DBB ball valves combine two isolation valves and a bleed valve into a single body, ensuring positive, verifiable double isolation for critical applications. Their design adheres to API 6D or API 608 for ball valve principles, while the overall pressure integrity is per ASME B16.34. Inspection and testing are exceptionally stringent: in addition to the shell test, testing must confirm zero leakage on the upstream seat, and then the bleed valve is used to verify zero leakage across the downstream seat. This rigorous sequential testing is essential for verifying the integrity of the total DBB system per API 6D.