- Published on Tuesday, 13 May 2014 16:19
- Written by Ron Manson and Tashika Varma
The term “double block-and-bleed (DBB)” carries a lot of misconception when it’s used to describe valve functionality. Every valve buyer and manufacturer seems to have a different idea of what the term means for valve selection, which can result in misguided specifications or the wrong choice of valve type. In the oil and gas industry, some of this confusion stems from the fact there are two credible sources that define the term differently. Another point of confusion comes because many people using the term double block-and-bleed really want a valve with double isolation-and-bleed (DIB) capabilities. However, the differences in definitions and terminologies involved are important when it comes to determining which valve capability is needed for what type of system.
DBB AND DIB DEFINITIONS
The most basic thing 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. All of this saves space, time and cost.
Two entities in the United States define DBB—the American Petroleum Institute (API) and the Occupational Safety and Health Administration (OSHA). API, as a trade organization, acts as an advocate as well as researcher for America’s oil and natural gas industry, outlining many industry guidelines for safe operation. OSHA, as a branch of the U.S. Department of Labor, is charged with enforcing health and safety legislation and outlining regulation that protects both people and the environment. The two group’s guidelines reflect these two purposes.
According to API 6D, Specification for Pipeline Valves, 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 notes in the definition that this valve does not provide positive double isolation when only one side is under pressure.
In contrast, OSHA 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 OSHA’s definition. API allows DBB valves to be one single valve with two unidirectional seats, while the OSHA standard can only be achieved through two separate valves with a method to bleed pressure in between. Some valves use a twin valve design. By combining two valves into one body, a twin valve design reduces weight and potential leak paths, while meeting the OSHA requirements for double block-and-bleed.
Valve associations usually choose to follow either the API or OSHA definition, and some have created their own handbooks with definitions for industry terms, including DBB. For example, the British Valve and 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 globe valve, into one assembly 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.”1
Double isolation-and-bleed confuses the debate about DBB. 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, then, 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. For applications that require an additional pressure barrier that seals separately from the main pressure barrier, it is important to use a DIB valve instead of a DBB to fulfill operational safety requirements. It’s also important in reference to the nature of the service, such as when low tolerance for leakage or the cleanliness of the fluid is in question.
Another big difference between DIB and DBB is based on 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 uses one or two bidirectional seats. The valve provides double isolation from pressure at both ends of the valve. However, there is an 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, then, an external relief system is needed to relieve pressure buildup.
A DBB or DIB valve can provide isolation in both upstream and downstream directions, even in high-pressure or high-temperature situations. This isolation is critical 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 or for integrity check situations where leakage can be monitored.
APPLICATIONS AND BENEFITS
Both DBB and DIB valves save space, reduce the need for costly multivalve systems and provide zero leakage capabilities from upstream to downstream. Such valves also allow for an integrity check of seat seals. In the oil and gas fields, both double block-and-bleed and double isolation-and-bleed valves can be used in a variety of applications and markets, such as liquefied natural gas, petrochemical, transmission and storage, natural gas industrial processes, mainline and manifold valves in liquid pipelines, and refined products transmission lines.
Both DBB and DIB are used in applications for which critical isolation is needed to ensure that leakage does not occur. The choice of what type depends on the application and type of service. For example, in liquid service near waterways or municipalities, double-expanding gate valves with DBB capabilities 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.
Another application in which DBB and DIB valves are used is meter calibration. Every closed valve in a 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 compresses both the upstream and the downstream seals firmly against the valve body, which means no help is needed from the line pressure to affect a positive seal.
Double block-and-bleed plug valves, which are heavily used in the transmission and storage markets, provide consistent and provable zero leakage of various fluids. When determining whether to use a DBB or DIB valve and which definition to follow (API or OSHA), it is important to have a clear understanding of the similarities and differences. It is also vital to take into account the specific application for the valve and ensure that the features required for isolation are fully tested during factory acceptance testing of the valve. VM
1Greenhalgh, Martin. Valve and Actuator Users’ Manual. 6th ed. Oxfordshire: British Valve & Actuator Association, 2010. Print.