Do I need vents?
Most of the time I do not use tile vents because somewhere in the system a tile lateral will be above the water table and be able to let plenty of air in so that it flows steady at the outlet. Often the tile is eventually 5, 10 or even 30 feet above the outlet at some point. There is a place however that we have used vents with great success. We had a really flat field with minimal elevation that needed a lift system. This meant that it was not unusual for every foot of tile in the whole system to be below the water table. There was no place in the system for air to get to the tile without first going through water. When we finished the install and started the lift pump, I watched the water flow into the lift pit from the large 15 or 18" main (it was long enough ago, I cannot remember what I installed for him). The pump quickly drained the pit, and I expected the main to keep feeding water in, but it paused and girgled. That did not make sense, because I knew from having just installed the tile, that the system was waterlogged (we were actually still in the process of finishing the install). I quickly dug down to the main farther up the system and installed a 4" PVC vent. It immediately flowed steady and the pump did not kick off because the pit stayed full as the water flowed in. Eventually the pump caught up because we had sized the pump a bit over the capacity of the main and as I said, the laterals were not all installed yet. Before installing the main, the water was barely flowing in. My guess is that a vented 6" tile would have flowed faster than that big main was flowing without the vent. I then installed another vent at the far end of the main and the system has been working excellent for years. I do not recommend vents for every install, but if you put in tile on very flat ground without much rise, this could really help a system to work like it is supposed to work.