Investigation and surface analysis on metal-to-metal sealing in globe valves

This thesis presents a detailed investigation into the sealing performance of metal-to metal globe valves, with a focus on how surface topography and contact mechanics influence leakage. The project work was done in collaboration with Meson Group AB. In industrial systems operating under high pressure, high temperature, and corrosive conditions, elastomeric seals tend to degrade quickly. As a result, metal-to-metal sealing solutions are preferred in such environments due to their durability and resistance to mechanical, thermal, and chemical stress. However, the success of these seals relies heavily on the interaction between hardened metallic surfaces, and particularly on their micro-scale topography and deformation behavior. The study involved a three-stage experimental approach. In the first stage, disc and seat surfaces with different finishes-no lapping, coarse grinding, and precision grinding were analyzed using Coherence Scanning Interferometry (CSI). In the second stage, valves from different manufacturers were examined. In the third stage, two full-scale DN80 valves, labeled sample D and sample E, were subjected to hydrostatic pressure testing to observe leakage behavior, followed by detailed surface characterization. Key areal surface parameters including Sq, Spk, Svk, Sk, and Sdr were extracted and analyzed using MountainsMap® software, applying Robust Gaussian filtering in accordance with ISO 25178. The results showed that lower surface roughness does not always correlate with better sealing performance. In particular, the valve sample D, which exhibited higher surface roughness, demonstrated lower leakage than sample E, likely due to better plastic deformation and real contact area under load. These findings emphasize that surface roughness alone is not a sufficient predictor of sealing quality. Factors such as material behavior, peak morphology, and applied torque are equally critical. The thesis provides practical recommendations for valve manufacturers regarding surface preparation, torque control, and quality assessment techniques to optimize the performance and reliability of metal-seated globe valves.