Pressure vessel nozzles experience two loads simultaneously –internal pressure and external loads due to piping reactions. A proper stress assessment of a nozzle requires the superposition of the stress systems from both loadings. Then the maximum stress intensities for the various stress categories are to be determined and compared with their specific stress limits. A proper stress assessment of a nozzle requires the superposition of the stress systems from both loadings.
The stresses due to external loads can be calculated by various analytical methods, e.g. WRC-107. In evaluating a pressure vessel for external nozzle loads, you should consider the source of the loads. The reason for this is sometimes the magnitude of the external nozzle loads provided are overly conservative. When this occurs, you can end up unnecessarily adding reinforcement to the pressure vessel or making other design modifications to accommodate these loads.
Standard Nozzle Load Tables
With standard nozzle load tables there is conservatism built into the values since they are to be used for all nozzles of the same size regardless of the function. For example, the same magnitude of loads will be applied to a 3-inch vent as a 3-inch inlet nozzle. Because of this, standard nozzle load tables will use the largest anticipated loads for any nozzle of a particular size for all of the nozzles of that size. Normally this would not be a problem because the thickness of the pressure vessel is usually the same at the location of the 3-inch vent and the 3-inch inlet nozzle. However, the problem is these standard nozzle load tables are often used for a variety of pressure vessels. It’s sort of a one-size-fits-all scenario where the standard nozzle load table is used for all vessels regardless of the type of pressure vessel, purpose of the nozzle, location of the vessel, and even project. And that’s where the conservatism comes in. In order to encompass all of the possible permutations, the magnitudes of the nozzle loads will be conservative.
Nozzle Loads from Piping System Stress Analysis
External nozzle loads from a piping system stress analysis are almost always more accurate than those from standard nozzle load tables. This is because these loads are project and nozzle specific. The magnitude of the nozzle loads from the piping system stress analysis, however, may be overly conservative depending on the boundary conditions used in the analysis of the piping system. Sometimes the analyst will completely constrain the ends of the model of the piping system where it connects to the pressure vessel. This is sometimes referred to as treating the pressure vessel as an anchor. While a pressure vessel can be considered relatively stiff, it is not infinitely rigid at the nozzles. Completely constraining the model of the piping system where it connects to the pressure vessel will result in external nozzle loads that may be an order of magnitude higher than reality. It is better for companies to have project-specific pressure vessel nozzle loading tables which can be used for relating allowable nozzle loads for vessels. Normally forces and moments at the nozzle and shell interconnection are provided in a tabular format. These force and moment values depends on following factors such as Nozzle diameter, Connected flange rating, Equipment and nozzle thicknesses and Equipment diameter etc.
While the magnitude of the nozzle loads may be overly conservative, it is unlikely you will be able to get your customer to remove some conservatism without first showing the design of the pressure vessel would have to be modified. Companies are not apt to change standard nozzle load tables, often because the company’s engineer you are dealing with does not know the details involved in defining the magnitude of the nozzle loads in the tables. Without knowing these details, there is a reluctance to reduce the loads.
If the external nozzle loads provided are from a piping system stress analysis and the ends of the model of the piping system were completely constrained, the loads can be reduced by more accurately modeling the boundary conditions of the piping system. This entails the analyst performing the piping system stress analysis defining stiffness values at the ends of the model of the piping system where it connects to the pressure vessel. Sometimes the analyst will estimate the stiffness based on experience using a rough order of magnitude value. Other times the pressure vessel engineer can provide values for the stiffness of the pressure vessel at the nozzles’ locations for the piping analyst to use. Both of these methods will produce more accurate loads at the nozzles.
In conclusion, evaluating pressure vessels for external nozzles loads is not overly complicated but does require either experience with WRC 107 or finite element analysis. This includes experience with any pertinent software, an understanding of the assumptions and limitations of WRC 107, and a thorough understanding of the applicable sections of the ASME Boiler & Pressure Vessel Code.
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