In the oil and gas industry, safety and isolation are among the first and highest priorities. When working with high-pressure pipelines, operators must ensure that the flow is completely stopped during maintenance.
This is where the double block and bleed valve comes into play. But, not all isolation valves are the same. The American Petroleum Institute (API) defines specific standards under API 6D to categorize these valves. The terms DIB-1, DIB-2, and DBB describe how valve seals and handles pressure.
This article breaks down these technical definitions to help you select the right valve for your piping system and avoid any failure or safety hazards.
What is Double Block and Bleed Valve (DBB)?
A Double Block and Bleed Valve (DBB) system isolates a pipeline by blocking pressure from both sides and safely venting the space between them.
However, there are two governing bodies in the United States that define DBB differently. These are the API (American Petroleum Institute) and OSHA (Occupational Safety and Health Administration).
Based on API, a DBB valve means “a single valve with two seating surfaces.” When the valve is closed, pressure from both ends is sealed off, and the cavity between the seats can be vented or bled. API also states that this valve does not provide positive double isolation if only one side is pressurized.
Meanwhile, OSHA described DBB as “the closure of a line, duct, or pipe by closing and locking or tagging two inline valves, and opening and locking or tagging a drain or vent valve between them.”
API’s DBB definition provides less isolation than OSHA’s. API permits a single valve with two unidirectional seats, while OSHA requires two separate valves with a bleed in between. Hence, there are some that utilize twin-valve designs by combining two valves in one body, reducing weight and leak paths while still meeting OSHA double block and bleed requirements.
Here is a clear summary table comparing API and OSHA’s DBB definitions:
| Aspects | API | OSHA |
| Valve Configuration | One valve with two unidirectional seats | Two separate isolation valves |
| Isolation Level | Provide no positive double isolation when only one side is pressurized | Provides positive double isolation |
| Bleed Requirement | Cavity venting or bleeding within the valve | Dedicated drain or vent between the two valves |
| Safety Compliance | Lower level of isolation | Higher safety and isolation standard |
Nonetheless, a DBB valve’s primary goal is to provide two inline isolation points, in which create a physical barrier between the upward and downward sections of the system to prevent any contamination or cross-contamination between the two flows. This setup is common in trunnion ball valve desig,n as they stay fixed while the seats move against them to create a seal.
What is Double Isolation and Bridge (DIB)?
The DIB valve is a unique component used in pipeline systems. It is a two-stage valve that allows a fast shut-off of hazardous fluids as it combines isolation and pressure relief in one. This is achieved using Double Piston Effect (DPE) seats. Meaning, if the upstream seat fails, the downstream seat can still hold the pressure, offering an extra layer of safety.
The DIB valve is typically used in the oil and gas industry, cold storage plants, different nuclear power plants, and industrial processes.
DIB-1 (Both Seats are DPE)
In a DIB-1 configuration, both the upstream and downstream seats have Double Piston Effect seats. This means both seats seal from both sides.
For DIB-1, over-pressurisation of the cavity is prevented by using an external safety relief valve. When the ball is closed, and upstream pressure is present, a failed upstream seat increases cavity pressure, which pushes the downstream seat to create a second seal on the ball.
DIB-2 (One DPE and One SPE)
DIB-2 is a hybrid design, featuring one bi-directional (DPE) seat and one uni-directional seal (SPE). An external relief valve is not needed as the bidirectional upstream seat relieves pressure upstream, while the unidirectional downstream seat blocks downstream relief. Moreover, DIB-2 must be marked on the flange edge to indicate the correct installation position.
A trunnion ball valve drawing for a DIB-2 will show one seat that can move under cavity pressure and one that stays locked.
The Role of Trunnion Ball Valve and API 6D’s Importance
The distinction between DBB and DIB comes down to the trunnion ball valve design, as the ball is supported at the top and bottom, allowing spring-loaded seats to seal against it. Engineers use it to determine whether a seat is SPE or DPE by where pressure acts on the seat rings: pressure on one side creates an SPE seat, while pressure on both sides creates a DPE seat.
API 6D is the gold standard for pipeline valves. While many api 608 ball valve manufacturers produce valves that adhere to industry standards, API 6D valves have proven isolation capabilities and are particularly built for the risks of long-distance pipelines through multiple rigorous testing and specific definitions for block and bleed functions.
When assessing a double block and bleed valve, always check and aim for API 6D. Following this minimizes the risk of environmental leaks and ensures workers’ safety during downstream repairs.
Conclusion
Accurately identifying the differences between DIB-1, DIB-2, and DBB is crucial for pipeline integrity.
DIB-1 and DIB-2 provide double isolation that is essential for high-risk conditions, while DBB uses self-relieving seats to regulate cavity pressure, making them suitable for standard liquid applications. Moreover, following the API 6D standards ensures your system remains safe, compliant, and efficient.
Onero Valve is a premier manufacturer specializing in high-performance flow control, including the double block and bleed valve. We provide custom isolation solutions that meet strict API 6D standards and ensure consistent, long-term performance for critical energy infrastructure. Visit our about page to learn how our engineering team can support your next project.
