Column diameter is determined based on the constraints imposed by flooding. Before beginning a diameter calculation, we want to know the vapour and liquid rates throughout the column. We then do a diameter calculation for each point where the loading might be an extreme: the top and bottom trays; above and below feeds, side draws, or heat addition or removal; and any other places where we suspect peak loads. Once we have calculated these diameters, we select one to use for the column, and then check it to make sure it will work.