Double block and bleed valves are used where positive isolation and assurance from flow leaking from upstream to downstream of a valve is required, nothing else. It has nothing to do with pressure class and little to do with safety. These valves are usually located in a piping path between two liquids that should not contaminate each other, as in a common suction header to a pump feeding a pipeline from either a diesel tank or a gasoline tank, where you will either transport gasoline or diesel in sequential batches. While transporting gasoline, you would want the diesel valve to have been closed, and bleed down, thus assuring that gasoline is not leaking into the diesel and making a mixture that is being feed into the pipeline. Likewise, while transporting diesel, you would want the gasoline valve to be closed and bleed down.
They are also used between or in front of meters at a custody transfer point where it would be unacceptable to have one meter working while the other meter was leaking product across it too. Likewise, it is also common to find these at any point or pipeline branch where a loss of product would be dangerous or could mean lost revenues. This would typically be at custody transfer points from a gas/oil producer to the meters as they enter a pipeline owned by others, between refineries, between process units where revenues are accounted for separately, etc.
