How to determine the design pressure of pressure vessels

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Design pressure: Let’s first take a look at the definition of design pressure: “It is the maximum pressure specified at the top of the container; together with the corresponding design temperature, it constitutes the basic design load condition for the container, and its value is not lower than the operating pressure.” ” --As referenced in GB150.1 \"Pressure Vessels\", the design pressure is denoted by the symbol P, while the operating pressure is denoted by Pw. Pressure unit: Generally MPa. Unless otherwise specified, pressure refers to gauge pressure (positive when higher than atmospheric pressure, negative when lower than atmospheric pressure). When determining the design pressure, the following situations can be considered: For vessels under internal pressure, (1) in the absence of any safety relief devices, that is, when there are no safety valves or burst discs on the equipment, safety devices must be considered in the pipelines. The design pressure shall be set at a level not lower than the operating pressure. Generally, the design pressure is slightly higher than the operating pressure, but it should not be too high either; otherwise, a large design margin is required, which is not economical. Try to keep the working pressure at no more than 1.1 times. For example, if the operating pressure is 1.0 Mpa, the design pressure can be set between 1.0 and 1.1 Mpa; for instance, the intermediate value of 1.05 Mpa can be used. Of course, it is also necessary to take into account the corresponding pressure rating when selecting the flange later on ； For example, if the operating pressure is 3.7 Mpa, multiplying it by 1.1 gives a design pressure of 4.07 Mpa; in this case, a PN63 flange should be used. If the operating pressure is multiplied by around 1.05, the design pressure will be 3.9 Mpa ； PN40 flanges can be used. (2) When a safety valve is installed, first determine the set pressure Pz of the safety valve; it is generally taken as 1.05 to 1.1 times the operating pressure ； When Pz < 0.18 MPa, it is possible to appropriately increase the ratio of the set pressure Pz to the operating pressure Pw, that is, to more than 1.1 times the operating pressure ； Then, select a design pressure that is not less than the set pressure Pz of the safety valve; in general, it is possible to use a design pressure that is slightly higher than this set pressure. For example, if the operating pressure is 0.6 Mpa, the set pressure is taken as 0.66 Mpa, and the design pressure is taken as 0.7 Mpa. (3) When a rupture disc is installed, first determine its minimum calibrated burst pressure Psmin; different types of metal rupture discs with varying structures have different values. For ordinary positive-arch-shaped rupture discs subjected to static loads, Psmin ≥ 1.43Pw ； Slitted positive arch, capable of withstanding static loads: Psmin≥1.25Pw ； Positive arch shape, subjected to pulsating loads: Psmin≥1.7Pw ； The inverse arch shape can withstand static and pulsating loads: Psmin≥1.1Pw. For inverse arch-shaped rupture discs, the required blasting pressure is the lowest; thus, a relatively low design pressure can be used. Then, the manufacturing range for the rupture disc is selected, as shown in the table below: Image. Subsequently, the design burst pressure Pb of the rupture disc is determined; this value equals the minimum calibrated burst pressure Psmin plus the lower limit of the selected manufacturing range for the rupture disc (taking the absolute value). ) Finally, the design pressure P is determined; this pressure is greater than or equal to the design burst pressure Pb plus the upper limit of the manufacturing range for the selected rupture disc. For example, if the operating pressure is 0.6 MPa, a full-range reverse-arch type rupture disc should be selected. The minimum calibrated blasting pressure Psmin is set at 0.66 MPa, the designed blasting pressure Pb is set at 0.73 MPa, and the design pressure P is set at 0.75 MPa. (4) When a safety valve is installed on the outlet pipeline, it shall be no lower than the set pressure of the safety valve plus the pressure drop of the fluid from the container to the safety valve. (5) The container is located on the pump inlet side and lacks a safety relief device. The design pressure P is taken as 1.0 to 1.1 times the operating pressure Pw, and verification is performed using an external pressure of -0.1 MPa. (6) The container is located on the pump outlet side and lacks a safety relief device. The operating pressure P must be no less than the maximum of the following two values: 1) the normal inlet pressure of the pump plus 1.2 times the normal operating head of the pump ； 2) The maximum inlet pressure of the pump plus the normal operating head of the pump ； 3) The normal inlet pressure of the pump plus the shut-off head (i.e., the head when the pump outlet is completely closed). (7) The container is located on the inlet side of the compressor, and there is no safety relief device. The operating pressure P should be taken as 1.0 to 1.1 times the operating pressure, with a negative external pressure of -0.1 MPa used for verification. (8) The container is located on the outlet side of the compressor and is without a safety relief device; the operating pressure P should be at least equal to the pressure at the compressor outlet. (9) The saturated vapor pressure at the highest operating temperature that can be achieved, when the critical temperature of the liquefied gas is 50°C or higher and reliable insulation facilities are available ； The saturation vapor pressure at 50°C is in the absence of insulation. (10) When the critical temperature of the liquefied gas is less than 50°C, a reliable insulation system shall be in place to measure the saturated vapor pressure at the highest operating temperature obtained through testing ； The gas pressure at 50°C when the maximum filling volume specified by the design is used without any insulation. (11) For mixed liquefied petroleum gases with a saturation vapor pressure at 50°C equal to or less than that of isobutane, there must be reliable insulation facilities to ensure the saturation vapor pressure of isobutane at the highest operating temperature possible ； The saturation vapor pressure of isobutane at 50°C is the value without any insulation. (12) For mixed liquefied petroleum gases with a saturation vapor pressure higher than that of isobutane at 50°C, reliable insulation facilities should be used to achieve the saturation vapor pressure of propane at the highest possible operating temperature. (13) For mixed liquefied petroleum gas with a pressure less than or equal to the saturated vapor pressure of propane at 50°C, in the absence of insulation, it is the saturated vapor pressure of propane at 50°C. (14) For mixed liquefied petroleum gases with a saturation vapor pressure higher than that of propane at 50°C, reliable insulation facilities are required to maintain the propane’s saturation vapor pressure at the highest operating temperature achievable ； For vessels without insulation, it is the saturated vapor pressure of propane at 50°C. Vacuum vessels: (1) Equipped with a safety relief device; the design external pressure P is taken as 1.25 times the maximum difference between internal and external pressures, or -0.1 MPa, whichever is smaller ； (2) Without a safety relief device: The design external pressure P is set at -0.1 MPa. For containers consisting of two or more pressure chambers, such as jacketed pressure vessels, the design pressure for each chamber is determined based on its respective operating pressure. The calculated pressure refers to the pressure used to determine the thickness of a component at the corresponding design temperature, including the hydrostatic pressure of the liquid column. Under normal circumstances, the calculated pressure is equal to the design pressure plus the hydrostatic pressure of the liquid column. When the hydrostatic pressure exerted on the component is less than 5% of the design pressure, it can be ignored.