When loading or unloading compressed gas _________.

Compressed Gas:
Keep liquid discharge valves on a compressed gas tank closed except when loading and unloading. Unless your engine runs a pump for product transfer, turn it off when loading or unloading. If you use the engine, turn it off after product transfer before unhooking the hose.
Unhook all loading/unloading connections before coupling, uncoupling or moving a chlorine cargo tank. Always chock trailers and semitrailers to prevent motion when uncoupled from the power unit.