Current Status of Research on Bolted Flanges

In recent decades, the work done by domestic and foreign researchers on bolted flanges has focused on the performance of gaskets and loosening of bolts, and significant progress has also been made in some areas. Bolted flanges are generally in the assembly structure, and accurate and timely judgment of the gasket's sealing condition is important for preventing accidents.
 
At present, domestic and foreign researchers have studied the status identification of the connecting structure, and the main research content is based on the analysis of the gasket's seal failure and selection of the gasket. Chunmei Zhao conducted a failure analysis of gaskets used in petrochemical production and the use principles of gaskets. The main reasons for the gasket's sealing failure are analyzed, including the physical and chemical properties of the medium, bolt's pre-tightening force, and material of the gasket. In order to prevent the failure of gaskets, relevant measures must be taken to ensure the safe and stable long-term operation of the production equipment, such as reasonable selection and correct installation of gaskets. Rong Liu and Boqin Gu simulated the three-dimensional finite element method on the bolted flange's structure, calculated the nonlinear characteristics of the gasket, analyzed the influence of the additional load on the strength and tightness of the bolted flange structure, and determined the relationship among the tightness of connecting systems, additional loads and residual pre-tightening stress of gaskets. The research results showed that the additional load had a greater impact on the tightness of the bolted flange structure. Yan Zhang and Guoying Zeng applied the random excitation signal to the model of the flange's thread structure. The bolt's loosening signal in the time domain was analyzed by applying different bolt forces to the threaded structure under the vibration environment. The standard deviation of the test signal and reference signal was calculated, and the study found that while the pre-tightening force was increasing, the standard deviation of the difference signal had a significant downward trend.
 
From this judgment, the looseness of the threaded connection can be based on the benefit of the signal difference. At present, the foreign research on flange connection mainly focuses on the research on performance of the bolted flange and the type of gasket.
 

Abid and D.H.Nash carried out the non-linear finite element analysis on the bolted flange with gaskets and without gaskets. According to the stress results of the flange and bolts and the rotation and displacement of the flange, the traditional flange gasket connection had a feasible transformation. For flange connections, it provided an optimal flange model for a static load environment with zero leakage. M.Murali Krishna, M.S.Shunmugam and others discussed the sealing performance of the three types of spiral wound gaskets. According to the width of the gasket and the outer diameter of the flange, the distribution of the gasket's contact stress was not uniform. The difference in contact stress between the inner and outer radius of the gasket depended on the type and elasticity of the gasket, and the standard did not consider these factors. William H. Semke, George D. Bibel and others studied the dynamic characteristics of the bolted flange through experimental research and numerical simulation. The flange was in a symmetrical position in the middle, and the pipeline system constrained in the form of a simple cantilever beam of equal length was studied. The results of the experiment and simulation showed that there was a good correlation between the fundamental frequencies of the two, especially the first two modes. The dynamic behavior of the simple piping system could be accurately replaced by a simplified beam model.

 
The bolted flange is a complex and important nonlinear structure. Researching the gasket's sealing characteristics of the connecting structure is of great significance to the identification of the connection state, and it also enriches the research results of dynamic characteristics. The research on the sealing characteristics of bolted flanges covers a wide range of subjects, but from the current research status at home and abroad, it mainly focuses on three aspects: The first is the reason for failures and selection of gaskets; the second is the research on the influence of loads on the structural sealing; the third is the research on the influence of the selection of gaskets on the structural stress. This article focuses on the second aspect.
 
The application prospects
In actual engineering projects, because the rigidity of the connection structure focuses on the weaker part of the entire structural system in most cases, the deformation energy of the structure is mainly concentrated in the connection part. The leakage of the gasket in the bolted flange's sealing system may cause the entire mechanism to fail, which in turn causes a major accident. The changes in the structural dynamics characteristic parameters reflect the changes in the structure of the bolted flange system, thereby predicting the changes in its sealing performance, which is helpful to identify the sealing performance of flange joints. At the same time, through the research on the sealing performance of bolted flange under dynamic loads, the sealing performance of the joints of pipeline equipment under complex conditions such as dynamic loads, wind loads and seismic loads can be predicted, and the reliable operation of the joint can be realized.