In other cases, an emulsion or “rag” layer will exist between the two liquids. Other interface situations include multiple interfaces between more than two products, or the interface between a liquid and a solid. In some cases, it may be necessary to measure the thickness of the upper layer.
No one sensor can solve all level interface measurement problems, but a user can often find more than one technology that can do the job. And, in some cases, two different level sensors can be used—such as an ultrasonic and capacitance transmitter—to measure the overall level and the interface.
Why measure interfaces?
The reason for wanting to find the interface between crude oil and water in a refinery tank is obvious: A refinery does not want any water to enter the distillation process. Once a user knows where the interface is, he or she can separate off the top crude, leaving only the water to be processed separately.
Accuracy is very important here because any oil in the water means product losses, and any water in the oil requires extra processing. This is quite often the case with other products. A process may require separating two different products and not have one product contain remnants of the other. In some applications, the separation may not be quite as obvious, such as methanol in water, diesel and green diesel, black liquor and soap, and so on.
In most cases, the products are separating due to a difference in the specific gravity of the products. Even though this difference is enough to cause the product to separate, it may be too small of a difference on which to base an interface measurement. What variables are required to control the process? Is the overall level value needed as well as the interface? Does the operator need to know the thickness of the upper layer to prevent cross contamination of the separate products?
The best advice is to consult with a local manufacturer’s representative to learn what the best technology is for a particular application. Most process instrumentation vendors have provided sensors for countless level interface applications involving dozens of different liquids, and a representative can tap into this accumulated knowledge. The world’s most advanced instrument will not work if it is installed in the wrong process or just not applied correctly.