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What is DBB Valve: Double Isolation and Bleed Valves or Double Block and Bleed Valves(1)

November 10, 2017


“Whenever a customer says they want DBB, most people may think it refer to monoflange double block and bleed valves, or integral type and modular type. But it is principally depending on the application and their previous experience with what they believe DBB valves to be,” states Wilia Wu, Technology Manager for Sino Valves. “Based on clients’ favorite valve type and application, we could then offer ball, gate, or enlarging plug valve options with DBB capabilities.”

There is a whole lot of misunderstanding surrounding the expression double block and bleed (DBB) as used to describe valve functionality. It appears every valve purchaser and manufacturer has a different notion about what the term means for valve choice, which may result in the specifications or valve type. Some of the confusion in the gas and oil industry originates from the fact that we now have two sources that define the term differently. “Another point of confusion comes when many people use the term double block-and-bleed when they really want a valve with capabilities of double isolation-and-bleed (DIB),” Wilia Wu explains. These differences in terms and definitions are significant in regards to which valve capability to utilize in what type of system. 

The most basic thing that a user is currently looking for when they specify a dual valve is a valve or valve system which supplies more reliable isolation in areas than a single valve could. This machine or valve unit functions to decrease the installation footprint, saves on piping requirements, and reduces weight. These conserve time, space, and price. 


Once the fluid is isolated, the bleed mechanism may drain the area between both valves or 2 seating surfaces. This is very important to maintenance and/or integrity assess scenarios where leakage could be tracked. The distinction between DIB and API’s DBB is though a double isolation and bleed valve supplies an additional seal against stress from 1 side, that a double block and bleed valve seals against pressures from both sides of the valve.

It is important to utilize a DIB valve instead of a DBB valve in applications that require another pressure barrier that seals of the pressure barrier. This is important to fulfill operational security requirements or according to the character of the service, for example the cleanliness of the fluid or low tolerance for leakage.

There are two entities in the United States that specify DBB — the American Petroleum Institute (API) and the Occupational Safety and Health Administration (OSHA). API is researcher for the oil and natural gas industry, outlining business guidelines for operation of America as well as a trade organization that acts as an advocate. OSHA is the division of the United States Department of Labor that’s charged with enforcing safety and health legislations, outlining legislation which protects our surroundings and both individuals. In contrast, the OSHA Federal regulation describes DBB as “the closure of a line, duct or pipe by closing and locking, or tagging, two in-line valves and by opening and locking, or tagging, a drain or vent valve in the line between the two closed valves.”

Valve associations usually opt to follow either API’s or the definition of OSHA, but a few have established their own handbook for industry conditions using their definitions, DBB included. DBB and DIB Definitions API also notes within this definition that this valve doesn’t offer positive double isolation when just 1 side is under pressure.

Isolation can be provided by A DIB or DBB valve in high-pressure or high-temperature situations, even in both the upstream and downstream directions. Isolation is important in cases where congestion through a valve might have major impacts. API’s DBB definition doesn’t achieve the same degree of isolation because the definition of the OSHA. API permits while the OSHA standard can only be achieved to bleed pressure in between DBB valves to be just one valve using two chairs. There are a few valves that utilize a valve layout. A double valve layout minimizes weight and potential leaks paths, while fulfilling the OSHA requirements for dual block-and-bleed by mixing two valves into one body. BVAA, such as API, says that just one valve is necessary, not a method. Based on BVAA, “DBB valves replace the previous traditional technique utilized by pipeline engineers to make a dual block-and-bleed configuration in the pipeline, usually by fabricating three valves using flanges, ‘Tee’ pieces, and related bolting.”

Another difference between DIB and DBB is the ability. Usually you will find two unidirectional chairs that are self-relieving. These seats don’t rely to ease pressure. On the other hand, a DIB valve utilizes one or two seats that are bidirectional. The valve offers isolation however there is usually one functional drawback, a DIB valve can’t relieve body cavity pressure meaning that its chairs are not self-relieving. When employing a DIB valve, an outside aid system is essential to relieve pressure buildup.

Double isolation-and-bleed (DIB) is another expression in the industry that makes the discussion about DBB even more perplexing. API defines DIB as a “single valve with two seating surfaces, all which, in the closed position, provides a seal against stress from a single source, with a way of venting/bleeding the pit between the seats surfaces.” This attribute can be supplied in one direction or in both directions.

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