The BDV, AFZ and HVV vent valves are designed to drain (blowdown) or vent (depressurize) pipework, steam traps, valves and others.
BDV – Blowdown valves – Manual discharge of strainer screen contents and generally to be
used as a drain and/or depressurization valve.
BDVL – Same as above, but also allows the discharge to be connected to a safe location.