Plant Services - August 2008 - (Page 47) Safety Valves some other mechanism, usually made of PTFE, which contains the O-ring under high pressure. This back-up ring reduces the extrusion gap of the O-ring gland and thereby keeps the O-ring contained. If the O-ring is permitted to extrude beyond specific limits, it might be sheared during actuation. The O-ring design is highly effective at high pressure. In terms of temperature, pressure and chemical attack, the design is limited by the properties of the elastomer. Pressure works for you Stem misalignment Beyond issues relating to stem seal design, there are some additional causes of stem leaks. These have to do with stem alignment. If for any reason the stem becomes tilted or forced to one side, there might be uneven wear on the stem seal, resulting in leakage. There are two basic causes of misalignment. In the first, misalignment might result from improper actuator installation. If the actuator and stem centerlines aren’t properly aligned, the stem tilts or skews, resulting in uneven stem seal wear. In the second case, damage to the seat seal inside the valve might cause the stem to tilt. To understand this issue, we must first review basic ball valve anatomy. Ball valves can have either a floating or trunnion ball design. Figure 3. A cross-section of a floating ball valve in the shut position shows downstream pressure pushing the ball to seal on the right-hand side. Arrows indicate the contact with the seat seals. The Original Blue Pipe for Compressed Air E n e rg y E ffi ci e nt Mo d u l a r D e si gn S i mp l e In sta l l a tion L o w e r In sta l l C ost P u sh -to -C o n n ect C o mp l e te l y R e u sable N o C o rro si o n Ava i l a b l e i n 1 /2 ”- 4” 7205 E. Hampton Ave. August 2008 Mesa, AZ 85209 Phone: 480-830-7764 www.transair-usa.com www.PLANTSERVICES.com 47 http://www.transair-usa.com http://www.transair-usa.com http://www.PLANTSERVICES.com
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