Failure Analysis and Improvement of Centrifuge Machine Seal

Chen Renlai Abstract: The reasons for the failure of the centrifuge mandrel seal are analyzed, and corresponding measures are taken according to the specific conditions.
Key words: centrifuge; bellows machine seal; end face specific pressure
1 Overview M701 centrifuge is the key equipment of the 1 # PTA unit refining section of our factory. It is a vertical structure, manufactured by ALFA LAVEL Company of the United Kingdom, model P -6100, a total of 4 sets. Under normal circumstances, three sets of equipment are used. 45MPa。 The speed is 2000r / min, the working pressure is 0. 45MPa. It centrifuges the slurry from the fifth crystallizer to remove PT acid from the material. The centrifuge itself has 3 oil lines, 1 for lubrication of the gear box, 150 oil, normal oil pressure is 0. 72MPa; 1 is used for lubrication of the support bearing, is No. 46 oil, normal oil pressure 0以上。 The pressure is 0. 07MPa; there is also one for the lubrication and cooling of the mandrel machine seal, is No. 46 oil, the normal oil pressure is about 0. 55MPa. The last two oil circuits share one fuel tank. The author mainly discusses the problem of the mandrel seal on the third oil road. Figure 1.

2 mandrel machine seal failure problem [1]
2. 1 Machine seal introduction The mandrel machine seal is a bellows machine seal, but it is very unstable during actual use. Some of them will leak when just used, and it can run for about 3 months. The leakage situation is also different. Some of them leak a large amount of lubricating oil into the process system, which affects the quality of the product; some of the process medium enters the lubricating oil, causing the emulsification of the lubricating oil to fail.
2. 2 Failure analysis is calculated based on data such as materials used by domestic manufacturers:
The inner diameter of the bellows: d1 = 217. 9mm; outer diameter: d2 = 251. 1mm;
Inner diameter of the graphite ring: D1 = 235. 2 mm; outer diameter: D2 = 241. 6 mm;
Sealing chamber pressure: P cavity = 0. 55MPa; medium pressure: P = 0. 45MPa;
Bellows stiffness: K = 10. 2N / mm; Compression: 8mm;
Effective diameter of bellows: De = 237. 1mm;
End face area: A = π (24. 162 - 23. 522) / 4 = 23. 97cm2
Bellows specific pressure: Pt = Ft / A = 10. 2 × 8 / 23. 97 × 10 - 4 = 0.034MPa; load factor: K = ( D22- De2) / ( D22 - D12) = 24. 162 - 23.712 / 24. 162 - 23. 522 = 0. 706.
According to the actual working conditions, take λ = 0. 4
Then, the end face specific pressure Pb = Pt + (P cavity - P) (K - λ) = 0. 034 + (0. 55 - 0. 45) (0. 706 - 0. 4) = 0. 065MPa
The starting pressure of the safety valve of the equipment is: P medium height = 0. 59MPa
Then, the end face specific pressure when the device is overpressured: Pb' = Pt + (P-high - P cavity) (1 - K - λ) = 0. 034 + (0. 59 - 0. 55) (1 - 0. 706 - 0. 4) = 0. 03MPa
If the oil system is out of service and the equipment is under working pressure, then the end face is pressed:
Pb′′= Pt + (P-P cavity) (1 - K - λ) = 0. 034 + (0. 45 - 0) (1 - 0. 706 - 0. 4) = - 0. 014MPa
It can be seen from the above calculation results:
1Pb > 0, Pb'> 0, that is, when the oil system is running, even if the machine seal is subjected to back pressure, the end face pressure of the machine seal is greater than zero. It is reasonable to say that the machine seal should not leak, but for the 2000r/min centrifuge, The recommended specific pressure of the end face is 0. 3MPa (recommended by the manufacturer; the recommended value of the literature 2 is 0.1 to 0.8 MPa). Obviously, the Pb value of the machine seal is low, which is why the machine seal is sometimes easy to leak oil.
2Pb′′< 0 , that is, after the equipment stops, the oil system is stopped, and the equipment maintains the working pressure. At this time, the end face specific pressure is less than zero, and the machine seal is sure to leak, which explains why the lubricating oil is easily emulsified.
3 Measures taken
3. 1 Increase the compression of the bellows In order to ensure that the oil system is out of service, the equipment fails to discharge normally and the machine seal leaks. By reverse calculation, the compression of the machine seal must not be lower than 11.2mm, in order to ensure The new machine seal has reached the compression amount. When the new machine seal is installed, the method of adding a metal gasket on the back of the moving ring is used (as shown in Fig. 2), and the normal compression is 11. 5 to 12 mm.
3. 2 Optimizing the process operation In order to prevent the emulsification of the lubricating oil, the technician must be relieved of pressure and flush the equipment before the oil stop system, and the operation is continued until there is no difference between the return oil temperature and the oil supply temperature. The oil system can be shut down.

3. 3 Increasing the pressure of the lubricating oil increases the compression of the bellows, which is equivalent to increasing the specific pressure of the end face of the machine seal. For increasing the pressure of the lubricating oil, it also increases the specific pressure of the end face of the machine seal. However, the specific pressure of the end face is not the more The better the big, the better the pressure of the lubricating oil can not be raised very high. Based on this point of view, it is decided to increase the pressure of the lubricating oil to 0.6 to 0. 7MPa, that is, during normal operation, the pressure difference between the two sides of the machine seal is 094至0. 125MPa。 The pressure is 0. 094~0. 125MPa.
The average speed of the sealed end faces is:
υ =π( D1 + D2) n/ 120=π(235. 2 + 241. 6) ×2000 ×10 - 3/ 120= 24. 97m/ s
Therefore, Pbυ= 2. 35~3. 12 < [ pυ] = 5. 67MPa·m/ s (recommended value)
The machine seal can be operated. From the actual use effect, the service life of the machine seal has also been greatly improved. Under normal circumstances, it can run for about 1 year, which can meet the requirements of normal production.

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