Sep. 30, 2024
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)."
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What is a Double Block & Bleed Valve?
A double block and bleed (DBB) valve is a type of industrial valve that provides two isolation barriers and a means of venting or bleeding the cavity between the barriers. This configuration is designed to enhance safety and simplify maintenance procedures in fluid or gas systems.
The main components of a double block and bleed valve typically include two block valves and a bleed (or vent) valve:
- Block Valves: These are the primary isolation valves. There are two of them in a double block and bleed valve. These valves can be closed to isolate a section of the pipeline or system.
- Bleed (Vent) Valve: This valve is situated between the two block valves. It is used to release or bleed the fluid or gas trapped between the two block valves. This ensures that any residual pressure is safely vented before maintenance activities.
The key benefits of a double block and bleed valve include:
- Enhanced Safety: By providing two separate barriers, a DBB valve reduces the risk of leakage or cross-contamination between different sections of a pipeline.
- Simplified Maintenance: The bleed valve allows for venting of the trapped fluid or gas, making it easier and safer to perform maintenance activities on the isolated section of the system.
- Space and Weight Savings: Compared to using multiple single valves and associated piping, a DBB valve can save space and reduce the overall weight of the system.
Double block and bleed valves are commonly used in industries such as oil and gas, petrochemical, and chemical processing where the isolation of fluids or gases is critical for safety and operational integrity. They are often employed in applications where a single point for isolation and venting is more convenient than multiple valves and fittings.
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