Double block and bleed valve (DBB ball valve) is double ball,which is used seat sealing structures for trunnion mounted ball valves. According to API 6D, DBB ball valve is a single valve with two sealed auxiliary, the closed position of which provides the pressure seal at both ends of the valve by means of bleed of the body cavity between the two seal surfaces, if the first seal leaks, the second will not seal in the same direction. 

The double block and bleed (DBB ball valve) pressure test :

The Double block and bleed valve (DBB ball valve) is partially opened so that the experimental flow is fully injected into the valve chamber, and then the valve is closed so that the bleeding of the valve body is open and the excess medium is allowed to overflow from the valve chamber test junction. Pressure should be applied simultaneously from both ends of the valve to monitor seat tightness through overflow at the valve chamber test junction. The figure below shows a typical Double block and bleed valve (DBB ball valve) configuration.

When the Double block and bleed valve (DBB ball valve)  is closed and the valve chamber test port is opened and both ends of the valve are pressurized (or pressurized separately), the valve chamber port detects leakage from each end to the valve chamber. Theoretically, DBB ball valve can not provide positive double isolation when only one side is under pressure, the vDouble block and bleed valve (DBB ball valve)  does not provide positive double isolation when only one side is under pressure.