Introduction
Nozzle local load analysis is a critical aspect of pressure vessel and piping design. External forces and moments from connected piping can significantly impact the integrity of the nozzle-shell junction, potentially leading to failure if not properly accounted for. One of the most widely used methods for assessing nozzle loads is the Welding Research Council (WRC) Bulletins 107/537 and 297. These methods evaluate local stress concentrations in the shell due to applied loads. To properly conduct WRC-based nozzle load analysis, it is essential to understand the three primary load categories: Weight Loads, Operating Loads, and Occasional Loads. This article explores their definitions and how they should be incorporated into WRC-based local load evaluations when performing pressure vessel calculations.
Types of Nozzle Loads
1. Weight Loads (Sustained Loads)
Weight loads consist of forces that are always present, regardless of operating conditions. These loads contribute to the sustained stress state in the nozzle-shell junction and are critical for primary stress evaluations. Weight Loads Include:
• Piping weight (including insulation and supports)
• Fluid weight (hydrostatic pressure inside the pipe)
• Nozzle and vessel weight
• Static reaction forces from supports
WRC Consideration: Weight loads are included as primary loads in WRC analysis and contribute directly to the local membrane and bending stresses.
2. Operating Loads
Operating loads are present only when the system is in operation. These loads typically result from thermal expansion and other dynamic effects that vary with system conditions.
Operating Loads Include:
• Thermal expansion forces and moments (from connected piping growth)
• Dynamic effects (pressure pulsations, vibration, cyclic loads)
WRC Consideration: While WRC does not explicitly separate operating loads, they must be included in the total load set used for analysis. These loads primarily contribute to secondary stresses.
When in operation the system is also typically under pressure, which creates additional circumferential or longitudinal membrane stresses. So the following is also considered as an operating load:
• Pressure effect
WRC Consideration: Pressure loads are included as primary loads in WRC analysis and contribute directly to the primary membrane and stress.
3. Occasional Loads
Occasional loads are those that occur under extreme conditions, such as seismic events or transient pressure surges. These loads are infrequent but must be accounted for to ensure structural integrity under rare but severe loading conditions. Occasional Loads Include:
• Seismic loads
• Wind loads
• Water hammer, slug flow, or other transient loads
WRC Consideration: Occasional loads should be considered in extreme condition evaluations. Since they are short-term, their allowable limits may be relaxed compared to sustained loads.
Combining Loads for WRC Analysis
Since WRC does not differentiate between sustained, operating, and occasional loads, the total applied loads must be considered in a single analysis. However, project specifications often provide predefined local loads that have already been adjusted based on industry best practices.
A common industry recommendation is that local loads provided in project specifications for WRC analysis are already factored down to 70% of the total applied loads. This means that when implementing these loads in WRC calculations, no further reduction should be applied. However, if the project specification states that “Local Loads for WRC analysis shall be 70% of the total applied loads”, then the values given are should be reduced by designers to by 70%. Many codes, including ASME B31.3, allow for occasional loads to be considered at 70% of their full value for stress analysis, since the probability of full sustained + full operating + full occasional load acting simultaneously is low. This approach ensures a conservative but not overly restrictive stress evaluation.
Conclusion
Proper evaluation of nozzle loads is essential to ensure the integrity of pressure vessel and piping systems. Weight, operating, and occasional loads must all be accounted for in WRC-based analysis, but project specifications often provide local loads that are already factored down to 70% of the total applied loads. In such cases, no further reduction should be applied in the WRC calculations. Understanding these load types and their proper inclusion in WRC analysis allows for safer and more efficient vessel and piping designs.
