20mm Single union ball valve-Socketed Solvent Weld

A double-union device, should always be used where the placement is not at an end of the line. That way if it ever needs to be removed for replacement or service, it can be done easily.

The single-union will not have enough clearance to be removed since it’s likely the pipes on either side of it will not be movable. As you unscrew the non-union side of the single union ball valve, the union side will be pushed forcibly into the pipe and likely prevent you from getting it off as the pipe joined to the union side will prevent enough clearance for the valve to be removed.