Discussion on the gas connection of naphtha floating roof storage tanks

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I. Concept for the modification to achieve phase connection in naphtha systems: The use of floating roof tanks and internal floating roof tanks is the primary method for controlling hydrocarbon contamination from crude oil and light oils such as gasoline; compared to dome tanks, they can significantly reduce hydrocarbon evaporation losses. The gas-connection process is employed to link the gas phases of multiple storage tanks holding the same type of oil, so that the gas emitted when one tank receives oil is captured by another tank that is simultaneously discharging oil. Practice of using gas interconnection in crude oil tank farms has proven to reduce large breathing losses by 85%. During the emission of oil and gas, components such as the vent valve and breather valve on the tank roof may carry away tiny oil droplets; or these droplets may form as a result of cooling by external cold air after the oil and gas are emitted, leading to localized contamination on the tank roof. Our tank area currently has nine naphtha tanks with a total capacity of 65,000 cubic meters. Each tank is equipped with an independent nitrogen sealing system. Over the past two years, technical upgrades have been carried out to seal the vents on these tanks in order to reduce vapor losses. Compared to before, the amount of breathing losses has decreased significantly, but the problem has not been resolved completely. First, each tank for storing gas operates as an independent system; during the discharge process, as the gas space within the tank increases, the nitrogen sealing control system must continuously supply nitrogen into the tank in order to maintain a slight positive pressure. In other words, the volume of material discharged is equal to the volume of nitrogen gas added. Second, while the feed tank is in the feeding state, as the volume of the liquid continues to increase and the gas space decreases, the gaseous portion inside the tank will be released into the atmosphere through the breather valve until feeding is complete. In other words, the volume of the inhaled air is equal to the volume of the exhaled air. It turns out that our naphtha tanks have been in use for over a decade now, and the sealing performance of their floating roofs is becoming increasingly poor. Even with internal floating roofs, some light hydrocarbons still manage to enter the upper layer of the floating roof through gaps, and are then released along with the nitrogen present in the tank. This phenomenon is most noticeable in summer; whenever feeding takes place, we can smell a strong odor of oil and gas. As the oil and gas carried in the gas discharged from the roof vent cool down upon contact with the outside cold air, they form tiny oil droplets that can cause local contamination on the tank roof. At the same time, as oil and gas evaporate into the atmosphere, in addition to causing air pollution, it also increases the fire risk in the tank area. The evaporation of light components reduces the quality of the raw materials and increases losses. If we adopt a gas-phase interconnection approach, connecting the gas systems of these nine naphtha tanks together, we can use just one set of nitrogen sealing control system located at the bottom of the tanks, on the main gas interconnection bus, to control the nitrogen sealing of this bus. (This makes it easier to regularly inspect the nitrogen sealing control system; in the previous setup, such systems were located at the top of each tank, making regular maintenance difficult. If the nitrogen sealing system fails, it could result in the tanks collapsing.) In this way, the changes in gas volume resulting from our daily receipt of raw materials and the dispatch of naphtha complement each other, thereby overcoming and reducing the gas volume losses and safety risks associated with large-scale breathing effects. Since the gas volumes in each tank are connected, the risk of equipment damage due to a failure in the nitrogen sealing control system in any individual tank is avoided, which in turn reduces environmental pollution. It reduces the amount of nitrogen used. Based on a current consumption of 65 tons of naphtha per hour, and assuming 330 days per year, this amounts to approximately 735,000 cubic meters. This does not yet include the loss of light hydrocarbons that are exhaled along with nitrogen. Someone may ask: Can the gas space in several tanks meet the gas replenishment requirement of the discharge tank? I say it’s more than sufficient; in each tank, the feed is generally raised to a safe height of 13.5–14 meters, which corresponds to 85% of the tank’s total capacity. There is also a 3-meter space above the floating disk, plus a 2-meter conical top, forming the gas space. The combined gas spaces in all tanks are enough to meet the gas requirements of the discharge tank. Currently, the pressure set for our nitrogen seal control system is ±50 Pa. As per the design, we can set the nitrogen seal control pressure between 500–1000 Pa, which allows us to meet the production requirements while ensuring safety, further reducing energy consumption through less gas emission, and promoting environmental protection. I would appreciate it if everyone here could share their thoughts on my ideas; discussions are welcome. Thank you!