When is it safe to remove the old packing from a valve under pressure?
If you are referring to the stem packing, you should be able to
do it with no problem under pressure. Shut the valve off, take off
the handle, then the nut with the packing in it, remove the old
packing, replace it and reinstall the nut and handle. Open the
faucet completely and that should be it.. Some valves use a rubber
formed packing instead of the graphite or Teflon rope packing.
Generally you can use either in place of the rubber or use a small
amount around the stem with the old rubber.