A chemical fertilizer company in Jiangsu completed its overall relocation at the end of 2006. Its ammonia synthesis equipment was put into operation at the end of June 2005 and the end of November 2006. The core operating equipment includes nine 6M50 nitrogen and hydrogen compressors, with a single unit annual output of 40,000 tons of ammonia.

There is a problem

Among them, when the 6M50 nitrogen and hydrogen compressor produced in Shenyang had been running for less than two years, the big end of its connecting rod often burned. During the disassembly and inspection of its parts, it was found that the other connecting rods did not burn. The babbitt alloy cast on the inner surface of the tile has also partially fallen off. The babbitt alloy on the inner surface of the large head tile that has burned has completely disappeared. The steel back of the tile directly rubs against the surface of the crankshaft journal, and the surface of the crankshaft journal is partially blue, indicating that it has occurred. When the local high temperature is exceeded, the surface of the crankshaft journal is cooled by the lubricating oil, which is equivalent to a "quenching" process, making the surface of the crankshaft journal easily prone to cracks, leading to the scrapping of the crankshaft, and directly affecting the safe operation of the compressor.

Cause analysis

The big head tile of the 6M50 nitrogen and hydrogen compressor is a Babbitt alloy tile, that is, a Babbitt alloy anti-friction layer with a thickness of about 0.5mm is poured on the inner surface of the steel back. During normal operation, the Babbitt alloy and the crankshaft journal are in the lubricating oil. Under the action of the oil film, the lubricating oil film is in a liquid contact operating state, but once the conditions change, the lubricating oil film is difficult to form or is destroyed, and it is very easy to cause tile burning. After analysis, the following reasons may exist:

1. The lubricating oil circuit of the connecting rod big head tile of the 6M50 nitrogen and hydrogen compressor: oil tank, oil pump, essential oil filter, oil cooler, middle body slide, cross head, cross head pin, connecting rod small head tile, connecting rod big head tile, crankcase, and oil tank. The connecting rod big head shoe is at the end of the lubricating oil circuit. The lubricating oil is released through pressure and flow in multiple lubricating parts. The amount of lubricating oil that finally reaches the surface of the connecting rod big head shoe is very limited. Once the circulating oil pressure is low, it may not be able to It effectively forms an oil film, causing direct friction between the big head tile and the journal surface. Furthermore, if the amount of lubricating oil is insufficient, the heat generated by friction cannot be fully taken away, so tile burning is very easy to occur.

2. The 6M50 nitrogen and hydrogen compressor connecting rod adopts the big end positioning method of the connecting rod, that is: there is not much gap between the two sides of the connecting rod's big head shoe and the crankshaft journal, while the two ends of the connecting rod's small head shoe are inside the crosshead There is a larger lateral distance. When the machining error is too large, there will be a certain deviation between the oil guide hole on the crosshead pin and the oil guide groove on the small head shoe. The overlapping area between the two will be reduced and its oil guide capacity will be weakened.

3. Other reasons: In terms of manufacturing quality of connecting rod big head tiles: there is a problem with the pouring quality of babbitt alloy and it is easy to fall off. Lubricating oil quality: After the lubricating oil is used for a period of time, the viscosity increases, the viscosity-temperature properties deteriorate, the lubricating oil film formation ability is poor, and it is easy to cause tile burning.

take improvement measures

In view of the above existing problems, corresponding improvement measures have been taken:

1. Regarding the problem of low lubricating oil volume caused by the lubricating oil circuit: try to increase the processing capacity of the lubricating system, increase the oil delivery volume and oil pressure of the lubricating oil pump, and increase the oil filter capacity of the lubricating system. The filtration area is matched with a cooling system to increase the circulating oil pressure and oil volume entering the compressor as much as possible to make up for the loss of lubricating oil leakage caused by too many lubricating parts and fully ensure the amount of oil entering each lubricating part.

2. Regarding the problem of installation deviation between the oil hole on the crosshead pin and the oil guide groove on the small head tile: Use an electric hand grinder to expand the oil guide hole on the crosshead pin into an oval shape along its axis, and increase the The contact overlap area between the two increases the oil conductivity.

3. Regarding the manufacturing quality of connecting rod big head tiles, contact its manufacturer to ensure the quality of supply, improve the pouring process, and improve the bonding force between babbitt alloy and steel backing.

