o-ring vs plastic insert

[danice]

I am currently studying on sealing of ball valve. May I know when will we choose o-ring as sealing? and when will we choose plastic insert as sealing? and what is the diference between gasket and o-ring? thank you.

 

[hydromech]

Hey...

Gaskets are static seals that rely on a clamping force to seal.

O-rings are dynamic seals that rely on pressure acting on them in order to become energised and create an effective seal.

By dynamic, I mean they move/deform under hydraulic or pneumatic pressure. O-rings aren't very good in rotary or reciprocating applications as they can dry out and stick to the sealing surface. And being round they roll very easily.

If you imagine the section of an o-ring as a clock face. Pressure applied at the 3 o'clock position will cause the o-ring to deform and move in all direction. The housing behind it and if fitted the backup rings cause the seal to apply a proportional force in the 6 o'clock and 12 o'clock directions.

Adrian

 

[mqas]

Danice,

To add to the info provided by hydromech....

It is customary to use plastic inserts in the mating faces of the body on low pressure floating ball valves. In these designs the main seat which contacts the ball is also a sold plastc insert and I assume it is not this aspect of the design to which you are referring. This seat insert fits into a cavity in the body (seat pocket) and seals against side wall or rear wall of the seat pocket. It also seals against the ball. In these applications the material is normally PTFE because of its high and low temperature properties and chemical inertness. These designs can also use PTFE seals in the stem area.

In higher pressure designs (trunnion mounted), it is customary to use orings to seal between the body members. In these designs the plastic seat insert is normally retained in a metal seat holder and there is normally an oring on the metal seat which seals the seat holder against the body (seat pocket). Again the plastic insert retained in the seat holder is also used to seal against the ball. Also, the stem and trunnion seal would be oring type.