Hi pdp123,
One of the reason, Pressure Seal was created in order to minimize the reduce the weight of conventional BoltedBonnet (BB) valve and or WeldedBonnet version.
Especially in BB type, normally the Bonnet is heavier to provide additional sufficient Seating Stress for BB gasket. Of course the main contributor is the torque figures of BB bolts.
Pressure Seal main concept is Bonnet Sealing against Plate (first picture) and or some sort of Wedge Ring (second picture). Those two components by means of internal pressure pushed against the Bonnet
If Pressure low enough (relative depending on design), there will not be enough force to "push to seal" the Plate or Wedge Ring against the Bonnet. And since most design have hole(s) between Bonnet and this sealing component (in order to make ambient pressure in between / positive pressure from internal), sometimes during StartUp/about to ShutDown/ or even during normal Operation its prone to leaking externally (not enough pressure to "push to seal").
Regarding your three main word
a. Steam, I cannot say much about this. Used both in Steam, Hydrogen, and other gaseous service. relatively High pressure (above Class 600#)
b. Clean and Non-corrosive. Corrosion attack from both internal and external side (either Plate or wedge type). Since it have holes, it is prone for (rain) water to settle on top of the Plate / Wedge and due to oxidation it will start corroded, cracking and so on. This is normally for wedge and or Plate that is made from Carbon steel and without heat tracing (if its heated, rain will evaporate spontaneously).
Some other point of view:
- Plate type normally comes in section which were welded. this is also prone for earlier oxidization, etc.
- Fatigue is a big issue, stressed (full pressurized) and non-stressed (not fully pressurized) condition on Wedge Ring and or Plate will end its lifetime after hundreds or sometimes thousands of cycle.
- For some design reason, the 8" and 6" model (I had with Gate valves) refer to FEA analysis and real life Plant condition more prone to faster fatigue cycle.
PS: At the end of the day, the weight is nor really a big difference for Piping to support compare to BB type. And for 25 years expected lifetime (for the same valve), I personally prefer Welded Bonnet.
Shall pressure Seal selected, ask vendor to provide FEA (and fatigue) analysis and om which pressure it is subject for leaking.
Regards,
MR
All valves will last for years, except the ones that were poorly manufactured; are still wrongly operated and or were wrongly selected
