Comprehensive Guide to DBB Valves and DIB Valves
7 min read
1. Overview
The development of DBB valves (Double Block and Bleed valves) and DIB valves (double isolating and bleeder valves) in pipeline systems began with the construction of the first 9 km oil pipeline in Central America in 1914, marking the beginning of the pipeline industry. The rapid development of the industry took place in the 1950s. According to statistics, the total length of global pipelines has reached 2.6 million kilometers, of which 800,000 kilometers of oil pipelines, 1.4 million kilometers of natural gas pipelines, and 400,000 kilometers of other petrochemical products. Pipeline is an essential energy supply line, and has become the five major transportation systems alongside railway, highway, waterway and aviation.
Pipe valves are rated up to PN42.0Mpa (Class2500), with a maximum diameter of 60 inches (DN1500). The cost of transporting liquids or gases by pipeline is only one-third of that by rail and two-thirds of that by sea, which has contributed to their rapid development over the past century. Valves used in long-distance pipelines are known as pipeline valves and include ball valves, gate valves, check valves, and plug valves. These valves meet the unique requirements of pipeline transport and possess special functions, such as the pipeline ball valve.
2. API Standards and Valve Definitions
The American Petroleum Institute (API) specified the characteristics of DBB valves and DIB valves in sections 3.1.15 and 3.1.16 of the 25th edition of API 6D (Specification for Valves) in November 2021. These definitions are critical for ensuring the correct application and functionality of the valves in various pipeline systems.
2.1 DBB and DIB Definitions
Double Block and Bleed (DBB) Valve
According to API 6D:2021:
- DBB Valve: A valve with two or more 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.
Double Isolation and Bleed (DIB) Valve
According to API 6D:2021:
- DIB Valve: A valve with two or more seating surfaces, each of which, in the closed position, provides a seal against pressure from a single source, with a means of venting/bleeding the cavity between the seating surfaces. This feature can be provided in one direction or both directions.
3. Valve Seat Structures: SPE and DPE
To understand the differences between DBB valves and DIB valves, it is essential to first explore the two common valve seat forms in fixed ball valves: Single Piston Effect (SPE) and Double Piston Effect (DPE).
3.1 Single Piston Effect (SPE)
Working Principle
The Single Piston Effect (SPE) valve seat operates by utilizing the pressure differential across the valve seat to achieve sealing. Here’s a detailed look at how the SPE valve seat functions:
- Valve Closure: When the valve is in the closed position, the medium pressure from the pipeline acts on the valve seat.
- Pressure Differential: If the pipeline pressure (right-side pressure) is greater than the pressure on the other side of the valve seat (left-side pressure), the medium pressure exerts a force on the valve seat. The area subjected to this pressure is denoted as Aa.
- Sealing Force: The force created by the medium pressure, combined with the spring force, pushes the valve seat against the ball, achieving a seal. The effective area (Aa) is greater than the internal area (Ai), resulting in a net sealing force.
- Pressure Relief: If the pressure inside the valve cavity (left-side pressure) exceeds the pipeline pressure (right-side pressure), the medium pressure on the internal area (Ai) overcomes the spring force, pushing the valve seat away from the ball, thus breaking the seal and allowing pressure to escape.
Schematic Illustration
3.2 Double Piston Effect (DPE)
Working Principle
The Double Piston Effect (DPE) valve seat provides bidirectional sealing, making it a more versatile solution compared to SPE. Here’s how the DPE valve seat functions:
- Valve Closure: Similar to the SPE, when the valve is in the closed position, the medium pressure from the pipeline acts on the valve seat.
- Bidirectional Sealing: The DPE seat is designed to seal against pressure from both directions. This is achieved by having two piston effect areas:
- First Sealing Area: When the right-side pressure is greater than the left-side pressure, the medium pressure acts on the valve seat, creating a sealing force as in the SPE configuration.
- Second Sealing Area: When the left-side pressure is greater than the right-side pressure, the medium pressure on the internal area (Ai) acts in conjunction with the spring force to push the valve seat against the ball, ensuring a seal.
- Sealing Efficiency: The DPE design ensures that both sealing surfaces (upstream and downstream) are engaged, providing a robust seal against pressure from either direction. The effective sealing area (Aa) is greater than the internal area (Ai), ensuring that the valve remains sealed under varying pressure conditions.
