Bolted flanges connections - TBSS Method (Tight Between Seal and Strength)

The previous article in this series, “ [BM 5 572] ,” presented the fundamentals of mechanical modeling of bolted flange assemblies. In particular, it introduced the design assumptions used in the major codes and standards, as well as the analytical methods for evaluating stresses in the various components of the assembly—specifically, the Taylor Forge method for flange sizing. The concepts of effective joint area, bolt preload, and the main loads applied to the assembly were also reviewed.

Once these elements have been established, the central question becomes determining the tightening torque to be applied during assembly. This torque must be sufficient to ensure the joint is leak-tight, while remaining within the mechanical strength limits of the various components. In practice, the tightening torque for a bolted flange joint cannot therefore be chosen arbitrarily: it must fall within an acceptable range defined by two fundamental limits.

The lower limit corresponds to the minimum conditions necessary to ensure the joint is leak-tight. It is generally related to the seating pressure of the gasket, the maintenance of sufficient residual compression during service, or, in some cases, the presence of metal-to-metal contact or a minimum preload in the bolting system. Conversely, the upper limit is dictated by the mechanical strength of the assembly’s components: flange strength, compression limits of the gasket, and bolt strength.

The TBSS (Tight Between Seal and Strength) method is based on this approach. It involves expressing the various mechanical conditions of the assembly in terms of equivalent tightening forces in order to directly define an acceptable tightening range between the sealing and mechanical strength limits. The operational tightening torque then corresponds to a target value within this range, taking into account the uncertainties associated with tightening processes and the variation in the forces actually applied.

Beyond simply determining the tightening value, this method also allows for the integration of the various tightening methods used in practice (applied torque, bolt elongation control, hydraulic methods) as well as the uncertainties associated with assembly procedures. It thus provides a coherent framework for defining a robust and reproducible tightening value under industrial conditions.

This article presents the formulation of the TBSS method and the various tightening limits...