Cause analysis and Countermeasures of the loosenes

2022-08-26
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Cause analysis and Countermeasures of high pressure regulating valve rod loosening

1. Introduction: Hebei Xibaipo Power Generation Co., Ltd. has an installed capacity of 4 × 300MW, Harbin steam turbine n.7/537 (538)/537 (538), subcritical, single shaft, condensing steam turbine. The steam inlet of high-pressure cylinder is controlled by two high-pressure main throttle valves and six high-pressure regulating valves (referred to as high-pressure regulating valves). The operation mode of high-pressure regulating valves adopts single valve or sequence valve according to the load. Among them, the #1 unit was put into operation in December 1993, and has undergone three major repairs and six minor repairs. The last major repair was in October 2002

2. Cause of the event

2.1. In October 2002 1 unit was overhauled, and 4 high-pressure regulating valve was disassembled and the head of the valve rod was found (there was no thread here, but there was a stop cross pin hole processed, ф 12mm, l=120mm). Because the stop horizontal pin (hereinafter referred to as the horizontal pin) did not come out, the 4 high-speed regulating valve operated stably before overhaul, and no abnormality was found

2.2. Cause analysis of door rod head fracture

2.2.1. Stress. During long-term operation, the steam produces axial periodic excitation force on the gate rod, and the head of the gate rod and the connecting thread are prone to fatigue damage

2.2.2. Processing technology. The cross pin hole of the door rod head is machined by drilling and then reaming. The edge of the hole presents a knife edge shape, forming a typical stress concentration point. Once the crack is formed, it will quickly develop into fracture

2.2.3. Material. The gate rod is made of 2Cr12NiMoWV, which works at high temperature. In order to increase its high-temperature oxidation resistance, the surface is subject to special nitriding treatment to form a layer of nitrogen iron compound with a thickness of about 0.5mm. Its hardness, wear resistance and oxidation resistance are greatly increased, the corresponding brittleness is increased, and the toughness is reduced

2.2.4 improper assembly process

process requirements of the assembly drawing of the high-pressure regulating valve of Harbin automobile factory: for cold installation, the valve rod should be screwed into the guide sleeve, and 152n*m torque should be applied to the valve rod, and then the horizontal pin should be installed. That is, it is required to install the rear door rod in a cold state and make full tight contact with the guide sleeve, and no gap is allowed. During hot operation, because the gate rod is wrapped in the guide sleeve, the temperature is higher than the guide sleeve, and its axial linear expansion is greater than the guide sleeve, the above tight fit can still be guaranteed. Therefore, when the gate rod is in hot operation, the gate rod receives the axial exciting force of steam, which will be directly transmitted to the guide sleeve, so that the connecting thread of the gate rod is not affected by the exciting force, because the connecting thread part is the stress concentration point where the local government attaches importance to the development and damage of lithium industry, which prevents the fatigue damage of the thread. The actual situation is not so. The six high-profile doors do not meet the technical requirements to varying degrees, that is, when the door rod is reinstalled in the cold state, after applying 152n*m torque, the horizontal pin hole of the door rod and the horizontal pin hole of the guide sleeve are misaligned, and the horizontal pin cannot be installed smoothly. In order to install the horizontal pin smoothly, the door rod must be slightly rotated back. If the horizontal pin hole is misaligned by 2mm and the connecting thread of the door rod is m50*2, the door rod needs to be withdrawn by about 0.025mm, that is, there is a gap of 0.025mm between the door rod head and the guide sleeve, which does not meet the tight fit of the technical requirements. If the maintenance personnel ignore the assembly technical requirements and install it in a labor-saving and hasty manner, there will be hidden safety hazards

there are two main reasons for the dislocation of the horizontal pin hole: A. when the gate thickness is small, the processing size of the parts does not meet the technical accuracy requirements; B the door rod operates under high temperature and stress for a long time, and the connecting thread is affected by tensile stress, compressive stress, vibration fatigue damage and high temperature oxidation, resulting in deformation and defects on the surface, and the assembly clearance changes accordingly

2.2.5. Transverse pin deformation. The horizontal pin was slightly bent and deformed after being stressed, and it was not replaced during maintenance. During forced installation, due to the large extrusion force of the non concentric horizontal pin on the edge of the pin hole, the stress state of the edge of the pin hole was further deteriorated, and the possibility of cracking increased sharply

