Fan Weijian, Shi Yongjun et al.: Application of vacuum-assisted light oil unloading technology, oil and gas storage and transportation, 2002, 21 (6) Empty assisted light oil unloading process to control the degree of vacuum in the gas collection tank, adjust the highest point of the oil discharge pipeline, and centrifugal pump inlet Absolute pressure in two places, elimination of air resistance in the pipeline, and pump evacuation. The vacuum-assisted light oil unloading process is compared with the positive pressure unloading process. The results show that the former has high safety, low energy consumption, convenient operation, and has the value of popularization and application.
Subject Headings Railway tanker vacuum unloader. Process 255411, Linzi District, Zibo City, Shandong Province, China; telephone “0533â€7523426. The light oil unloading system using centrifugal pumps for unloading and unloading railway tank trucks often causes air resistance and unloading pump evacuation during the operation process, which makes the unloading speed slow down. The cavitation of unloading pumps or unloading pumps With the continuous improvement of production technology, the positive pressure assisted unloading and material cooling techniques have been applied in some large light oil unloading systems, which have overcome the difficulties of unloading vehicles to different extents, and have been solved. The problem of slow unloading speed is still existed, but problems such as poor safety and high energy consumption still exist.The vacuum-assisted light oil unloading process can solve these problems, which can ensure the normal operation of the unloading operation and can significantly reduce the unloading cost.
When it is too small, the tongue tape is softened by being immersed in oil, and the sealing performance is lost. For this reason, the installation height of the sealing tape was increased to 220 mm, which improved the life and sealing performance of the tongue tape.
(4) Adjust the ventilation height of the safety vent valve to ensure that the aeration area is equal to the cross-sectional area of ​​the short oil pipe in and out of the oil, eliminating the drawbacks of increasing the oil and gas loss due to damage to the floating plate due to the small aeration area and excessive area.
The original float uses foam as the float, because the performance of the float is unstable, and it is easy to absorb oil, which causes the floating plate to sink from significant buoyancy. Therefore, the float material is changed from foam to aluminum float to solve the problem that the buoyancy becomes smaller.
In general, the difficulty of light oil unloading by centrifugal pumps is related to the unloading system. First, the absolute pressure somewhere in the unloading pipeline is less than the saturated vapor pressure of the oil at the operating temperature, forming a pipeline resistance; second, the absolute pressure at the centrifugal pump inlet is less than Pumps are allowed to inhale pressure and produce cavitation; third, a large amount of gas enters the pump body, causing the unloading pump to pump out. Gas resistance is a phenomenon in which the liquid in the pipeline is blocked by the gas space. The evacuation pump is evacuated because the density of gas entering the pump body is small and the gas cannot be discharged in time, which affects the entry of the liquid into the centrifugal pump. The phenomenon of both pipeline resistance and pumping is due to the presence of gas and the formation of gas zones.
Second, the transformation effect is good sealing effect, after testing, the upper part of the floating disk oil and gas concentration is far below the lower limit of the explosion limit.
The system error of sampling test and oil height measurement is avoided, and the accuracy of the test and measurement is improved.
C3) The safety factor is improved, especially the installation of the spare respiratory system, perfecting the fault emergency function of the floating plate.
(Revision date received = edit: Chou Bin 1, pipeline gas causes the pipeline resistance and the pump pumping air mainly comes from the following three aspects.
Precipitation of dissolved air in oil products.
In the process of using crane unloading, when the absolute pressure of a pump population siphon pipe somewhere (usually the highest point of the unloading crane pipe, see point C) is less than the saturated vapor pressure of the oil, the oil product begins to gasify Form gas. In the actual production, the process of gasification of oil products is complex, and the saturated vapor pressure of single-component liquids is generally only a function of temperature. Light oil is generally a complex mixture composed of mutually dissolved multi-component hydrocarbons. The saturated vapor pressure is related to the concentration of each component at a certain temperature and is not constant. The saturation vapor pressure of the light components in the oil is very high. The saturated vapor pressure of the light component Q in the naphtha is 20 MPa at 20C, while the saturation vapor pressure of the heavy components is less than zero. During the unloading process, when the absolute When the pressure is reduced to the saturated vapor pressure of light components, the light components first vaporize and gradually increase, and the gasification components gradually increase. In other words, when the unloading vehicle drops, the absolute pressure at point C at the top of the crane pipe decreases with the decrease of the liquid level in the tank, and a considerable degree of gasification occurs before the saturated vapor pressure of the oil is reduced. The nature of light oil is determined.
