Applications of differential pressure transmitters in oil depot metering

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1. Introduction 　　　Currently, the commonly used methods for measuring the liquid level in oil storage tanks are radar level gauges, or float, buoy, and steel-band type level gauges. Although radar level gauges offer high precision, they are also expensive. Level gauges such as floats and balls are rather troublesome to install and maintain. Differential pressure level gauges are widely used in sealed vessels such as boiler drums, but their measurement results do not represent the actual liquid level; therefore, they are rarely used in the design of oil tank level measurement systems. In fact, the exact liquid level in oil storage tanks is not that important; what users really need to know is not the level itself, but rather the actual amount of oil in the tank – in terms of tons – so as to prevent overfilling. Based on this analysis, using the differential pressure method to measure liquid level (actually in terms of tons) is also a good option. Since differential pressure transmitters are now highly mature in terms of application, models such as the FG700 differential pressure transmitter produced by Changhui Instrument Co., Ltd. feature excellent technology, with an accuracy level of up to 0.25; moreover, they offer favorable performance-to-price ratios. 2. Design principle 　　　 As the name implies, a differential pressure level gauge measures the pressure difference, that is, △P=ρg△h. Since oil tanks are usually cylindrical, the area S of their circular cross-section remains constant. Therefore, the weight G = △P·S = ρg△h·S; with S being constant, G is directly proportional to △P. In other words, as long as the △P value is accurately detected, the actual inventory level G of the oil can be determined. The formula also shows that its density ρ is inversely proportional to the height △h; although the volume of the oil expands or contracts with changes in temperature, causing the actual liquid level to rise or fall, the pressure detected remains constant. If the user needs to display the actual liquid level, medium temperature compensation can also be introduced to address this issue. 　　3. Practical Application 　　　In the Beijing oil depot project, the author applied this approach to the actual design. 　　 　　Design conditions: 2000 m3 oil tank, diameter d=14.5 m, height h=14 m. 　　 　　Primary gauge: The FG700 flanged flameproof differential pressure transmitter from Anhui Changhui Instrument Co., Ltd. is used; the flanged design is intended to prevent dirt at the bottom of the tank from accumulating and blocking the pressure transfer tubes. The transmitter’s range is 0–140 kPa. 　　 　　Secondary display: The FXT series intelligent light-column alarm indicator is used, featuring a universal signal input that allows for arbitrary range adjustments. The liquid level is displayed via light columns, while the amount of oil in tons is shown numerically. Taking Tank No. 6 as an example, S=π×r2=3.14×7.252=165 m2, with a height of 14 m. 　　 　　At the top of the oil tank, a level alarm system is designed to prevent the oil from overflowing, serving as an extra safety measure. In the application, since the measurement value is given directly in tons, regardless of the type of oil stored in the tank, the value displayed on the secondary meter represents the tons of oil present in the tank, eliminating the need to determine the density for conversion. 　　Generally, the transfer of oil products in and out of storage is carried out using pumps, with flow measurement being done via elliptical gear flow meters. Due to the limited accuracy of these flow meters, which is at most 0.25 grade, it is also necessary to measure the density; as a result, there can be some discrepancies, leading to measurement-related disputes. Now, since the measurement results for oil tanks are in tons and the accuracy can reach 0.25 grade, the measurement results are more accurate compared to volumetric flowmeters. Although when a small quantity of oil is received or dispatched, the absolute error in the measurement is relatively large due to resolution limitations, when a large volume of oil is handled, its high precision and low relative error are unmatched by other measurement methods, making it particularly suitable for monthly, quarterly, and annual inventory checks. Practice has shown that its main advantages include: ① Simple and convenient installation and maintenance ； ②The readings are intuitive, direct, and clear, allowing for an immediate determination of the oil inventory level ； ③The determination and conversion of density are eliminated. 4. Points to note 　　　　(1) During design and installation, consideration should be given to placing the pressure tapping openings at the bottom of the oil tank as low as possible, in order to eliminate errors caused by temperature changes; temperature compensation should be introduced if necessary. 　　 　　(2) When the horizontal cross-section of the oil tank is not uniform (e.g., smaller at the top and larger at the bottom), compensation measures need to be considered. For secondary meters, the FGST series level-capacity controller is selected. 　　 　　(3) To achieve a certain level of accuracy, if a breather valve is installed on the top of the oil tank, a differential pressure transmitter must be used instead of a pressure transmitter. For open-top oil tanks or situations where high precision is not required, a pressure transmitter can be used directly to facilitate installation. 　　 　　(4) Where possible, use smart meters for secondary instruments, as they facilitate range adjustment and enable temperature compensation, among other functions. 　　 　　(5) During installation, a water collector should be installed in the negative pressure chamber of the differential pressure transmitter, and the water should be drained regularly to prevent accumulated water from affecting accuracy.