Schematic Illustration
4. Categorization of DBB Valves and DIB Valves
Based on the upstream and downstream applications of SPE and DPE valve seats, API 6D defines specific types of DBB valves and DIB valves:
Upstream Seat | Downstream Seat | API 6D Type | Performance Characteristics |
---|---|---|---|
SPE | SPE | DBB Valve | No installation direction requirement; valve cavity pressure automatically releases through the low-pressure side; risk of medium leakage; internal leakage check possible via valve body relief valve (if installed). |
DPE | DPE | DIB-1 Valve | No installation direction requirement; includes DBB functionality; valve cavity pressure must be manually released to prevent overpressure; provides bidirectional isolation. |
SPE | DPE | DIB-2 Valve | Installation direction required; includes DBB functionality; upstream pressure automatically releases through the seat; downstream provides double isolation. |
DPE | SPE | DIB-2 Valve | Installation direction required; includes DBB functionality; downstream pressure automatically releases through the seat; upstream provides double isolation. |
A special form of the DDB valve, combining two ball valves, is known as the double ball valve.
5. Recommended Selection Guide
Selecting the appropriate valve seat structure for DBB valves and DIB valves depends on the medium being transported and specific operational requirements. Here is a detailed guide:
- For Gas Medium: Use the DPE (Double Piston Effect) structure for valve seats. The DPE structure provides bidirectional sealing, which is essential for gas mediums to ensure no leakage occurs in either direction.
- For Liquid Medium: Use the SPE (Single Piston Effect) structure for valve seats. The SPE structure allows for automatic pressure relief within the valve cavity, which is particularly useful for liquid mediums.
- DPE (DIB-1) or SPE on Both Ends: When both ends use either DPE (DIB-1) or SPE seats, there is no specific requirement for medium flow direction. This configuration provides flexibility in installation and operation.
- SPE and DPE Combination (DIB-2): When one end uses an SPE seat and the other end uses a DPE seat (DIB-2), the valve must be installed with a specific flow direction. Ensure the medium flows in the direction where the upstream seat is SPE and the downstream seat is DPE, allowing for automatic pressure relief upstream and double isolation downstream.
- Upstream SPE, Downstream DPE: In this configuration, if the pressure in the valve cavity becomes too high, it can automatically release to the upstream pipeline through the SPE seat. This setup is ideal for systems where pressure buildup needs to be managed efficiently.
Safety and Maintenance Considerations
- For DIB-1 Valves: Both seats are bidirectional and do not have self-relief functions. If the medium has characteristics prone to sudden pressure increases (e.g., gasification), a safety relief valve must be installed to release pressure when necessary.
- For Non-Pressure Sensitive Gas Mediums: For gases like natural gas that do not exhibit significant pressure increases, a safety relief valve may not be required, simplifying management and reducing potential leak points.
Conclusion
The evolution of DBB valves and DIB valves in pipeline systems has been significant since their inception, driven by the expanding pipeline industry and advancements in technology. Understanding the distinctions between DBB valves and DIB valves and their appropriate applications ensures effective pipeline operation and safety.
FAQs
- What is the highest pressure rating for pipeline valves?
- The highest pressure rating can reach PN42.0Mpa (Class2500).
- What are the two main types of valve seats in fixed ball valves?
- The two main types are Single Piston Effect (SPE) and Double Piston Effect (DPE).
- How does the SPE valve seat function?
- The SPE valve seat provides unidirectional sealing by pushing against the ball under pressure.
- What is the difference between DBB valves and DIB valves?
- DBB valves provide a seal against pressure from both ends with venting capability, while DIB valves provide isolation from a single pressure source with venting capability in one or both directions.
- When should DPE structure be used for valve seats?
- The DPE structure is recommended for gaseous media to ensure bidirectional sealing.
- What is the primary function of a DBB valve?
- The primary function of a DBB (Double Block and Bleed) valve is to isolate a section of the pipeline by sealing against pressure from both ends and providing a means to bleed the cavity between the two sealing surfaces. This ensures no fluid or gas passes through, allowing safe maintenance and operation.
- Why is it important to consider the medium when selecting a valve seat structure?
- It is important to consider the medium (gas or liquid) because different mediums have varying pressure characteristics. For instance, liquids benefit from the SPE (Single Piston Effect) structure for automatic pressure relief, while gases require the DPE (Double Piston Effect) structure for bidirectional sealing to prevent leakage and ensure safety.