Many products from IMI-Flow Design have an integrated ball valve for isolating the terminal. Under the handle of these ball valves there's a nut. This is called the "packing nut".
Concealed under the packing nut is... wait for it... packing. Specifically, there's a piece of PTFE which is squeezed by the nut to form a seal against the body and stem.
Here the packing is shown un-deformed, overlapping the bottom portion of the packing nut. In the actual valve it re-shapes itself and flows as the nut is tightened.
O-rings are great. As can be seen in the cross-section, we use one of them on our ball valve stem. Unfortunately all elastomers have an Achilles heel: it's always possible for some combination of temperature and chemistry to make them fail. PTFE is very resistant to heat and chemicals, but it isn't "rubbery". Thus the PTFE makes it possible to stop any stem leak, but the nut must be tightened to squeeze it against the surfaces. If the PTFE gets worn from turning the handle many times, the nut can simply be tightened a little more.
It's important to note in the cross section above that the stem on IMI-Flow Design ball valves is "blowout proof". That is, there's no way that someone can loosen something and have the stem come out. This is an important safety feature which is widely accepted, but some of our competition has introduced valves lacking this feature.
The end connection design can also be seen. Again an o-ring is used to provide the primary seal. Again there is a secondary seal: in this case it is metal to metal. This end connection allows IMI-Flow Design to customize a valve so that both ends match their mating devices directly with no adapters. The dual seals on this end connection ensure that it will not leak.