safety in the plant 33
temperatures and pressures fluctuate. The result is easy actuation and minimal wear to the packing.
O-ring seal
Another effective stem seal technology is the O-ring design. When properly designed, this technology provides flexibility for applications requiring high pressure, low pressure, or a broad pressure range. The O-ring is usually made from a highly elastic material, such as fluorocarbon FKM. A proper stem design with an O-ring configuration requires a back-up ring or some other mechanism, usually made of PTFE, which will contain the O-ring under high pressure. In terms of temperature, pressure, and chemical attack, the design is limited by the specifications of the elastomer. Beyond issues relating to stem seal design, there are some additional causes of leaks from the stem. These have to do with alignment of the stem. There are two basic causes of misalignment.
In the first case, misalignment may result from improper installation of the actuator. If the centre line of the actuator and the centre line of the stem are not properly aligned, the stem will become tilted, resulting in uneven wear of the stem seal. In the second case, damage to the seat seal inside the valve may cause the stem to tilt. Ball valves can employ either a floating or trunnion ball design.
In a floating ball design, the ball is not fixed inside the housing but floats between two seats. In the shutoff position (Fig. 3), the ball seals against the seat on the low-pressure side, pushed downstream by a positive pressure differential.
By contrast, the trunnion design employs a ball, but the ball is not a discrete sphere. Rather, its geometry includes two cylinders, the ‘trunnions’, affixed to the ball at the top and bottom (Fig. 4). The unit fits into a space in the valve body and cannot move along the flow axis.
Fig. 3 A cross-section of a floating ball valve in the shut position, with downstream pressure pushing the ball to seal on the right hand side. Arrows point to the seat seals.
In the case of high differential pressure across the seat, a free floating ball can be pushed too far downstream. In the absence of an advanced seat design, such as a spring energised seat, the ball may not return to the center position. As a result, the stem will tilt to one side, and uneven stem wear will occur. The trunnion design prevents excessive movement of the ball downstream. The trunnions, which are fitted in place, keep the ball centered and the stem properly aligned.
Conclusion
This article is intended to show that different designs have different strengths and relative merits, and these factors have, therefore, a direct impact on fugitive
Fig. 4. A trunnion ball valve.
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Fig. 2. Cross-sections of two different types of packing. 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.
emissions. The real cost of a valve is not just the purchase price but that of the overall cost of ownership. With raw material feedstock prices increasing, as well as the frequency and severity of environmental non- compliance fines, direct and indirect costs associated with regular and on-going maintenance, failure and replacement must be considered at all times. m
Michael Adkins is general industrial valve product manager and Peter Ehlers is alternative fuels market manager for Swagelok Company, Solon, Ohio, USA.
www.swagelok.com
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