Most commonly used water treating equipment items rely on the forces of gravity to separate the oil droplets from the water continuous phase. The oil droplets, being lighter than the volume of water they displace, have a buoyant force exerted upon them. This is resisted by a drag force caused by their vertical movement through the water. When the two forces are equal, a constant velocity is reached, which can be computed from Stokes’ Law as:
Several conclusions can be drawn from this simple equation:
1. The larger the size of an oil droplet, the larger the square of its diameter, and, thus, the greater its vertical velocity. That is, the bigger the droplet size, the less time it takes for the droplet to rise to a collection surface and thus the easier it is to treat the water.
2. The greater the difference in density between the oil droplet and the water phase, the greater the vertical velocity. That is, the lighter the crude, the easier it is to treat the water.
3. The higher the temperature, the lower the viscosity of the water, and thus the greater the vertical velocity. That is, it is easier to treat the water at high temperatures than at low temperatures.