Double Block and Bleed valves have evolved to replace the process of bolting together individual valves to provide dual isolation.

This new assembly provides great savings in weight, space and installation times especially in instrument or instrument cage isolation. These savings can be as much as 60% in weight and studies have shown that a 70% installation time saving is also possible. However the greatest savings are to be seen in the reduction of leak paths to atmosphere, therefore reducing the risk of the potential hazards this entails.

Dual isolation is a necessary requirement when maintenance is taking place down stream of the first isolation valve. Cavity venting is provided by either a ball or globe OS&Y vent valve so that trapped pressure between the two isolation valves is safely vented.

These valves have also evolved to encompass the function of chemical injection (using a suitable quill) and sample points. In-built check valves are often integrated in these valves.