What are ferrules used for in plumbing?

Answer 1

A ferrule is used on a compression type joint. Care should be taken to properly tighten the nut, but typically it goes between the nut and a fitting. You should use plastic ferrules on plastic pipe but also be sure to use a brass or stainless sleeve inside the open end of the pipe so you don't crush it and it maintains therigidity of the pipe. You should use a brass ferrule on copper pipe which are stronger and give a better squeeze on the pipe sealing the joint. Usually hand tight plus a 1/2 turn is good to start with. water test the fitting and if it leaks you can always shut it back off and tighten more. One rule of Plumbing is tight is tight, too tight is broke!

There is no need to apply pipe sealant, pipe dope, or Teflon tape to the threads of the compression nut. None of these things will hurt, but they won't help either, since the seal depends ONLY on how well the ferrule is compressed against the pipe and the taper of the fitting. If the fitting leaks, it is because something is wrong. You could have over- or under-tightened the nut, the ferrule might have been damaged, the taper might have a nick in it, etc.