Source: Steam Turbine Man
The rotating part of a steam turbine is commonly known as a rotor, which is composed of a spindle, impeller, blades and other parts. The shaft of a large steam turbine resembles an umbrella that is only half open.
The blades are embedded in the impeller, and the steam turbine relies on high-pressure and high-temperature steam to impulse the blades to drive the rotating shaft to rotate. Therefore, the quality of the blade is very high, and high-grade alloy steel should be used as raw material.
1. Why is the whole rotor shaped like a half-open umbrella?
Originally, the 125,000-watt intermediate reheat steam turbine had a total of 4,800 blades. The blades vary in size, with the smallest being half the length of a thumb (23 mm) and the largest being longer than an arm (700 mm).
So, why do the blades have to be made of different sizes? A circle of nozzles and a circle of blades is called a first stage, and there are 25 stages of 125,000 rounds of intermediate reheat steam turbines. The vapor pressure of each level is not the same, the first stage is subject to the steam inlet pressure, so it has the highest pressure, and the last stage is the exhaust pressure of the steam turbine, which is the lowest. The blades of the steam turbine are arranged in the opposite direction of the steam pressure, the first stage is the smallest, and the blades of the next stage gradually increase until the last stage is the largest, so that the whole rotor looks like a half-open umbrella.
2. Why is the steam pressure high, but the blades should be smaller?
This is so that the steam impact force received by each part of the turbine shaft can be consistent. We know that the pressure of steam is inversely proportional to the dissolution. The pressure of the first stage of steam is the largest, and the steam dissolution at this time is the smallest, so the blade is the smallest; when the steam passes through the first stage, its pressure gradually decreases, and then its volume gradually increases, so the conduction area of the steam has to increase, so the blade has to be larger. If each stage of the blades is made of the same size, then the impact force of the steam on each part of the shaft will be large and small, and the rotation of the shaft will be unstable. This is the reason why steam turbine blades grow from small to large.
The blades are an important part of the rotor. The working conditions of the blades are very difficult, the temperature is high and constantly changing, and it is also subject to considerable centrifugal and bending forces. During operation, if the blades are broken, it will cause serious damage to the equipment.
Therefore, the material of the blade, as well as the manufacturing process, have quite high requirements, such as the blade of the 125,000-watt intermediate reheat turbine, the error of precision during processing cannot exceed one-third of the hair.
3. Why do equal-section blades be used in the high-pressure section of the steam turbine?
Generally, in the high-pressure section of the steam turbine, the steam volume flow rate is relatively small, the blades are short, the blade height ratio d/L (d is the average diameter of the blade, L is the blade height) is larger, and the difference between the circumferential velocity and steam flow parameters along the entire blade height is relatively small. At this time, the work efficiency of the blade can not be significantly improved by changing the section profile of different leaf heights, so the section profile of the blade body is made the same along the leaf height, that is, the equal section blade is made. Although the efficiency is slightly affected, the processing is convenient, the manufacturing cost is low, and the strength can also be guaranteed, which is conducive to the generalization of some blades.
Fourth, why should the low-pressure blades of the steam turbine be designed as torsion blades?
In order to increase the power, the blades of large units are often made very long. With the increase of blade height, when the leaf height ratio has a small value (generally less than 10), the difference of circumferential velocity and vapor flow parameters at different blade heights can no longer be ignored. At this time, the cross-sectional profile of the blade must change accordingly along the blade height, so that the blade is twisted and deformed, so as to adapt to the variation law of vapor flow parameters along the blade height and reduce the flow loss. At the same time, in order to improve the distribution of tensile stress caused by centrifugal force along the leaf height, the cross-sectional area of the leaf body should also be gradually reduced from the root to the top.