Centrifugal compressor seals

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1． What is the role of sealing? What forms can it be divided into according to its structural characteristics? If a centrifugal compressor wants to achieve good operating results, a certain gap must be reserved between the rotor and the stator to avoid friction, wear, collision, damage and other faults. At the same time, due to the existence of the gap, it will naturally cause leakage between stages and at the shaft end. Leakage not only reduces the working efficiency of the compressor, but will also cause environmental pollution and even fire and explosion accidents. Therefore, leakage is not allowed. Sealing is an effective measure to avoid leakage between compressor stages and shaft ends while retaining an appropriate gap between the rotor and stator. Depending on the operating temperature, pressure of the compressor and whether the gas medium is polluted or not, different structural forms can be used for sealing, and it is commonly called a sealing device. According to the structural characteristics, sealing devices can be divided into five types: air-extraction type, labyrinth type, floating ring type, mechanical type and spiral type. For generally toxic, flammable and explosive gases, floating ring type, mechanical type, spiral type and air-extraction type sealing devices should be used. If the gas is non-toxic, harmless and has a low pressure rise, labyrinth sealing devices can be used. 2． What are the structural characteristics of the labyrinth seal device? The labyrinth seal is the most basic sealing form between stages and shaft ends of centrifugal compressors. According to different structural characteristics, it can be divided into 4 types: smooth, concave and convex, stepped and honeycomb. ① Smooth labyrinth seal: The smooth labyrinth seal has two structures: integral and panel, as shown in Figure 3-1. It has a simple structure and is easy to manufacture, but the sealing effect is poor. ② Zigzag labyrinth seal: The structural feature of this labyrinth seal is that the sealing teeth have different protrusion heights, and the high and low teeth are arranged alternately, as shown in Figure 3-2. The matching shaft surface is a special concave and convex groove. This structure of high and low teeth and concave and convex grooves makes the smooth sealing gap into a zigzag type, thus increasing the flow resistance and improving the sealing efficiency. ③ Step labyrinth seal: Figure 3-3 shows a stepped labyrinth seal, which is commonly used at the impeller cover and balance plate. ④ Honeycomb labyrinth seal: Figure 3-4 shows the schematic diagram of the honeycomb labyrinth seal. It is pressed from a 0.5mm thick stainless steel sheet. The manufacturing process is complex, the sealing sheet has high strength and good sealing effect. 3. What is the sealing principle of the labyrinth seal device? In order to explain the sealing principle of the labyrinth seal device, we first analyze the flow state of gas in the seal. When the gas flows through the gap formed by the sealing teeth and the shaft surface, the airflow is throttled. The pressure and temperature of the airflow decrease, while the flow rate increases. After passing through the gap, there is a larger cavity formed by the two sealing teeth. As shown in Figure 3-5, the volume of the gas in this cavity increases, the speed decreases, and a vortex flow is formed. Therefore, the gas moves in this cavity, causing the temperature to return to before throttling. Every time the gas passes through a gap and a subsequent larger cavity, the air flow is throttled and expanded. As the number of gas flows through gaps and cavities increases, and the gap value decreases, the flow rate and pressure drop of the gas become larger and larger. When the pressure drops to approximately the back pressure, the gas no longer continues to flow out, thus achieving gas sealing. From the above analysis, it can be seen that the sealing effect will be better when the sealing gap is reduced and the number of sealing teeth is increased. However, the increase in the number of sealing teeth will lead to an increase in the axial size. At the same time, as the number of sealing teeth increases, the sealing effect will gradually decrease. Therefore, the interstage seal before and after the impeller generally only has 3 to 6 teeth, and the shaft end seal has 6 to 35 teeth. If the tooth tip clearance is too large, the sealing effect will be poor. If it is too small, it will cause friction and wear between the rotor and the sealing teeth. Therefore, generally the minimum radius clearance shall not be less than the value calculated by the following formula.: S=0.2(0.3～0.6)D/1000 where S: Seal radius gap, mm ； D: Nominal diameter of sealing part, mm. It is difficult to achieve zero leakage with a single labyrinth seal. When the gas is a toxic, flammable and explosive medium, the sealing device is required to have better reliability and safety. For this reason, it is more effective to use a sealing form that combines a labyrinth seal with other seals. For example, combined with a floating ring seal and an exhaust seal, a better sealing effect can be obtained. 