The difference between Double Block & Bleed (DBB) and Double Isolate & Bleed (DIB)

Much confusion exists over the terminology and the differences between Double Block & Bleed Valves and Double Isolation & Bleed Valves. One point of confusion comes because many people using the term Double Block & Bleed really want a valve with Double Isolation & Bleed capabilities.

However, the differences in definitions and terminologies involved are important when it comes to determining which valve capability is needed for what type of system. The term DBB is defined by different organizations, such as API, OSHA and BVAA but also associations like EEMUA and companies like Shell.

For the Oil & Gas Industry API 6D / ISO 14313 is one of the most important specifications. API 6D defines a Double Block & Bleed Valve as a “single valve with two seating surfaces that, in the closed position, provides a seal against pressure from both ends of the valve with a means of venting/bleeding the cavity between the seating surfaces”. The 2008 note points out that this valve does not provide positive double isolation when only one side is under pressure.

By contrast, API 6D defines a Double Isolation & Bleed Valve (DIB) as a “single valve with two seating surfaces, each of which, in the closed position, provides a seal against pressure from a single source, with a means of venting/bleeding the cavity between the seating surfaces. The note adds that this feature can be provided in one or in both directions.”

The important distinction between the DBB and DIB is, that on a DBB if there is somebody working downstream on the line and the first seal leaks the second seal will not seal in that same direction. However, it is important to consider that API is talking about a single valve with two seating surfaces.

In contrast, the OSHA definition can only be achieved with two separate valves with a method to bleed pressure in between.