Cause Analysis of Prelubrication Pump Coupling Fracture in Emergency Diesel Generator Set of Nuclear Power Plant
An emergency diesel generator (LHP \ Q) mechanical failure alarm occurred in the main control of Unit 2 of a nuclear power plant. On-site inspection found that the 2LHP152PO pre-lubricating oil pump was stopped, all the coupling teeth were broken, and the rubber elastic block on the force side disappeared. On February 19, 2017, another 2LHQ152PO pre-lubricating oil pump unit was expanded and inspected based on feedback. It was found that the rubber elastic block of the coupling was also worn and cracked. The pre-lubrication pump set consists of a motor, a coupling, and a gear oil pump. The coupling mainly includes two cast-iron half couplings and a rubber elastic block (see Figure 1). 1 Coupling fracture cause analysis process First list the possible failure modes, and then through laboratory analysis, data analysis, mechanical calculation analysis, field installation investigation and other analysis, finally determine the root cause and contributing factors of coupling fracture . 1.1 Coupling Fracture Laboratory Analysis 1.1.1 Rubber Elastic Block Hardness Test The hardness test of the rubber elastic block of the coupling is based on the measurement results: 2LHP152PO, 2LHQ152PO elastic block hardness values are high, compared to the requirements of the brand, not The average hardness of the rubber block on the load side is 7 points higher, and the average hardness of the rubber block on the load side is 13 points higher. The average hardness measurement result of the new rubber block spare part is 77.4, which is very close to the Shore hardness 78 required by the grade. The new rubber block and 2LHP152PO and 2LHQ152PO rubber blocks are produced in 2013. The analysis results show that the hardness of the 2LHP152PO and 2LHQ152PO rubber blocks has increased to varying degrees during use, the hardness of the load side has increased significantly, and there is aging. 1.1.2 Fracture analysis of elastic rubber block The macroscopic observation of the rubber block of 2LHQ152PO elastomer was performed using a stereo microscope. It was found that the frontal shape of the unstressed rubber block was normal, and the shape was basically the same. It can be clearly seen that the rubber block is deformed by compression, and it can be seen that the cracks opened by compression and a large number of small lateral micro cracks in the same direction. Observing the microscopic morphology of the front of the rubber block under a scanning electron microscope, it can be seen that a large number of nuclear power plant emergency diesel generator sets of pre-lube pump couplings are broken and analyzed. In the event of a breakage of the pre-lube pump coupling of an emergency diesel generator set, starting from the failure mode of the possible coupling breakage, a comprehensive analysis of laboratory analysis, centering \ vibration data analysis, and mechanical calculations is used, combined with field feedback, and one pair of couplings is used. The possible causes of the fracture were analyzed and demonstrated. Finally, the root cause and contributing factors of the fracture of the coupling were found out, and effective improvement measures were proposed. Keywords: cause analysis; pre-lubricating oil pump; coupling; fracture 5 Evaluation of synthetic standard uncertainty The above-mentioned standard uncertainty components are independent of each other, so the synthetic standard uncertainty is: 6 Extended uncertainty The determination of u is mainly due to u (Foi) and the normal distribution in the main component. Therefore, the higher the 95% of P, the more the factor k can be taken to be 2, then: 7 Uncertainty Report Calibration and Measurement Capability (CMC): According to JJF1194- According to the 2008 "Tire Strength and Tripping Tester Calibration Specification", 5 points with approximately uniform distribution are used for calibration within the measurement range. This article calibrates a total of 5 points of 10kN, 20kN, 30kN, 40kN, and 50kN. The measurement uncertainty is shown in Table 3. The CMC of this project is: (0-50) kN, Urel = 0.4%, k = 2. Reference Literature Institute  Ni Yucai. Evaluation of Uncertainty in Practical Measurements [M]. 5th Edition. Beijing: China Quality Inspection Press, 2016: 155.  Li Wei. Measurement Error of Indication Error of Electronic Universal Testing Machine Evaluation of Certainty [M]. Modern Measurement and Laboratory Management, 2014: 04.  Zhao Yuan, Chen Haozhao. Brief introduction of horizontal microcomputer detection and control of tire strength decoupling static load testing machine [M]. Rubber Technology and Equipment , 1986: 04  JJF1194-2008, Calibration specifications for tire strength and knockout testing machine [S].  JJF1059.1-2012, Evaluation and expression of measurement uncertainty [S]. Calibration point (kN) is not Degree of certainty component (kN) u (c kN) U (kN) (k 2) Urel (%) u (Foi) 10 20 30 40 50 0.017 0.035 0.052 0.069 0.087 0.0035 0.0035 0.0035 0.0035 0.0035 0.018 0.035 0.052 0.069 0.087 0.035 0.070 0.104 0.139 0.1730.4 0.4 0.4 0.4 0.4 . To sum up, the transverse cracks of the rubber block extruded and expanded originated from the surface stress aging cracks. 1.1.3 Tensile performance test The tensile strength test of 2LHQ152PO rubber block. The tensile strength test of three groups of samples. The tensile strength of 2LHQ152PO rubber block is about 14-16MPa, and the elongation at break is about 120% -140. %. A comparative test of the tensile properties of the new spare parts rubber blocks shows that the elongation at break of the new spare parts are all above 300%, which is much higher than the elongation of 120% and 140% of the failed rubber parts. Tests have shown that the rubber of 2LHQ152PO coupling has obvious aging phenomenon. 