Analysis and Solution of Valve Failure Mode

The valve is the control component in the fluid conveying system, which has the functions of stopping and dredging the fluid, adjusting the flow, dredging, checking, stabilizing the pressure, dispersing the fluid, and relieving the overflow pressure. The types and specifications of valves are complex, including globe valves and ball valves with simple structures, and complex valves suitable for automatic control systems.
 
With the expansion of product demand and the improvement of industrial automation technology, more and more attention has been paid to valve failure analysis and solutions, especially in aerospace, nuclear power, thermal power, nuclear power, high temperature and high pressure and other fields, in order to ensure the smooth production without accidents. Valves play an important role in industrial production, so it is very important to study the failure forms and causes of valves for practical applications such as stress, strength and fatigue. At present, many scholars have done a great deal of research on the reliability analysis of valves, some of which are from individual to whole, that is, to conduct a complete analysis of valves with specific environments and requirements, study their failure forms and principles, and then expand the whole field. The other part starts with the basic theory, first making a comprehensive design and analysis of valve reliability from different directions such as strength and sealing, and then conducting experimental research on specific valves according to different situations. Due to the variety of valves and complex working conditions, there is no complete specification for design guidance. Therefore, based on the analysis of valve failure types, this paper briefly describes the causes of different failure modes of valves and puts forward solutions from the perspectives of materials, design, testing and manufacturing.
 
Failure mode and cause analysis
 
Seep

Leakage is mainly divided into external leakage and internal leakage. External leakage mainly refers to the fluid flowing out of the system due to sealing failure, otherwise, it is called internal leakage. The main reason for valve leakage is seal failure. Wu Weizhi et al. found that the valve of a 300 MW steam turbine in a thermal power plant was leaking. After systematic inspection, it was found that the valve seat was worn after many times of openings and closings, which led to a decrease in the sealing performance of the valve, and the lax closing resulted in the leakage. In a low-temperature environment, with the decrease in temperature, the tensile and compressive properties of rubber sealing materials will drop sharply and harden, which will lead to the decrease of contact surface preload and cause leakage. Under the condition of high temperature and high pressure, the valve body will be deformed, and the valve stem may drop and jam. No matter what the working condition, once the valve seal fails and causes leakage, especially involving high temperature and high pressure, corrosive, radioactive, flammable and explosive media, poisoning, fire, explosion, personal injury and other accidents will occur.
 
Leakage location: sealing surface
Cause of leakage:
① The surface of the sealing surface is uneven due to material making or strong extrusion.
② Bending of valve stem causes deflection of assembly position.
③ Not selected according to the demand.
 
Packing:
① Packing is not selected according to the requirements or age, resulting in reduced sealing performance.
② The packing is damaged due to the valve stem problem.
 
Sealing ring:
① The material is not selected according to the requirements, and its fatigue resistance is not good.
② Corrosion and problems in heat treatment methods.
 
Valve body and bonnet:
① Loose bolts and fasteners or different tightness.
② The material strength is insufficient or the processing technology is defective.
 
The causes of valve leakage due to seal failure are as follows:
 
(1) The design is unreasonable. Huang Hanhui used ANSYS and MSC. Fatigue to jointly simulate, and found that the fatigue life of the steam drum tandem valve was prolonged with the decrease of load, the improvement of surface smoothness and good heat treatment. The analysis results were consistent with the basic engineering knowledge, which verified the feasibility of using analysis software to analyze the fatigue life of the tandem valve. Wu Zongping made a finite element analysis of the bellows, established the valve leakage model, and calculated that when the surface roughness Ra was 0.1~0.4 μm, it not only met the service life requirements but also solved the problem of valve leakage.
 
(2) The structure is unreasonable. Xiao Dinghao designed the sealing structure of the slag lock valve on the basis of the working environment, sealing specific pressure, structure size, etc. The finite element model is established for simulation, and the flow model and leakage model are used for analysis. The effective sealing interval of design parameters and the valve seat leakage grade under different parameters are obtained and verified. After analyzing the U-shaped seal, Mao Feng found that the maximum creep strain occurred in the upper and lower arms of the U-shaped seal and the root of the U-shaped groove, and there was no leakage.
 
