_
valve stem-> |=|
| |
|--@ @--|
| | | |
---/ | | \---
( | | ) <-bellows
--- | | ---
) | | (
--- | | ---
( | | )
--- | | ---
) | | (
|--- | | ---|
| | | |
+---@----++| |++----@--+
| |x| |x| | <-valve body
| ++| |++ |
@ indicate welds.
X indicate valve packing
A bellows seal is used to create an impermiable seal around the valve bonnet to prevent the emission or leakage of the media in the valve to the surrounding atmosphere. A typical valve packing arrangement uses an elastomer under compression to prevent leakage. However, elastomers wear with use or can be easily damaged by media that have particulates. Although there is some elastomeric valve packing in a bellows seal arrangement, this is done to prolong the useful life of the bellows seal rather than prevent leakage.
Since the bellows is welded to the stem and the valve body, any leak path has to cross metal. Bellows seals are used on globe valves rather than rotary valves. The valve stem in a globe valve moves up and down and the folds in the bellows act like a spring to allow this motion.