Some thoughts away from mainstream:-
Check valves installed the wrong way around. There was a famous plane crash where the arrow on the hydraulic check valve was cast incorrectly the wrong way around.
Check valves selected without design for dynamic conditions. In a surge analysis the reverse velocity vs the dv/dT needs to be determined and the valve selected accordingly.
Air valve selection can be too large or small. The selection and location needs to consider filling, draining, pipe burst and dynamic behaviour of the system. Also if buried in a bolted cover pit they cant vent of fill.
No maintenance on sewage air valves and they clog up.
Valves selected for solids service need to be considered for wear of components. Sewage is a nightmare as stringy parts catch up in the system.
Wafer butterfly valves stcuk in the open position unable to be removed without massive excavation of the pipeline. Wafer valves selected without considering removal of pipe or equipment downstream. Select full flange type.
Incorrect or no drawings of the valve installation so cannot be found when you need to isolate or maintain. Sydney Water had to get someone out of a retirement village hoping his alzheimers hadnt kicked in as this guy was the only one who knew where a particular isolation valve was buried.
Knife gate valves where fastners in the chest bear onto the slide plate and stop it operating.
Inside screw gate valve in dirty or corrosive service.
Relief valves unsupported so discharge force stresses pipe.
Suggest you get hold of the book " What went wrong!"
"Sharing knowledge is the way to immortality"
His Holiness the Dalai Lama.
