Application of Differential Pressure Transmitter in Oil Depot Metering
In the current oil tank level measurement design of oil depots, radar level gauges or floats, buoys, steel belt level gauges, etc. are more popular. Although radar level gauges have high accuracy, they are also costly, while level gauges such as buoys and floats are troublesome to install and maintain. Differential pressure level gauges are widely used in closed containers such as boiler drums, but the measurement results are not the real level. Therefore, in the current oil tank level measurement design of oil depots, it is more popular to use radar level gauges or Floats, buoys, steel belt level gauges, etc. Although radar level gauges are highly accurate, they are also costly, while level gauges such as buoys and floats are troublesome to install and maintain. Differential pressure level gauges are widely used in closed containers such as boiler drums, but the measurement results are not the real level, so they are rarely applied in the design of oil tank level measurement. In fact, the precise liquid level of the oil tank in the oil depot is not very important. What the user actually needs to understand is not the liquid level, but to understand the actual quantity of oil in the tank (that is, tonnage) by measuring the liquid level, so as to prevent overflow. Therefore, it is also a good choice to use the differential pressure method to measure the liquid level (actually in tons). Because the application of differential pressure transmitters is very mature, such as the 1151, 3051 and EJA differential pressure transmitters, the technology is very complete, the accuracy can reach 0.075 level, and the price has fallen sharply, and the performance price is higher. 2 The design principle, as the name suggests, is the pressure difference measured by the differential pressure level gauge, that is, △Pu003dρg△h. And because the oil tank is often cylindrical, the area S of the cross-section circle is constant, then the weight Gu003d△P?Su003dρg△h?S, S is unchanged, and G is proportional to △P. That is to say, as long as the △P value is accurately detected, the actual oil inventory G can be obtained. From the formula, it can be seen that its density ρ is inversely proportional to the height △h. When the temperature changes, although the volume of the oil expands or shrinks, the actual liquid As the position increases or decreases, the detected pressure always remains the same. If the user needs to display the actual liquid level, it can also be solved by introducing medium temperature compensation. 3 Practical application in the Wenzhou New Century Oil Depot project, the author applied this idea to the actual design. Design conditions: 2000m3 oil tank, diameter du003d14.5m, height hu003d14m. On the top of the oil tank, a set of liquid level alarm device is designed to prevent the oil from overflowing as a double insurance. In the application, the measured value is directly the tonnage, so no matter what kind of oil is stored in the tank, the value displayed by the secondary meter is the tonnage of the oil in the tank, which avoids the need to determine the density for conversion. In general, oil products entering and leaving the warehouse are often measured by pumping through an elliptical gear flowmeter. Due to the limited accuracy of the flowmeter, the highest is only 0.2, and the density calculation is required. The results are often somewhat different, which causes measurement disputes. Now because the measurement result of the oil tank is in tons, and the accuracy can reach 0.2 or even 0.1, the measurement result is more accurate compared with the positive displacement flowmeter. Although the absolute error of the measurement result is relatively large due to resolution when a small number of oil products are in and out of the warehouse, when a large number of oil products are in and out of the warehouse, the higher accuracy and smaller relative error are other factors. Incomparable measurement methods, especially suitable for monthly, quarterly, and annual inventory. Practice has shown that its main advantages are: ① Simple and convenient installation and maintenance; ② Intuitive, direct and clear readings, which can directly read the inventory of oil products; ③ Elimination of density measurement and conversion. 4 Attention issues (1) When designing and installing, consider that the pressure opening at the bottom of the oil tank should be placed as low as possible to eliminate errors caused by temperature changes, and temperature compensation should be introduced when necessary. (2) When the horizontal section of the tank body of the oil tank is unequal (for example, the upper part is small and the lower part is large), compensation measures should be considered. For example, the secondary meter selects WP-H80 series liquid level-capacity controller. (3) In order to achieve a certain accuracy, if a breathing valve is installed on the top of the oil tank, a differential pressure transmitter must be used instead of a pressure transmitter. When the requirements for open oil tanks or accuracy are not high, pressure transmitters can be used directly to facilitate installation. (4) The secondary meter should use smart meter as far as possible, which can easily change the range and realize temperature compensation. (5) When installing, the negative pressure chamber of the differential pressure transmitter should be installed with a water collector, and the sewage should be drained frequently, so as not to affect the accuracy of the accumulation of water.
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