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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.
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, like this one from J-Flow Controls, 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.
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.
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:
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.