4. For lubricating oil quality issues: Contact the compressor manufacturer and change the original designed lubricating oil type to: compressor oil L-DAB150 to improve the bearing capacity of the oil film. At the same time, vacuum filtration is strengthened, oil quality is analyzed regularly, and replacement is performed as appropriate to ensure that the lubricating oil quality reaches the specified indicators.

Usage after first improvement

Through the above improvement measures, the operation of the 6M50 compressor has been stabilized to a certain extent. However, after using it for a period of time, tile burning still occurs frequently.

Analyze the reason again: Although the surface of some crankshaft journals where tile burning occurred has been manually polished and smoothed by sandpaper, the ovality and cylindricity of the journal surface have deteriorated, resulting in a deterioration of the wedge-shaped contact with the inner wall of the big head tile. The ability to form an oil film will be correspondingly worse, and there may be local contact friction. In addition, the contact pressure on the surface of the connecting rod's big head tile is large during operation, which easily generates more heat, which can easily cause the babbitt alloy on the surface of the big head tile. Melting and falling off will quickly reduce the load-bearing capacity of large-head tiles in a short period of time, so tile burning occurs repeatedly.

Improvement measures again

In view of the above-mentioned recurring problems, corresponding improvement measures have been taken:

1. Regarding the problem of crankshaft journal surface wear: After comparing various repair methods of the crankshaft journal, the "low-temperature iron plating" process was adopted to repair the burned and worn crankshaft journal to the original drawing size, and the journal surface was processed by a grinder. , ensure its proper cylindricity and parallelism between the crankshaft journal and the main axis of the crankshaft, and try to restore the crankshaft to its original manufacturing standards.

2. For connecting rod big head tiles: try to use new materials to make them. Since the 6M50 compressor made in Shanghai uses aluminum-magnesium alloy to make the connecting rod big head tiles, we contacted the Shanghai compressor spare parts supplier to process a set of aluminum-magnesium alloy big head tiles according to the Shenyang connecting rod big head tiles for trial use. The reason for using aluminum-magnesium alloy: Even if tile burning occurs, there will be no damage to the crankshaft journal due to friction between the same metals.

Usage conditions and improvement measures after further improvements

Through the implementation of further improvement measures, the operation of the 6M50 compressor has been stabilized to a large extent. However, after using it for a period of time (about a year), cracks or even cracking damage occurred in the large-head tiles along the direction of the oil tank.



Analyze the reason: the large-head tile is made with reference to the original babbitt alloy large-head tile, and its lubricating oil groove is opened on the back of the large-head tile. After the force analysis of the bearing bush is carried out according to the sliding bearing principle when it is running, It was found that this is the part in the middle of the big head tile that bears the greatest load, but due to the existence of the lubricating oil guide groove, it is also the weakest part of the big head tile. The original Babbitt tiles are supported by steel backing, while the aluminum-magnesium alloy tiles are supported by the overall aluminum-magnesium alloy. Since the strength of the two materials is very different, there is no aluminum-magnesium alloy tile in the Babbitt tiles. Cracks and cracks appear in the oil tank.

Three improvement measures

After stress analysis based on the principle of sliding bearings, the lubricant oil guide groove of the aluminum-magnesium alloy tile was changed from the outer surface of the big head tile to its inner surface, so that the back of the big head tile closely fits the inner wall of the big head hole of the connecting rod, reducing the oil guide groove The stress on the large head tile is reduced to reduce the possibility of cracks. Since the load-bearing capacity of the sliding bearing is related to the contact width of the bearing pad, after the lubricating oil guide groove is changed from the outer surface of the big head pad to its inner surface, the load-bearing width is reduced by one oil guide groove width on the surface. However, due to the strength of the aluminum-magnesium alloy Limited, the middle part of the original type of aluminum-magnesium alloy tile does not play an actual load-bearing role, so this improvement measure is theoretically feasible.



Conclusion

The improved aluminum-magnesium alloy large-head tiles have been in use to this day, and they have also been inspected during maintenance opportunities. No cracks and damage to the oil guide groove have been found again, which has greatly stabilized the normal operation of the 6M50 compressor. This time The transformation was very successful and achieved huge economic benefits.