Flanged Joints: Bolt Load, Gasket Compression and Separation

Flange Geometry and Bolt Patterns

A flanged joint consists of two flanges — one on each pipe or vessel — a ring of bolts that clamps the flanges together, and a gasket that seals between the flange faces. The flange geometry includes the flange outer diameter, the bolt circle diameter (the pitch circle on which the bolts are arranged), the flange thickness, the hub (the transition from the pipe to the flange ring), and the gasket contact face (the raised face, flat face, or ring joint groove). The bolt pattern is typically a circular array of equally spaced bolts — the number and size of the bolts are determined by the pressure rating and the flange size. The bolt circle is positioned between the gasket and the flange outer diameter, and the bolt spacing must be close enough to maintain a uniform gasket compression around the circumference. A wide bolt spacing produces non-uniform compression — the gasket is more compressed at the bolts and less compressed between them. The flange geometry is standardised by codes (ASME B16.5, EN 1092) for standard pipe flanges, and custom flanges are designed by analysis (ASME Boiler and Pressure Vessel Code, EN 13445).

The flange geometry and bolt pattern are a system: the bolt circle position, the bolt spacing and the flange thickness all affect the gasket compression uniformity. Wide bolt spacing produces non-uniform compression — more at the bolts, less between them — which can cause leakage between the bolts.

Preload and Gasket Compression

The bolt preload compresses the gasket to a stress that is sufficient to seal the joint. The gasket compression stress must be high enough to deform the gasket into the flange face surface irregularities (seating the gasket) and to maintain the seal under the design pressure and temperature. The minimum gasket seating stress (y) and the gasket factor (m) — the ratio of gasket residual stress to the pressure — are gasket-specific properties defined in the flange design codes. The bolt preload must be sufficient to seat the gasket (achieve the y stress) and to maintain the gasket residual stress above m × P (the minimum operating stress) under all design conditions. The preload is distributed among the bolts, and each bolt contributes to the gasket compression through the flange bending. The gasket compression is not uniform — it is highest near the bolts and lowest between the bolts, and the flange rotation (see below) further non-uniformises the compression through the radial direction (inner edge vs outer edge).

The bolt preload must seat the gasket (achieve the minimum seating stress y) and maintain the residual gasket stress above m × P under all design conditions. The gasket compression is not uniform — it varies circumferentially (between bolts) and radially (inner edge vs outer edge).

Local Flange Bending and Flange Rotation

The flange ring bends under the combined action of the bolt preload (pulling the flange outward, toward the bolt circle) and the gasket reaction and pressure load (pushing the flange inward, toward the pipe axis). The bending causes the flange ring to rotate — the outer edge moves toward the mating flange and the inner edge moves away. This rotation has a critical effect on the gasket compression: it opens the gasket contact at the inner edge (the pressure-side edge) and concentrates the compression at the outer edge. The non-uniform gasket pressure from flange rotation is the primary cause of leakage in flanged joints — if the inner edge pressure drops below the minimum sealing stress, the joint leaks. The flange rotation is reduced by increasing the flange thickness (a thicker flange is stiffer and rotates less), by reducing the bolt circle diameter (less moment arm), and by using a stronger hub (the hub stiffens the flange ring). The flange design codes include formulas for the flange rotation and the resulting stress, but for critical flanges, FEA is used to compute the rotation and the gasket pressure distribution accurately.

Flange rotation opens the gasket at the inner edge and concentrates compression at the outer edge. If the inner edge pressure drops below the minimum sealing stress, the joint leaks. Reduce rotation with thicker flanges, smaller bolt circle diameter, or a stronger hub. For critical flanges, use FEA to compute the gasket pressure distribution.

