Discussion on the failure of the secondary air compressor punching body
Filling plate fracture micro-morphology packing plate is an important part of compressor cylinder, with complex alternating stress, working conditions are very demanding, material performance requirements are high, shape is complex, and machining precision is high. During normal operation, due to the throttling action of the packing ring, the adjacent two packing discs are subjected to different radial pressures, resulting in different strains. Therefore, when the compressor is running, the pressure will cause the two packing discs to contact each time. One slip, resulting in wear and tear for a long time. Because the radial deformation of the packing disc is small, the slip between two adjacent packing discs is only a few tens of micrometers, which is fretting wear.
Through the above analysis, it can be seen that the packing disc will produce fretting wear during operation. However, during the operation of the secondary compressor of Maoming ethylene high-pressure device, cracks often appear on the surface of the packing plate, and even the crack penetrates the entire sealing surface, resulting in the service life of the packing plate being far lower than the design value. This case is the same type of compressor. Very rare in the middle. Therefore, it is necessary to further analyze the reasons for the premature cracking failure of the packing disc.
The abnormal failure of the packing can be derived from the failure-cracking packing disc. The sealing surface of the packing disc has only a few parts at the inner side, causing severe wear, cracks also occur at the outer edges of these areas, and the wear area is circumferentially There is no regular distribution. Analysis of the failure of the packing ring and the wear of the packing disc and the failure record of the cracking can reveal a phenomenon worthy of attention, that is, the wear and fracture frequency of the packing ring is higher than the cracking of the packing disc.
Therefore, it can be considered that the short life of the packing ring accelerates the failure of the packing disc, because the grinding debris of the packing ring penetrates into the contact surface of the packing disc under the working internal pressure, accelerates the fretting wear of the contact surface, and promotes crack initiation and rapidity. Expansion.
The packing ring of the ultra-high pressure compressor is usually made of a copper alloy, and its main components are elements such as copper and tin. Under normal circumstances, the filler of the compressor can generally be used for 153 years, but the Maoming ethylene ultra-high pressure compressor mostly occurs after half a year of operation. The disassembly of the compressor cylinder found that the packing ring was seriously worn, the base body had been ground smooth and shiny, and many packing rings had broken. The grinding ring of the packing ring could be seen everywhere in the cavity of the packing plate.
Corrosion of the packing ring Corrosion of the packing ring During the disassembly and repair of the second stage cylinder of the compressor, it is found that some small pits appear on the surface of the packing. The depth of the small pit is different. The more the inner ring is, the more the outer ring is gradually reduced. . There are black deposits in the pit.
The use of a long time filler increases the depth and diameter of the small pits, causing the filler to often break. In order to analyze the cause of the fracture of the filler and find out the reason for its short service life, the author conducted a sample analysis of the black sediment in the small pit.
IR Spectra of Sediment The black sediment sample was dissolved in CdCl3 and shaken to make a 1H NMR spectrum of the dissolved material.
The band in the figure indicates an oxidizing substance. There is no unoxidized polyethylene in the figure, because the complete polyethylene band is roughly 720 cm-1. The IR spectrum detects that the main components are some aliphatic hydrocarbons and some PAG-based substances. A small amount of aromatic hydrocarbons was also found. Nuclear magnetic resonance results showed that the aliphatic hydrocarbon material was oxidized prepolymerized ethylene. PAG lubricants have been dissolved in oxidized polymers but have not been oxidized by themselves. A small amount of aromatic hydrocarbon material is basically a polymerization inhibitor in PAG.
The post-scattered electron map deposits of the deposits were washed and dried to make a post-separation scattering electron map. Comparing the electron maps, we can see the difference in the average atomic number. The brighter area is because of the element with higher atomic mass. (a) is a typical area of ​​sediment and (b) shows a deposit that looks orange. The composition of the elements can be known from the X-ray spectra obtained from different regions of the sediment. X-ray spectrum. It indicates that the main area of ​​the sediment sample is carbon and oxygen, and the smaller bright areas are silicon and oxygen. This also means that most of the oxidized polyethylene. Some trace elements, including Na, Mg, Al, K, Ca, Cl, Fe, Cu, and Zn, were also detected in the X-ray spectrum. These trace elements are estimated to be components of the filler ring.
The medium orange block is mainly carbon, oxygen (oxidized polyethylene) and copper, and copper is the main component of the packing ring, which is also the reason why the orange block is observed.
Analysis of the X-ray Spectra of the Sediment by the Ring Corrosion According to the above analysis, the main component of the deposit is an oxygenated hydrocarbon, indicating that ethylene has been polymerized, and the polyethylene deposited on the packing ring has been under compression (high temperature, high pressure). Oxidation; PAG (polyethylene glycol oil) has been dissolved in the sediment, but some PAG has not been oxidized; the deposit contains metal components, especially copper (from the packing ring), because the oxidation process of the prepolymer will form The acidic substance causes corrosion of the packing ring, thus causing metal components in the sediment; the oxygen in the sediment is derived from the initiator air that needs to be injected in the process, and the air is injected in front of the compressor and enters the compressor along with the ethylene gas. Reacted.
From the composition analysis of the black deposits, it can be concluded that the filler ring (copper alloy) is corroded by the reaction product of the lubricating oil (PAG) and the working medium (ethylene, etc.) under the working conditions of the compressor, and the surface forms a small pit and gradually expands. Eventually the strength of the packing ring is reduced to cause breakage.
Regarding the corrosion mechanism of copper alloys, many papers <5,6> are discussed. In the aqueous solution of ammonia, ammonium salt and cyanide, in the presence of aerobic and oxidizing agents, complex ions are formed by dissolving Cu2+. From the morphology and corrosion products of small pits on the corrosion surface of the filler, the filler undergoes selective corrosion, and the corrosion is de-tin corrosion. Analysis of the cause of premature cracking of the packing plate The black sediment analysis report indicates that an orange substance can be seen during microscopic amplification, the main component of which is copper. This indicates that the main component of the filler ring matrix, copper, is worn away. Combined with the corrosion of the packing ring, it is not difficult to infer that since the packing ring matrix has been corroded, and corrosion is a localized corrosion, a small pit is formed on the surface of the packing ring, and the corrosion is more serious near the inside of the packing ring, so that the packing ring is in the column. Under the reciprocating friction of the plug, the wear resistance is greatly reduced, and the generated wear products are also greatly increased. Under normal conditions, the wear of the packing ring is very slight, and the particles generated by the friction can be discharged out of the cylinder with the lubricating oil. However, when the packing ring is corroded to cause abnormal wear, the product of the wear is greatly increased and cannot be completely discharged from the compressor, so that a part of the wear product remains in the cavity between the compressor packing disks. Since the compressor is in operation, the adjacent two packing discs slide relative to each other, and the pressure of the contact surface is also cyclically changed, and the wear product may gradually penetrate into the sealing surface of the packing disc under the action of the gas pressure, so that the seal is sealed. The sliding of the surface changes from simple sliding friction to fretting wear with abrasive particles, which greatly accelerates the wear of the packing disc. Due to the accelerated wear and the large positive pressure of the packing disc, the speed at which the micro-cracks are formed on the worn surface is greatly increased, causing the material to produce fretting fatigue cracks and further expanding, causing the packing disc to crack.
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