Ingolf's insight: Part 3

摘要

Do end-users really care about fugitive emission, or do they only want cheaper valves? So what about high density die formed graphite packings? To answer that, we have to take a look back to Part 2 of this article. The main issue of getting a good seal is contact between the packings and the spindle/bonnet before the compression of the packing rings starts. This is more important with high-density rings since they have a lower compression grade (10-12%). There are other elements besides the packing rings that have to be addressed, and that is the shape, dimensions and the tolerances of the spindle and packing groove in the valve bonnet If we look at Figure 12, this is a graph from one packing manufacturer This graph shows us some important details: The surface finish on the stem should be Ra ≤ 0,5 μm and the surface finish in the groove should be Ra ≤ 2,0μm. This is understandable because the stem is a dynamic seal, and the wall of the groove is a stationary seal. We don't want a coarse stem that tears away the packing material when operated. The depth of the groove L is approx. five times S, which is the cross-section of the packing. Then, one very important small detail: The distance between the stem/bonnet and stem/packing compression ring, indicated with two red arrows must not be more then 0,03 millimetre times S (cross-section of the packing). This is extremely important when dealing with high density Grafoil rings. If the distance between the stem and the packing compression ring is too great, you may end up with packing extruded by the system pressure as shown in Figures 13 and 14. Both packings used are high density Grafoil rings in high pressure gas applications. Both valves had a clearing between the spindle and the packing compression ring that was way too large. Both cases led to major gas leaks and total shut down of the plants. These are only two examples of a situation that, unfortunately, is not totally uncommon. These were major gas leaks, not fugitive emission, you could say. But there is a link here, a link between tolerances, packing material, packing treatment and small or larger leaks.