Gravity separator basics.

Gravity separation is a very simple principle, and most people have a basic understanding of it. However, there are a number of aspects that people are seldom aware of, and very frequently basic rules are violated.

This, by means of a series of animations, explains oil separation in a simple tank, in a classic separator, in a separator with baffles and in a Suparator® separation system.

The information in this section allows the reader to compare oil separators of various constructions and clearly shows how conditions improve when using the Suparator® technology.

About density, gravity and flow.

Typically, oil, and comparable substances, have a lower density than water, and gravity will cause the lighter oil to rise to the surface and float on top of the water. That's what gravity separation is.

Ideally, for an oil water mixture to separate, there should be no flow or agitation at all. That is what settling tanks are about. Normally though, the amount of fluid to be treated dictates flowing it through a device to separate the oil.
In that case the flow inside the separation device should be horizontal and laminar. Horizontal so that the direction of the flow is perpendicular to the direction of the gravity forces; the flow does not interfere with the upward traveling of oil drops. Laminar to avoid re-mixing of oil and water.

Oil separation in a simple tank.

When feeding an oil/water mixture through a tank or separator, oil, due to its lower density, will tend to rise to the surface and accumulate on top. This is what is called gravity separation.
Important thereby is the flow in the separator. A strong turbulent flow will mix oil and water; the oil can not escape to the surface. A horizontal laminar flow is ideal as this gives the oil the best change to reach the surface.

Observe the flow through the tank in the sketch at the left. The inlet is on the left, the outlet on the right. In the middle of the tank, the flow uses the full height of the tank and, provided the flow rate is not too high, the flow pattern will be laminar.

Notice that the corner sections of the tank do not really participate. There is no flow through the corners, which reduces the effective space and thereby the performance.

Click the image to let the oil in. In a simple tank, little will happen at the entrance side. The flow will push the oil layer towards the outlet side, and the downward flow at the end will pull along oil and dirt.

Oil separation in a classic separator.

To improve the design, the separator on the left is provide with a baffle at the inlet side, to divert the flow, and with under and over flow weirs at the right end to maintain a constant high level and minimize the oil that escapes.

The available space is used more effectively and with a laminar flow pattern the oil will rise to the surface well enough. To create a laminar flow, the separator should be dimensioned correctly ( L x W = 2 : 1 and a height of 80 to 100 cm) and the flow rate must not be too high for the size separator.

Click the image to let the oil in. The standard separator arrangement is a significant improvement. Most of the tank is used and less oil can be pulled down, below the underflow weir. The floating oil will initially be pushed towards the underflow weir, but as the layer gets thicker, it will cover the entire surface of the separator.

Putting baffles in an oil separator.

There are many oil separators on the market that have all kinds of baffling inside. The idea is to force the flow to follow a certain path making the oil/water mixture stay inside the separator longer. Unfortunately the result is the opposite of what one is trying to achieve. The path may be longer, but the velocity of the flow increases significantly. The result is a turbulent flow and "dead zones" where no flow occurs and which do not contribute to the separation.

Look at the image on the left. In the middle of the separator is a baffle plate with an opening near the bottom. See how the flow is forced to follow a path that leaves large sections of the tank unused. This single baffle plate has a very strong negative effect on the separation.

What is true for the single baffle plate is true for any construction that prevents or interferes with a horizontal, laminar flow. Even coalescing plates, often installed to improve separation, can have a negative effect on the flow pattern.

Click the image again. See what the effect of the baffle is. Only large oil drops will succeed in reaching the surface and getting out of the flow. The unfavorable flow pattern and the high velocity underneath the baffle make is impossible for the finer oil drops to escape. A lot of oil is carried off with the effluent.

Adding Supartor® to an oil separator.

Now look at the tank shown at the left. The box, shown inside the tank, represents a Suparator® unit.

Part of the total flow will pass through this unit and the result is a significant improvement of the flow pattern. There are hardly any dead zones in the separator now and a laminar flow is achieved over practically the full length of the tank.

The amount of oil that can escape via the underflow weir in reduced to an absolute minimum which is a big improvement compared to any other type of gravity separator.

Click the image for oil. Using the Suparator® technology changes the entire picture. There are no floating layers; oil can not accumulate as it is removed instantly by the Suparator® unit. And with the oil, all the dirt is carried off and removed.

Notice how the flow creates optimal conditions for all the oil to be captured and carried into the Suparator® unit.

Suparator® separation systems perform better than any other oil separation device, simply by creating optimal conditions for oil separation.