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Standard valve sealing: relying on packing (such as graphite or PTFE) to wrap the valve stem and form a seal. When the valve stem rotates or moves up and down, it will rub against the packing. Over time, the packing will wear and age, causing the medium to leak along the valve stem (external leakage).
Bellows sealed valve: One end of the bellows is welded to the valve stem, and the other end is welded to the valve cover or body, forming a sealed metal barrier. When the valve is opened or closed, the movement of the valve stem is converted into axial compression or extension of the bellows. The valve stem itself does not have dynamic sealing points with the external environment.
2、 Working principle: How to achieve "zero leakage"?
Transforming dynamic sealing into static sealing: In standard valves, the reciprocating or rotating motion of the valve stem is "dynamic", making it difficult to ensure permanent sealing. The bellows seal transforms this critical dynamic sealing point into a static seal of the bellows component itself (i.e. the welded end of the bellows). The reliability of static sealing is far superior to that of dynamic sealing.
Bellows act as barriers: The process medium is completely contained within the inner cavity of the valve body and bellows. No matter how the valve stem moves, the medium cannot come into contact with the valve stem, let alone leak into the atmosphere.
Double sealing guarantee: As a safety redundancy, bellows sealed valves typically retain a set of traditional packing located above the bellows assembly. This packaging has two purposes:
Main function: Serve as a secondary seal. In rare cases of bellows failure, it provides a temporary safety barrier to prevent sudden large leaks of the medium.
Secondary function: Protect the bellows and valve stem from external pollutants such as dust and corrosive substances.
Therefore, the bellows seal provides a dual safety system of "main seal (bellows)+backup seal (packing)", achieving the highest level of sealing safety.
III、 Core advantages and application scenarios
Advantage:
Zero external leakage: This is the main advantage, completely eliminating leakage at the valve stem and meeting the strictest environmental and safety standards.
High reliability and long service life: Reduce frequent maintenance and tightening required due to packing wear.
Suitable for extreme media:
Toxic, flammable, and explosive media such as chlorine, phosgene, hydrofluoric acid, and light hydrocarbons in petrochemical products, leakage can be catastrophic.
Extremely valuable medium: preventing the loss of precious materials.
High vacuum or ultra-high purity media: prevent air from entering or product contamination (e.g. in the semiconductor and polycrystalline silicon industries).
Adhere to strict regulations: comply with strict leak rate standards such as TA Luft (German air purity standard) and ISO 15848.
Typical application industries:
Petrochemical products: hazardous media in refineries, olefin and aromatic hydrocarbon plants.
Chemical industry: involves the production of highly toxic and corrosive chemicals such as pesticides and pharmaceutical intermediates.
Nuclear industry: absolute containment of radioactive media.
Polycrystalline silicon and semiconductor industry: zero leakage treatment of specialty gases such as silane, phosphine, and chlorine. This is the standard configuration for almost all key natural gas pipelines in the industry.
IV、 limitation
Higher costs: More complex manufacturing processes (especially high-quality hydraulic forming and automatic welding) result in significantly higher prices than standard valves.
Travel restriction: The expansion/compression stroke of the bellows is limited, which restricts the valve lift. Usually not suitable for valves that require high lift.
Pressure and temperature limitations: The pressure and temperature resistance of corrugated pipes are limited within a certain range and need to be carefully selected according to the usage conditions.
Fatigue life: Bellows will experience metal fatigue under repeated cycling. High quality products that have undergone rigorous testing are crucial.
summary
You can think of it this way: bellows sealing is the "nuclear grade" sealing solution for valves. It uses flexible metal barriers, sacrificing the ease of maintenance and low cost of standard packaging, to achieve unparalleled absolute safety and reliability when handling hazardous, precious, or ultra pure media. When selecting valves for critical/hazardous media, bellows sealing is often an important technology to consider.