Regarding the design pressure of liquefied petroleum gas storage tanks
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I’m working on design verification recently, and I have a diagram of a liquefied petroleum gas storage tank. The design temperature is 50 degrees Celsius, and the design pressure is 2.2 MPa. If someone asks me why the design pressure was set so high, how should I answer? The main components of this liquefied petroleum gas are propane; its saturated vapor pressure at 50 degrees Celsius is around 1.7, which is certainly not high enough to generate such high pressures. As for propylene, its vapor pressure is higher than that of propane, but it still isn’t that high either. There can’t be a high content of methane either. . . . . . Also, the tank shell is required to be made of Q345 after normalizing, but the notes state that the hydrogen sulfide partial pressure should be less than 0.00035 MPa – so is stress corrosion not taken into consideration? ), I’m not sure if this is a contradiction or not. The drawings they gave me to work on are really confusing me,,,,, Also, they suggested giving a corrosion margin of 1, but the experts want me to use 3,,,,,
Pressure vessels containing mixed liquefied petroleum gas at room temperature shall have a design temperature of 50°C. When its saturated vapor pressure at 50°C is lower than or equal to that of isobutane at 50°C, the saturated vapor pressure of isobutane at 50°C is taken as the design pressure ； When its saturated vapor pressure at 50°C is higher than that of isobutane at 50°C and lower than or equal to that of propane at 50°C, the saturated vapor pressure of propane at 50°C is taken as the design pressure ； When its saturated vapor pressure at 50°C is higher than that of propane at 50°C, the saturated vapor pressure of propylene at 50°C is taken as the design pressure ； Note: The saturated vapor pressures of common liquefied gases at 50°C are as follows: Propylene: 2.16 MPa; Propane: 1.77 MPa; Cyclopropane: 1.303 MPa; Isobutane: 0.79 MPa. Normalizing is required due to H2S, as a precautionary measure in the design. Of course, it makes sense to say it’s not at the right temperature either. A 1mm corrosion margin is a bit small; usually it’s 2–3
This post was last edited by ylb913 on 2016-1-11 21:22. Liquefied gas is primarily composed of C3 and C4 compounds, but C2 components are inevitably present as well. 1. The saturated vapor pressure of propylene at 50 degrees is relatively high; the value I found is 2.06 MPa. 2. Public utility systems have a relatively long service life, which means that the design pressure for them is also tend to be higher.
In the past, the design pressure was set at 1.8 MPa; later, the standard specified it as 1.77 MPa
In the past, the design pressure was set at 1.8 MPa; later, the standard specified it as 1.77 MPa :)
According to HG/T20580, the saturated vapor pressures at 50 degrees Celsius are 2.03 for propylene, 1.71 for propane, and 0.69 for isobutane
Liquefied petroleum gas is a mixture of C3 and C4 compounds, and the operating pressure depends on the partial pressures of these components. The saturated vapor pressure of propylene at 50°C is 2.16 MPa; therefore, a design pressure of 2.2 MPa is adopted for conservative purposes. The Code of Practice stipulates that containers for storing liquefied petroleum gas must be made of normalized steel plates.
The corrosion margin is related to the design service life...
When the components cannot be determined, the design pressure P is set at 2.16 MPa (G) based on propylene, the design temperature is 50 degrees, and Q345R hot-rolled steel plate can be used for the material. It is also advisable to carry out overall stress-relief heat treatment to prevent the H2S impurity level from exceeding the specified limits.
Does the regulatory standard specify that containers for storing liquefied petroleum gas must be made of normalized steel plates? Which one is it?
Liquefied petroleum gas generally contains hydrogen sulfide; in cases of stress corrosion caused by hydrogen sulfide, normalized steel plates should be used, as specified in clause 2, item 3 of section 7.8.2 of HG/T20581-2011