Pressure Loading and Separation

The internal pressure exerts an axial force on the flange — the pressure acting on the end of the pipe or vessel creates an end force that tries to separate the flanges. This end force is F_end = P × A_internal, where P is the internal pressure and A_internal is the internal cross-sectional area at the gasket. The end force unloads the gasket (reduces the gasket compression) and increases the bolt load (the bolts must resist the end force). The gasket residual stress under pressure is (F_preload − F_end) / A_gasket, where A_gasket is the gasket contact area. If the end force exceeds the preload, the gasket unloads completely, the joint separates, and the joint leaks. The flange design must ensure that the residual gasket stress under the design pressure is above the minimum sealing stress (m × P). The separation risk is highest at the design pressure combined with the external piping loads (see below) — the combined loading can unload the gasket on one side of the flange even if the average gasket stress is adequate.

  Pressure end force:

  F_end = P × A_internal

  Gasket residual stress under pressure:

  σ_gasket = (F_preload − F_end) / A_gasket

  Sealing requirement:

  σ_gasket ≥ m × P   (gasket factor × design pressure)

  →  If F_end > F_preload, the joint separates and leaks
  →  Check the residual gasket stress at the design pressure

External Piping Loads

External piping loads — axial forces, bending moments and torsion transmitted from the connected piping — add to the pressure loading and can significantly affect the bolt load and the gasket compression. An axial tensile force from the piping adds to the pressure end force and unloads the gasket further. A bending moment from the piping (thermal expansion, weight, seismic) loads the bolts on one side of the flange in tension and unloads the bolts on the other side — the bolt load is non-uniform around the circumference. The gasket compression is correspondingly non-uniform: more compressed on the side where the bolts are loaded in tension, less compressed (or open) on the side where the bolts are unloaded. The torsion from the piping adds a shear load to the bolts, which must be checked in combination with the tension. The external piping loads are often the governing load case for flange design — a flange that is adequate for the pressure alone may be inadequate when the piping loads are included. The flange must be designed for the combined loading: pressure plus axial force plus bending moment plus torsion, with the bolts checked at the maximum combined load and the gasket checked at the minimum residual stress (on the unloaded side).

External piping loads — axial force, bending moment and torsion — are often the governing load case for flange design. A flange adequate for pressure alone may be inadequate with piping loads. The bending moment non-uniformises the bolt load and the gasket compression — check the bolts at the maximum combined load and the gasket at the minimum residual stress.

Bolt Load Redistribution

In a flanged joint under combined pressure and bending, the bolt load is not uniform around the circumference. The bolts on the side of the flange where the bending moment adds to the pressure (the tension side) carry more load; the bolts on the opposite side (the compression side) carry less. The redistribution depends on the flange stiffness: a stiff flange distributes the load more evenly (the flange ring spans between the bolts and shares the load), while a flexible flange concentrates the load at the individual bolts. The bolt load redistribution also depends on the gasket stiffness: a stiff gasket transfers more load to the bolts on the tension side, while a compliant gasket absorbs some of the load. The maximum bolt load — at the tension-side bolt — must not exceed the bolt allowable, and the minimum bolt load — at the compression-side bolt — must be sufficient to maintain the gasket compression above the sealing stress. The redistribution is a three-dimensional problem that is best analysed with FEA — the flange, the bolts, the gasket and the pressure are all included, and the bolt loads and gasket pressures are extracted at each bolt and at each gasket segment.

Gasket Stiffness and Nonlinear Contact

The gasket stiffness is nonlinear. The gasket material (compressed fibre, spiral-wound, metal ring, PTFE) has a load-compression curve that is stiffer in compression than in unloading — the gasket does not fully recover when the load is removed. The nonlinearity is important because the gasket stiffness affects the bolt load redistribution and the flange rotation. A stiffer gasket (high compression stiffness) attracts more load from the bolts and increases the bolt load; a softer gasket absorbs the load and reduces the bolt load. The nonlinear gasket behaviour also affects the operating cycle: during the first pressurisation, the gasket stiffens as it is further compressed by the pressure end force; during depressurisation, the gasket does not fully recover, and the residual compression is lower than the initial. This means the joint may seal on the first pressurisation but leak on subsequent cycles — the gasket has taken a compression set. The gasket nonlinearity must be included in the flange analysis for accurate prediction of the bolt load and the gasket pressure. The gasket manufacturer provides the load-compression curve, which is input to the FEA as a nonlinear material model or a nonlinear contact pressure-overclosure relationship.

