Safety codes globally have a requirement to provide a positive means to isolate energy sources and hazardous substances prior to performing maintenance. For gaseous hydrogen systems, methods such as a blind flange, a double block valve arrangement or a double block and bleed valve arrangement can provide that positive isolation.
Installing a blind flange requires breaking the supply line and inserting a solid insert that blocks the flow. The disadvantage of this approach is that it is more laborious than the other options and a method of isolation is needed to safely install the blind flange. The components involved are a lower cost than the other options but that cost is offset by the additional labor and system down-time required. For these reasons, they typically are only used for long term isolation.
A double valve arrangement is an effective approach that can be implemented quickly. A disadvantage of the double valve is that hydrogen may leak through the first valve and allow pressure to build between the valves without any indication. Aside from leading to a false sense of security, the pressure may also push its way through the second valve into the downstream plumbing and work area. While it may seem unlikely for two valves to leak, there sometimes is a common mode failure where both valves are damaged at the same time.
A double-block-and-bleed valve arrangement has a third valve to act as a means to vent, or "bleed" pressure between the two block valves. . In this configuration, leak through of the first valve cannot pressurize the second blocking valve, thereby eliminating the leak-through failure mode of the double block valve arrangement. For hydrogen systems, the outlet of the bleed valve should be routed to a safe venting location. A double block and bleed system can also be automated. In that situation the block valves are designed to fail closed and the bleed valve to fail open. Double block and bleed valves can also be used to safety depressurize and vent the downstream section prior to the isolation.