Double Block and Bleed Isolation Double block and bleed (DBB) isolation is a safety and maintenance practice used in piping and process systems to achieve a secure and verifiable isolation of fluid flow. It involves the use of two isolation valves arranged in series with a bleed valve positioned between them. When both block valves are closed and the bleed valve is opened, any leakage past the first v valve can be safely released or monitored, confirming a complete seal before performing mainten