Hello . Im a newbie to the boards . Being in Chicago we have relief valves on all systems that have more than thirty pounds of refrigerant in the seal system . Third year in the trade I was sent out to pump down a trane 40 ton condenser . Having my gauges after the liquid line valve i didn't realize that the pressure got above the 450 pd setting of the valve . My hearing didn't come back for a few hours . Only happen once . Live and learn .