Cause Analysis And Solution Measures For Bolt Bite Of Main Steam Isolation Valve Of A Nuclear Power Unit

Valves are the most widely used key accessories for fluid transmission and control in nuclear power systems.They have the functions of regulating flow,stabilizing pressure,diversion,blocking interception,preventing reverse flow and releasing pressure.The most common connection method for nuclear power valves is threaded connection.Because of its simple form,stable connection,and easy disassembly,it is widely used in detachable and fixed connection environments.However,in the disassembly and maintenance of equipment,jamming or biting of valve thread pairs often occur.The cost of valve bolts and nuts for most large nuclear power equipment is tens of thousands of yuan.If it loses efficacy,it will not only affect the normal operation of the equipment body or the entire production unit,but also severely affect the operation safety of the nuclear power system.Therefore,it is of great engineering significance to carry out analysis and research on bolt jamming and seizure of large-scale key valves of nuclear power,and propose on-site solutions.This article takes the bolt jamming and seizure failure of the main steam isolation valve of a domestic nuclear power unit as an example.The main research work and results obtained are as follows:(1)The internal causes of bolt jamming and seizure were studied and obtained,through the analysis of bolt material composition and metallographic structure,combined with experimental tests of mechanical properties such as torsion,shear,tensile and hardness.(2)Based on the elastoplastic theory to calculate the additional stress and strain of the thread,and use the finite element method to numerically analyze the Mises equivalent stress analysis of the thread under the effect of the offset load,and determine the torque value and standard of the thread moment,combined with material composition analysis and mechanical performance experimental test and analysis results,as well as thread wear analysis and installation operation analysis under high temperature conditions,determine the external cause of thread seizure.(3)Combined with the actual production conditions on the site,a fast and efficient disassembly method for bolts that have failed due to jamming is proposed,that is,the method of disassembling bolts by using additional pre-tensioning force of tensile bolts;regarding the severely bitten bolts,a disassembly method by destroying the thread pair is proposed,and the hole is re-expanded to deepen the installation of the reducing bolt to meet the safety bearing requirements;combined with the current advanced technology and the theoretical calculation methods and experimental test analysis results in this paper,the surface treatment of threads,the change of bolt fastening methods,Improvement measures such as adding quality control points during gasket replacement and reassembly,practical results show that the above measures can effectively prevent the bolts from becoming jammed or even biting.The above research results can provide technical guidance for preventing problems such as jamming or seizure of bolts for large-scale equipment similar to nuclear power and related process industry bolts,reducing the failure of threaded mating pairs,and achieving rapid disassembly of bolts and replacement of jammed bolts.