DBB definitions
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)."
To me, double block and bleed always sounded like a move out of the Cobra Kai karate dojo. Of course a double block and bleed valve has nothing to do with karate, it has more to do with controlling process flow to allow for maintenance, sampling, and flow diversion. This post describes the attributes of a double block and bleed valve, its function, and application.
What is a double block and bleed valve?
A double block and bleed valve is like having three valves in one.
API 6D defines a double block and 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 vending/bleeding the cavity between the seating surfaces."
There are a couple different styles of double block and bleed valves. One style has two independently seating valves inside, with a bleed or drain valve. This configuration is referred to as a double isolation and bleed (DIB). The DIB valve can isolate either side of the valve (bi-directionally) to vent or bleed the cavity and isolate piping.
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This trunnion style ball valve from Val-Matic provides double block and bleed in a single valve. This style can isolate piping on both sides of the valve (uni-directionally) to vent/bleed the valve cavity between the seats.
Using a single double block and bleed valve system versus 3 separate valves saves installation time, weight on the piping system, and space. It also cuts down the number of leak paths to the atmosphere, reducing the risk of hazards when transferring liquids dangerous to humans or the environment.
Which applications require a double bock and bleed valve?
Double block and bleed valves are most commonly used in the oil and gas industry, but can also be helpful in many other industries.
It's typically used where bleeding the valve cavity is required, where piping needs isolation for maintenance, or for any of these other scenarios:
- Prevent product contamination
- Remove equipment from service for cleaning or repair
- Meter calibration
- Liquid service near waterways or municipalities
- Transmission and storage
- Chemical injection and sampling
- Isolate instrumentation such as pressure indicators and lever gauges
- Primary process steam
What to look for when selecting a double block and bleed valve?
As with any valve, during selection, one should consider the fluid properties, pressure, temperature. Perhaps most importantly, however, is how the valve will be used. With a number of configurations offered, consider which of these will fit your process and needs best.
If you're unsure which double block and bleed valve is right for you process, talk to an engineer. They can help determine which one will achieve your objectives best.
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