Plant Services - August 2008 - (Page 46) Lapp Group's New Full-Line Product Catalog Safety Valves Make a choice The Lapp Group of North America is pleased to announce the release of our new 728 page product catalog. Included in the new catalog is our full range of OLFLEX®, UNITRONIC®, and ETHERLINE® cables, EPIC® connectors, SKINTOP®, SILVYN®, and FLEXIMARK® accessories, value-added and custom services. Many enhancements have been made to this new edition. New features include: • Complete your installation guides • Standard put-ups • Technical comparison charts in the front of each section. New products include: • OLFLEX® Tubing & Sleeving • EPIC® M23 Connectors • EAB, EAC & EPT Mil Style connectors • NFPA 79 2007 compliant cables. For additional information or to request a catalog visit www.lappusa.com. One-piece packing Two-piece chevron packing Figure 2. On the left is the standard one-piece packing. On the right are the two ferrules that make up a chevron stem packing. In the chevron packing, minimal pressure from the packing nut pushes down on the two triangular shapes, resulting in outward, even pressure between the stem and the housing. 888-456-3539 www.lappusa.com pressure ranges, as well as regular and easy actuation without excessive wear. A chevron packing consists of two matched gaskets, one fitting inside the other. The cross section of the gaskets is triangular. Fitted together, the two gaskets form a rectangular cross-section (Figure 2). As force is applied from the stem’s packing nut, the two gaskets are pushed against each other along the diagonal point where they meet, which distributes the force horizontally and evenly against the stem and body housing. A minimal pressure from the packing nut produces a substantial seal between the stem and the body housing. For the chevron seal to work correctly, the two PTFE gaskets – the packing – must be held in place to reduce cold flow during thermal cycling. The packing in the chevron design, therefore, must be adequately contained and supported by packing support rings and glands, which evenly distribute pressure to the packing. To increase the interval between inspections and adjustments, the chevron design also might include Belleville washers, which are springs that produce a live load on the packing. Live loading places uniform force on the www.PLANTSERVICES.com packing: As temperatures and pressures fluctuate, the springs provide a constant bias force against the seal and the body to maintain the appropriate amount of sealing force. O-ring seal Another effective stem seal technology is the O-ring. When properly designed, O-rings provide flexibility for applications requiring high pressure, low pressure, or a broad pressure range, such as a cylinder where, for example, pressure might drop from 2,300 psig (158.5 bar) when full to 100 psig (6.9 bar) as it nears empty. The O-ring usually is made from a highly elastic material, such as fluorocarbon FKM. Like the two-piece chevron design, the O-ring design doesn’t require excessive packing nut pressure. Rather, the O-ring is energized by pressure in the media stream. As that pressure increases, the O-ring further deforms and increases pressure on the stem. Conversely, as pressure in the gas stream decreases, the O-ring relaxes, filling the space between the stem and the body. A proper stem design with an O-ring configuration requires a back-up ring or August 2008 46 http://www.lappusa.com http://www.lappusa.com http://www.PLANTSERVICES.com
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