There are two entities in the United States that define DBB&#;API and OSHA. According to API, a DBB valve is 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." API also notes in this definition that this valve does not provide positive double isolation when only one side is under pressure.
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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 leaks paths while meeting the OSHA requirements for double block and bleed.
Valve associations usually choose to follow either API's 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)."
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.
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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 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 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.
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