Optimization of Stuffing Box Stem Sealings in Valves by Means of Surface Treatment and Coating

The optimization of the frictional behavior of valve stems in stuffing box sealing systems by means of stem surface treatment and coating is subject of a research project carried out at MPA Stuttgart. Low friction should lead to a high compression and densification of the packing already during assembly resulting in a reduced drop of packing stress with service time and improved tightness. Several surface treatment and coating technologies (Nitrogen and Boron Hardening, inductive coat, Si- and Me-DLC, AlTiN, Chromium-Nitride Multilayer, Tungsten-Carbide and Chromium-Carbide) were examined. Friction tests were carried out at 400 °C followed by leakage tests (160 bar, test fluid Nitrogen) on simulated stuffing box sealings with usual graphite packings and coated or surface treated stems in comparison to a “standard” stem without coating and surface treatment. These combined friction and leakage tests were accompanied by mechanical, technological and metallographical investigations. Visual inspection of the stems after the friction tests suggests a classification in 3 categories depending on the graphite adhesion to the stem surface. Some coatings caused a deterioration (increase of leakage rate) compared to the standard stem (without any coating or surface treatment). In the other cases the leakage rates were comparable to that of the standard stem. Most favorable behavior was observed for the stem with inductive coat. This technology can be seen as a repair technology for damaged stems.