Finding an oil / water interface level

09/03/2010


Dear Control Engineering: I was reading the article on differential pressure sensors, and I was wondering if I could use that density calculating feature to determine the interface point between oil and water in the same tank?

Let’s think about how that feature works and see how to solve your problem. As the article pointed out, if you have two taps on the side of a tank some known distance apart, you can calculate the density of the contents, regardless of the depth, as long as both taps are submerged. This works because density is mass per unit of volume and the pressure device is giving you, for all practical purposes, the weight of the volume of liquid between the two taps. It takes some math to get the final answer, but nothing all that complicated. Of course this assumes you have only one homogenous product in the tank. Two products with different densities that don’t mix will complicate things.

If you install one of these density measuring setups near the bottom of the tank, it will give you the average density of the liquid at that level, but it won’t necessarily help you find the interface layer. It can tell you if the interface layer is above, below, or even in between the taps, but no more detail than that. This suggests two possible solutions:

First, if you don’t mind the idea of drilling lots of holes in the tank and mounting multiple sensors, you will be able to determine the interface, at least to the extent that it falls between two given sensors. It’s sort of like trying to determine the level in a tank by using multiple point level sensors. The number of measuring points will determine your precision.

Second, if the liquid level in the tank doesn’t change much, you could spread out the two taps. As long as both taps are submerged and the interface level is between them, you can determine where it is because the reading is actually an average density for the volume that is between the taps. As long as you know the density of both components, you can calculate the interface point relative to the two taps.

If you need a more precise measurement, the simplest approach would be to use a technology that can find the interface level, such as a magnetostrictive device that uses two floats on the same shaft. The lower float is designed to sink in oil but float on water. The upper float will float on the surface of the oil. This will give you a very precise measurement of both levels.

If you’re determined to use the density measuring feature but can’t really use the two suggestions mentioned earlier, a hybrid approach could work. You will need some sort of top-down level sensor that will give you the volume in the tank. This could be radar, ultrasonic, single-float magnetostrictive, or a few others. The DP density sensor will give you the average density for the total tank contents, or at least the contents above the taps. If you know the total volume in the tank and have the density reading from the bottom to give you a true average for the whole tank, you can calculate the proportion of oil and water, assuming you know the actual density of both.

Ultimately determining the answer depends on analyzing what information you have and filling in the blanks. The combination of factors will give you the answer.

Peter Welander, pwelander@cfemedia.com

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