A common question we hear from many of MRC Global’s customers concerns the definition of a double block-and-bleed valve. There is confusion because there are many interpretations of the term double block-and-bleed (DBB) when describing valve functionality. It seems that almost every end user and manufacturer has a different idea of what the term means for valve selection, which can result in the wrong specifications or valve type. The issue gets even more confusing because the term double block-and-bleed is commonly used when requesting a valve with capabilities of double isolation-and-bleed (DIB), which is a different design. American Petroleum Institute (API) specifications now refer to and describe both. Based on a customer’s preferred valve type and application, MRC Global can then offer ball, gate or expanding plug valve options with DBB capabilities. But the first challenge is to determine which valve is the best solution for the application.
According to API 6D “Specification for Pipeline Valves” standards, a double block-and-bleed (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.” API also notes in this definition that this valve does not provide positive double isolation when only one side is under pressure.
At MRC Global, we teach our sales force and customers about double block-and-bleed using the API 6D definition. Since DIB and DBB 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. Additionally, it is important that the features required for isolation are fully tested during factory acceptance testing of the valve by a quality reputable manufacturer.
If you have questions about which valve design is best for your specific application, please contact us. With a staff of engineers throughout our company and even more technically trained representatives, we can help you review your options and choose the best overall solution for your need.
About the Author
Greg Peterson has been with MRC Global since 2006, and in the valve industry on both the manufacturing and distribution side for nearly 25 years. Greg’s understanding of technical valve and valve automation solutions, combined with his passion for sharing that knowledge, has earned him a reputation as an expert throughout our industry. He is a regular speaker and attendee at API, VMA and Valve World events and conferences.