1 Scope

1.1 General

This European Standard defines a Calculation method for bolted, gasketed, circular flange joints. Its purpose is to ensure structural integrity and control of leaktightness. The following equations use gasket parameters based on definitions and test methods specified in EN 13555.

1.2 Requirement for use of the Calculation method

Where permitted, the Calculation method is an alternative to design validation by other means e.g.

– special testing;
– proven practice;
– use of standard flanges within permitted conditions.

1.3 Validity

1.3.1 Geometry

The Calculation method is applicable to the configurations having:

— flanges whose section is given or may be assimilated to those given in Figures 4 to 12;
— four or more identical bolts uniformly distributed;
— gasket whose section and configuration after loading can be assimilated by one of those given in Figure 3;
— flange dimension which meet the following conditions:

a) 0,2 ≤ b_F/e_F ≤ 5,0; 0,2 ≤ b_L/e_L ≤ 5,0
b) e_F ≤ max {e₂; d_B0; p_B x { e 2 ; d B 0 ; p B × ( 0 , 01 … 0 , 10 ) × p B / b F 3 }
c) cos φ ≥ 1 / ( 1 + 0 , 01 d s e s )

NOTE For explanations of symbols see clause 3.

NOTE The condition b_F/e_F ≤ 5,0 need not be met for collar in combination with loose flange.

NOTE The condition e F ≥ p B × ( 0 , 01 … 0 , 10 ) p B b F 3 is for limitation of non-uniformity of gasket pressure due to spacing of bolts. The values 0,01 and 0,10 are to be applied for soft (non-metallic) and hard (metallic) gaskets respectively. A more precise criterion is given in annex A.

NOTE Attention may need to be given to the effects of tolerances and corrosion on dimensions; reference should be made to other codes under which the calculation is made, for example values are given in EN 13445 and EN 13480.

The following configurations are outside the scope of the Calculation method:

– flanges of essentially non-axisymmetric geometry, e.g. split loose flanges, web reinforced flanges;
– flange connections having direct or indirect metal to metal contact between flanges inside and/or outside the gasket, inside and/or outside the bolt circle, except the special case of spacer-seated flanges, which is covered in annex G.

1.3.2 Materials

Values of nominal design stresses are not specified in this Calculation method. They depend on other codes which are applied, for example these values are given in EN 13445 and EN 13480.

Design stresses for bolts are to be determined as for flanges and shells. The model of the gaskets is modelled by elastic behaviour with a plastic correction.

For gaskets in incompressible materials which permit large deformations (for example: flat gaskets with rubber as the major component), the results given by the Calculation method can be excessively conservative (i.e. required bolting load too high, allowable pressure of the fluid too low, required flange thickness too large, etc.) because it does not take account of such properties.

1.3.3 Loads

This Calculation method applies to the following load types:

– fluid pressure: internal or external;
– external loads: axial forces and bending moments;
– axial expansion of flanges, bolts and gasket, in particular due to thermal effects.

1.3.4 Mechanical model

The Calculation method is based on the following mechanical model:

a) Geometry of both flanges and gasket is axisymmetric. Small deviations such as those due to a finite number of bolts, are permitted. Application to split loose flanges or oval flanges is not permitted.
b) The flange ring cross-section (radial cut) remains undeformed. Only circumferential stresses and strains in the ring are treated; radial and axial stresses and strains are neglected. This presupposition requires compliance with condition1.3.1a).
c) The flange ring is connected to a cylindrical shell. A tapered hub is treated as being an equivalent cylindrical shell of calculated wall thickness, which is different for elastic and plastic behaviour, but always between the actual minimum and maximum thickness. Conical and spherical shells are treated as being equivalent cylindrical shells with the same wall thickness; differences from cylindrical shell are explicity taken into account in the calculation formula.

This presupposition requires compliance with1.3.1c).

At the connection of the flange ring and shell, the continuity of radial displacement and rotation is accounted for in the calculation.

d) The gasket contacts the flange faces over a (calculated) annular area. The effective gasket width (radial) bGe may be less than the true width of gasket. This effective width bGe is calculated for the assembly condition (I = 0) and is assumed to be unchanged for all subsequent load conditions (I = 1,2 ...). The calculation of bGe includes the elastic rotation of both flanges as well as the elastic and plastic deformations of the gasket (approximately) in assembly condition.
e) The modulus of elasticity of the gasket may increase with the compressive stress Q on the gasket. The modulus of elasticity is the unloading elasto-plastic secant modulus measured between 100 % and 33 % for several gasket stress levels. The calculation method uses the highest stress (Q) in assembly condition.
f) Creep of the gasket under compression is approximated by a creep factor PQR.
g) Thermal and mechanical axial deformations of flanges, bolts and gasket are taken into account.
h) Loading of the flange joint is axisymmetric. Any non-axisymmetric bending moment is replaced by an equivalent axial force, which is axisymmetric according to equation (44).
i) load changes between load conditions cause internal changes of bolt and gasket forces. These are calculated with account taken of elastic deformations of all components. To ensure leaktightness, the required initial assembly force is calculated (see 5.4) to ensure that the required forces on the gasket are achieved under all conditions (see 5.3 and 5.5).
j) load limit proofs are based on limit loads for each component. This approach prevents excessive deformations. The limits used for gaskets, which depend on Qmax are only approximations.

The model does not take account of the following:

k) Bolt bending stiffness and bending strength. This is a conservative simplification. However the tensile stiffness of the bolts includes (approximately) the deformation within the threaded part in contact with the nut or threaded hole (see equation (34)).
l) Creep of flanges and bolts.
m) Different radial deformations at the gasket (this simplification has no effect for identical flanges).
n) Fatigue proofs (usually not taken into account by codes like this).
o) external torsional moments and external shear loads, e.g. those due to pipework.

BS EN 1591-1:2001+A1:2009

Flanges and their joints. Design rules for gasketed circular flange connections - Calculation method

Published:

15 Sep 2001

Withdrawn:

31 Dec 2013