Double block and bleed valves, or DBB ball valves, are specialized ball valves that provide double isolation and bleeding capabilities. They contain two ball valves with a bleed valve in between.  The ball valves isolate the upstream and downstream sections of the pipeline, while the bleed valve releases any trapped fluid between the valves.

DBB valves are used when performing maintenance on a pipeline section. The two ball valves are closed to isolate the target section, then the bleed valve is opened to depressurize it by draining the trapped fluid. This ensures the safe and environmentally compliant isolation of the pipeline. 

DBB valves provide extra assurance of zero leakage compared to single valves. For any flow to occur, both ball valves would have to fail, which is highly unlikely if the valves are properly selected and tested. The ball valves allow quick shutoff and re-opening with 90-degree turns.

DBB valves are used in oil & gas, chemical and other industries that require frequent operations on live pipelines. They maximize safety, environmental protection and uptime. DBB valves come in various types like 3-way (2 ball valves and 1 bleed) or 4-way (2 ball valves and 2 bleeds) configurations. Sizes, pressure ratings, and ends like flanged or threaded are chosen based on application needs. Manual or automated operation is available depending on the required level of control and integration.

In summary, DBB ball valves provide the vital functions of double isolation, bleeding, and de-pressurization required to safely perform work on live pressurized pipelines while mitigating environmental and safety hazards. They come in a wide range with many options to suit application needs. Overall, DBB ball valves are an indispensable tool for industries that manage hazardous fluids and gases.