Talk about single block and bleed ball valve,With regards to single block and bleed valves, you can either install as two separate valves (i.e. a block valve with a bleed drain downstream in the pipework) or you could install an integral single block and bleed unit.Recommanding the use of a single Gate valve with a "split" wedge and the tap at the bottom canter of valve body drilled out and fitted with a bleed valve.
This was done on one of my projects long ago.  I think it was a Crane valve.
The downstream vent version is commonly used to vent stored energy in pneumatic systems.  When the valve is closed - air to the tool or hose is vented so that it can be disconnected or worked on safely. One application of the upstream vent version is as a steam trap test valve. When valve is closed, upstream steam/condensate is vented and observed.
In the plumbing industry, they're referred to as "Stop and Waste" valves.

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