Application and consideration of metering tonnage in oil depot using differential pressure transmitter
In the current measurement of the oil level of the oil tank, the magnetic flap level gauge, the float level gauge, the radar level gauge, the differential pressure level transmitter and the like are used. Although the radar level gauge has high precision, it also has affected factors and high purchase cost. However, the magnetic flap level gauge and the float level gauge are not expensive, and the accuracy is acceptable, but the installation is troublesome and the failure rate is relatively high. a little. The differential pressure type liquid level transmitter is also widely used in the liquid level measurement in the closed tanks such as the oil level of the oil tank and the bubble of the boiler, but the direct measurement result is not the liquid level value, but a difference of the corresponding conversion. Pressure value.
In fact, the ultimate goal of measuring the level of oil tanks in many oil tanks is to understand the weight of the oil in the tank, that is, the tonnage. Because each tank of the oil depot is large and has a large tonnage, it is only known that the tank level does not accurately grasp the existing capacity of the tank. Since the measured level still has to calculate the tonnage, the differential pressure measured by the differential pressure type liquid level transmitter using the differential pressure method is very suitable. Like the differential pressure transmitters of type 1151 and 3051, the technology is perfect, and the accuracy and stability are all in the air.
2, the design principle
As mentioned above, the result of the differential pressure level transmitter measurement is the high and low pressure difference, which is converted into the formula (1): ΔP = ρg ¡÷ h. Under normal circumstances, the oil tank is a cylinder, then the bottom cross-sectional area S is constant, according to formula (2): weight G = ¡÷ P · S = ρg ¡÷ h · S, if the bottom cross-sectional area S does not change, Then the weight G is proportional to the differential pressure ΔP. Since the ΔP high and low differential pressure values are measured, the actual amount of oil G in the tank can be obtained. (According to this formula (2), the weight G is constant at this moment, and the density ρ is inversely proportional to Δh, that is, when the temperature changes, although the volume of the oil may expand or contract, the actual liquid level rises or Decrease, but the ΔP obtained according to formula (1) is constant, that is, the measured pressure difference is constant.
3, an example
In the Xinjiang Wanji Petrochemical Project, the author applied this idea to the actual design.
Design conditions: 3000m3 oil tank, diameter d=16m, height h=15m.
One time table: Select SC3851/3351LT series flange type differential pressure transmitter (single flange liquid level transmitter) of Huai'an Sanchang Instrument Co., Ltd., use flange type to prevent sedimentation of tank bottom and block pressure Tube, transmitter range 0 ~ 150kPa.
Secondary table: Select SC-600 intelligent light column adjuster, universal signal input, can change the range arbitrarily, display the liquid level with light column, and display the tons of oil by digital. Taking the No. 4 tank as an example, S = π × r2 = 3.14 × 8.6 square = 232 m2, and the height is 15 m.
The secondary meter is used in the remote control room. It can set the full liquid level alarm to prevent the liquid tank from overflowing. However, in order to prevent the liquid level alarm device on the top of the field tank, a liquid level alarm device is also installed. Alarm) to prevent oil from overflowing as double insurance. In the application, since the measured value is directly in the tonnage, regardless of the oil stored in the tank, the value shown in the secondary meter is the tonnage of the oil in the tank, which avoids the trouble of requiring the density to be converted.
In general, the oil out of the warehouse is often pumped through the elliptical gear flowmeter. Due to the limited accuracy of the flowmeter, the maximum is only 0.2, and the density calculation is required. The results are often somewhat different, resulting in measurement disputes. Now because the results of the tank measurement are in tons and the accuracy can reach 0.2 or even 0.1, the measurement results are more accurate than the volumetric flowmeter. Although the absolute error of the measurement result is large due to the resolution when the small amount of oil is stored in the warehouse, the higher precision and smaller relative error when the large amount of oil is stored in the warehouse is other. It is unmatched by measurement methods, and is especially suitable for monthly, quarterly, and annual inventory. Practice shows that its main advantages are:
1 Installation and maintenance is simple and convenient; 2 The reading is intuitive and clear, and the stock of oil can be read directly; 3 The density measurement and conversion are eliminated.
4 attention to the problem
(1) When designing and installing, the pressure tapping hole at the bottom of the tank should be considered as low as possible to eliminate the error caused by temperature change, and temperature compensation should be introduced if necessary.
(2) When the horizontal section of the tank of the tank is not equal (as small as above), compensation measures should be considered. For example, the secondary meter uses the 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 tank, a differential pressure transmitter must be used instead of a pressure transmitter. When the open tank or accuracy requirements are not high, the pressure transmitter can be used directly to facilitate installation.
(4) The secondary meter should use the smart meter as much as possible, which can easily change the range and realize temperature compensation.
(5) Install the water collector in the negative pressure chamber of the differential pressure transmitter during installation, and drain the water frequently to avoid the accumulation of water affecting the accuracy.