Determination of Compressive Stress to Ensure Plastic Deformation of Octagonal Rings of Different Metallurgy

Abstract This paper is a follow up to PVP2024-124123, which identified inaccuracies in calculation spreadsheets used in the North American refining industry to calculate the torque values for octagonal ring joints, and set forth a more accurate computational model. However, that paper did not suggest an appropriate target seating stress for octagonal ring joints. To fill this void, this paper reports on in-house testing performed on the most commonly used ring joint gasket materials to identify the deflection point at which plastic deformation begins to increase. Because sealing at a metal-to-metal interface only occurs with plastic deformation of the softer material, this paper investigated how this attribute changed as a function of ring material and hardness. It also examined the relationship between this point of plastic deformation and the Minimum Yield Strength of the material, to see if the Minimum Yield Strength could be a useful proxy in our model. The goal of this data collection is to set forth general guidelines for the minimum gasket stress that should be applied on octagonal rings – based merely on these mechanical properties. Further validation testing will be conducted to determine the additional stress (delta S) that is required to achieve reliable sealing.