DBB Valve vs Double isolation and bleed valve

The "Double Block and Bleed” is normally used at some point where you need to shut off a line temporarily for some task. You also want this operation to be absolutely safe. So, you install two valves with a short flanged spool piece between the valves. In the spool piece you install a Bleed (a vent valve for gas services or a drain valve for liquid service). To operate this you shut off both line valves and open the Bleed valve.

A "DBB Valve" ( www.vovalve.com/DBB-valves.html ) is a single unit item that has higher cost than a grouping common piping components to do the same thing.

Most of the "DBB Valves" you will find today are "Ball" valves. If your line class requires Gate valves (does not allow Ball valves) then you would need to assemble the components.