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Natural gas processing station orbit valve due to perennial operation coupled with the inspection and maintenance of quality, but off, often a valve failure, the normal operation of the molecular sieve system has had a serious impact. The failure of the most influential technology system is the orbit valve stem fracture failure, the valve stem can not open after the fracture, the entire process is blocked, will cause the molecular sieve front-end pressure rise, the compressor pressure suppression shutdown. In case of failure in winter to meet the timely and effective treatment, just off the zeolite of less than timely regeneration will cause molecular sieve freeze blocking. Orbit valve failure cause Orbit valve is located in natural gas processing plant natural gas processing technology molecular sieve process import and export, the process medium is natural gas, the normal operating pressure of 3.OMPa, pressure fluctuations; temperature fluctuations in the range of 50-200 ° C. The device put into operation from 1997 to August 2006 before the maintenance of the valve stem fracture accident. August 2006 overhaul of the replacement of a number of stems, replaced stem material for the ASTM420, the original stem material replaced by ASTM8620. To break the valve stem only serve less than six months. By 2012, there were 7 valve stem fracture accidents in succession. Gas station to produce a greater impact on the safe and stable production, analysis of the cause of the stem rupture and solve the valve stem without delay. The problem is ultimately focused on stem quality issues and assembly quality issues. 2, the stem fracture reasons In order to find out whether the stem material out of the question, natural gas processing station commissioned a qualified unit on the stem of a professional analysis of the results are as follows: (a) from the hardness test shows that are consistent with or Exceeding technical requirements (HRC 33 to 40) From the hardness values, the replaced 8620 stem performs better than the fractured 420 stem; (b) Both stem materials meet the standards by chemical composition analysis; (c) SEM Fracture analysis. SEM fracture analysis shows that a large number of micro-cracks and fatigue cracks perpendicular to the micro-cracks exist on the cross-section, indicating the brittle fatigue fracture characteristics of the valve stem. The results of the comprehensive analysis conclude that the quality of the stems is in line with AS T M42 0 stems or ASTM 8620 stems, with ASTM 8620 stems slightly superior to ASTM 420 stems. Stem breakage is due to the valve stem in the role of low frequency alternating stress under the surface stress concentration fatigue cracks, resulting in reduced fatigue life of the material, leading to brittle fracture of the stem. 3, the causes of fatigue cracks After a professional analysis, concluded that the cause of stem fracture is fatigue fracture, fatigue pattern and how it has become a challenge before us. Although the stem material has a useful life, but this fatigue too fast, fatigue spent in the past 10 years without fracture, now spent less than half a year on the fracture. Certainly related to the quality of inspection and maintenance, the quality of the valve stem check the quality of the parameters of the valve stem stroke and valve sealing surface installation direction. According to the working principle of the orbit valve, the valve stroke assembly is too short: the stem seal bevel and the spool pin pressure are not enough, resulting in tight seal between the valve spool and the seat ring and easy leakage of the valve. Valve stroke assembly is too long: the stem seal bevel and valve pin pressure increases, resulting in valve plug and seat ring seal is too tight, the valve stem increases the fatigue easily. Valve stem mainly A, B, C and D function section. A section for the valve stem linear motion control section, B section of the valve stem rotation control section, C section ball loose pressure section, D section for the ball rotation section. For my station 6 inch orbit valve, in which A + B = 81mm; 3 inch orbit valve A + B = 61mm. In the assembly process 6-inch valve assembly stroke range of options: 6-inch valve stroke: 85.73 --- 90.49mm, 3-inch valve stroke: 48.42 --- 53.18 mm It is clear that 6-inch valve stroke is too long. 3-inch valve stroke in a reasonable range, which is why the 3-inch valve has never occurred stem failure. ORBIT track ball valve structure for the single-sided seal, while requiring non-sealing direction that the reverse pressure can not exceed 0.2Mpa, or ORBIT rail valve unsealed side pressure higher than the sealing side of the pressure, the sealing effect is best, or may exceed requirements The leakage, reach the use effect. According to the sealing requirements listed in my station molecular sieve in various working conditions orbit valve sealing surface installation diagram: Figure 2 Theoretical valve sealing surface installation figure The greater the figure the higher the pressure, the smaller the number on behalf of the lower pressure, the red part of the theoretical seal Face side. In theory, 6-inch orbit valve sealing surface should be away from the zeolite side, so the sealing effect will be better. The actual gas station adsorption inlet sealing surface is toward the molecular sieve side, there is reverse pressure differential about 200kpa, although it does not affect the body seal, but the impact stem force. This reverse pressure differential increases the fatigue stress on the stem. So far, the broken orbit stems are all adsorbed, and when the switchover from the cold blow to the adsorbed state, the stem is broken and the valve stem is in the closed state at breakage. Orbit valve is basically open when the force. Force mainly in the closure, the stem of the functional section C bias valve plug pin, so that the valve plug and seat ring seal tightly. From this it can be concluded that the fatigue pattern of the stem is caused during the closing period and also breaks when it is closed. Figure 3 adsorption Orbit valve in the molecular sieve blowing cold force analysis Orbit valve in the molecular sieve cold blow force analysis, can be drawn. Stem in this period the main force for the diaphragm to the stem downward force: F = P * S = instrument wind pressure * diaphragm area = 4.5 kg * 3.14 * 20 * 20 = 5652 kg. And reverse pressure force: F2 = sealing surface S1 * pipeline pressure P = 3.14 * 6 * 6 * 200kpa = 226.08 kg. Valve spool to the valve stem of the reverse force: F reverse = F * tanφ1 + F2, with the stem stroke increases, the valve stem and valve seat ring ring the more tight, the angle φ1 will increase, The reverse force will also increase. Under the action of alternating alternating low frequency stress, fatigue cracks occur in the stress concentration area to reduce the fatigue strength of the material, resulting in brittle fracture of the valve stem. Because the upper half of the valve rod packing box tightly wrapped, so the stem crack at the junction box and the spool valve and in Figure 3, the red part of the stress concentration. In summary, the stem of the reasons for the fracture: 1, stem stroke assembly is too long, resulting in stem seal bevel into the spool too deep. Stem by increasing force, fatigue. 2, the valve seal installation direction is not reasonable, work in the sealing surface has a reverse force. Increase the valve stem load. 4, the solution (1) for the stem fracture, taken to replace all the original 420 stem. Stem and stroke adjustment to a reasonable range. Stem stroke should be less than A + B = 81mm, but also sealed tight; (2) adjust the valve position seal facing the direction of low pressure, the valve when working there is a positive pressure.