Necessities for Measuring the Interface Level

The reason for measurement of interface level in a refinery tank is obvious that separate off the top crude and any water, then process the separated water only to reduce cost and difficult in processing. Accuracy is critical here, because any oil in water means costly losses; on the contrary, water in oil calls for premium processing for further refining and purification.

Other products may face similar situations in processing, in which two different mixture needed to be separated completely, namely excluding any remnants of the other. Many separations of chemical liquids like methanol in water, diesel and green diesel and even soap are not obvious in a tank or vessel. Although the gravity difference is enough to cause separation, such difference may be too small to base an interface measurement.

Devices for Level Measurement

In spite of which industry is applied in, there are recommended level sensors to solve the tricky technical problems.

Inline Density Meter: When wet oil is injected into a sedimentation tank or oil-water separator, the oil phase and the water phase are gradually separated for the sake of due to different densities, after sedimentation, and an oil-water interface is gradually formed. The oil layer and the water layer belong to two different media. The production process requires accurate and timely knowledge of the location of the oil-water interface so that when the water level reaches a certain limited height, the valve can be opened in time to drain water.

In case of the complicated situation in which water and oil fall to separated successfully, it's necessary to monitor the one-meter liquid above the drainage hole with online density meter. The drainage valve should be opened upon the density of liquid reaches 1g/ml; otherwise, the drainage valve should be closed when detected density lower than 1 g/ml, regardless of its separation status.

At the same time, the water level changes must be monitored in real time during the drainage process. When the water level reaches the lower limit, the valve is closed in time to avoid waste and environmental pollution caused by oil loss.

Floats and Displacers: A float sensor floats on the top level of liquids, somewhat different with what it sounds like. A displacer sensor adjusted to a certain gravity of the bottom fluid is able to floated on the top level of the target fluid. The tiny difference between floats and displacer lies in a displacer is designed to be submerged in total. They could be used to measure level interfaces of multiple fluids.

Floats and displacers are the least expensive devices to measure the level of interfaces, while it's shortcomings rest on restrictions on the single liquid for which they are calibrated. Besides, they are prone to be affected by turbulence in the tank or vessel, then stilling wells are needed to be installed to solve the problem.

Another drawback of using floats and displacers regards to their mechanical float themselves. The weight of floats may be impacted by additional coat or stick. The ability of the float to float on the top surface of the liquid will be altered accordingly. The same would hold true in case the gravity of the product varies.

Capacitance: A capacitance transmitter features a rod or cable that contact with material directly. The coated rod or cable could be taken as one plate of a capacitor, while the metallic metal wall could be regarded the other plate. Readings on the probe may vary for different materials between two plates.

Capacitance transmitter raises requirements on the conductivity of two liquids -- one should be conductive and the other should be non-conductive. Conductive liquid drives the reading and the other leave small effect on the output. Nevertheless, a capacitance transmitter is independent of effects from emulsions or rag layers.