Double Block and Bleed VS Double Isolation and Bleed Valve

In the oil and gas industry, the definitions of Double Block and Bleed Valve (DBB) and Double Isolation and Bleed (DIB) valves vary by source. Differences in definitions and terminology are important when it comes to which valve capability is used in which type of system.

DBB definitions

There are two organizations in the United States that define DBB—API and OSHA.

According to API 6D Twenty-second Edition, a DBB valve is a “valve with two seating surfaces which, in the closed position, blocks flow from both valve ends when the cavity between the seating surfaces is vented through a bleed connection provided on the body cavity”

In contrast, OSHA describes DBB as “the closure of a line, duct, or pipe by closing and locking or tagging two inline valves and by opening and locking or tagging a drain or vent valve in the line between the two closed valves.”

API’s DBB definition does not achieve the same level of isolation as OSHA’s. API allows DBB valves to be one single valve with two unidirectional seats, while the OSHA standard can only be achieved with two separate valves with a method to bleed pressure in between. There are some valves that utilize a twin-valve design. By combining two valves into one body, a twin-valve design reduces weight and potential leak paths while meeting the OSHA requirements for double block and bleed.

Valve associations usually choose to follow either APIs or OSHA’s definition, but some have created their own handbook with their own definitions for industry terms. For example, the British Valve and Actuator Association (BVAA) defines DBB as “a manifold that combines one or more isolation valves, usually ball valves, and one or more bleed/vent, usually a needle-style global valve, into one assembly for interface with other components (e.g., pressure measurement transmitters, pressure gauges, and switches).”

DIB definition

API defines 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 direction or in both directions.

The difference between API’s DBB and DIB definitions is that double block and bleed valves seal against pressure on both sides of the valve, while double isolation and bleed valves provide an additional seal against pressure on one side only. It is important to use a DIB valve instead of a DBB valve in applications that require an additional pressure barrier that seals the main pressure barrier separately. This is important to meet certain operational safety requirements or according to the nature of the service, such as low leak tolerance or fluid cleanliness.

Differences of DIB and DBB

Both DBB and DIB valves provide isolation in both upstream and downstream directions, even under high-pressure or low-pressure conditions.

For DBB valves, there are usually two one-way self-bleeding seats. These seats do not rely on external mechanisms to relieve “trapped” pressure in the ball or disc.

In contrast, DIB valves use one or two bidirectional seats. These valves provide double isolation from pressure across the valve, but cannot relieve body cavity pressure through the valve seat. DIB valves require an external pressure relief system to relieve pressure build-up.

For DIB valves, each seat can cut the block of the flow from upstream and downstream, even if one seat is damaged, the other seat can work efficiently. DIB valves are used in applications where critical isolation is needed to ensure that leakage does not occur.

Applications

Both DBB & DIB valves can be used in a variety of applications and markets, such as LNG, petrochemical, transmission and storage, natural gas industrial processes, mainline and manifold valves in liquid pipelines, and refined product transmission lines.

Relia Valve offers a wide range of DBB or DIB valves and options to fit specific needs. For example, in liquid service near waterways or municipalities, double block and bleed ball valves with DBB capabilities, provide a tight mechanical seal simultaneously upstream and downstream, which is normally unaffected by pressure variations or vibrations.

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