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Double block & bleed valve (DBBV) is a non-lubricated, resilient seal, plug-type valve which has a mechanical means of freeing the plug before it is rotated from the closed to the open position.
In opening the valve, the plug is raised, thus retracting the seating segments or slips through their tapered dovetail connections. Only after the slips are fully retracted from the body seat is the plug rotated to the open position.
Conversely, in closing the valve, the plug and slips are rotated freely, with no seal-to-body contact until the slips are positioned over the ports.
Then the plug is driven down between the slips and the tapered surfaces wedge out the slips for a positive upstream as well as downstream shut-off.

The small DBB general plug valve is handwheel operated, and require up to 3 turns to open or close. Up to 2 3/4 turns expand or retract the slips, while turn rotates the plug. Large valves operate in a similar manner, except that they have enclosed weather-proof worm gearing.
At the top of the valve, a position indicator flag shows the exact plug position. It appears in line with the flow when the valve is open and perpendicular to the flow when the valve is closed.
Since DBBV hold bubble-tight, for ease of opening in liquid service, it is important to prevent trapped body pressure from exceeding the working pressure of the valve.
Therefore, a relief system is required to prevent pressure buildup in the body cavity.
For maximum upstream sealing, do not back off the hand wheel.
Warning: Do not over torque by using cheater bars.