Steam turbines: how hot steam turns into electricity
Scientists are still struggling to find the most effective ways to generate current - progress has rushed from galvanic cells to the first dynamos, steam, nuclear, and now solar, wind and hydrogen power plants. Nowadays, the most massive and convenient way to generate electricity remains a generator driven by a steam turbine.
Steam turbines were invented long before man understood the nature of electricity. In this post we will simplistically talk about the structure and operation of a steam turbine, and at the same time recall how the ancient Greek scientist was fifteen centuries ahead of time, how the coup took place in turbine engineering, and why Toshiba believes that a thirty-meter turbine should be manufactured with an accuracy of 0.005 mm.
How a steam turbine works
The principle of operation of a steam turbine is relatively simple, and its internal structure has not fundamentally changed for more than a century. To understand the principle of operation of a turbine, we will consider how a thermal power plant works - a place where fossil fuels (gas, coal, fuel oil) are converted into electricity.
The steam turbine itself does not work, it needs steam to function. Therefore, the power plant begins with a boiler in which fuel is burned, giving off heat to the pipes with distilled water penetrating the boiler. In these thin pipes, water turns into steam.
A clear scheme of the operation of a thermal power plant, generating both electricity and heat for heating homes. Source: Mosenergo
The turbine is a shaft (rotor) with radially spaced blades, like a large fan. A stator is installed behind each such disk - a similar disk with blades of a different shape, which is mounted not on the shaft, but on the turbine body, and therefore remains stationary (hence the name stator).
A pair of one rotating disk with blades and a stator is called a step. There are dozens of stages in one steam turbine - do not untwist the heavy shaft of a turbine with a mass of 3 to 150 tons by passing steam through just one stage, so the stages are grouped in series to extract the maximum potential energy of the steam.
Steam at a very high temperature and under high pressure is supplied to the turbine inlet. The steam pressure distinguishes turbines of low (up to 1.2 MPa), medium (up to 5 MPa), high (up to 15 MPa), ultra-high (15-22.5 MPa) and supercritical (over 22.5 MPa) pressure. For comparison, the pressure inside the bottle of champagne is about 0.63 MPa, in the car tire of a passenger car - 0.2 MPa.
The higher the pressure, the higher the boiling point of water, which means the temperature of steam. Steam superheated to 550-560 ° C is supplied to the turbine inlet! Why so many? As it passes through the turbine, the steam expands to maintain the flow rate and loses temperature, so you need to have a supply. Why not overheat the steam above? Until recently, this was considered extremely complex and meaningless — loading on the turbine and boiler became critical.
Steam turbines for power plants traditionally have several cylinders with blades, into which high, medium and low pressure steam is supplied. First, the steam passes through the high-pressure cylinder, spins the turbine, and at the same time changes its parameters at the outlet (pressure and temperature decrease), after which it goes into the medium-pressure cylinder, and from there low pressure. The fact is that the steps for steam with different parameters have different sizes and shapes of the blades in order to more efficiently extract the energy of the steam.
But there is a problem - when the temperature drops to the saturation point, the steam begins to saturate, and this reduces the efficiency of the turbine. To prevent this, steam is reheated in the boiler after the high cylinder and before it enters the low pressure cylinder. This process is called intermediate overheating (industrial overheating).
There can be several cylinders of medium and low pressure in one turbine. Steam can be supplied to them both from the edge of the cylinder, passing all the blades sequentially, and in the center, diverging to the edges, which evens out the load on the shaft.
The rotating shaft of the turbine is connected to an electric generator. In order for the electricity in the network to have the required frequency, the generator and turbine shafts must rotate at a strictly defined speed - in Russia, the current in the network has a frequency of 50 Hz, and the turbines operate at 1500 or 3000 rpm.
To put it simply, the higher the consumption of electricity produced by a power plant, the stronger the generator resists rotation, so you have to supply a larger flow of steam to the turbine. Turbine speed controllers instantly respond to load changes and control the steam flow so that the turbine maintains constant speed. If the load drops in the network and the regulator does not reduce the amount of steam supplied, the turbine will rapidly increase speed and collapse - in the event of such an accident, the blades easily penetrate the turbine body, the roof of the thermal power plant and fly apart several kilometers away.
