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How Does Double Block and Bleed Work? Isolation Safeguards for Your Pipeline and the Environment
by Aaron Madden, Senior Product Manager,
As regulatory scrutiny increases, here’s how double block and bleed isolation helps ensure pipeline integrity and reduces risk.
The regulatory landscape for pipeline operators is evolving amid heightened scrutiny from agencies and the public alike. have shifted their approach to oversight, no longer establishing prescriptive rules but setting performance-based standards instead. And while this change is largely meant to encourage innovation and give operators the flexibility to tailor compliance to their specific circumstances, it’s a double-edged sword: The burden on operators to demonstrate compliance and manage risks effectively is greater than ever. Today, meeting expectations for safer pipelines — especially in high consequence areas (HCA) where a failure could result in significant loss of life, property damage or environmental harm — requires an elevated emphasis on pipeline integrity. That includes identifying and addressing defects.
To ensure a safe work zone while a pressurized pipeline is being repaired, modified or maintained, operators often use plugging tools, like the հʱʳ® Train, classified as double block and bleed (DBB) to isolate the affected section. DBB is considered the most secure form of isolation, whether via mainline block valves or by isolation technologies.
Dual plugging heads of a հʱʳ® Train Isolation System
Double Block and Bleed Fundamentals
In general, a DBB procedure includes:
- Determining where the pipeline requires work.
- Installing a bleed port between the two plugging heads.
- Installing an isolation fitting on the pipeline on either side of the section to be isolated.
- Hot tapping, or cutting, into the pipeline through the newly added fitting.
- Inserting an isolation tool into the pipeline through the fitting and isolating the segment of pipeline.
- Opening bleed port to depressurize the isolated section in order to confirm seal quality of the plugging technology/tool.
- Bleed ports are also used to depressurize the anulus between the individual plugging heads for each Isolation Tool. At this step, the operator has the option to blow down to atmosphere or utilize cross compression technology to reduce emissions.
- Monitoring, flaring, or cross compressing any pressure increase during the isolation period.
- With the pipeline section isolated, maintenance or repair work can begin safely.
The importance of DBB technology:
- Provides a fully-rated redundant layer of protection against accidental release of hazardous materials.
- Ensures that the isolated section is completely free from hydrocarbons.
- Reduces the risk of human error and minimizes downtime. It also eliminates up to 97% of carbon emissions compared to traditionally blowing down large segments of in-service systems.
An illustration of a double block and bleed isolation.
What is Double Block and Bleed? It Depends Who You Ask
Despite the prevalence of DBB, there’s no standard definition for it in the pipeline industry.
For example, the focuses on valve design and functionality: In API 6D, it says DBB 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.”
, on the other hand, provides general safety principles and basic procedural steps for applying DBB in multiple industries, not just oil and gas, including LNG, petrochemical, transmission and storage, and water and wastewater. This underscores the technology’s broad applicability. Specifically, 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."
As for the Pipeline and Hazardous Materials Safety Administration (PHMSA), it often implies or references DBB in regulations related to pipeline integrity, maintenance and emergency shutdown procedures, but doesn’t offer a definition. PHMSA’s Pipeline Safety Regulations (49 CFR Part 195) involve the principles of DBB related to valve closures, pressure relief and verification procedures, while Hazardous Liquid Pipeline Regulations (49 CFR Part 192) address requirements for isolating pipelines containing hazardous liquids, which often necessitates DBB procedures.
Given the lack of an industry-wide definition that outlines specific parameters, requirements or limitations for DBB implementation, it’s not surprising that many companies have established their own . That means there can be variations in how even within the same industry.
Double Block and Bleed Versus Double Block and Monitor
DBB is not the only double barrier isolation method, operators also commonly use double block and monitor (DBM), particularly in offshore pipelines. There are many similarities between DBB and DBM, but there is one distinct difference.
A DBB isolation features two sealing elements with a bleed valve between them, while a DBM isolation has a similar set of two sealing elements, it does not require bleeding whatsoever. Instead, the isolation tool, like the ٱʱܲ®, has the ability to monitor the pressure in the annulus between the two sealing elements.
This monitoring capability provides an additional layer of safety and assurance during pipeline operations. By continuously observing the pressure in the annulus, operators can quickly detect any potential integrity issues with the isolation, allowing for timely mitigation. This feature makes DBM particularly valuable in challenging offshore environments, where maintaining a secure isolation is critical to both safety and operational efficiency.
A chart highlighting the different double barrier options for isolating a pipeline.