2.2.6 impact stress. In the later stage of major and minor maintenance, the unit often performs DEH mixed prevention test, large interlock test, overspeed test when starting up, and emergency shutdown during operation. The high-speed regulating valve will close quickly in 0.3 ~ 0.5 seconds. In design, because there is no buffer spring structure between the high-speed regulating valve rod and the valve disc (such as the one installed inside the high-pressure main throttle valve core), the valve rod is strongly impacted many times, and the impact stress of the valve rod head is particularly serious, Further aggravates the crack of the cross pin hole

3. Countermeasures: replace the door rod. In view of the fact that if the cross pin hole of the old guide sleeve is drilled again, the inner diameter may be expanded by the drill or the drill deviates, making the cross pin hole invalid, resulting in the assembly size of the cross pin not meeting the requirements, it is decided to re process the cross pin hole, which is located 20mm below the original hole. After the cold installation of the new gate rod and the guide sleeve, the drilling and processing are completed at one time

4. Development of the event

4.1 on April 8, 2003, the operator found that the local valve of #1 unit #4 high-pressure regulating valve had an opening (the main control monitoring screen showed that there was an indication of LVDT valve opening feedback), but the valve did not respond after the closing command was issued. It was suspected that the servo valve of the actuator had a fault, and the maintenance personnel were required to deal with it. The machine inspector closed the oil inlet stop valve and found that the hydraulic servo motor did not close with it. Then he opened the quick unloading valve to unload the oil, but the hydraulic servo motor still did not close. When checking the steam side actuator, he accidentally prepared to measure the elastic modulus and the specified non proportional extension strength; It is found that there is a pin on the cover of #4 high-pressure regulating valve. Confirm that the horizontal pin of the valve rod and the guide sleeve is disengaged, and the valve rod moves downward from the guide sleeve after rotation to close the steam inlet

4.2. Cause analysis

4.2.1. After the installation of the horizontal pin, the two ends of the horizontal pin hole must be sealed and riveted with a sample punch. Because the material hardness of the door rod sleeve is high, it is difficult to seal and rivet, and the deformation of the sealing and riveting point is small, which fails to play a good sealing role

4.2.2. There is little difference between the length of the horizontal pin and the length of the pin hole, and there is little room for riveting, resulting in difficulties in riveting. If the horizontal pin is longer than the pin hole, it cannot be riveted

4.2.3. Due to multiple sealing and riveting disassembly around the pin hole of the guide sleeve, there are many defects in the edge base metal, which makes it difficult to seal and rivet

4.2.4. The key reason is that in the original design, the pin hole of the guide sleeve is clamped by the lever arms on both sides, that is, the pin is observed from the outside. Due to the cover of the lever arm, only half of the cross-section of the pin can be seen. Its design idea can be understood as that when the pin looses for various reasons and attempts to move to one side, it will bump against the lever arm and stop after a small displacement. The cross pin hole reprocessed in the overhaul was moved down 20mm as a whole, and the cross pin just got out of the protection of the lever arm. During operation, the cross pin could be released smoothly, causing the valve rod to move down and close the valve

5. Treatment steps

5.1. Keep the unit running in single valve operation mode, and the load is limited to below 220MW. Close the #4 high-pressure regulating valve, the oil inlet stop valve of the hydraulic servo motor, and cut off the oil circuit. At the same time

5.2 remove the connecting pin between the hydraulic servo motor and the lever arm, first support the lever arm with a jack, and insert a suitable cushion block (square steel) into the lower clearance of the pressure storage spring

5.3 remove the pull-down spring, remove the jack, remove the connecting pin between the lever arm and the guide sleeve and the rear support, and remove the lever arm to make the guide sleeve completely exposed

5.4 because the valve disc is in the closed state, there is a huge steam pressure difference between the front and rear of the valve, the valve disc is tightly pressed on the bottom opening, and the gate rod will not move easily. At this time, rotate the guide sleeve clockwise until the pin hole is aligned, reinstall the pin and weld the sealing point firmly, and restore the rest of the connection

5.5. Closely monitor the load change, open the oil inlet stop valve of the hydraulic servo motor, 4 put the high-pressure regulating valve into operation, and remove the cushion block under the pressure storage spring

6. Reflection

6.1. The maintenance personnel must have a comprehensive understanding and mastery of the structure and principle of the equipment, which is required to be specific to every detail. Any change or change of equipment (especially products produced by imported or imported foreign technology) can only be carried out after the demonstration of professional technicians. Any behavior without technical demonstration will pay an undue price

6.2. The perfect maintenance process of high-profile door is the prerequisite to ensure its long-term safe operation, in which the quality of personnel plays a leading role. Further strengthening the technical training of maintenance personnel and improving the overall skill level should be an important focus at present. Only in this way can the maintenance quality of equipment be ensured

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