1.1.4 Analysis of the surface wear morphology of the rubber block By observing the surface wear morphology of the rubber elastic block of the 2LHQ152PO coupling through a stereo microscope, it can be found that the surface of the rubber elastic block in contact with the metal claws has wear marks in the same wear direction and is stressed. The surface of the side rubber block has oblique wear marks, and the directions are consistent, indicating that the wear of the rubber block is mainly caused by the misalignment of the coupling. 1.1.5 Metal tooth claw material analysis The metal tooth claw is analyzed for fracture and metallographic analysis. The fracture is a brittle fracture. The metallurgical structure is evaluated according to GB / T 7216: the graphite type is type A + D, and the graphite length is level 6. The number of pearlite is 70 and the metallographic structure is normal. 1.2 Analysis of centering data 1.2.1 About centering Standards The centering standards for couplings by oil pump manufacturers are cross section clearance of 5mm, angular deviation of 0.5mm (about 0.8mm), and radial deviation of 0.1mm. 1.2.2 Analysis of actual alignment data After inspection, 2LHP150PO has a section clearance of 5.47mm, an angular deviation of 0.73mm, and a radial deviation of 0.1mm. 2LHQ150PO has a section clearance of 3.97mm, an angular deviation of 0.44mm, and a radial deviation of 0.14. mm, compared with the required alignment standards, it can be found that none of them meet the alignment requirements of the oil pump. In summary, it can be known that the installation is not strictly in accordance with the alignment standards, which results in poor alignment. 1.3 Vibration data analysis Looking at the historical vibration trend of 2LHQ152PO, it can be seen that the vibration at the non-driving end of the pump meets the requirements of the standard, but the vibration at the driving end is high, and the vibration value decreases after replacing the new coupling. At the same time, the vibration measurement spectrum of the 2LHQ152PO before and after the replacement of the coupling at the site is retrieved. According to the comparison before and after replacing the coupling, it can be seen that the vibration spectrum of the pump drive end is 11 times the frequency (meshing frequency) as the main component. To sum up: The high vibration at the drive end of the pump is not caused by poor alignment of the coupling, but mainly due to gear meshing. After replacing the new coupling, the vibration at the drive end is mainly due to the new coupling being more compact and improved. The stiffness of the shafting. 1.4 Coupling Selection Analysis 1.4.1 Coupling Torque Check Calculation According to the gear pump EOMM manual, the motor power is 7.5kW and the speed is 720r / min. According to the coupling installation technical manual provided by the manufacturer, the permissible torque of the POLY-NORM AR coupling used on site is 150N · M, and the maximum instantaneous torque is 300N · M. According to JB / T 7511-1994 "Calculation of Mechanical Coupling Selection", for a gear pump driven by a motor, the calculated torque Tc is as follows (calculated by taking the motor power Pw): Tc = TKwKKzKt where: Kw = 1, K = 1.5 ， Kz = 1 ， Kt = 1 T = 9550Pw / n Pw ———— Motor power n ———— Motor speed is calculated: 2LHP152PO The minimum required torque of the coupling needs to be greater than 149.2N · M, the actual selection of POLY -NORM AR coupling requires a torque of 150 N · M, which has just met the standard requirements. 1.4.2 Maintenance requirements for POLY-NORM couplings POLY-NORM AR couplings are rubber-elastic block couplings, which compensate for radial and angular deviations of the coupling mainly through the deformation of the elastic block. During the operation of the equipment, the elastic block will inevitably wear out. Therefore, for this type of coupling, frequent inspection and maintenance are needed to ensure the stable operation of the coupling. According to the inspection and maintenance requirements of the POLY-NORM AR coupling, the wear thickness of the rubber block needs to be measured every two months to determine whether replace. 1.4.3 Comprehensive analysis of type selection The allowable torque of the KTR POLY-NORM AR coupling meets the standard requirements, but the torque selection margin is seriously small. The rubber blocks used in POLY-NORM AR couplings are prone to wear and are sensitive to changes in alignment and thinning of the rubber blocks. This type of coupling requires frequent inspection and maintenance. According to comprehensive analysis, the selection of POLY-NORM AR couplings is not suitable for the needs of field operation. 2 Cause analysis and improvement measures 2.1 Root cause The torque selection margin of the pre-lubrication pump coupling is too small, and the alignment is not performed according to the alignment standard during installation, resulting in 2LHP152PO's rubber blocks occurring quickly during long-term continuous operation Wear and stress aging phenomena. 2.2 Contributing Factors Due to the long-term operation of the diesel pre-lubrication pump, it is difficult to carry out frequent inspections and maintenance on the site, and the elastic block of the coupling is gradually consumed, resulting in direct contact with the metal tooth claws and causing fracture. 2.3 Improvement measures According to analysis and test, replace this type of coupling with a diaphragm coupling, and adjust the pump that does not meet the alignment standard. Align and check the maintenance outline for the upgraded coupling. Yusheng version daily inspection list, increase the vibration measurement of the pump.