L&T Valves - Double Block & Bleed Plug Valves 220420

L&T Valves Double Block & Bleed Plug Valve provides online-verifiable bubble-tight sealing and is a safe and reliable replacement to the conventional two-valve system.
Options: High Temperature, Cryogenic, FBE-lined, Steam-jacketed valves
DBB plug valve working mechanism.

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Explanation is not clear. Not able to understand the concept clearly.