The skim pile is a type of disposal pile. As shown in Figure 7-18, flow through the multiple series of baffle plates creates zones of no flow that reduce the distance a given oil droplet must rise to be separated from the main flow. Once in this zone, there is plenty of time for coalescence and gravity separation. The larger droplets then migrate up the underside of the baffle to an oil collection system.
Besides being more efficient than standard disposal piles, from an oil separation standpoint, skim piles have the added benefit of providing for some degree of sand cleaning. Most authorities having jurisdiction state that produced sand must be disposed of without “free oil.” It is doubtful that sand from a vessel drain meets this criterion when disposed of in a standard disposal pile.
Sand traversing the length of a skim pile will abrade on the baffles and be water washed. This can be said to remove the free oil that is then captured in a quiescent zone.