Effect of Surface Characteristics on Compressive Stress and Leakage Rate in Gasketed Flanged Joints

Abstract

The effect of surface characteristics on compressive stress and leakage rate in gasketed flanged joints is discussed qualitatively and quantitatively based on experimental verifications. The sensitivity of the sealing phenomenon to the sealing surface characteristics, gas pressure and stress level is presented. Sealing surfaces produced with grinding, turning and milling procedures of different roughness values were the subject of the tests with three types of gasket, namely, PTFE (polytetrafluoroethylene), graphite, and spiral wound. The experimental results indicated that the roughness value Ra (average arithmetic height) has no effect on the sealing performance of the gasket, except for the ground type sealing surfaces. The platen (flange) surface forms were found to be the determining factor on the leakage rate and flow regime for gasketed flanged joints. It is shown that the surface characteristics have no effect on the leakage rate and on the gasket performance for the PTFE gasket at high stress levels. However, at low stress levels the rougher ground and milled sealing surfaces having radial channels on the surface produce larger leakage rates. The effect of surface characteristics, stress level, and gas pressure was more evident on the graphite and spiral wound gasket types, at both low and high stress levels. It is also shown that the leakage rate follows an exponential law as a factor of surface stress and gas pressure, for all sealing surface characteristics and gasket types. While there is little difference in the leakage rate for the PTFE gasket when the gasket stress level is increased by a factor of two, from S3 to S5, a substantial leakage rate reduction by a ratio of 17.5 times was observed for the graphite and spiral wound gaskets for most sealing surfaces. On the other hand, when increasing the helium gas pressure from 200 to 800 p.s.i., the leakage rate increased by 5 times for the PTFE, and up to10 times for the graphite and the spiral wound gaskets. It was determined that while molecular flow regimes can be achieved for the PTFE gasket for all sealing surfaces, the two other gasket types provided laminar flow under the same operating conditions.