Double Block and Bleed Versus Double Isolate and Bleed
While DBB is the preferred isolation method in high-hazard industries, it’s not the only one. Operators also use double isolate and bleed (DIB), although mostly in lower-pressure systems.
What’s the difference? There are a few key dissimilarities.
For example, while a DBB valve typically has two independent sealing seats with a bleed valve between them, a DIB valve is a single valve with two seating surfaces. Each seating surface provides a seal against pressure from a single direction.
Beyond that, things get a little more complex when it comes to valves, because there are two types of DIB seat configurations. In DIB-1, both seats rely primarily on upstream pressure for sealing. This configuration requires an external relief mechanism to prevent pressure buildup in the valve body, something DBB does not. That’s because DBB has self-relieving seats — a design feature in valves that allows the seat to lift slightly under high-pressure conditions, preventing excessive pressure buildup within the valve body and reducing the risk of damage or leakage.
Like DBB, DIB-2 also has a self-relieving seat. It relies primarily on upstream pressure for sealing but can also be assisted by downstream pressure. DIB-2 also has a double piston effect (DPE) seat that relies on both upstream and downstream pressure to maintain a seal. The combination of these two seat types provides a robust sealing mechanism.
Perhaps most important, DBB can be deployed following a hot tap, which is performed while the pipeline remains in service.
DBB and DIB Require Direct Access
Both DBB and DIB are intrusive operations requiring direct physical access to the pipeline.
For DBB, that’s provided through hot tapping — using a hot tap machine to cut into the pipeline while it remains in service. While most DBB technology requires two taps, certain technologies allow it to be done with just a single tap. Two independent temporary plugging heads connected in a train formation are inserted into the tap, providing two elastomeric seals. A bleed port between the two seals evacuates remaining product. The use of a single tap reduces potential leak paths, and there’s less time and expense associated with welding, tapping, and other jobsite activities.
Hot tap machines are designed to handle specific pressure ranges and pipeline materials and are available in various sizes to accommodate different pipe diameters. Accurate machine positioning and cutting are crucial for a successful hot tap.
The tapping machine connects securely to the pipeline through fittings that are welded onto the pipe, becoming a permanent part of the system. High-pressure հʱʳ® fittings are considered the industry standard. (The registered trademark հʱʳ®, which is applied to plugging machines and its DBB train system in addition to fittings, has become a generic term in the oil and gas industry, much like Xerox or Kleenex.)
Once the հʱʳ® fittings are in place and the hot tap has been made, crews can deploy and set the DBB technology.
Hot Tapping or Shutdown?
There’s no question that cutting into a pressurized pipeline carries with it certain risks. Fortunately, proper planning and the use of specialized equipment by experienced, highly skilled personnel mitigates those concerns.
Still, under certain conditions, some operators prefer shutting down their entire system prior to pipeline repair.
Blowing down a large section of pipeline is not a budget- or environment-friendly alternative.
For one thing, it results in significant economic impacts related to supply disruptions, lost revenue and the high costs of planning, labor, equipment and logistics. In addition, the process of taking the line out of service and bringing it back is not only difficult, time-consuming and costly, but depressurization is a leading source of emissions during pipeline maintenance and repair. Especially since you can utilize isolation technologies to reduce up to 97% of emissions per mile of piping system.
Using isolation technologies minimizes downtime, production loss and environmental impact compared to traditional system shutdowns. The hot tapping procedure is designed to be efficient, reducing the total time required for pipeline repair or modification. Following best practices can minimize, and virtually eliminate product venting, negating much of the fugitive emissions associated with purging and packing processes during shutdown and restart and minimizes operational interruptions.
Ultimately, though, the choice between leveraging isolation technologies and system shutdown depends on a variety of factors, including specific project requirements, pipeline characteristics and risk assessment.
Conclusion
In the evolving landscape of pipeline operations, where safety, regulatory compliance, and environmental considerations are paramount, choosing the right isolation method is critical. Double Block and Bleed (DBB) technology stands out for its ability to provide a secure, reliable solution that minimizes risk during pipeline maintenance and repairs, particularly in high-exposure environments. While other methods like Double Block and Monitor (DBM) and Double Isolate and Bleed (DIB) offer valuable alternatives, DBB's proven track record and versatility—especially when combined with advanced hot tapping techniques—make it the preferred choice for operators aiming to maintain pipeline integrity without compromising operational efficiency. As the industry continues to adapt to increasing regulatory demands, innovations in isolation technology will remain key to ensuring both the safety of pipelines and the protection of the environment.