Positive isolation is a method of flow control that involves physically disconnecting or removing a spool, which is a short piece of pipe or tubing that connects two sections of a system. This is considered the most reliable method of isolation, as it physically separates the two sections and prevents any accidental flow. Positive isolation typically involves the use of blind flanges, which are special connectors that seal the connection between two pipes or valves. These flanges are held in place by studs and nuts, which apply mechanical force to create a seal. This mechanically energized seal is the core of positive isolation, as it prevents any substance from leaking through the connection.

However, the process of switching between positive isolation and reconnection can be complex and costly. It typically involves re-assembling the flanges, bolts and, nuts that were used for the isolation, which can be time-consuming and require specialized tools and skills. In addition, any substances that have been isolated may need to be purged or drained before the system can be safely reconnected.