Proper Valve Stem Sealing Best Practices

To achieve and maintain low emissions, packing must be “live loaded” to keep constant pressure on the sealing rings (Figure 2). This is usually accomplished using compressed Belleville-type springs. These springs maintain a constant force on the packing, ensuring it seals over time, even as the rings wear from stem movement. Unfortunately, the increased pressure tends to restrict valve movement, so the sealing materials and valve stem finish must be carefully chosen to minimize fugitive emissions, while allowing valve stem movement.

Sealing valve stems with bellows

An alternative to valve packing is a valve bellows seal. A bellows seal uses a welded or mechanically formed metal barrier around the valve stem that can compress and stretch like an accordion (Figure 3). Because the seal is made of metal with a very low rate of deformation in critical areas, bellows seals achieve virtually zero leakage.

Packing versus bellows

Each method of valve stem sealing has pros and cons, so the best choice depends on the application. Perhaps the biggest advantage of standard or environmental packing is its comparatively low cost, along with a wide variety of valve packing materials and designs to suit most applications. Valve packings can also be adjusted and replaced without disassembling the valve.

The biggest advantage of a bellows design is its ability to deliver zero leakage. Such a specification is critical for lethal service applications. The bellow materials can also be chosen to handle higher temperatures and corrosive applications. Because the operational life of a bellows seal is based on the number and length of strokes, the estimated time to failure can be predicted with some accuracy, so replacement can be planned.

Each design has disadvantages as well. The performance and lifetime of packing is based on many variables, which are not always easily predicted. Small leaks usually can be addressed by tightening the packing, but at some point, the packing must be replaced. Also, the surface finish of the valve stem can have a big impact on the life and performance of a packing design. Regardless, all valve packing will leak to some extent, and this may not be acceptable in certain applications.

As mentioned previously, bellows seals will fatigue and eventually fail. When that occurs, the valve must be fully disassembled to replace the bellows seal. For this reason, the total cost of ownership for a bellows seal is typically higher than that of packing.

Application examples

When properly selected and applied, both packing and bellows seals can handle challenging applications. In one liquified natural gas application in Australia, a 24-inch by 30-inch letdown valve used a specially designed environmental packing arrangement and had very low valve stem leakage, despite operating at cryogenic temperatures around –300°F (Figure 5). Any fugitive emission from this valve translated into lost product, lost energy, and environmental damage—so it was critical to minimize leaks.