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Effective degasser control provides accurate metering with 80 GHz radar technology

15 January 2019

Dutch company Sime Darby Unimills (SDU) in Zwijndrecht, refines raw vegetable oils and fats for food and animal feed markets. The raw materials are delivered by inland tanker-ships to the plant and then processed, after which they are distributed to buyers in industry across the Netherlands.

Until recently, the amount of fat and oil delivered was only randomly sampled and checked. Gerrit van den Burg is the Instrumentation project leader at SDU: "We had a good relationship with the transport company, but we wanted to get rid of those sampling processes. That cost us quite a lot of time and money. Even more important to us was that we wanted to have a real-time continuous insight into the quantity of our incoming and outgoing raw materials." This can only be done by properly controlling the degasser stage of the process. 

Air bubbles
The reliable and accurate measurement of the flow from the ship to the storage tanks may seem like a simple job, but it is somewhat more complex. "The problem is that what you pump out of that ship is not just liquid," says the VEGA Account Manager. When you pump you get air bubbles entrained in the oil and you want to get rid of it before the liquid passes through the meter. Otherwise, the measurement is simply not accurate. "By first sending the liquid through a degasser, consisting of a standing cylindrical vessel, the air is effectively removed from the liquid thus ensuring what you measure is just pure product.”

The effective operation of the degasser can only be guaranteed if the liquid level in the vessel is maintained within a certain working area. Too high a level can disrupt the functioning of the degasser, too low a level slows down the unloading process unnecessarily. It is therefore important to ensure that the liquid level is monitored and controlled.  The liquid density is of course not constant because of the entrained air and this rules out many technologies that need consistent density or electrical properties, such as pressure, float, capacitance or displacer based techniques. Pressure inside the tank can also change rapidly, ruling out using ultrasonic level sensors. This means a radar level measurement is the most reliable way to effectively measure and control the level in such a vessel. The level control also needs to be almost real-time, because the vessel is not particularly large.

However, the ability to perform a reliable level measurement with a contactless (or through air) radar depends on a number of factors. Roosendaal: "The degassing tank is rather narrow and the inlet is halfway up the tank. Conventional wide beam radar measurement would register so much disturbance by the inlet alone that the actual level would hardly be measurable. This inlet creates a powerful liquid flow along the entire surface. The level meter must be able to measure the actual surface level in the confines of tank, close to the wall and turbulent inlet. In other words, you need a meter with a narrow beam angle."

The transmission angle of a radar sensor is determined by the area of its antenna and the frequency. The VEGA engineer added: "Increasing the surface area of the antenna is not possible in such an application. What remains is to use a level meter that has a sufficiently high frequency. Using VEGAPULS 64 80GHz radar, you can make a focused measurement with which you can really measure the level right to the bottom. With this very small 1 ½" version it achieves a transmission angle of seven degrees and thanks to this strong focusing, the false echo from the inlet remains very small.” He continued, "This is evident from the echo diagnostics in this application. Thanks in part to the high dynamic range of the sensor, our readings show us there is a very good measurement reliability of 35dB. That's a signal strength figure where we know there is plenty in hand  to be able to determine and follow the level properly", adds Roosendaal. "You have to add in the fact that in this application we are also dealing with a low reflectivity (dielectric constant/dk) liquid and a turbulent surface. High frequency radar is better suited for liquids with a low dK than a lower frequency radar.  It is the various properties of this particular radar level meter that combine to offer a very good solution." summarises the Engineer. "High frequency, a very small transmission angle and a large dynamic range. This means that you can still reliably measure level in a small, narrow tank, a product with both a low dK and a turbulent surface."

The customer is very satisfied with the application. "It all works excellently, I cannot say anything else. Now we know exactly what is coming into our factory thanks to reliable equipment and measurements. The outgoing stream we have well mapped long ago. If you are talking about traceability, we are now well prepared for the future." For Mr. Van den Burg, the choice of VEGAPULS 64 was fairly straightforward. "We have been working with VEGA for a long time and I have a lot of confidence in that company. They know what they are doing. The field of ultra high-frequency radar measurement is still relatively new, but VEGA is the only party with a lot of experience with that technique. For a reliable flow measurement, a quick and accurate level measurement is required. We see that in practice that the measurement is very reliable. In fact, now that we have such good experience with this non contact radar, it could also be interesting to replace our guided radar in our large storage tanks."

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