1. Introduction

        In the current design of oil tank level measurement in oil depots, radar level gauges or floats, buoys, steel belt level gauges, etc. are more popular. Although radar level gauges are highly accurate, they are also costly, while level gauges such as floats and floats 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 the true liquid level, so they are rarely used 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 users actually want to know is not the liquid level, but to understand the actual amount of oil in the oil tank (i.e., tonnage) by measuring the liquid level, so as to prevent overflow. Based on this analysis, it is also a good choice to use the differential pressure method to measure the liquid level (actually tonnage). Because the application of differential pressure transmitters is very mature at present, differential pressure transmitters such as 1151, 3051 and EJA have very complete technology, accuracy can reach 0.075 level, and the price has dropped significantly, with high performance and price.

 2. Design principle

        As the name implies, 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 its cross-sectional circle is constant, then 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. From the formula, it can also be known that its density is inversely proportional to the height h. When the temperature changes, although the volume of the oil expands or shrinks, the actual liquid level rises or decreases, the detected pressure always remains unchanged. If the user needs to display the actual liquid level, medium temperature compensation can also be introduced to solve it.

 Three, practical application

Differential pressure transmitter in a certain oil depot project, the author applies this idea to the actual design. Design conditions: 2000m3 oil tank, diameter d=14.5m, height h=14m.

Primary meter: flange type explosion-proof differential pressure transmitter is selected. The flange type is selected to prevent the sedimentation of dirt at the bottom of the tank and block the pressure pipe. The transmitter range is 0~140kPa.

Secondary meter: intelligent light column display alarm instrument is selected, universal signal input, the range can be changed arbitrarily, the light column is used to display the liquid level, and the number of tons of oil products is displayed digitally. Take the 6# tank as an example, the height is 14m.

On the top of the oil tank, a set of liquid level alarm device is designed to prevent oil overflow as a double insurance. In the application, since the measured value is directly in tons, no matter what kind of oil is stored in the oil tank, the value displayed on the secondary meter is the tonnage of the oil in the tank, avoiding the trouble of measuring density for conversion.

Generally, the oil in and out of the warehouse is often measured by pumps through oval gear flowmeters. Due to the limited accuracy of the flowmeter, the highest is only 0.2 level, and the density calculation is required. The results are often inconsistent, causing measurement disputes. Now because the result of the oil tank measurement is in tons, and the accuracy can reach 0.2 or even 0.1 level, the measurement result is more accurate than that of the volumetric flowmeter. Although the absolute error of the measurement result is large when a small amount of oil is in and out of the warehouse due to the resolution, its higher accuracy and smaller relative error when a large amount of oil is in and out of the warehouse are unmatched by other measurement methods, and are particularly 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 The density measurement and conversion are eliminated.

IV. Notes

(1) When designing and installing, the pressure tapping hole at the bottom of the oil tank should be placed as low as possible to eliminate the error caused by temperature changes, and temperature compensation should be introduced when necessary.

(2) When the horizontal cross-section of the tank body is unequal (such as small at the top and large at the bottom), compensation measures should be considered. For example, the secondary meter uses the XWP-C803 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. For open oil tanks or when the accuracy requirement is not high, a pressure transmitter can be directly used for easy installation.

(4) Secondary meters should use smart meters as much as possible, which can easily change the range and achieve temperature compensation, etc.

(5) When installing the differential pressure transmitter, the negative pressure chamber should be equipped with a water collector and the sewage should be drained frequently to prevent water accumulation from affecting the accuracy.