Gasket parameters – Part III – Gasket factors Qsmin(L), Qsmax, Qmin(L), PQR and EG
The approval of the new standard EN 1591-1 – ”Flanges and their joints – Design rules for gasketed circular flange connections – Part I: Calculation method” – is done within the framework of Directive 97/23/CE PED which requires the standardization of everything related to pressure equipment. The norm defines the calculation method for gasketed circular flang joints and this calculation requires certain gasket parameters. Thus, EN 13555 provides the test procedure to determine the new generation of parameters used in EN 1514-1 equations.
These rules form the basis of Appendix G of EN 13445-3, representing an alternative to the method in Chapter 11 of the same norm. In 2013 a new version of 1591 was approved.
The test methods in EN 13555 include loading and unloading cycles in order to simulate service conditions. In service the gasket will be loaded during the tightening stages of the assembly process, but as the pipe line is pressurised, the gasket surface pressure will reduce due to the hydrostatic end thrust created by the pressurised fluid in the pipeline.
| Qmin(L) | – Minimum gasket surface pressure on assembly required at ambient temperature in order to seat the gasket into the flange facing roughness and close the internal leakage channels so that the tightness class is to the required level L for the internal test pressure [MPa].
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| QS min(L) | – Minimum gasket surface pressure required under service pressure conditions, so that the required tightness class L is maintained for the internal test pressure [MPa].
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| QS max | – Maximum gasket surface pressure that may be imposed on the gasket at the indicated temperatures without collapse or compressive failure of the gasket [MPa].
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| EG | – The unloading modulus determined from the thickness recovery of the gasket between the initial compression surface pressure and unloading to a third of this initial surface pressure [MPa].
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| PQR | – A factor to allow for the effect on the imposed load of the relaxation of the gasket between the completion of bolt up and after long term experience of the service temperature.
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| LN | – Tightness classes – the L index indicates the maximum leak rate for the tightness class [mg s-2 m-2]. |
The tightness classes are also defined. The tightness class implies the continuous compliance of a maximum leakage rate. The series of tightness classes can be continued for better tightness classes and lower leak rates.
| Tightness classes | L1,0 | L0,1 | L0,01 |
| Specific leak rates [mg/s∙m] | ≤1,0 | ≤0,1 | ≤0,01 |
Currently, this is the calculation method recommended by international sealing entities, as ESA – European Sealing Association and FSA – Fluid Sealing Association.