Gasket stiffness is nonlinear — stiffer in compression than in unloading. The gasket does not fully recover when the load is removed, and the residual compression after a pressure cycle can be lower than the initial. This can cause a joint to seal on the first pressurisation but leak on subsequent cycles. Include the gasket nonlinearity in the flange analysis.

Thermal Effects

Temperature changes affect the flanged joint through differential thermal expansion and through changes in the gasket and bolt material properties. If the bolts and the flange have different coefficients of thermal expansion, a temperature change produces a differential expansion that changes the bolt preload — the same mechanism as in a general bolted joint (Article 05). A steel bolt in a steel flange at elevated temperature gains or loses preload depending on the relative expansion and the joint stiffness. The gasket material properties also change with temperature — the gasket may creep (stress relaxation) at elevated temperature, reducing the gasket compression and the preload. The flange material properties (yield strength, elastic modulus) decrease at elevated temperature, reducing the flange stiffness and increasing the flange rotation. The combined thermal effects can be significant: a flange that seals at room temperature may leak at elevated temperature because the gasket has relaxed, the bolt preload has changed, and the flange rotation has increased. The flange design must include the thermal effects — the gasket stress, the bolt load and the flange rotation must be checked at the design temperature, not just at ambient.

Thermal effects can cause a flange that seals at room temperature to leak at elevated temperature. The gasket creeps (relaxes), the bolt preload changes with differential expansion, and the flange rotation increases as the flange material softens. Check the gasket stress, bolt load and flange rotation at the design temperature.

Modelling Methods for Flanged Joints

The analysis of flanged joints ranges from the code formula method to full three-dimensional FEA. The code method (ASME Boiler and Pressure Vessel Code, Section VIII, Appendix 2; EN 13445) uses simplified formulas for the flange stress, the gasket stress and the bolt load, based on the flange geometry, the gasket properties and the design pressure. The code method is the standard approach for standard flanges and is required for code-stamped vessels. The axisymmetric FEA method models the flange as an axisymmetric structure (the flange and the gasket are axisymmetric, the bolts are smeared into an equivalent axisymmetric load) — this captures the flange rotation and the gasket pressure distribution in the radial direction but not the circumferential variation from the bolt spacing or the external bending. The full three-dimensional FEA method models the flange, the bolts, the gasket and the pressure with solid elements, including the individual bolts, the bolt holes, the gasket contact and the non-uniform loading from the external piping. The 3D FEA captures all the effects: the flange rotation, the circumferential bolt load variation, the radial gasket pressure distribution, the nonlinear gasket behaviour and the thermal effects. The 3D FEA is used for critical flanges, custom flanges and flanges with complex loading — but it requires significant modelling effort and computational cost.

The code formula method is the standard for code-stamped vessels. Axisymmetric FEA captures the radial behaviour. Full 3D FEA captures everything — including the circumferential bolt load variation from external bending — and is used for critical flanges with complex loading. Select the method that captures the relevant effects at the minimum cost.

Key takeaways

• A flanged joint is a system: the bolts, the flanges, the gasket and the pressure load all interact. The gasket must remain compressed under all design conditions — pressure, external loads, thermal — or the joint leaks.
• Flange rotation is the bending of the flange ring under bolt preload and pressure loading. The rotation opens the gasket contact on the inner edge and concentrates the compression on the outer edge — a non-uniform gasket pressure that can cause leakage if the inner edge pressure drops below the minimum sealing stress.
• External piping loads — axial forces, bending moments and torsion from the connected piping — add to the pressure loading and can significantly increase the bolt load on one side of the flange while reducing it on the other. The flange must be designed for the combined loading, not just the pressure.
• Gasket stiffness is nonlinear and depends on the compression history. The gasket stiffens as it is compressed, and it does not fully recover when the load is removed. The nonlinear gasket behaviour must be included in the flange analysis for accurate bolt load and gasket pressure prediction.