Documents - EN1591 Foreword

EN 1591 consists of two parts:

  • EN 1591-1 Flanges and their joints – Design rules for gasketed circular flange connections – Part 1: Calculation method
  • ENV 1591-2 Flanges and their joints – Design rules for gasketed circular flange connections – Part 2: Gasket parameters

The Calculation method satisfies both leaktightness and strength criteria. The behaviour of the complete flanges-bolts-gasket system is considered. Parameters taken into account include not only basic ones such as:

  • fluid pressure;
  • material strength values of flanges, bolts and gaskets;
  • gasket compression factors;
  • nominal bolt load;

but also:

  • possible scatter due to bolting up procedure;
  • changes in gasket force due to deformation of all components of the joint;
  • influence of connected shell or pipe;
  • effect of external axial forces and bending moments;
  • effect of temperature difference between bolts and flange ring

Calculation for sealing performance is based on elastic analysis of the load/deformation relations between all parts of the flange connection, corrected by a possible plastic behaviour of the gasket material. Calculation for mechanical resistance is based on (plastic) limit analysis of the flange-shell combination. Both internal and external loads are considered. Load conditions covered include initial assembly, hydrostatic test, and all significant subsequent operating conditions. The calculation steps are broadly as follows:

  1. First, the required minimum initial bolt load (to be reached at bolting-up) is determined, so that in any subsequent specified load condition, the residual force on the gasket will never be less than the minimum mean value required for the gasket (value is gasket data from ENV 1591-2, for instance). The determination of this load is iterative, because it depends on the effective gasket width, which itself depends on the initial bolt load.
  2. Then, the internal forces that result from the selected value of initial bolt load are derived for all load conditions, and the admissibility of combined external and internal forces is checked as follows:
  • bolting-up condition: the check is performed against the maximum possible bolt force that may result from the bolting-up procedure;
  • test and operating conditions: checks are performed against the minimum necessary forces, to ensure that the connection will be able to develop these minimum forces without risk of yielding, except in highly localized areas.
    Higher actual initial bolting results in (limited) plastic deformation in subsequent conditions (test, operation) But the checks so defined assure that these deformations will not reduce the bolt force to a value less than the minimum required.

If necessary, the flange rotations may be estimated in all load conditions, using annex E, and the values obtained, compared with the relevant gasket limits which could apply.

Checks for admissibility of loads imply safety factors which are those applied to material yield stress or strength in the determination of the nominal design stresses used in the Calculation method.

NOTE: Where flanges are used to comply with other codes the Calculation method does not specify values for nominal stresses.
Nevertheless, since all significant design parameters are accounted for, the use of low safety factors is made possible by special use of nominal design stresses:

  • for assembly conditions the nominal design stresses have the same values as for the hydraulic pressure tests (normally higher than for operating conditions);
  • the nominal design stresses for the bolts are determined by the same rules as relevant for the flange and shell material e.g. same safety factor on yield stress.

The minimum force required on the gasket for leak tightness considerations may be established by two different ways:

  1. Use of tabulated gasket factors, for example those given in ENV 1591-2, which are based on industrial experience and correspond to mainly gas and steam leak rates.
  2. Derivation from measured leak rate versus gasket stress data, if available for the gasket, for example as in ENV 1591-2. This permits design to be based on any specified maximum leak rate.

The use of this Calculation method is particularly useful for joints where the bolt load is monitored when bolting up.
The greater the precision of this, the more benefit can be gained from application of the Calculation method.


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