According to the API 6D definition, 'a single valve with two seat surfaces, in the closed position, seals against pressure from both directions, and the flow in the body cavity between the seat surfaces is discharged by means of a hole opened into the body cavity and an associated relief valve'. Block & Bleed valves. Especially when maintenance is required on pipelines, it allows maintenance without the need to stop the line or plant completely. It should be noted here that Double Block & Bleed valves are not equivalent to Double Isolation & Bleed valves. API 6D Double Isolation & Bleed definition; 'Valves with a single valve having two bearing surfaces, in the closed position, are sealed against pressure from a single direction, and the flow in the body cavity between the bearing surfaces is discharged by means of a hole drilled into the body cavity and a discharge main connected to it'. As can be understood from the name, while DBB valves provide 'Block' feature, they do not provide isolation like DIB. For example, a line needed maintenance and our DBB valve was closed. Let's assume pressure is coming from one direction. It is not abnormal for the valves to have tightness problems after a certain operation. While one of the bearing surfaces is under pressure, the other is under the influence of ATM pressure. Normally, even if the valve leaks, it may still not be a problem, as the flow will be constantly thrown out through the relief valve. However, let's assume that the relief valve does not work due to any malfunction and cannot discharge the flow. In this case, the body can be exposed to pressure up to 13-14 bar. This means that the bed surface open to the atmosphere is exposed to a reverse flow. If the pressure displaces the bearing, there can be a serious safety hazard if there are people working on the line at the time. For these reasons, the requirement of the line should not be analyzed well and the operation and personnel should not be endangered by using DIB where necessary!