4. What are the structural characteristics of the floating ring seal device? The floating ring seal, also known as the floating ring oil film seal, is a type of liquid seal. It is developed from the fixed sleeve sealing structure as shown in Figure 3-6. In the fixed sleeve sealing structure, the sleeve is fixed. Therefore, the rotor and the sleeve are prone to friction and wear. If the gap between the sleeve and the rotor is increased, the amount of sealing oil leakage will increase, especially in high-pressure sealing situations. This not only requires an increase in the energy of the sealed oil pump, but also increases the recovery and treatment facilities and costs for the dirty oil. Obviously, the economy is poor. In order to overcome the shortcomings of the fixed sleeve sealing structure, the floating ring sealing structure as shown in Figure 3-7 has been improved and designed. This sealing structure is generally composed of an inner floating ring (high pressure ring), an outer floating ring (low pressure ring), a spring, a sealing ring and an anti-rotation pin. During the operation of the equipment, the floating ring is in a floating state under the action of the oil film pressure, which overcomes the friction and wear phenomenon that is easy to occur with the fixed sleeve. ； The sealing gap can be appropriately reduced, and the sealing oil pump capacity and waste oil recovery and treatment facilities can be reduced and simplified accordingly. Therefore, it is an ideal sealing device. If the pressure of the sealed gas is high and the sealing capacity of the floating ring seal device needs to be improved, this goal can be achieved by increasing the number of floating rings. The sealing device shown in Figure 3-8 is an internally cooled floating ring sealing structure that adds three floating rings to two floating rings, which improves the sealing efficiency and also increases the difficulty of manufacturing and maintenance work. When the number of floating rings increases, the working environment of the inner floating ring is harsher. Its sealing gap is smaller and the oil leakage is less. Therefore, the temperature at the inner floating ring is higher. In order to improve the working conditions of the inner floating ring and extend its service life, some cooling grooves or cooling holes are often opened on the floating ring, as shown in Figure 3-9, so that part of the sealing oil enters the floating ring gap after passing through the cooling groove, so as to reduce the temperature of the floating ring, improve the working conditions of the floating ring, and improve the reliability of the floating ring. Practical experience has proven that this technical measure is effective and feasible. 5. What is the sealing principle of the floating ring seal device? The floating ring seal, its oil supply equipment and control instruments together form a complete sealing system. The control process of this system is shown in Figure 3-10. During the normal operation of the unit, the sealing system provides sealing gas 0.05 to 0.07MPa higher than the first-stage inlet gas pressure, and also provides sealing oil 0.05 to 0.07MPa higher than the sealing gas pressure. The pressure difference between the sealing oil and the sealing gas is the external condition for the floating ring seal to achieve good sealing effect. Its value is assisted by the functions of the high-level oil tank and the control instrument in the system. After the sealing oil is injected into the sealing chamber of the floating ring, it leaks along the floating ring gap to the inside of the inner floating ring and the outside of the outer floating ring. Since the rotor is rotating at high speed, the sealing oil flowing into the floating ring gap forms an oil film with a certain load-bearing capacity under the action of the rotating shaft. On the one hand, the oil film lifts the floating ring to achieve liquid lubrication between the floating ring and the journal, thereby reducing friction and reducing wear. On the other hand, since the oil film fills the entire floating ring gap, it can prevent the leakage of the gas medium and play a sealing role. Therefore, the oil film is also called a sealing oil film. The mixture of sealing oil and sealing gas flows through the gap in the inner floating ring to the cavity A of the inner floating ring (as shown in Figure 3-11). It flows along the oil return pipeline in the sealing oil to the oil and gas separator. The separated oil returns to the tank, and the gas is vented or flared. The sealing oil flows through the outer floating ring gap to the outer floating ring cavity B (Figure 3-11), and flows directly back to the fuel tank from this cavity. The sealing effect of the floating ring seal device is directly related to the floating ring gap. From the perspective of reducing sealing oil leakage and improving sealing efficiency, the floating ring gap should be minimized. However, if the gap is too small, it will lead to the deterioration of the floating ring working conditions and even the occurrence of floating ring shaft failure. Therefore, the floating ring gap is generally selected within the following range.: The inner floating ring radius gap S=(0.0005～0.001)×D (3-1) The outer floating ring radius gap S=(0.001～0.002)×D (3-2) where D: Nominal diameter of floating ring, mm. 6． What are the structural characteristics of a mechanical seal device? Mechanical seals are also called end face seals. According to their structural characteristics, they can be divided into two basic forms: single end face and double end face. As shown in Figure 3-12, it is sealed by the close fit of the two end faces, so it is also called a contact seal. This is its significant feature that distinguishes it from non-contact seals such as labyrinth seals and floating ring seals. This seal has the advantages of small leakage, low energy consumption, and long service life. From the structural characteristics, there are many forms such as single end face, double end face, balanced type and unbalanced type. However, in terms of composition, it mainly consists of the following four basic units.: ① sealed unit: That is, the sealing end face composed of a moving ring and a static ring, which is the core of the mechanical seal. ； ② buffer compensation unit: The