The definitions of double block and bleed DBB and double isolation and bleed DIB differ between sources in the oil and gas industry. Differences in definitions and terms are important when it comes to which valve capability to use in what type of system. Because DBB and DIB have become such generic terms in the industry, it is important to take into consideration the application, media, and various environmental challenges when choosing the appropriate solution and valve type. 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.
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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 note points out that this valve does not provide positive double isolation when only one side is under pressure. 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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Share on twitter. Share on linkedin. Written by Tim-Frederik Kohler on January 19, Technical regulations. Practical application.
How Does a Double Block and Bleed Ball Valve Work?
Double block and bleed valves are used for isolation in critical applications. They also prevent product contamination and maintain system integrity. DBB valves make it easy to remove machinery for cleaning, repair, and maintenance. Before DBB valves were used, isolating pipelines and machinery was a difficult process.
The difference between Double Block & Bleed (DBB) and Double Isolate & Bleed (DIB)
What is Double Block and Bleed Valves? Why use a Double Block and Bleed valve? Double Block and Bleed valves have evolved to replace the process of bolting together individual valves to provide dual isolation. This new assembly provides great savings in weight, space and installation times especially in instrument or instrument cage isolation. However the greatest savings are to be seen in the reduction of leak paths to atmosphere, therefore reducing the risk of the potential hazards this entails. Dual isolation is a necessary requirement when maintenance is taking place down stream of the first isolation valve. Cavity venting is provided by either a ball or globe vent valve so that trapped pressure between the two isolation valves is safely vented.
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The primary function of a double block and bleed system is for isolation and the secondary function is for intervention. Under certain conditions double block and bleed systems are needed to prevent product contamination or where it is necessary to remove essential equipment from service for cleaning or repairs while the unit continues in operation. Of course, such equipment must be provided with a spare or it must be possible to bypass it temporarily without shutting down the unit. The nature of the fluid, its pressure and temperature, and many other factors must be considered when determining the need for double block and bleed systems. Generally, block Valves should be considered for the onstream isolation of equipment if the fluid is flammable or otherwise hazardous, or if the fluid is in high-pressure or high-temperature service. The purpose of the bleed Valve is twofold.