I am going to make a guess on this. The foam is formed by the water dropping over the bioballs. The foam needs to exit the fractionator, to be skimmed. If the water has to come up and over, then the foam will stay in the tower portion and not flow out, as it is lighter than the water that will be pushed outward. The design is such that the amount of water entering the system is not as great as the capacity of the exit pipe, so foam rides out on the surface. The little skimmer plate causes the foam to build in height to the point of flowing out the exit pipe. To assist in getting the foam to exit, a valve is put in the line to cause some reduction of flow, increasing the depth of the water within the exit pipe, so the foam doesn't have to be as deep to start the exit/
Zone 7 A/B
Keep your words sweet. You never know when you may have to eat them.
Richard