Investigation On Settlement Stratification And Measurement Of Long-stay Oil In Large Floating Roof Tank

In the process of static storage,heavier components like wax and asphaltene in oil will separate out and precipitate.The precipitated particles such as wax crystal lead to density stratification,and the amount of precipitation differs depending on factors such as temperature,composition.However,the oil stratification phenomenon in tank causes different properties in different locations of tank,and the phenomenon will affect the daily management and operation of oil depot.Storage tanks in our country reserves are mostly large double floating roof tanks which have the characteristic such as big capacity,large diameter.When the measurement is carried out with the metrological specification,phenomenon of deposits and layering can cause inaccurate measurement of oil quality,also affect oil transporting,heating and unloading quality.Therefore,mastering delamination properties of oil is of great significance to the safety management and efficient operation.On the basis of theoretical study of thermodynamic phase equilibrium,the author establishes crude oil solid deposition model to predict the organic solid deposition,make component test result of crude oil as input data and predict the amount of organic solid deposition in crude oil through the simulation calculation of model.The author sets up indoor experiment system and gose to the scene for many times to measure oil temperature,sample at different height and test density in different seasons.Obtain valuable production datas in the scene,modify the solid deposition model and design software with Visual Studio software to calculate the solid deposition.The software can realize quick calculation of crude oil solid deposition.In addition,the effect of sedimentary layer on oil metering is studied and accurate measurement method is proposed.Using the solid deposition model established to calculate the solid precipitation points of three kinds of light oil which are respectively 20?,9?,19?.As the temperature of oil reduces,more solid sediment would be deposited out.Increment of solid deposition would increase with each degree reduction in temperature.It is found that the lowest temperature of three kinds of light oil is about-5?when the oil is storaged without heating in three years and the mass percentage of solid organic precipitation quality is about 2%~3%.Through simulation calculation,it is found that the model is relatively accurate to study the sedimentary of wax melting system and degassed oil system.Through the field sampling and density test,it is found that as storage time goes on,the density of crude oil more than 2 meters far from the bottom of tank has been decreasing and the difference of oil density at different locations is about 0~1.0 kg/m~3.Within the scope of 2meters above the bottom of tank,the density of oil increases with the storage time going on and the density of oil increases from up to down.The density of oil at the bottom of tank is larger than the density of oil in the middle and upper part,and the increment is from 0.4kg/m~3to 8.3 kg/m~3,which is density stratification.At present,the domestic measurements of oil in tank mainly rely on manual gauging and the average of multipoint density is usually regarded as the average density in the metering tube,which doesn't consider the effects of horizontal nonlinear density differences,density changes and density stratification on measurement.Due to density stratification,the density of oil that is more than 2 meters high verticaly from the bottom of tank is larger.The density stratification influences the measurement of oil density.Tank 1 has capacity of 62.3 thousand tons and density stratification could make an effect on the total metering quality of oil,and the value of influence is 69 t.Meamwhile,the domestic measurements of oil temperature doesn't consider the effects of horizontal nonlinear temperature differences and temperature changes on measurement.Through long-time temperature tests of oil in tanks,the results show that the distribution of temperature field is mainly affected by the ambient temperature.While the ambient temperature is lower than the oil temperature,the natural convection is reinforced and the oil temperature is basically the same.When the ambient temperature is higher than the oil temperature,the natural convection is restrained,and there is an apparent vertical gradient of oil temperature.The oil temperature rises from bottom to top in the central and upper,the upper longitudinal temperature gradient is about 0.2~0.4?/m and temperature is uniform in the underpart.Horizontal temperature differences is small,which is near to 0.2~1.6?.Oil temperature of underpart is higher because of the sediments in the bottom of tank.When crude oil is measured in large tanks,this paper advices that oil density should be measured in at least five points and oil temperature should be measured in at least ten points or density and oil temperature could be measured in two levels.The aim of these solutions is to reduce amount gauging differences caused by uneven distribution of density and temperature,improve the measuring accuracy of oil temperature,lay the foundation of oil metering of crude oil and safe efficient production,work for better storage management.