The truth of using DBB

The truth of using DBB

How does it Work: Double Block-and-Bleed & Double Isolation-and-Bleed

Introduction

There is a lot of misunderstanding surrounding the term double block-and-bleed (DBB) as used to describe valve functionality. It seems almost every valve buyer and manufacturer has a different idea of what the term means for valve selection, which can result in the wrong specifications or valve type. Some of this confusion in the oil and gas industry stems from the fact that there are two credible 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),” Ron Manson, Cameron’s Director of Applications Engineering, explains. These differences in definitions and terms are important when it comes to which valve capability to use in what type of system.

“Whenever a customer says they want DBB, it is mainly based on the application and their past experience with what they consider DBB valves to be,” says Mike Hintz, Cameron’s District Manager for Engineered & Process Valves. “Based on customers’ preferred valve type and application, we can then offer ball, gate, or expanding plug valve options with DBB capabilities.”

The most basic thing that a user is looking for when they specify a double block-and-bleed valve is a compact valve or valve system that provides more reliable isolation in critical areas than a standard, single valve would. This smaller system or single valve unit serves to reduce the installation footprint, saves on extra piping requirements, and reduces weight in critical areas. These all save space, time, and cost.

DBB and DIB Definitions

There are two entities in the United States that define DBB – the American Petroleum Institute (API) and the Occupational Safety and Health Administration (OSHA). API is a trade organization that acts as an advocate as well as researcher for America’s oil and natural gas industry, outlining many industry guidelines for safe operation. OSHA is the branch of the United States Department of Labor that is charged with enforcing health and safety legislations, outlining legislation that protects both people and our environment.

According to API 6D “Specification for Pipeline Valves” standards, a double block-and-bleed 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.” API also notes in this definition that this valve does not provide positive double isolation when only one side is under pressure.

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.”

API’s DBB definition does not achieve the same level of isolation as the OSHA’s definition. API allows DBB valves to be one single valve with two unidirectional seats, while the OSHA standard can only be achieved with two separate valves with a method to bleed pressure in between. There are some valves that utilize a twin valve design. By combining two valves into one body, a twin valve design reduces weight and potential leaks paths, while meeting the OSHA requirements for double block-and-bleed.

Valve associations usually choose to follow either API’s or OSHA’s definition, but some have created their own handbook with their own definitions for industry terms, DBB included. For example, the British Valve & Actuator Association (BVAA) defines double block-and-bleed as “a manifold that combines one or more isolation valves, usually ball valves, and one or more bleed/vent, usually a needle style global valve, into one assemble for interface with other components (e.g., pressure measurement transmitters, pressure gauges, and switches).”

BVAA, like API, says that for DBB capabilities, only one valve is required, not a system. According to BVAA, “DBB valves replace the previous traditional technique employed by pipeline engineers to create a double block-and-bleed configuration in the pipeline, usually by fabricating three valves using flanges, ‘Tee’ pieces, and associated bolting.”

Double isolation-and-bleed (DIB) is another term in the industry that makes the debate about DBB even more confusing. API defines DIB as a “single valve with two 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 in both directions.

The difference between API’s DBB and DIB is that a double block-and-bleed valve seals against pressures from both sides of the valve, while a double isolation-and-bleed valve provides an additional seal against pressure from only one side. It is important to use a DIB valve instead of a DBB valve in applications that require an additional pressure barrier that seals separately of the main pressure barrier. This is important to fulfill certain operational safety requirements or according to the nature of the service, such as low tolerance for leakage or the cleanliness of the fluid.

Another big difference between DIB and DBB is the ability to relieve pressure. Usually with a DBB valve, there are two unidirectional self-relieving seats. These seats do not rely on an outside mechanism to relieve pressure. On the other hand, a DIB valve utilizes one or two bidirectional seats. The valve provides double isolation from pressure at both ends of the valve, but there is usually one operational drawback, a DIB valve cannot relieve body cavity pressure past the seats, meaning its seats are not self-relieving. When using a DIB valve, an external relief system is necessary to relieve pressure buildup.

A DBB or DIB valve can provide isolation in both the upstream and downstream directions, even in high-pressure or high-temperature situations. Isolation is important in cases where leakage through a valve could have major consequences.

Once the fluid is isolated, the bleed mechanism can drain the area between the two valves or two seating surfaces. This is important for maintenance and/or integrity check situations where leakage can be monitored.

Applications and Benefits of DBB and DIB

Both DBB and DIB valves save space, reduce the need for costly multivalve systems, and provide zero leakage capabilities from upstream to downstream. The valve also allows for an integrity check of seat seals. Both double block-and-bleed and double isolation-and-bleed valves can be used in a variety of applications and markets, such as LNG, petrochemical, transmission and storage, natural gas industrial processes, mainline and manifold valves in liquid pipelines, and refined products transmission lines.

DBB and DIB valves are used in applications where critical isolation is needed to ensure that leakage does not occur. Depending on the application and type of service, a wide range of DBB or DIB valve types and options are available to fit specific needs. For example, in liquid service near waterways or municipalities, double-expanding gate valves with DBB capabilities, like Cameron's WKM® Pow-R-Seal™ gate valves, are preferred for critical isolation because they provide a tight mechanical seal, upstream and downstream simultaneously, which is normally unaffected by pressure variations or vibrations.

“When I teach our sales force and customers about double block-and-bleed, I use the API 6D definition,” says Cameron’s gate valve product manager, Anthony Carrea. “In my presentations, I explain that a DBB valve is a valve with two seats and when closed, will seal against pressure from both ends of the valve allowing the body cavity to be bled.”

Another application in which DBB and DIB valves are used is the meter calibration market. Every closed valve in the meter system must seal drop tight. Even a small leak will cause errors in the meter calibration and the incorrect meter factor will persist until the next proving operation. This can cost end users huge sums of money. Choosing the correct API verified DBB or DIB valve can help ensure correct calibration almost every time.

The mechanical wedge action of a double block-and-bleed plug valve, like Cameron's GENERAL VALVE® Twin Seal™, compresses both the upstream and the downstream seals firmly against the valve body, needing no help from the line pressure to affect a positive seal. Double block-and-bleed plug valves, which are used a great deal in the transmission and storage markets, provides consistent and provable zero leakage of various fluids.

Conclusion

“When determining whether to use a DBB or DIB valve and which definition to follow, API or OSHA,” says Manson. “It is important to have a clear understanding of the similarities and differences of the types of block-and-bleed valves and the specific application the valve will play in, additionally it is important the features required for isolation are fully tested during factory acceptance testing of the valve.”

Since DBB and DIB have become such generic terms in the industry, it is important to take into consideration the application, media, and various environmental challenges when choosing the appropriate solution and valve type.

The following table will be part of an Informative Appendix in the next edition of API 6D which is currently out for ballot. The table has been augmented with a column showing where the valve types comply with OSHA regulations.