Category: Process Design

External Pressure

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Most vessels are designed to contain positive internal pressure, but it is possible to develop a partial vacuum inside of the vessel during steam-out cleaning, when draining liquids, or during abnormal process conditions. Therefore, it is general practice to check all vessels that are designed for an internal pressure for their resistance to collapse under […]

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Design Pressure: ASME Code, Section VIII, Division 2

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Under Division 2, the pressures used for the design of each component are very similar to those discussed for Division 1, although terminology differs. The maximum pressure permissible at the top of the vessel at the design temperature is defined as the design pressure (Pd), and is displayed as such on the vessel’s nameplate. However, […]

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Calculation of Maximum Allowable Working Pressure

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The actual thicknesses of the various vessel components will usually be thicker than the thickness calculated using the component design pressure (P). It is usually more economic to obtain the required thickness plus corrosion allowance by purchasing the next thicker commercial size of plate, pipe, or ANSI B16.5 flange, than to have the components specially […]

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Pressure Drop

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Pressure Drop

The process design may indicate a large pressure drop (DP) through the vessel that should be considered in the design of each vessel component. If DP is significant, it should be added to establish the component design pressure (P):

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Design Pressure and Operating Pressure

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The maximum operating pressure (Po), specified by process design engineers, is the maximum internal pressure that will occur under normal process conditions. The design pressure (Pd) is determined by adding a margin to the maximum operating pressure (Po) to allow for pressure surges above Po (without lifting the pressure safety relief valve). Po should be […]

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Maximum Allowable Working Pressure

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The maximum allowable working pressure (MAWP) is required to be displayed on a pressure vessel’s nameplate, and is defined in the Code as, “the maximum pressure permissible at the top of the vessel in its normal operating position at the (design) temperature.” The MAWP is not the same as the design pressure (Pd), which provides […]

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Design Pressure

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Design Pressure

The following pressures must be considered in the design of a pressure vessel: • Operating pressure • Design pressure • Maximum allowable working pressure (MAWP) Figure 400-1 shows a typical pressure vessel with the pressures that must be considered, and the relationships among these pressures. These pressures are also illustrated in Figure 400-2. The terminology […]

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Determining Design Conditions

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The following design conditions for a pressure vessel must be established before the actual design begins: • Design pressure • Design temperature • Wind and earthquake loads • Corrosion allowance • External loads • Internal loads Pressure and temperature are the factors that often govern the mechanical design of a pressure vessel. The ASME Code […]

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