Oil-free lubricating compressors generally refer to components that are in contact with the gas (excluding the transmission components) and are oil-free. The key to solving the oil-free lubrication technology is to use the ideal self-lubricating material and reasonable structure to achieve oil-free lubrication of the compressor. Good quality self-lubricating material guarantees reliable service life of sealing friction parts (such as piston rings, guide rings, sealing rings, etc.). Reasonable structural design plays an important role in achieving oil-free lubrication of piston compressors.
2 self-lubricating materials
The premise of oil-free lubrication of piston compressors is to use various self-lubricating materials to manufacture friction rings such as piston rings, guide rings and stuffing box sealing elements to eliminate the need between them and cylinders (or cylinder liners) and piston rods. Lubricating oil, generally a material with a small friction factor is called a self-lubricating material, that is, a friction reducing material. Currently, the self-lubricating materials used on compressors are mainly polymers and polymer-based composites DD are commonly referred to as plastics. Such as filled with polytetrafluoroethylene, polyamide (nylon), polyimide.
Pure polytetrafluoroethylene is a well-known self-lubricating material. It is non-toxic, tasteless, and chemically stable at a certain temperature, but has poor wear resistance. With the development of science and technology, the use of materials is becoming more and more demanding. In order to meet the requirements of engineering performance for materials, various composite materials have emerged. Commonly used filler materials are: glass fiber, graphite, molybdenum disulfide, copper, and the like. The wear rate of filled polytetrafluoroethylene is more than one thousand times lower than that of the pure state, so it has been widely used in frictionless parts of oil-free lubricating compressors.
3 plastic friction parts defects
3.1 poor heat resistance
Sensitivity to temperature is one of the weaknesses of plastics. The effect of temperature on the properties of plastics is much more obvious than other materials, and it is resistant to high temperatures, and the range of low temperatures is narrower than other materials, which limits the use of plastics. In the compressor, the temperature of the compressed gas is generally used as a control index for the maximum use temperature of the self-lubricating material, and the commonly used filling polytetrafluoroethylene has a maximum use temperature of <160 °C.
3.2 low thermal conductivity
Plastics have much lower thermal conductivity than metals. The thermal conductivity of plastics is 0.17 to 0.35 W/(m? °C), while that of steel is 46 to 70 W/(m? °C). The thermal conductivity of plastic is only 1/300 of steel, which is 1/500~1/800 of copper.
3.3 linear expansion coefficient
The coefficient of linear expansion of plastic is much larger than that of metal, which is 2 to 10 times that of metal, and is easily affected by temperature to cause dimensional change. This not only increases the difficulty of designing friction components, but also affects the normal operation and dimensional accuracy of components.
3.4 creep
Creep of metal occurs only at high temperatures, and plastics creep under long-term load even at normal temperatures, which is detrimental to the dimensional stability of plastic parts. The filled polytetrafluoroethylene is easy to be destroyed by cold flow due to its low strength.
4 multi-level compression
The gas temperature at the end of compression increases with the increase of the compression ratio, but the temperature of the compressed gas is not allowed to rise arbitrarily. The excessive temperature will affect the working performance of the plastic seal (limited to 160 ° C). At higher gas pressures, only the multi-stage compression can be used to control the exhaust temperature of the gas compressor within the specified range.
Multi-stage compression is beneficial to reduce the exhaust gas temperature, save power, reduce the force on the piston (acting on the transport mechanism), and improve the volumetric efficiency of the cylinder.
5 piston ring repair
Piston oil-free lubrication compressor cylinder seal repair piston ring is one of the key parts in the piston compressor, its quality directly affects the compressor displacement, power and its sealing and reliability, that is, the impact The economics of the compressor. The service life of the piston ring is highly dependent on the material and structure of the piston ring.
