The EN 1591-1: 2013 standard, which defines algorithm for calculating clamping forces and stresses in a flange connection, requires the knowledge of the friction coefficient/factor between the gasket and flange faces. In real conditions the flange joint is not only affected by the internal pressure of the vessel and forces causes by thermal expansion of joints elements. There are also external forces and bending moments acting on the flange connection (like forces coming from the elements to which the flange joint is fitted - such elements are for example pipes or walls of tanks). These external loads acting on the flange joint may cause significant gasket displacement or even lead to scenario that the gasket slips out from between flange surfaces, which altogether may lead to further negative failures eventually. In order to prevent from such an unwanted situation, it is necessary to include during the calculation process, the input values ??of the forces, external moments and the friction coefficient µG.

The value of friction factor coefficient is determined in our laboratory on the PowerPress #1 test stand with usage of special Friction Factor testing device. This special Friction Factor device is designed in accordance with the provisions of the EN 13555 test standard. For testing purpose, it is placed together with two gaskets samples between the seating surfaces of the test stand. This device, as well as the flange surfaces, is also capable of warm up the seals to the test temperature. The surface pressure acting on the gaskets is initially equal to the QA value, which is the initial value of the gasket clamp - the “installation load value”. Then, the surface pressure is reduced to the “minimum load value” Qsmin (L), which value is determined in the previously conducted tightness tests of the tested gasket. Such a testing procedure simulates the operation of a flange connection in real installation conditions. Loads decrease from the installation load to the minimum load still ensuring the assumed tightness.

The value of the friction coefficient is calculated according to the appropriate formula. Moreover, the µG value depends on: gasket material / type, test temperature, roughness of the contacting surfaces. The PowerPress #1 test stand with additional friction testing equipment can operate at temperatures up to 500°C.

Above method to determine Friction Factor µG is applicable to all flat types of gaskets dedicated for bolted flange joints. These are: solid metal gaskets, metal&soft material gaskets and soft material gaskets.

It does not apply however to gaskets installed by putting inside deep grooves (such as RTJ) or inside especially prefabricated conical surfaces (such as Lens gasket). Usage of special RTJ or Lens constructions eliminate risk that gasket could slips out from between flange surfaces. That is also why in the algorithm of the EN 1591-1: 2013 standard, such gaskets as RTJ or Lens are not subject to the calculation of the load force needed to keep it between the flanges. Thus, it is not required to know the value of the friction coefficient.