The Importance of the Sealing of the Flange Connection at High Temperatures

There are many detachable sealing connection forms for containers and pipes, such as threaded connections, socket connections, and flange connections. Among them, the flange connection is the most common in the industry due to its simple structure, easy assembly, and disassembly. The flange connection is mainly composed of flanges, bolts, and gaskets. The flange rings of the two parts of devices or pipelines are connected by relying on the pre-tightening force of the bolt. At the same time, the gasket was pressed to achieve the sealing effect at the connecting part. The flange connection is the general term for bolt gasket flange sealing system, which has easy disassembly and assembly, high strength, and good sealing performance.
 
In the process industry, the flange connection as an important connection form is widely used in equipment and pipelines. As a connecting and sealing structure, it is required to have reliable sealing and sufficient strength. Affected by a load of bolt pre-tightening force, medium pressure, temperature under operating conditions, additional bending moment and additional axial force generated by the piping system, leakage at the connecting part is prone to occur, causing the failure of the sealing. This can waste energy, cause serious environmental pollution, do harm to health, or even lead to catastrophic accidents such as explosions. For example, the chlorine leakage incident at the United Insecticide Plant in Bhopal, India in 1984 was the most serious industrial accident in history. It had a great impact. It caused a tragedy of 25,000 direct deaths, 550,000 indirect deaths, and more than 200,000 people being permanently disabled.
 

For the units in the process industry such as devices of oil refining, chemical industry, and nuclear energy, there are many devices or flanges of pipelines that operate under high temperature for a long time. The compression resilience of the gasket will decrease due to the creep relaxation and accelerated aging of the gasket at high temperatures. In addition, the temperature and pressure fluctuations of the working conditions cause the sealing specific pressure on the gasket to gradually decrease, and finally, it may cause the sealing failure of the flange connecting system. The high temperature will also cause the creep deformation of bolts and flanges, which will lead to a failure of the bolt's pre-tightening force, warping and deflection of flanges, sealing failure, and leakage. According to records, leakage of the flue gas's flange joints of oil refining equipment, a fire caused by leakage of flanges in the high temperature and high-pressure hydrogenation equipment in petrochemical enterprises, and leakage of the overheated steam's flange joints of power plants often occur. These flanges work at high temperatures and some work at a temperature over 500 ℃. The sealing failure of the flange connection not only directly affects the high-load operation of the device, but also causes considerable economic losses due to closing and maintenance.

 
In the face of harsh working conditions such as high temperature, high pressure, pressure and temperature fluctuations, the urgent requirements for the development of new sealing materials, optimization of flange connections, strength and sealing of flanges and pipelines are also higher and higher. As a result, countries, industry organizations and enterprises have attached great importance to the research on flange connections and sealing gaskets in the world. Therefore, understanding the working mechanism of the bolted flange connection, optimizing the relevant standards and regulations and designing bolted flange connections can not only meet the strength requirements but also have good sealing performance, which becomes an unfailing subject currently. Although the manufacture of most flanges has been standardized, there are still many problems in the analysis of designed flange connections. Researchers in many countries continue to study and the main difficulty in the research lies in the material nonlinearity and stress of gaskets, that is the strain time lag effect. The strength and sealing of bolted flange connections are inseparable, and they cannot be analyzed and studied separately. Their strength and sealing must be analyzed as a whole through the connecting part.
 
The influence of temperature on the sealing of flange connections doesn't mention in the current design specifications, especially the research on flange connections at high temperatures is quite rare. Therefore, it is of great significance to analyze the sealing of flange connections at high temperatures. The sealing study of the flange connecting system at high temperature is a problem of the thermal-solid coupled field. The influence of temperature on the change of the performance of each flange connection and coordination of deformation is very complicated. At present, numerical simulation methods are used to study the influence of temperature on the sealing of flange connections.