Where the valve shaft comes out the top of the shutoff valve on a steam system, the cottony/fabricy gasket underneath the retention nut has deteriorated and allows steam through. What is this material, and what can I use as a new seal to prevent the steam escaping.
Most hardware stores carry a teflon string-like material that you can twist around the valve shaft under the packing nut that will seal the nut and shaft hole when you tighten it down. Or you can take the packing nut itself to the hardware and they can fit a seal-washer inside the nut. They come in many different sizes. Shut-down the system and take off the handle and remove the nut.