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The double-seal structure of the DBB valve is not just about having two valve discs; more importantly, each disc has both soft and hard sealing surfaces. The precision of the hard seal is no less than grade 4, which ensures zero leakage in the valve while also making it fire-resistant. As shown in Figure 1, Figure a shows the soft seal shape when open, and Figure b shows it when closed. From the figures, you can see that the soft seal (rubber) is embedded in the groove of the valve disc, raised in the middle and recessed on both sides. This design helps the soft seal get compressed and flow into the side grooves when the valve is closing, eventually forming a hard seal. The advantage is that it protects the soft seal from being crushed while ensuring the hard seal is achieved.