API describes Double Isolation & Block 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.” This feature can be provided in one or both directions.

The main difference is that the double block and bleed valve protects against pressure from either side of the valve while the double isolation and bleed valve offers additional seal against pressure from just one side. It is necessary to use the DIB valve instead of the DBB valve in applications requiring an external pressure barrier to be sealed separately from the main pressure barrier. This is critical to meet such operational safety standards or depending on the type of the operation, such as low leakage tolerance of fluid cleanliness.

There are normally two unidirectional self-relieving seats with a DBB valve. These seats do not depend on an external pressure relief system. On the other hand, the DIB valve uses one or two bi-directional seats. The valve allows dual isolation from the pressure at both ends of the valve, but cannot ease the pressure of the body cavity past the seats. DIB valves need an external relief mechanism to ease the build-up of pressure.

Both valves could protect in both the upstream and downstream directions, even in high-pressure or high-temperature conditions. Isolation is critical in situations where leakage through the valve can have significant implications. Once the fluid is sealed, the bleed system may drain the area between the two valves or two seating surfaces. This is critical for maintenance and integrity checks in cases where leakage can be tracked.