Double Block and Bleed Valve
Double Block and Bleed Valve
※ Fire Safe Design
※ Anti Blow Out Stem
※ Size Range: NPS 2 to NPS 28
※ Class Range: 150LB to 2500LB
※ Design STD.: API6D/ API608/ ISO17292
※ Double block and bleed Valve Manufacturer & Supplier
The primary issue with the use of valves in industrial applications is the downtime period for isolating and carrying out any maintenance in the system. Is it possible not to shut down the whole facility and just isolate the area you’re working on? The answer is yes; all you need is to install a double block and bleed valve.
Traditionally, engineers would install two normal isolation valves and a separate facility for bleeding the space in between to produce a double block and bleed system. The single compact block design by the double block and bleed valve manufacturers replaces the existing conventional approach used by pipeline engineers to achieve this arrangement in the pipeline.
Continue reading this article to know more about the construction and working of double block and bleed valves, their classification, special features, strengths, and the vast range of applications.
What Is A Double Block And Bleed Valve?
The Double block and bleed valve is designed to shut off the flow in a section of pipe on both sides of the valve rather than just one. In simple words, when you close the valve, it blocks both the upstream and downstream sides of your working area and then bleed any pressure that remains in the pipeline and valve.
API and OSHA are two US entities that have separately defined the double block and bleed valve or the DBB valve. According to API, the DBB valve is 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 venting/bleeding the cavity between the seating surfaces.”
While OSHA describes the 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.”
Most valve associations and block and bleed valve manufacturers follow and adopt the API’s or OSHA’s definition of DBB valves. However, some have created their handbook with their own definitions for industry terms.
Construction & Working Of Double Block And Bleed Valve
The double block and bleed valve or the DBB valve is a combination of two ordinary ball valves linked together by a common inlet end. These valves can perform the task of 3 separate valves, i.e., 2 separate valves for isolation and 1 valve as a drain.
DBB valves operate on the principle that isolation can be achieved from both the upstream and downstream process flow / pressures. This is achieved by placing two ball valves back-to-back separately or placed as a single unit.
A third valve, known as the bleed valve, is positioned in the center cavity of the two-block valves. After the isolation is achieved in the main process, the cavity that is created between these isolations can be drained using the bleed valve.
Bleed valves may be either used to directly vent to the atmosphere, or a hose connection/piping system can be used in case of a closed disposal system. According to double block and bleed valve manufacturers, the bleed valve basically serves four significant functions:
- Drain/vent the cavity created between two isolation valves
- Diversion of fluid flow
- Connection point for sampling and injection
- Connection for integrity or maintenance check to monitor leakage
Classification Of Double Block And Bleed Valve
DBB valves can be classified according to their body structure, bore type, and ball support. Some of the most common classification as per double block and bleed valve manufacturers is listed below.
Classification as per valve configuration
- Single unit DBB valve
- Cartridge type standard length DBB valve
- Three-piece non-standard length DBB valve
Classification as per body structure
- Modular or Split Body Type DBB valve
- Integral DBB valve
Classification as per bore type
- Full bore DBB valve
- Reduced bore DBB valve
Classification as per ball support
- Trunnion type DBB valve
- Floating type DBB valve
Double Block And Bleed Valve: Technical Specifications
- Size Range : NPS 2 to NPS 28
- Class Range: 150LB to 2500LB
- Design Standard : API6D / API608 / ISO17292
- Fire Standard : API 607 & API SPEC 6FA
- End Connection : Flanged; BW; Threaded
- Operation : Manual Lever / Pneumatic / Electrical
Double Block And Bleed Valve: Special Features
Double block and bleed valve manufacturers have included various design elements to improve the operating and safety standards of these valves to meet industrial requirements. Some of the unique features of the DBB valve includes
- DBB valves are equipped with two body gaskets, one is a soft material, and the second is graphite. This double body gasket arrangement ensures the best sealing even in a corrosive environment and provides a fire-safe design.
- A static-conduction spring is set between the stem and the ball of the DBB valve to prevent static electricity from igniting the fluid. So, these valves are anti-static, as well as fire safe.
- The stem of the DBB valve has an integrated T-Type shoulder for effective blow-out resistance. This design prevents the valve stem from getting blown out of the valve when working under pressure.
- The stem is made by cold rolling for the best surface finish to reduce friction during stem movement, and it also assists in effective sealing.
- Double block and bleed valve manufacturers pay careful attention to the valve’s life and operation during the design, development, testing, and manufacturing stages. This starts with the selection of the best quality material for the valve body and its parts.
- The seat design, stem-bearing system, and stem seals are arranged all together to provide low torque output.
- DBB valves are also provided with a locking arrangement to lock the valve in either the open or closed position.
Advantages of Double Block And Bleed Valve
The double block and bleed valve is a compact valve design that provides reliable isolation in a hazardous and critical environment. Some of the advantages of this valve include
- Can provide effective sealing against both upstream and downstream end pressure with zero leakage
- Easy venting of the cavity in between the isolation block valves
- Reduced weight and simple installation
- Improved operational safety in a hazardous environment
- Significant savings in space as compared to a conventional arrangement
- Provisions for chemical injection and sampling point
- Savings on additional piping requirements
- Prevents contamination of flowing media
- Permits equipment for easy servicing and repair
- Uninterrupted flow with negligible pressure drop
Applications Of Double Block And Bleed Valve
DBB valves are typically used where bleeding of the valve cavity is required. These valves find their application in scenarios like
- Prevention of product contamination is critical
- Equipment removal from service for cleaning or repair
- Hazardous liquid system, pipelines, and its manifolds
- Meter calibration
- Liquid service near waterways or municipalities
- Transmission and storage
- Chemical injection and sampling
- Instrument isolation such as pressure indicators and lever gauges
- Primary steam processes
Some of the major industrial applications of Double Block and Bleed Valve (DBB valve) includes:
- Oil & Gas industry
- Chemical and Petrochemical industries
- Industrial processes for LNG and Natural Gas
- Transmission lines for liquid and refined products
Double Block And Bleed Valve Selection Parameters
Depending on the requirement, various types of DBB Valve configuration is available in the market. The selection of double block and bleed valves depends on the following parameters.
- Process media and application
- Temperature and pressure
- Toxicity and operating environment
- Material of valve parts
How Is DBB Valve Different From DIB Valve?
The double block and bleed valve may appear quite similar to double isolation and bleed valve or DIB, which may be confusing for users. So, let us look at some basic differences between the two valves.
- A DBB valve seals against pressure from both sides of the valve, while a DIB offers additional sealing against pressure from only one side.
- A DBB Valve contains two unidirectional self-relieving seats that are pressure-relieving without the need for an external mechanism. On the other hand, a DIB valve employs one or two bi-directional seats that are not self-relieving. So an external pressure relief device needs to be installed in this design of the valve.
- If the first seal leaks in a DBB valve, the second will not seal in the same way, but a DIB valve offers an extra pressure barrier in addition to the primary pressure barrier.
As discussed in the article, the main reason to use a double block and bleed valve is to ensure that the fluid from upstream and downstream in a system does not reach the other components of the system. This allows the users to easily bleed off or drain the remaining fluid from the intermediate section in order to execute any maintenance, repair, or replacement work.
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