Bolted flanges connections - Mechanical model and calculation

Bolted flange assemblies ensure both mechanical continuity between two pieces of equipment and the leak-tight containment of a fluid subjected to varying conditions of pressure, temperature, and load. Their design is based on a structured functional approach, centered around three main roles, each fulfilled by a specific component (see [BM 5 571] ):

• the flanges, which hold the gasket in place and absorb mechanical forces;

• the gasket, which ensures the connection is leak-tight;

• the bolts, which apply and maintain the necessary contact pressure on the gasket.

The previous article in this series presented the characteristics and components of these assemblies. In particular, it described the main types of flanges and flange faces defined in the standards ( EN 1092-1 and EN 1759-1 ), as well as the different families of gaskets and their characteristic parameters m and y. These parameters, derived from the technical literature, provide standard values for analyzing assemblies when detailed manufacturer data is not available. The function of the bolts and the main components of clamping systems were also reviewed.

Once these elements have been defined, the central issue in the design of a bolted flange joint is to determine the clamping forces that simultaneously ensure the joint’s leak tightness and the mechanical strength of the various components. This problem boils down to identifying the loading conditions that ensure sufficient compression of the gasket, while remaining within the allowable limits of the flanges and bolting.

The calculation methods adopted by the major codes and standards ( RCC-M , EN 13480-3 and EN 13445-3 ) generally rely on a two-phase approach. The first phase involves determining the forces required to ensure the joint’s tightness, distinguishing between seating conditions during assembly and service conditions under pressure. The second phase aims to analyze the mechanical strength of the joint by evaluating the stresses induced in the flanges, the gasket, and the bolts....