5.1 Sealing principle of piston ring
The main function of the piston ring is to seal the gap between the cylinder mirror and the piston to prevent gas from leaking from one side of the compression volume to the other. The piston ring is an open ring. In the free state, its outer diameter is larger than the cylinder diameter. After the cylinder is installed, the ring diameter is reduced, leaving only a linear expansion gap e at the opening. As shown in Figure 5-1. The piston ring is sealed by throttling and blocking. The sealing principle is shown in Figure 5-2. When the ring is loaded into the cylinder, a preloading pressure Pk is generated due to the elasticity of the ring, so that the ring abuts against the cylinder wall. When the gas passes through the uneven gap of the ring surface, it is throttled and blocked, and the pressure drops from P1 to P2. At the same time, because there is a side gap between the piston ring and the ring groove, the ring is close to the side with low pressure, so The gap between the inner surface of the piston ring and the ring groove (commonly referred to as the back gap) has a gas pressure (back pressure) approximately equal to P1, and the gas pressure acting along the outer surface of the piston ring varies from P1. Change to P2, the average of which is approximately equal to 1/2 (P1 + P2). Thus, a pressure difference ΔP ≈ P1 - 1/2 (P1 + P2) = 1/2 (P1 - P2) is generated in the radial direction, and this pressure difference causes the piston ring to abut against the cylinder wall to achieve a sealing effect. Similarly, there is also a pressure difference in the axial direction, and the ring is pressed against the side of the ring groove to seal. The greater the pressure inside the cylinder, the greater the sealing pressure, which indicates that the piston ring has the characteristics of self-tight sealing.
5.2 Structural form of the piston ring
So far, the piston rings of the incisions have been completely circled by a rectangular or square section at home and abroad. The original metal piston ring has a slit form, a diagonal cut, and a straight cut. From the analysis of the gas leaking through the incision, the incision is the least and the straight incision is the most. However, since the filled polytetrafluoroethylene has low strength and is easy to be cold-flowed, the small-diameter ring is easily damaged by the lap or oblique cut, and the processing of the lap is complicated. Therefore, there are currently more straight incisions. Although the relative leakage of such slits is larger, under the same operating conditions, the number of rings filled with polytetrafluoroethylene is much less than that of metal rings. Therefore, the total amount of incision leakage can also be reduced accordingly.
In addition, there are two rings placed in a ring groove, the slits are staggered from each other. This structure can greatly reduce the amount of leakage through the slit, but also causes the load to be more concentrated on the first group of rings at both ends of the piston. , resulting in a wide disparity in the life of each ring.
As the wear increases, the slit of the piston ring also gradually increases. The volumetric efficiency is reduced until it is finally scrapped.
In order to overcome the phenomenon that the piston ring is affected by the uneven load and affect the service life, the compensation measures shown in Figure 5-6 can be taken. Figure 5-6 shows the piston ring with the relief hole, that is, the first ring with the largest load or In addition, the second ring opens some radial holes, consciously releasing the gas and transferring the load to the next few rings.
6 maintenance of the guide ring.
In oil-free lubricated pistons, guide rings should be provided for both horizontal and vertical compressors. The role of the guiding ring is to bear the lateral force caused by the quality of the piston component and other reasons, to ensure the linearity of the piston movement, to improve the sealing effect, and to avoid direct contact between the piston and the cylinder to prevent the cylinder wall from being pulled. The axial height of the guide ring should be higher than the piston ring, and an appropriate dynamic fit should be taken between the cylinders. Two guide rings can be used, which are arranged at both ends of the piston, and a guide ring can also be used, which is arranged at the middle or the end of the piston, and the structure of the filled Teflon guide ring is as follows.
6.1 Piston ring guide ring
This structure is the same as the rectangular section piston ring, but its axial dimension is larger than the piston ring. Generally, the guide ring on the vertical compressor adopts this structure, and its axial dimension is twice that of the piston ring. The structure is simple and easy to install, but the ring is subjected to the same gas radial force as the piston ring, which causes the specific pressure to increase and accelerate wear. For horizontal compressors, the piston is often sunk due to wear of the guide ring. Therefore, for horizontal compressors, especially the unloading measures of the compressed gas by the guide ring should be considered.
