A sealing system in the valve body which prevents the process fluid coming out through the valve stem. Commonly used gland packing in PDO are Teflon, Graphite Asbestos…etc
Teflon is the better of the two with regard to fluid sealing, stem friction, and stem wear also quite resistant to attack from a wide variety of chemical substances. Unfortunately, it has a limited temperature range and cannot withstand intense nuclear radiation (making it unsuitable for use near reactors in nuclear power plants). Graphite is another self-lubricating packing material, and it has a far greater temperature range than Teflon as well as the ability to withstand harsh nuclear radiation, but creates much more stem friction than Teflon. Graphite packing also has the unfortunate property of permitting galvanic corrosion between the stem and bonnet metals due to its electrical conductivity.