buffer compensation mechanism is composed of a spring as the main component. It is an important condition for maintaining the normal operation of the mechanical seal. ； ③ Transmission unit: A transmission mechanism composed of sleeves, keys or fixed pins. It is a reliable guarantee for the dynamic ring to rotate with the shaft and is also a prerequisite for dynamic sealing. ； ④ Auxiliary sealing unit: It is composed of moving ring seals, static ring seals and other components. It is an auxiliary sealing mechanism to solve the possible leakage areas other than the sealing end face. It is an indispensable component of the mechanical seal. Mechanical seal is a promising sealing form in fluid sealing technology. It is currently used in situations where the pressure is 5.9MPa and the end face sliding speed is 80m/s. This application range can basically meet the requirements of most production process conditions. 7. What is the sealing principle of the mechanical seal device? In order to explain the sealing principle of the mechanical seal device, we first analyze the possible leakage routes of the mechanical seal. If the shaft end seal of the compressor is a single-end mechanical seal as shown in Figure 3-12, the gas medium can leak out of the machine along the following routes. ① Leakage along the matching gap between the sleeve and the shaft to the outside of the machine ； ② It leaks to the outside of the machine along the gap formed by the sealing sleeve and the casing. ； ③ It leaks to the outside of the machine along the gap formed between the floating seal ring and the shaft sleeve and the gap formed between the side of the floating seal ring and the sealing sleeve. From the above analysis, it can be seen that in order for the gas medium to leak out of the machine, it must pass through the above three routes. Therefore, to prevent the gas medium from leaking, effective sealing measures should be taken to seal these three leakage routes. As can be seen from the figure, the first and second leakage routes are sealed by the "O" ring. Because the sealing elements have no relative movement, they are called static seals. There is relative motion between the floating seal ring and the shaft sleeve, as well as between the floating seal ring and the seal sleeve, so it is called a dynamic seal. The function of the dynamic seal is achieved under the following conditions: ① Sealing oil is continuously supplied from outside the machine to the sealing chamber, and its pressure is 0.02MPa higher than that of the gas medium. ； ② The floating sealing ring is tightly attached to the side of the sealing sleeve under the action of the sealing oil pressure, so that the sealing between the floating sealing ring and the side of the sealing sleeve is achieved. ； ③ The rotating shaft sleeve drives the sealing oil in the gap of the floating seal ring to rotate together and forms a sealing oil film with load-bearing capacity. The gap between the shaft sleeve and the floating sealing ring is thus sealed. ； ④ The sealing surface composed of a dynamic seal ring, a carbon ring and a static seal ring mainly seals the leakage of seal oil and improves its operating conditions under the action of the lubrication performance and cooling effect of the seal oil. From the above, it can be seen that the end face of this form of single-ended mechanical seal is not sealed by the medium gas, but by the sealing oil. The gas medium is sealed by a floating sealing ring. The sealing task of the end face of the double-end mechanical seal has two aspects. It has the end face on the gas medium side. On the one hand, it must seal the sealing oil from leaking into the gas medium, and at the same time, it also prevents the gas medium from leaking from here. For details, readers can combine Figure 3-12 for further analysis. 8. What is the composition and sealing principle of the exhaust seal device? The exhaust seal is often used in conjunction with the labyrinth seal. Its structure is shown in Figure 3-13. The shaft-end labyrinth seal is designed and combined into three sealing chambers C, B, and A. The C and B sealing chambers are connected to the gas inlet through a balance tube to keep the pressure in the corresponding sealing chambers at both ends of the inlet and outlet balanced. The gas medium leaking into the outer sealing chamber A will be sent to a designated location for discharge or other processing through the air extraction system, thereby achieving a sealing effect of zero leakage at the shaft end. The normal operation of the air extraction sealing device requires a power gas source with parameters that meet the requirements. This gas source can come from compressed air, steam and compressed gas media. The rich gas compressor of the catalytic cracking unit of Shengli Refinery uses rich gas as the power source for its shaft end seal, and has gained successful experience. 9. What is the structure and sealing principle of the spiral seal device? Spiral seals are dynamic seals, also called spiral viscous seals. They are processed with spiral grooves on the surface of the sealing part of a high-speed rotating shaft, as shown in Figure 3-14. When the shaft rotates at high speed, the spiral grooves pump the viscous fluid filled in it, establishing a certain sealing pressure. This pressure is opposite to the pressure of the fluid medium and is the resistance to leakage of the medium. When this resistance is balanced with the pressure of the medium, it can prevent the fluid medium from leaking out of the machine. If the sealed medium is liquid, spiral viscous sealing can be used directly. When the sealed fluid is gas, the following measures need to be taken to be effective. ① On the surface of the sealing part of the shaft, two sections of