I. Introduction
In the current design of measuring the level of the oil tank in the oil depot, it is more popular to use radar level gauge or float, buoy, steel belt level gauge, etc. Although the radar level gauge has high accuracy, the cost is also high, while the buoy, float and other level gauges are more 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 real liquid levels, so they are rarely used in the design of tank level measurement. In fact, the exact level of the tank in the oil depot is not very important, the user actually needs to know not the liquid level, but by measuring the liquid level to understand the actual quantity (i.e. tonnage) of oil in the tank, so as to prevent overflow. For this analysis, 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 1151, 3051 and EJA and other differential pressure transmitters, the technology is very perfect, the accuracy can reach 0.075 level, and the price has dropped sharply, and the performance price is higher.
2. the design principle
As the name suggests, the result measured by the differential pressure level gauge is the pressure difference, and since the oil tank is often cylindrical, the area S of the cross-sectional circle is constant, so the weight is proportional to the height. That is, as long as the P value is accurately detected, the actual oil inventory G can be obtained, and its density is inversely proportional to the height h from the formula, and when the temperature changes, although the oil volume expands or shrinks, the actual liquid level rises or decreases, and 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
The differential pressure transmitter is in an oil depot project, and the author applies this idea to the actual design. Design conditions: 2000m3 oil tank, diameter d=14.5m, height h=14m.
Primary table: the selection of flanged flameproof differential pressure transmitter, the selection of flange type is to prevent the sedimentation of dirt at the bottom of the tank and block the impulse pipe, the transmitter range of 0~140kPa.
Secondary meter: the use of intelligent light column display alarm, universal signal input, can change the range at will, use the light column to display the liquid level, and use the number to display the tonnage of oil. Take the 6# tank as an example, the height is 14m.
At 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, because the measured value is directly in tons, no matter what kind of oil is stored in the tank, the value displayed by the secondary meter is the tonnage of oil in the tank, which avoids the trouble of measuring the density for conversion.
In general, the oil in and out of the warehouse is often measured by the pump through the elliptical gear flowmeter, due to the limited accuracy of the flowmeter, the highest is only 0.2 grade, and the density needs to be calculated, and the results are often some discrepancies, resulting in measurement disputes. Now because the tank measures in tonnes with an accuracy of 0.2 or even 0.1, the measurement results are more accurate than volumetric flow meters. Although the absolute error of the measurement results is large due to the resolution of a small amount of oil in and out of storage, its higher accuracy and smaller relative error are incomparable to other measurement methods when a large number of oil products are in and out of storage, and are especially suitable for monthly, quarterly and annual inventory. Practice shows that its main advantages are: (1) simple and convenient installation and maintenance; (2) The reading is intuitive, direct and clear, and the inventory of oil products can be directly read; (3) Density measurement and conversion are exempted.
4, pay attention to the problem
(1) In the design and installation, the pressure opening at the bottom of the tank should be lowered as low as possible to eliminate the error caused by temperature changes, and temperature compensation should be introduced if necessary.
(2) In the case that the horizontal cross-section of the tank is unequal (such as small at the top and large at the bottom), compensation measures should be considered. For example, the secondary table uses XWP-C803 series liquid level-volume controller.
(3) In order to achieve a certain accuracy, if the top of the oil tank is equipped with a breathing valve, a differential pressure transmitter must be used instead of a pressure transmitter. When the open oil tank or accuracy is not required, the pressure transmitter can be directly used to facilitate installation.
(4) The quadratic meter should be used as much as possible to use the smart meter, 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 discharged frequently, so as not to affect the accuracy of water accumulation.
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