Leakage caused by the sealing performance and operation of the sealing points in the unloading pipeline cannot be ignored. In the operation of the large-scale light oil unloading system, the number of valves, crane pipe joints, and pipe sealing points of the pump population pipeline are large, and gas infiltration is inevitable under negative pressure conditions. The valve of the crane that did not participate in the operation is not closed tightly, or the unloaded crane position valve cannot be closed in time, and the amount of air carried by the oil flow is also increased.
There is dissolved air in different degrees in the crane unloading map. Under normal temperature and pressure, the solubility of gasoline to air is approximately 20% (volume), and its solubility decreases with the decrease of the external pressure. At the same time, gasoline will also carry air in the form of tiny bubbles. During unloading. When the absolute pressure at a certain point is lower than the atmospheric pressure, the air partially dissolved in the oil and the carrying air bubbles will be separated, which accelerates the gasification of the oil.
With the centrifugal pump's direct unloading process system, the highest point of the unloading crane pipe is the area where the absolute pressure is lower in the oil discharge pipeline at the pump inlet. When the temperature rises, the light oil components can easily gasify and dissolve the air. It will also be here. At the time of unloading, the absolute pressure in the siphon pipe at the top of the highest point of the crane tube continuously decreases as the liquid level in the vehicle continues to drop, as the light components of the oil product continuously vaporize, the dissolved air in the oil product continuously precipitates, and the system leaks. A lot of bubbles are produced. These bubbles, under the combined action of buoyancy and flexural resistance, are continuously trapped and accumulated at the highest point of the crane tube, forming a gas zone. The gas zone gradually occupies the entire over-current section, resulting in a decrease in the amount of oil passed until the final stop of flow, forming an air resistance.
Due to the formation of a low-pressure zone at the entrance of the centrifugal pump impeller during unloading, when the highest point of the pipe or other gas volume is cut off due to air resistance, the pump population oil flow cannot be replenished in time, and the pressure drops sharply, causing the oil to fall. The demographics are rapidly gasified, together with the gas carried in the pump's population pipeline, into the pump body. Gas can not be discharged in the pump body, resulting in pumping out of the chestnut, so that the oil can not enter the pump, affecting the operation of the pump, or even stop the pump operation.
It can be seen from the above situation that the common cause of air resistance and pumping problems is the formation of gas. The generation of air resistance further exacerbates the evacuation of the unloading pump. Pumping is another form of air resistance.
Third, the vacuum-assisted light oil unloading process The vacuum-assisted light oil unloading process is based on the direct unloading of centrifugal pumps. A series gas collection tank is added to the pump inlet line. The upper opening of the gas collection tank is connected to the vacuum system. Vacuum is maintained through the gas collection tank to maintain a certain negative pressure. Looking at the population flow of the entire pump, the gas collection tank is equivalent to a balloon on the pump population line. The key to the vacuum-assisted light oil unloading process is to control the vacuum level of the gas collection tank and adjust the absolute pressure of the high point of the crane pipe and the inlet of the centrifugal pump so as to avoid the air resistance and the pumping of the unloading pump. See the process flow.
The vacuum assisted light oil unloading process performs in the following three aspects.
(1) Control the vacuum level of the gas collection tank and reduce the absolute pressure at the highest point of the crane pipe. It can be seen that when the gas collection tank is evacuated, its operating pressure is reduced, so that the pressure at the highest point of the crane pipe is also reduced (equivalent to increasing the pressure difference between the liquid level in the tank car and the highest point of the crane pipe). Increases the energy flow in the siphon conduit of the crane tube and increases the flow rate. The increase of the flow rate increases the deflection resistance of the oil flow, enhances the ability to carry air bubbles, reduces the accumulation of air bubbles, and avoids the formation of air resistance.
