The rotor and stator of a centrifugal compressor, one rotating at high speed and the other stationary, must have a certain gap between the two, so there must be gas leakage from one part to another inside the machine, and at the same time leak to the outside (or inside) of the machine. In order to reduce or prevent these leaks of gases, a sealing device is required.
There are many types of seals for centrifugal compressors. According to its position, it can be divided into internal seal (interstage seal, intermediate seal) and external seal (shaft end seal)., the former prevents leakage between the cavities of the internal flow part of the machine, such as the seal on the wheel cover, the fixed distance sleeve and the balance plate; The latter prevents or reduces the leakage of gases from the machine to the outside or from the outside to the inside of the machine (the pressure of the gas inside the machine is lower than the air pressure outside), such as the first impeller seal on the suction side and the last impeller outlet seal.
According to its sealing principle, it can be divided into air seal and liquid seal. In the air seal, there are labyrinth seals and inflatable seals; Among liquid seals, there are fixed seals, floating ring seals, fixed built-in mechanical seals, and other liquid seals.
The structural form of the seal is related to the pressure, the medium and the part of its seal, the general interstage seal adopts the labyrinth seal, and the air seal on the balance plate often adopts a honeycomb-shaped labyrinth seal. The sealing of toxic, flammable and explosive media in chemical compressors mostly adopts liquid seals, pumping seals or inflatable seals. For high-pressure, toxic, flammable and explosive gases such as ammonia, methane, propane, petroleum gas and hydrogen, etc., leakage is not allowed, and the shaft end seal adopts floating ring seal, mechanical seal, pumping seal or inflatable seal. When the compressed gas is non-toxic, such as air, nitrogen, etc., and a small amount of gas leakage is allowed, a labyrinth shaft end seal can also be used.
The commonly used seals of centrifugal compressors in chemical production include labyrinth seals, floating ring seals, mechanical seals and dry gas seals, etc., in addition, a new type of magnetic fluid seal has emerged in recent years.
Labyrinth sealing
Labyrinth seals are generally comb-like structures, so they are also called comb seals. The flow of gas in a labyrinth seal is shown in Figure 1. As the gas flows through the comb-like sealing gap, the pressure and temperature of the gas decrease and the velocity increases due to the narrow flow channel, i.e., a part of the static pressure energy is converted into kinetic energy. When the gas enters the cavity between the comb teeth, due to the sudden expansion of the cross-sectional area of the flow channel, the air flow forms a very strong vortex, the velocity almost completely disappears, and the kinetic energy is converted into heat energy, so that the gas rises to the original temperature, while the pressure behind the gap is still maintained in the cavity. The gas passes through each comb in turn, and the pressure is continuously reduced, so as to achieve the purpose of sealing. Therefore, the labyrinth seal converts the gas pressure into speed, and then reduces the speed to achieve the balance between the internal and external pressure, thereby reducing the leakage of gas from high pressure to low pressure.
Fig.1. Flow of gas in a labyrinth seal.
There are various structures of labyrinth seals, and the following are used in compressors.
1) Tortuous type(Fig. 2) It is characterized by a groove on the shaft in addition to a sealing tooth (or sealing sheet) on the sealing body. There are two types of seals for zigzag labyrinths: integral type and mosaic type. The disadvantage of the integral type is that the spacing of the sealing teeth cannot be too short, so the axial size is long; The axial size can be greatly reduced by using the inlay type.
Fig.2. Zigzag labyrinth seal.
2) Smooth type(Fig. 3) This kind of seal is either made of a smooth shaft or a smooth inner surface of the sealing body, which can be divided into an overall smooth type and an inlay smooth type.
Fig.3. Smooth labyrinth seal.
3) Step type(Fig. 4) It is mostly used for sealing on wheel covers or balance plates.
Fig.4. Step seal.
4) Honeycomb type(Fig. 5) The honeycomb sealing process is complex, but the sealing effect is good, and the sealing sheet has high structural strength.
Fig.5. Honeycomb seal.
The number of comb teeth in the labyrinth seal is generally 4 35 pieces. The material of the comb teeth should be softer than the corresponding part of the rotor to prevent damage to the rotor when the seal comes into contact with the rotor. Its commonly used materials are generally bronze, copper, antimony, tin alloy, aluminum and aluminum alloy. When the temperature exceeds 393K, nickel-copper-iron Monel alloy can be used, or stainless steel strips can be used. When the gas is the most sparkable, it should be made of materials that do not produce sparks, such as silver, nickel, aluminum or aluminum alloys, and PTFE materials can also be used. Labyrinth sealing is a relatively simple sealing device, which can be used for sealing at both ends of the casing and between stages (the sealing between intermediate stages is almost always labyrinth sealing).
Floating ring sealing
The basic structure of the floating ring seal is shown in Figure 6. The seal is mainly made up of several floating rings, and the high-pressure oil is injected into the sealing body by the oil inlet hole 12, and then overflows to the left and right sides, the left side is the high-pressure side, the right side is the low-pressure side, and the oil that flows into the high-pressure side is discharged by the oil return hole 11 by the high-pressure floating ring, the oil baffle ring 6 and the oil throwing ring 7. Because the oil pressure is generally controlled at a pressure slightly higher than that of the gas, the pressure difference is small, so the amount of oil leakage to the high pressure side is small. The oil flowing into the low-pressure side passes through several floating rings (three in the picture) and then out of the sealing body. Because the pressure difference between the high-pressure oil and the atmosphere is large, the amount of oil leakage to the low-pressure side is large. The floating ring is hung on the axle sleeve 5 and is movable in the radial direction. When the shaft rotates, the floating ring is floated by the oil film, in order to prevent the floating ring from rotating, generally add pin 3 to control, and the oil film formed at this time closes the gap to prevent gas leakage.
Fig.6. The basic structure of the floating ring seal.
1-floating ring; 2-fixed ring; 3-pin; 4-spring; 5-shaft sleeve; 6-oil retaining ring; 7-oil throwing ring; 8-shaft; 9-labyrinth seal; 10-seal; 11-oil return hole; 12-oil inlet hole.
The floating ring seal is mainly the formation of an oil film between the floating ring and the shaft sleeve by high-pressure oil, which produces throttling and pressure reduction, preventing the gas communication between the inside and outside of the machine. Because the oil film plays the main sealing role, it is also called oil film sealing.
In order to facilitate assembly, several L-shaped fixing rings are generally made, and the floating rings are installed in the middle of the L-shaped fixing rings. The pressure difference of the high-pressure ring is small, and only one is generally used. The low pressure ring has a large pressure difference, and several are generally used. In order to make the gap between the floating ring and the L-shaped fixing ring not too large, the floating ring is flattened with a spring 4.
Floating ring seals are mainly used in the shaft seals of centrifugal compressors to prevent gas escape or air intake into the machine. If the device is in good working order, the sealing performance can be "absolutely sealed". It is especially suitable for high-pressure, high-speed centrifugal compressors, so it is widely used in petrochemical plants to seal all kinds of expensive high-pressure gases and all kinds of flammable, explosive and toxic gases.