Why must you lock a spring hanger during the hydro test?

The spring hanger must be locked because in most instances, the hydro test exceeds the installed load and without the stops, the spring coil would deflect when subjected to that higher load. Secondarily, the travel stops act to bypass the coil so that the rigid components that make up the entire assembly are subjected to that hydro test load and it is not “seen” by the spring coil.