6.2 Guide ring with unloading groove
The structure is configured to open and unload the groove on the outer circumferential surface of the guide ring in parallel or at an angle to the axis, so that the gas passes through the unloading groove and directly acts on the rear piston ring. When the unloading groove has an angle with the axis and is parallel to each other, the tangential component generated by the gas pressure can cause the guiding ring to rotate slowly, and the surface of each part of the guiding ring can reach the lower pressure receiving position. Make the ring evenly worn. It is also possible to open a herringbone groove, at which point the circumferential rotational force generated by the gas pressure will balance each other. The actual effect of the guide ring with unloading groove and unloading hole after the application of the enterprise proves that its service life is several times longer than the traditional guide ring.
6.3 Piston ring guide ring design and maintenance
The design of the guide ring should also consider the characteristics of filling PTFE, such as large coefficient of thermal expansion and easy wear. The guide ring for the slit is suitable for the integral piston or the combined piston, but due to the slit, the guide ring can withstand the same specific pressure (equal to one more piston ring) as the piston ring under the radial action of the gas force, and also supports The weight of the piston and piston rod. In horizontal compressors, the piston and cylinder are not concentric due to the wear of the guide ring, which affects the normal operation of the piston ring and the stuffing box. Therefore, it is necessary for the guide ring to take unloading measures to release the pressure of the gas.
6.4 overall guide ring
The integral guide ring has an integral combined guide ring, an interference heat-pressing integral guide ring and a non-interference integral guide ring. The integral combined guide ring is pressed into a metal backing ring by filling in the filled polytetrafluoroethylene ring. Since the guide ring is no longer affected by the gas back pressure, the service life is greatly improved. At present, most oil-free lubricating compressors in China use an over-pressure hot-pressing integral guide ring. The structure is to heat the guide ring and press it into the piston body heated by the interference fit. The piston of the guide ring can be used. It is both integrated and combined. The non-interference integral guide ring is generally used on the combined piston, which is characterized by convenient installation and gas back pressure, and is often used in low temperature applications.
7 stuffing box
The stuffing box is one of the important components of the compressor. Its function is to prevent the gas in the compression volume from leaking along the piston rod by the gap between the sealing piston rod and the cylinder. The sealing function of the stuffing box directly affects the normal production of the compressor, and requires good sealing performance and wear resistance. Oil-free lubricating compressors are mostly used to compress flammable, explosive, toxic gases and precious rare gases, and have higher sealing requirements for stuffing boxes. The leakage of the stuffing box is an external leakage of the compressor. In addition to reducing the production capacity of the compressor (ie, the amount of exhaust gas), it also brings a series of adverse or serious consequences.
7.1 Structure of the stuffing box
Depending on the application, the filled Teflon stuffing box has different structures, such as herringbone stuffing box, butterfly stuffing box, dimensional stuffing box, and plane stuffing box, currently in high pressure, medium pressure, low pressure oil-free lubrication. Planar stuffing box structures are commonly used in stuffing boxes (with piston rod diameters above 40 mm).
8 Conclusion
Oil-free lubricating compressors are used to compress flammable, explosive, toxic and valuable rare gases, so the requirements for sealing are higher than for general compressors. Filled with PTFE seals, it has good self-lubricating properties and is widely used, but it also has poor wear resistance, easy cold flow damage, small thermal conductivity and large linear expansion coefficient. There are many types of piston oil-free lubrication compressors, and the working conditions are different. Therefore, the rationality of the structural dimensions of the cylinder seal will directly affect the volumetric efficiency of the cylinder and the service life of the seal. Based on the summary of practical experience, this paper describes the precautions for the maintenance of the cylinder seal of the piston-type oil-free lubrication compressor. And the determination of the reasonable size of the seal structure. Thereby, the quality of maintenance is improved.
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