LIVE LOADED VALVE STEM PACKING WITH SHAPE MEMORY ALLOY OPTION

摘要

Live loaded valve stem packing wherein grafoil packing (40) is molded into a metal cap (42) which not only prevents grafoil packing from extruding after it is compressed into the packing box (46), but also has the feature of live loading built into the metal cap. This metal cap can be made from two different materials. The first one is the conventional flat spring materials (17-7PH S.S. or 302 S.S.) used by the Belleville washer manufacturers. The second one is a Shape Memory Alloy {commonly known as SMA materials, e.g. Ni-Ti (Nickel-Titanium alloy) or Fe-Si-Mn (Iron-Silicon-Manganese alloy). The conventional or SMA material metal cap is designed with a singular hump (43), entirely along the top surface of the metal cap. On this hump, there are two slots (48), 180 degrees apart, through the hump for the purpose of removing the packing after it is installed. This hump is the live loading feature of the invention. Upon complete assembly of the packing box, the hump is elastically flattened out due to the controlled (bolt torque) axial loading through the fasteners (36). As the packing consolidates with the valve in service or the packing box components relax during thermal cycling, the hump deflects to maintain a relatively constant load on the packing. The invention optionally employs SMA material metal caps which "senses" increase in temperature and reacts to these changes by reverting back to it as manufactured state (in this case the hump deflects) to provide a constant or enhanced axial load on the packing so it can continue to provide its sealing function even during a fire.