spiral grooves in opposite directions are processed, as shown in Figure 3-15. In the middle of the two sections of spiral grooves, a smooth cylinder with a diameter smaller than the outer diameter of the spiral groove is processed. ； ② The sealing body is equipped with sealing oil inlet and outlet ； ③ The design is equipped with a sealing oil supply system to provide sealing oil with suitable pressure, temperature and flow rate for the sealing chamber. After the above measures are completed, when the rotor rotates at high speed, the two spiral grooves with opposite directions on the shaft surface produce opposite pumping effects on the sealing oil and gather it towards the middle smooth section. As a result, a seal with a certain sealing ability is formed in the smooth section between the two spiral grooves. For oil rings, when the sealing capacity △Pout of the oil ring is equal to the sum of the gas medium pressure P plus the pressurization ΔPin generated by the inner spiral groove (that is, △Pout = P + △Pin), a dynamic balance is formed. This dynamic balance is an important condition for the spiral seal to be used for gas sealing. 10. What new technologies are there for shaft end seals? What are their structural characteristics and working principles? Shaft end seals are a technical issue that needs to be carefully considered by the design, manufacture and use units of centrifugal compressors. Labyrinth seals and floating ring seals are mature technologies for shaft end sealing of centrifugal compressors. However, this kind of seal requires a long axial position, a complex oil supply system, and has the disadvantages of greater interference with the rotor operation. In response to the above problems, several new seals with good reliability, low leakage and low energy consumption have been designed and tried. They are introduced as follows. (1) Gas seal Gas seal is a non-contact seal that uses gas medium as lubricant. Through the ingenious design of the sealing element structure and its performance, leakage can be reduced to a minimum. The structure is shown in Figure 3-16. Its characteristics and sealing principles are:: ① The sealing seat and the rotor are relatively fixed. On the end face of the sealing seat opposite the primary ring (i.e., the primary sealing surface), a sealing block and a sealing dam are designed as shown in Figure 3-17. These sealing blocks are of different sizes and shapes. When the rotor rotates at high speed, the gas injected into them generates a pressure, thereby pushing the primary ring away, forming gas lubrication, reducing the wear of the primary sealing surface, and preventing the gas medium from leaking to a minimum. The sealing dam is used to prevent gas leakage when parking. ； ② The primary ring, primary ring retainer and primary ring support form a static seal assembly. Under the action of spring force, this assembly can follow the seal seat in the axial direction to maintain the minimum gap between the primary sealing surfaces. ； ③ This sealing requires a stable sealing gas source, which can be medium gas or inert gas. No matter what kind of gas is used, it must be filtered and become clean gas. (2) Magnetic fluid seal Magnetic fluid seal is a non-contact seal. This sealing device not only has a sealing function when the rotor is running, but also can maintain sealing performance after the rotor is stopped. The structure is shown in Figure 3-18. Its structural features and sealing principles are as follows. ① The sealing device consists of 12 permanent magnets evenly distributed along the sealing journal, and a certain gap is maintained between the magnet blocks and the journal. ② Inject magnetic fluid into the gap, and the injection amount is 10 times the volume of the gap. ③ The magnetic fluid forms a sealing film in the gap under the action of magnetic poles and centrifugal force, thereby achieving magnetic fluid sealing. (3) Spiral groove gas seal Spiral groove gas seal is also a new sealing structure. As shown in Figure 3-19. The structural characteristics and sealing principle of the sealing device are as follows:: ① The sealing device consists of sealing elements such as sealing rings and rotating rings. ② The sliding surface of the sealing ring has a specially made circumferential groove, as shown in Figure 3-20. The cavity of the circumferential groove communicates with the outside of the sealing ring through a small hole. The sealing groove divides the sliding surface of the sealing ring into two parts. The outer diameter of the circumferential groove is the supporting part, and the inner diameter of the circumferential groove is the sealing part. ③ The matching part between the rotating ring and the sealing ring support part is made into a spiral groove as shown in Figure 3-19. ④ When the rotor rotates at high speed, air is supplied from the air supply hole to the circumferential groove of the seal ring. The gas flows from one direction to the outside under the action of the spiral groove of the rotating ring, and forms a fluid film at the spiral groove. The fluid film exerts force on the seal ring, causing the seal ring to be slightly separated from the rotating ring. The micro gap is the spiral groove thrust bearing gap and also the sealing gap. The size of this gap is determined by the balance between the spring force acting on both sides of the sealing ring and the reaction force of the fluid film. The higher the rotational speed, the greater the reaction force of the fluid film and the larger the gap. Therefore, the sealing ring is required to move freely along the axial direction to track the axial micro-movement of the rotating ring, thereby ensuring an appropriate sealing gap and achieving an ideal effect of spiral groove gas sealing.