperature.
The resulting condensate is collected in a condenser unit, where it enters the pump tubes and is poured into the deaerator.
From there, the liquid is again transferred to the heater, converted into gas and launched into a new cycle.

In addition to these main elements and a simple operating algorithm, there is a list of other devices - turbocharging and a heater.