I think we have established that trunk mains should have a minimum slope, to and from air valves, to aid air evacuation. But this is not necessary on distribution pipelines where air is easily evacuated through hydrants and connections.
For main transmission pipelines the rate of filling and air valve sizing need to be selected to avoid high velocities under air pockets and air valve slam causing transients. This is usually of less concern with high pressure pipelines because the pipes are not pressurised to their design pressure until after they are filled.
Also with steep pipes it may be necessary to limit the filling rate to avoid high velocity free surface flow and possible cavitation damage to the cement mortar lining.