The vacuum-assisted light oil unloading process regulates the vacuum level of the gas collection tank so that the absolute pressure at the centrifugal pump inlet is not lower than the allowable suction pressure to ensure the suction performance of the centrifugal pump. Centrifugal pumps can only avoid the occurrence of cavitation under the conditions that ensure that the suction pressure meets the requirements. It can be seen that because the gas collection tank is connected to the vacuum system, to ensure the normal operation of the centrifugal pump, the following formula should be satisfied: h, the friction between the gas collection tank and the pump into the Q; the centrifugal pump allows inhalation pressure.
The operation pressure P of the gas collection tank has the greatest impact on the height difference H. Under the premise of guaranteeing the exhaust effect, the vacuum degree of the gas collection tank is reduced, so that h is maintained at a relatively high level, and the height difference can be reduced Qilu Petrochemical Company The height difference of the naphtha unloading system was controlled within the range of 2.34.2m. To this end, the unloading centrifugal pump was installed underground.
In actual production, by controlling the height difference between the lowest operating liquid level of the gas collection tank and the population axis of the unloading pump, it can be ensured that the unloading pump runs under normal suction. In order to facilitate the operation and reduce the operating frequency, so that the operation can easily meet the requirements of the height difference H, the volume of the gas collection tank should be increased and the time to control the height of the liquid level should be extended.
The flow of oil in the pump's population pipeline is carried at higher flow rates and pressures through the gas collection tank. The flow rate is rapidly reduced, so that the carried gas is analyzed, floated, accumulated in the gas collection tank, and then discharged from the vacuum line so that the gas cannot enter the pump. Avoid unloading pump pumping.
Since the pumping of the pump is closely related to the pipe resistance, the elimination of the air resistance largely avoids pumping out. The vacuum-assisted light oil unloading process increases the vacuum energy of the siphon line, increasing the liquid energy and increasing the flow rate, thus avoiding the formation of high-point air resistance.
IV. Comparison of vacuum-assisted light oil unloading technology and positive pressure unloading technology Currently, the positive pressure unloading technology commonly used in train trestle bridges generally adds a submersible pump to the front end of the crane pipe, and drives the submersible pump as a pre-discharge centrifugal pump through compressed gas. Power source, but there are many shortcomings, the use of vacuum-assisted unloading process can effectively compensate for the lack of positive pressure unloading process.
The positive pressure unloading process adopts compressed gas to drive the submersible pump to unload. The sealing effect of the gas source pipeline is difficult to ensure. The air motor and the leaking gas increase the stirring and volatilization of the light oil in the tank car, and the electrostatic accumulation increases. Some systems with unqualified public works conditions even use compressed air instead of nitrogen, increasing the risk of fire. The vacuum-assisted light oil unloading process effectively solves this problem.
A pre-pressurized unloading system is implemented with a submersible pump. Generally, about 0.6 MPa of nitrogen is used as the gas source. Taking naphtha unloading as an example, the unloading cost due to nitrogen consumption and maintenance and renewal of submersible pumps is approximately 2.57 yuan/t. Since the vacuum-assisted light oil unloading process employs a vacuum system for pumping bottom oil throughout the unloading process, The consumption can be reduced and the unloading cost can be drastically reduced.
The vacuum-assisted light oil unloading process, in addition to the other unloading processes commonly used in the current storage industry, effectively overcomes the unloading difficulties caused by air resistance and pump evacuation, and ensures the unloading speed, thereby improving the safety of the unloading process. There have been significant breakthroughs in reducing energy consumption and facilitating operations. The production practice of this process has proved this.
The process only achieves the dynamic balance of oil and gas volatilization by evacuating the gas collection tank, and does not fundamentally eliminate the root cause of gas resistance. On the contrary, due to the reduction of the highest point of the crane pipe and the pressure in the gas collection tank, the oil product volatilizes more. . At the same time, this process only uses the vacuum of the gas collection tank to prevent the system from leaking, and it does not fundamentally eliminate the negative effects of gas infiltration outside the system. Therefore, this issue should be further studied.
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