A high-pressure flanged connection is a precision mechanical closed-loop system: bolts provide the clamping force, flanges transmit and amplify this force, and the gasket receives this force, deforming to block leak paths. Understanding the mechanical behavior of each element in this closed loop is fundamental to designing reliable high-pressure joints .
Gasket Stress: The Physical Threshold for Sealing
For any gasket to achieve a seal, the compressive stress it experiences must exceed two critical values: the initial “seating stress” during installation, which causes the gasket material to flow and fill the microscopic irregularities of the flange faces; and after internal pressure is applied, the residual stress on the gasket must always remain higher than the internal pressure multiplied by the “gasket factor” (m). This factor, varying with gasket type and material, is the core basis for selection.
Bolt Load: The Source of Force Input
Bolt preload is the sole energy input to the system. However, only about 10% of the reading on a torque wrench is ultimately converted into useful gasket compression force; the remaining 90% is consumed by thread friction and nut face friction. This is why using precise lubrication and methods like “torque-plus-angle” or hydraulic tensioners is far more reliable than torque-only control.
Flange Rigidity: The Distributor of Force
Under bolt force, flanges undergo slight bending deformation. If flange rigidity is insufficient, the edges will lift, causing excessive stress on the outer edge of the gasket and insufficient stress on the inner edge, creating a risk of “internal leakage.” The tapered hub design of weld neck flanges is precisely intended to increase root rigidity, distributing force more uniformly to the gasket .
Mechanical Evolution Under Thermal Cycling
When a system undergoes heating and cooling cycles, the mechanical balance initially established is disrupted due to differential thermal expansion coefficients of materials and bolt creep relaxation. Bolt load can decrease, and gasket stress decreases accordingly. This is the fundamental reason why periodic “hot torquing” or the use of disc springs is necessary in high-pressure, high-temperature services.
When procuring high-pressure flanges, do not only focus on static indicators. You must require suppliers to provide stiffness calculation results based on finite element analysis, along with fatigue assessment reports for the expected number of thermal cycles.
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