Turbine steam seal type, classification, comparison

 Large turbines are composed of several stages, each of which is composed of two parts: dynamic and static, so the whole turbine is also composed of two parts: dynamic and static. There must be a certain gap between the rotating and stationary parts of the turbine to prevent mutual friction. Because of the pressure difference between the inside and outside of the cylinder, the front and rear of the partition and both sides of the dynamic lobe with reaction, and the corresponding dynamic and static parts must maintain a certain gap between them so that they do not touch each other, therefore, steam sealing devices must be set.
   The steam seal of the turbine is divided into different locations according to the installation: shaft end seal (referred to as shaft seal), bulkhead seal, and through-flow part of the steam seal, respectively, used to prevent the shaft end of the turbine, the bulkhead and the top of the moving leaves, the root of the steam leakage, its role is to prevent the outside air into the turbine, and the steam mixture in the turbine, reduce the amount of steam leakage, thereby reducing the chemical make-up water and prevent the high level of energy of the working medium to The steam turbine is a vulnerable part of the turbine. As a wearable and necessary part of the turbine, the steam seal of the turbine is attracting more and more attention from the engineers and technicians engaged in the design of the turbine. The steam seal of a steam turbine is of increasing concern to engineers and technicians involved in the design of steam turbines, because the test results of steam turbine operation show that the steam leakage loss of a steam turbine accounts for about 1/3 of the internal loss. With the continuous development of steam sealing technology in recent years, the safety and reliability of turbine operation and the thermal efficiency of the turbine have been improved accordingly.
   1、Traditional steam seal
   At present, the traditional steam seal which is widely used in large, medium and small turbines is mainly the labyrinth type steam seal. The labyrinth type steam seal is commonly used according to the shape of the cross section, including fir-type steam seal and comb-tooth steam seal. The comb seal is widely used due to its low cost, simple structure, safety and reliability and easy installation. The sealing mechanism of the comb-tooth labyrinth seal is that the inner circle of the seal ring and the outer circle of the seal sleeve (high temperature part of the rotor shaft) have many matching comb teeth and concave and convex shoulders, forming a small ring-shaped gap (called the seal gap) and steam expansion chamber to stop the leakage of steam. The steam seal ring is installed in the T-slot of the inner circle of the shaft seal sleeve (spacer) by means of two convex shoulders on the outer circle. Each steam seal ring is divided into six arc blocks (called steam seal blocks), and each block is fitted with two spring pieces between the shaft seal sleeve (bulkhead), so that the block is pressed elastically towards the centre, thus maintaining a minimum gap between the static and dynamic parts.
   Once the conventional comb-tooth labyrinth seal has been installed, the seal gap remains constant during operation without friction. After normal installation, if the steam seal gap is too large, it will cause an increase in steam leakage, leading to a reduction in the thermal efficiency of the turbine set; if the gap is set too small, the rotor may vibrate more during the start-up and shutdown process past the critical speed, leading to friction between the steam seal teeth and the rotor. Although the spring plate on the back of the block can make the block give way when friction occurs between the block and the rotor, reducing the friction pressure, it cannot avoid friction, which will cause wear of the teeth and increase the gap between the block and the rotor, thus increasing the heat consumption. When the friction is serious, the teeth of the seal will be deformed, brittle or even ruptured. At the same time, the friction between the radial seal of the spacer and the rotor will cause the rotor to be locally heated, resulting in thermal deformation and bending of the rotor, leading to further wear of the seal teeth and increasing the gap between the radial seal and the rotor. Therefore, taking into account the vibration of the rotor over the critical speed, the radial clearance of the traditional comb-tooth labyrinth seal is generally 0.60~0.80mm when installed, and some gaps are larger according to the different conditions of the rotor.
    2, honeycomb steam seal the so-called honeycomb seal, is in the static sub-seal ring on the inner surface of the regular honeycomb diamond-shaped positive hexahedral small honeycomb hole-shaped seal strip, its material is only 0.05 ~ 0.10mm thick nickel-based temperature-resistant thin plate made on special molding equipment positive hexahedral grid profile, and then after special welding equipment welding, according to the seal ring size made of honeycomb strip in The honeycomb strip made according to the seal ring dimensions is welded to the parent seal by vacuum brazing technology in a vacuum brazing furnace, and a honeycomb seal is formed, as shown in Figure 1.

                              Figure 1 Honeycomb seal
   The structure of the honeycomb seal is characterised by the fact that the traditional low teeth of the seal are turned and replaced by a honeycomb seal, which is made up of hexagonal holes with a single hexagonal side size of 2 to 3 mm. the honeycomb seal has a wider sealing band, which changes the disadvantage that the number of teeth of the traditional straight seal can only be arranged very little (generally 1 to 2 teeth) due to structural limitations, and still retains the high teeth of the seal. This is equivalent to increasing the number of teeth of the seal, increasing the resistance to vapour flow and improving the sealing effect. The honeycomb seal still adopts the back plate spring structure of the traditional seal, so the installation clearance is generally taken as the upper limit of the design value of the radial clearance of the traditional seal. Compared with the existing traditional high and low tooth structure steam seal, the number of teeth is relatively increased a lot, thus the sealing effect is greatly improved. Due to the short size of the structure of the turbine, it is not possible to design a retreating steam seal, but a fixed structure is used, so that the honeycomb seal will be damaged when there is friction and will not affect the shaft.
    3、Bryden vapour seal
    Due to the structural sealing characteristics of the traditional steam seal and considering the vibration of the rotor, the steam seal gap cannot be adjusted to a smaller size, thus the steam leakage loss of the unit cannot be further reduced and the thermal efficiency of the unit can hardly be further improved. With the development of vapour seal technology, a new type of adjustable vapour seal was introduced from the USA - the Braden vapour seal. This steam seal can make the steam seal gap to a smaller 0.35~0.45mm, which can avoid the rotor from vibrating too much and rubbing against the steam seal when it is over critical in operation. The Braden seal eliminates the plate spring at the back of the conventional seal and replaces it with four coil springs at the end of each seal arc.

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