Then they bleed off or vent the remaining fluid from the system on the downstream side of the manifold.
This design also has operational advantages, there are significantly fewer potential leak paths within the double block and bleed section of the pipeline.
Because the valves are full bore with an uninterrupted flow orifice they have got a negligible pressure drop across the unit.
Significantly, because all the valve components are housed in a single unit, the space required for the installation is dramatically reduced thus freeing up room for other pieces of essential equipment.
Due to the volume of the cavity between the two balls being so small, the operator is afforded the opportunity to evacuate this space efficiently thereby quickly establishing a safe working environment.