(3) Unreasonable material selection. A439D-2C austenitic nodular cast iron was trial-produced in Zhang Tao. The spheroidizing rate of austenitic cast iron is higher than 95%, its mechanical properties and composition are better than those of the same kind of cast iron in China, and its plasticity index is increased by three times. After the material is applied to the double-sided sealing structure, the service life of the valve is obviously prolonged, and the manufacturing cost and manufacturing difficulty are reduced. Wei Hongpu et al. researched and developed a cobalt-free iron-based alloy powder, which was coated on the stainless steel substrate by laser cladding technology. The results show that the hardness of the stainless steel substrate covered with the powder is obviously improved, which is about twice that of the uncoated substrate. A large number of scholars have analyzed and improved the sealing structure of the valve from the aspects of design, structure and material, but there are still some problems. First, the sealing is qualitatively analyzed under specific conditions, and the calculation method of quantitative analysis under most conditions cannot be given. Secondly, sealing involves many microscopic phenomena in the working environment, and the principles and manifestations of microscopic phenomena caused by different influencing factors are different. Therefore, it is very important to study the sealing failure caused by various factors and the superposition of different factors, which can provide the direction and way to further improve the sealing structure and performance.
 
Rupture of the valve body
 
The main causes of valve body rupture are as follows:
 
(1) Improper selection of valve body material or material defects, such as sand holes, air bubbles, shrinkage cavities, etc. When installing the wafer butterfly valve, if rubber pads or other gaskets are added to increase the sealing surface at the end of the valve body, the valve body will be cracked and cracked during use. Zou Shengwu analyzed the force, material, torque and other aspects of the valve body in this state, and believed that nodular cast iron should be selected as the valve body material, and there should be no defects, so as to ensure the normal use and safe operation of the valve body. Su Liang analyzed the influence of casting defects on the strength of the hydraulic valve body and established a casting defect model with ProCAST software. By comparing the stress values of the ideal valve body and the defective valve body under different pressures, it was found that the valve body with shrinkage defects would produce greater local stress. That is, the shrinkage cavity will reduce the local effective wall thickness of the valve body, resulting in stress concentration, resulting in the reduction of the bearing capacity, strength, fatigue and service life of the valve body.
 
(2) External environmental factors lead to cracks in the valve body. Low temperature, high temperature, high pressure and other external environments will cause the valve body to creep, fatigue, impact and other phenomena, and then lead to the valve body rupture. It has been mentioned in the literature that the main reason for the rupture of the valve body of the coke oven riser is the long-term combined corrosion of high temperature and other working conditions, resulting in deformation, hole-out, jamming and other phenomena. Ma Wenju found that the valve body of the check valve at the end of the slurry lift pump was cracked due to repeated forward and reverse water hammer impacts of the fluid with a large amount of sediment and a 30-meter drop, and its strength was greatly damaged, and its service life was reduced.
 
(3) Strong vibration, resonance or violent installation causes uneven stress on the valve and cracks. During use, the cracked valve body caused by vibration and other factors will be impacted by the medium, which will prolong the crack and eventually lead to damage to the valve body. Niu Chuangui investigated the influence of vibration and noise on the main steam isolation valve of the nuclear power plant and found that the steam flow rate exceeded the recommended 40~60 m/s, reaching 87 m/s, resulting in acoustic noise, forming an acoustic resonance cavity, causing fretting fatigue damage to the valve, and cavitation accelerated the valve's destruction. Zhao Yanan mentioned that high-cycle fatigue vibration and low-cycle fatigue vibration will shorten the service life of the valve, and will also cause some damage to the power generation system, pipelines and instruments.
 
Clamping stagnation
 
(1) The material selection and stress superposition cause deformation and bending, or the valve action are stuck due to physical reasons such as excessive and tight packing. When Jiang Xiaohong and others conducted the fatigue life test of the valve, the valve failed many times. By disassembling the valve, it was found that the main reasons were as follows: the material strength of the valve stem was not enough, the thread was seriously damaged, and the valve stem "snapped" with the valve stem nut; The surface finish of parts does not meet the requirements; Due to the thermal stress, the bolt pair is welded, which leads to the failure of the valve fatigue life test. Zhang Wenbin et al. found that the opening and closing pressure of the valve in the stuck state was higher than that in the normal state by comparing the normal state of the hydraulic gate valve with the stuck state. After disassembling and assembling the valve, it is known that the larger distance error between the rams will also lead to the valve sticking.
 
(2) The valve is stuck due to pollution caused by the working environment or chemical corrosion and other reasons. Wei Guojian and others mentioned that the liquid oxygen control valve in the main engine of the space shuttle was covered and replaced by the stuck oil film due to the corrosion of the valve stem, and the remaining oil film deteriorated and cemented under the action of the hot air in the transmission box for a long time, resulting in the corrosion of the valve stem and the stuck valve. Vibration and noise of valves In the actual production process, the vibration and noise of valves usually accompany each other. Vibration, unstable flow and vortex caused by diameter reduction will cause the valve to vibrate, reduce the control performance, affect the service life of the valve, and even cause accidents such as valve leakage. The main causes of valve vibration and noise are as follows.

