Double Block and Bleed and the Effects of Its Definitions on Pipeline Safety

Double Block and Bleed (DBB) is the term commonly used to describe the systems or valving arrangements that provide double barrier pressure isolation of those performing work on a pipeline system downstream of the barrier. The consequences of releasing pipeline pressure downstream to an unsuspecting maintenance crew go without stating. If the risks are so high as to demand not single, but double barrier isolation between the fury of pipeline pressure and the safety of personnel and equipment, why then has it been so difficult to develop a consensus for the definition or description of DBB systems? This paper will explore in detail the internationally published definitions for DBB and analyze their merits with respect to pipeline safety. Definitions from organizations such as the American Petroleum Institute (API), Occupational Health and Safety (OHSA), the International Standards Organization (ISO) will be reviewed to determine what they contribute or fail to contribute to the intent of worker protection. Some Provincial Governments have written specific requirements for pipeline isolation into law, while others do not address the matter. These issues will also be explored and compared to the practices of companies operating oil and gas pipelines. Some insight on the division of definitions is offered through an understanding of the purposes or needs of the definer. For example, a valve manufacturer with a need to demonstrate that his valve will indeed block flow from both directions and thereby permit maintenance of the valve without removal from the pipeline may not fully appreciate the pipeline maintenance employee’s need to isolate himself from pipeline pressure by two independent barriers. It will be shown that standards and specifications are available to support both perspectives and that there are compelling reasons for deriving commonality between these perspectives. With the development of International Standards for oil and gas pipelines and increasing emphasis on the harmonization of various national standards, matters such as operator safety must not be compromised. Interesting opportunities exist to revisit topics such as definitions for DBB and perhaps derive a set of words that allows consensus and encompasses the true intent of the concept.