In most valves, the stem is sealed with packing that presses against it as it moves. That packing works well, but it is a wear part, and over time it can become a leak path, which is a problem when even a trace of the process media must not escape. A bellows-sealed valve removes that leak path altogether. This article explains how a bellows seal works, why it matters for zero-leak and low-emission service, and where these valves are the right choice.
How a bellows seal works
A bellows-sealed valve, sometimes called a packless valve, uses a thin-walled metal bellows as the stem seal instead of packing. The bellows is welded into the valve so that it forms a sealed, flexible barrier around the stem. As the stem moves to open or close the valve, the bellows compresses and extends with it, so the stem never has to slide against a packing seal. Because there is no dynamic packing rubbing against the stem, there is no packing wear and no gradual path for the media to leak out along the stem. Many designs also close off external leakage at the body with a metal-to-metal gasket seal, so the whole valve stays tight.
The bellows itself is the heart of the valve, and its quality determines how long the valve lasts. A well-made bellows has uniform wall thickness and controlled internal stresses, which gives it the flexibility and fatigue resistance to survive many open-and-close cycles.
Why choose one
The main reason to specify a bellows-sealed valve is leak integrity over the life of the valve. With no stem packing to wear, fugitive emissions along the stem are kept extremely low, which matters where the media is toxic, flammable, or otherwise hazardous, and where emissions are regulated. A second reason is reliability and service life, since the sealed bellows keeps performing where packing would need adjustment or replacement. Some designs go a step further and add packing above the bellows as a secondary containment layer, so that if the bellows ever fails, the media is still held back rather than released suddenly. For the most purity-sensitive systems, bellows-sealed valves are also available with the clean internal finishes and cleaning processes that ultra-high purity work demands.
FITOK’s bellows-sealed valve range
FITOK offers a family of bellows-sealed valves, which lets you match the valve to the pressure, temperature, and purity of your system rather than compromise on one design. The range runs from low-pressure valves built for high purity and ultra-high purity service, through a secondary-packed design that adds a second containment layer above the bellows, to higher-pressure pneumatically actuated valves for more demanding duties. There is also a bellows-sealed metering valve with a micrometer handle for precise, repeatable flow settings in zero-leak gas systems. Across the range you will find non-rotating stem tips to extend shutoff cycle life, replaceable bellows and stem-tip assemblies, and manual or actuated options. The exact pressure and temperature ratings depend on the series, so it is worth confirming these against the datasheet for the valve you intend to use.
Where they are used
Bellows-sealed valves are widely used in semiconductor and ultra-high purity gas systems, in chemical and analytical processes, and anywhere the media is hazardous or the system must meet strict emission limits. In short, they suit any application where a leak along the stem is simply not acceptable, and where a packed valve would carry too much risk over its service life.
Choosing the right bellows-sealed valve
The choice comes down to your pressure and temperature, the purity of the system, whether you need secondary containment, and whether the valve is for shutoff or precise metering. If you are specifying bellows-sealed valves for a zero-leak or low-emission application, we can help you select the right FITOK series and supply it.
Specifying valves for a zero-leak or low-emission system? Our engineers can help you match a bellows-sealed valve to your service, and prepare a tailored quotation. Contact us to get started.