Mechanical vibration
Unreasonable number and spacing layout of process pipes, elbows and valves will lead to vibration and pressure pulsation of medium fluid in the pipes. The external environment resonates with the medium fluid, which will also cause the valve to vibrate. The causes of mechanical vibration of the valve also include the vibration of the valve flap. When the medium flows through the valve flap, the pressure difference before and after the inflow greatly increases, resulting in the vibration of the valve flap, thus causing the valve vibration. When Liu Li and others studied pipeline vibration, they found that fluid pulsation was the main cause of pipeline vibration. Qiao et al. studied the dynamics of a cantilever pipeline under different flow rates and concluded that when the medium velocity exceeds the critical velocity, the pipeline will vibrate periodically, and the valve will also vibrate accordingly.
 
Cavitation vibration
Cavitation vibration is mainly aimed at the phenomenon that occurs when the medium is liquid. When the fluid speed is increased, the pressure is reduced and lower than the saturated vapor pressure, the liquid will generate bubbles, and when the saturated vapor pressure is restored, the bubbles will burst, which will cause the valve to vibrate. Ma Qiaochun and others confirmed through theoretical analysis and diagnostic experiments that the vibration of the regulating valve of the boiler reheater was mainly caused by serious cavitation caused by valve deviation from design.
 
Hydrodynamic vibration
The large flow rate, unstable flow and diameter reduction of the medium will lead to the vortex. Because of the random occurrence probability, complicated vortex calculation method and uncertain energy, resonance will occur when the fluid frequency is consistent with the valve frequency. Xu Dengwei [20] mentioned that although it is very complicated to calculate the vortex frequency, subjectively, the vortex will fall off at a specific frequency. When the falling-off frequency is consistent with the natural frequency of the valve, it will lead to resonance and noise. Xú Zhēng et al. used FLUENT to simulate and analyze the main steam isolation valve channel with 3D modeling and finite element method. Based on time-averaged flow field calculation and large eddy simulation, they studied the flow characteristics of a 3D turbulent flow field and found out the flow field inducement of vibration and noise.
 
Of the main failure forms of the valve body, such as rupture, sticking, vibration and noise, most of them are caused by vibration except for design and material factors. As for valve vibration, although a lot of research has been conducted at home and abroad, most of them only analyzes and calculates the failure of specific valves, and there is no unified method or criterion. Moreover, it is difficult to fundamentally solve the vibration problem because of the complicated fluid flow, the complicated calculation of vortex energy and frequency, the random occurrence of resonance phenomenon, and the superposition of environmental corrosion and destruction.
 
Solution
 
Aiming at several important forms of valve failure, it can be solved from several aspects, such as material, design, test and production.
 
material
The stress, strength, corrosion resistance and other characteristics of the material in practical application are the key factors in solving the valve failure problem and improving the reliability of the valve.
 
① Make use of advanced technology to develop composite materials.
 
② Heat-treat valve fittings and components to improve their performance and internal metal structure, so as to improve the corrosion resistance and strength of the valve.
 
③ Advanced surface technologies such as spray welding and cladding are adopted to improve the wear resistance and corrosion resistance of parts.
 
Design
Advanced dynamic simulation, parametric method and finite element analysis are used to calculate the strength and design the structure of the product. In addition to the special process requirements, the influence of environmental factors, such as chemical corrosion, thermal stress corrosion, resonance, frequency, pressure fluctuation, etc., should be considered, and the stress limit calculation and fatigue performance analysis related to system operation and working condition conversion should be carried out.
 
Test and production
According to the requirements of valve characteristics and actual functions, a test platform meeting the working environment of the valve is simulated and established. The action and performance tests of some key valves are carried out under the simulated actual working conditions, so as to master their reliability under the actual operating environment. Improve valve manufacturing process and equipment, and avoid material defects, strength reduction, size deviation and other problems caused by insufficient manufacturing level. After Zhang Wenbin corrected the distance between the valve rams, the problem of valve sticking was solved.
 
Conclusion
 
(1) Through summarizing the common valve failure forms, analyze their causes and mechanisms.
 
(2) Investment in research and development of new materials or composite high-strength materials should be increased, strength calculation and structural design of products should be carried out by using advanced design means and methods, and process level and equipment manufacturing level should be improved to improve the quality of valves.
 
(3) Establish the valve reliability database and reliability analysis system, and discover and eliminate the factors that affect the reliability in the early design stage as soon as possible.