How steam turbines came about
Around the XVIII century BC, mankind had already tamed the energy of the elements, turning it into mechanical energy to perform useful work - these were Babylonian windmills. By the II century BC. e. water mills appeared in the Roman Empire, whose wheels were driven by an endless stream of water from rivers and streams. And already in the 1st century AD e. man tamed the potential energy of water vapor, with its help setting in motion the man-made system.
The Eolipilus of Geron of Alexandria is the first and only reactive steam turbine for the next 15 centuries. Source: American Mechanical Dictionary / Wikimedia
The Greek mathematician and mechanic Geron of Alexandria described the bizarre mechanism of eolipil, which is a ball fixed to the axis with tubes emanating from it at an angle. The steam that came into the ball from the boiling boiler exited the tubes with force, causing the ball to rotate. The machine invented by Heron at that time seemed like a useless toy, but in fact, the ancient scientist constructed the first steam jet turbine, whose potential was estimated only after fifteen centuries. A modern eolipil replica has a speed of up to 1,500 rpm.
In the 16th century, the forgotten invention of Heron was partially repeated by the Syrian astronomer Takiyuddin al-Shami, but instead of a ball, a wheel was driven into motion, on which steam blew directly from the boiler. In 1629, a similar idea was proposed by the Italian architect Giovanni Branca: a stream of steam rotated a paddle wheel, which could be adapted to mechanize the sawmill.
The Branca active steam turbine did at least some useful work - it “automated” two mortars.
Despite the description by several inventors of machines that convert steam energy to work, it was still far from a useful implementation - the technologies of that time did not allow creating a steam turbine with practically applicable power.
For many years, the Swedish inventor Gustaf Laval hatched the idea of creating a certain engine that could rotate the axis with great speed - this was required for the operation of the Laval milk separator. While the separator was working from a “manual drive”: a gear system converted 40 rpm on the handle to 7000 rpm in the separator. In 1883, Laval managed to adapt Heron's eolipil, providing the engine with a milk separator. The idea was good, but the vibrations, terrible high cost and uneconomicalness of the steam turbine forced the inventor to return to calculations.
The Laval turbine wheel appeared in 1889, but its design has survived to this day almost unchanged.
After years of painful trials, Laval was able to create an active single-disc steam turbine. Steam was supplied to a disk with blades of four pipes with nozzles under pressure. Expanding and accelerating in the nozzles, the steam hit the blade of the disk and thereby set the disk in motion. Subsequently, the inventor produced the first commercially available turbines with a power of 3.6 kW, connected turbines with dynamos to generate electricity, and also patented many innovations in the design of turbines, including such an integral part of them as the steam condenser in our time. Despite a difficult start, later things with Gustaf Laval went well: leaving his previous company for the production of separators, he founded a joint-stock company and began to increase the capacity of the units.
In parallel with Laval, his research in the field of steam turbines was conducted by the Englishman Sir Charles Parsons, who was able to rethink and successfully complement the ideas of Laval. If the first one used one disk with blades in his turbine, then Parsons patented a multistage turbine with several disks in series, and a little later he added stators to the structure to equalize the flow.
The Parsons turbine had three successive cylinders for high, medium, and low pressure steam with different blade geometry. If Laval relied on active turbines, then Parsons created reactive groups.
In 1889, Parsons sold several hundred of its turbines for the electrification of cities, and another five years later, the experimental ship Turbinia was built, developing a speed of 63 km / h previously unattainable for steam engines. By the beginning of the 20th century, steam turbines became one of the main engines of the fast electrification of the planet.
