The fundamental basis for selecting atmospheric-pressure storage tanks versus pressure storage tanks – the actual vapor pressure of the material at storage temperature and the hazardous properties of the material

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The selection between atmospheric storage tanks and pressure storage tanks is one of the most critical decisions in the design of petrochemical and storage and transportation systems. The fundamental basis for selection is the true vapor pressure (TVP) of the material at storage temperature, as well as its hazardous properties. 1. Key selection threshold: Saturated vapor pressure and boiling point. Vapor is continuously generated at the surface of any liquid. The higher the temperature, the greater the vapor pressure. Atmospheric Tanks: Application conditions: The actual vapor pressure of the material at its maximum storage temperature is lower than the local atmospheric pressure (usually, a TVP of < 88 kPa absolute pressure is required). This means that the material is a stable liquid at normal temperature and pressure, and it will not boil. Typical materials: water, crude oil, diesel, lubricating oils, heavy naphtha, as well as most chemical substances that are liquid at room temperature (such as methanol and toluene). Design standards: Primarily in accordance with API 650 or GB 50341. The design pressure is usually between atmospheric pressure and 18 kPa (slightly positive pressure). Pressurized Tanks: Application conditions: The actual vapor pressure of the material at the maximum storage temperature is higher than the local atmospheric pressure. If no pressure is applied, the material will instantly boil and vaporize at room temperature. Typical materials: liquefied gases such as liquefied petroleum gas (LPG), liquid ammonia, propylene, butadiene, chlorine, etc. Design standards: Mainly in accordance with ASME Section VIII or GB 150 (pressure vessels). The design pressure is usually greater than 0.1 MPa (gauge pressure), and can even reach several MPa. 2. Detailed selection decision logic: In actual engineering design, the choice of storage tank is not determined solely by vapor pressure; four other factors must also be taken into consideration. Factor 1: Flash point and volatility control (for atmospheric pressure storage tanks). For liquids with low vapor pressure but high volatility and flammability, such as gasoline and aviation kerosene, using conventional atmospheric pressure storage tanks with fixed roofs can result in significant \"breathing losses\" and pose a fire hazard. Solution: Normal-pressure storage tanks are still used, but they are upgraded to floating roof tanks. By floating a float on the liquid surface, the gas phase space is completely eliminated, thereby suppressing volatilization. Dimension 2: The trade-off between operating temperature and economic efficiency (storage of liquefied gases). For materials that are gases at room temperature (such as propane), there are two engineering-based storage strategies: storage at room temperature under high pressure: using pressure vessels (such as spherical tanks or horizontal bullet tanks). Rely on extremely thick steel plates to withstand the high vapor pressure of materials. The advantage is that no refrigeration system is required, but the disadvantage is that the volume of a single tank is limited by the thickness of the material (usually not exceeding 10,000 m³). Storage at low temperature and normal pressure (frozen storage tanks): Normal-pressure storage tanks are used, but equipped with a large refrigeration compression system to lower the temperature of the material below its boiling point at normal pressure (for example, reducing liquid ammonia to -33°C). The advantage is the low cost of constructing storage tanks, with each tank having a capacity of hundreds of thousands of cubic meters, making them suitable for ultra-large-scale storage. Dimension 3: Material toxicity and environmental requirements. If a material is highly toxic (such as phosgene or hydrocyanic acid) or poses a severe threat to the environment, even if its vapor pressure at room temperature is not high, pressure storage tanks of a higher grade may be used in order to achieve \"absolute zero leakage\". Extreme strict manufacturing, flaw detection, and airtightness standards for pressure vessels are employed to ensure an absolute safety baseline. Dimension 4: Spatial geometry. Atmospheric pressure storage tanks are usually flat-bottomed cylindrical shapes (vertical); they are governed by the hydrostatic pressure of the liquid column, are suitable for being built in a laid-out manner, and require a large amount of space. Pressure storage tank: To resist the stress resulting from internal high pressure trying to cause expansion, a geometric shape with the most even distribution of forces is required; this is usually spherical or in the form of a horizontal cylinder with elliptical ends.