Now "Turbinia" is exhibited in a museum in Newcastle. Pay attention to the number of screws. Source: TWAMWIR / Wikimedia
Toshiba Turbines - A Century-Long Way
The rapid development of electrified railways and the textile industry in Japan forced the state to respond to increased electricity consumption by building new power plants. At the same time, work began on the design and production of Japanese steam turbines, the first of which were delivered to the needs of the country in the 1920s. Toshiba (in those years: Tokyo Denki and Shibaura Seisaku-sho) also joined the case.
The first Toshiba turbine was released in 1927, it had a modest power of 23 kW. Within two years, all steam turbines manufactured in Japan left Toshiba factories, and units with a total capacity of 7,500 kW were launched. By the way, for the first Japanese geothermal stationOpened in 1966, Toshiba also supplied steam turbines. By 1997, all Toshiba turbines had a total capacity of 100,000 MW, and by 2017, deliveries had grown so much that the equivalent capacity was 200,000 MW.
This demand is due to manufacturing accuracy. A rotor weighing up to 150 tons rotates at a speed of 3600 rpm, any imbalance will lead to vibrations and accidents. The rotor is balanced with an accuracy of 1 gram, and geometric deviations should not exceed 0.01 mm from the target values. CNC equipment helps to reduce deviations in turbine production to 0.005 mm - this is the difference with the target parameters among Toshiba employees is considered a good form, although the permissible safe error is an order of magnitude greater. Also, each turbine must undergo a stress test at increased speeds - for units at 3600 rpm, the test provides acceleration to 4320 rpm.
A good photo for understanding the dimensions of the low pressure stages of a steam turbine. Here is a team of the best masters of the Toshiba Keihin Product Operations factory. Source: Toshiba
Steam Turbine Efficiency
Steam turbines are good in that with an increase in their size, the generated power and efficiency increase significantly. It is economically much more profitable to install one or several units at a large thermal power plant, from which electricity can be distributed over long-distance networks than to build local thermal power plants with small turbines with power from hundreds of kilowatts to several megawatts. The fact is that with a decrease in size and power, the cost of a turbine in terms of kilowatts increases at times, and the efficiency drops by two or three times.
The electrical efficiency of condensing turbines with superheating varies at the level of 35-40%. The efficiency of modern thermal power plants can reach 45%.
If you compare these indicators with the results from the table, it turns out that a steam turbine is one of the best ways to meet the large needs for electricity. Diesels are a “home” story, windmills are costly and low-power, hydropower plants are very costly and geo-referenced, and hydrogen fuel cells , which we have already written about, are a new and, rather, mobile way to generate electricity.
The most powerful steam turbine: such a title can rightfully be worn by two products at once - the German Siemens SST5-9000 and the turbine manufactured by ARABELLE, owned by the American General Electric. Both condensing turbines deliver up to 1900 MW of power. To realize such a potential is possible only at nuclear power plants.
Record turbine Siemens SST5-9000 with a capacity of 1900 MW. A record, but the demand for such power is very small, so Toshiba specializes in units with half the power. Source: Siemens
Smallesta steam turbine was created in Russia just a couple of years ago by engineers of the Ural Federal University - PTM-30 is only half a meter in diameter, it has a capacity of 30 kW. The baby can be used for local production of electricity by utilizing the excess steam remaining from other processes in order to derive economic benefit from it, and not to release it into the atmosphere.
The Russian PTM-30 is the smallest steam turbine in the world to generate electricity. Source: UrFU
The most unsuccessfulsteam turbines - steam locomotives in which steam from the boiler enters the turbine, and then the locomotive moves on electric motors or due to mechanical transmission is considered the use of a steam turbine. Theoretically, a steam turbine provided many times more efficiency than a conventional steam locomotive. In fact, it turned out that the steam turbo locomotive displays its advantages, such as high speed and reliability, only at speeds above 60 km / h. At lower speeds, the turbine consumes too much steam and fuel. The US and European countries experimented with steam turbines on locomotives, but the terrible reliability and dubious efficiency shortened the life of steam turbines as a class to 10-20 years.
The coal steam turbine locomotive C&O 500 broke almost every trip, which is why a year after its release it was sent for scrap. Source: Wikimedia