Strength Calculation of Venturi Scrubber Elbow with Opened Nozzle

Literature Overview

The paper by He Xiaoxiang, published in Petroleum and Chemical Machinery (2015, Vol. 18, No. 5, pp. 14-16), addresses a specific engineering challenge: the strength calculation of a nozzle opening on a Venturi scrubber elbow. The conventional design methods are insufficient for this non-standard configuration, and the author employs finite element analysis (FEA) using ANSYS software to perform a stress analysis in accordance with JB 4732-1995 (Steel Pressure Vessel Design Standard - Analytical Design). The study concludes with recommended wall thicknesses for the elbow and nozzle that satisfy stress requirements. This work is relevant to engineers involved in pressure vessel design, particularly those dealing with non-standard geometries where analytical methods are inadequate.

Core Technical Content

The Venturi scrubber is a common equipment in the chemical and petrochemical industry used for gas purification. The elbow section with an opened nozzle presents a complex stress state due to the combination of internal pressure, external loads, and geometric discontinuities. The conventional design methods, such as the membrane stress method or the boundary stress method described in ASME Section VIII Div. 1, are not applicable to this configuration because the nozzle is located on a curved surface rather than a cylindrical or spherical surface.

Component	Designed Wall Thickness	Notes
Elbow	28 mm	Increased from initial estimate due to stress concentration
Nozzle	40 mm	Thicker due to higher stress at the nozzle-to-elbow intersection
Nozzle extension length	858 mm	Provides adequate support and reduces stress at the intersection

The FEA model was created in ANSYS, and the stress results were processed using the linearization method as specified in JB 4732-1995. The linearization procedure separates the stress into membrane, bending, and peak components, each of which is evaluated against its respective allowable stress limit:

Membrane stress: Evaluated against the material's allowable stress at design temperature. This component represents the average stress across the thickness and is responsible for gross deformation.
Bending stress: Evaluated against a higher allowable stress (typically 1.5 times the membrane allowable). This component represents the linearly varying stress across the thickness and is responsible for local bending.
Peak stress: Evaluated against a further increased allowable stress (typically 2.0 to 3.0 times the membrane allowable). This component represents stress concentrations and is responsible for fatigue and fracture.

The linearization method is a critical step in FEA-based stress analysis because raw FEA results include all stress components and cannot be directly compared with allowable stress limits. The linearization procedure provides a systematic way to separate the stress components and apply the appropriate design criteria.

Standards and Design Methodology

The design methodology follows JB 4732-1995, which is the Chinese analytical design standard for steel pressure vessels. This standard is broadly equivalent to ASME Section VIII Division 2, which provides similar stress analysis and design-by-analysis procedures. The key provisions of the standard relevant to this study include:

Standard Provision	Description	Application in This Study
Stress linearization	Separation of stress into membrane, bending, and peak components	Used to process FEA results for compliance evaluation
Allowable stress limits	Different limits for membrane, bending, and peak stress	Used to evaluate each stress component
Load cases	Definition of design load combinations	Used to define the FEA loading conditions
Material properties	Allowable stress, yield strength, fatigue data	Used in the stress evaluation

The use of FEA for non-standard geometries is a well-established practice in pressure vessel design. However, the accuracy of the results depends on several factors:

Mesh quality: The mesh must be fine enough to capture stress concentrations, particularly at the nozzle-to-elbow intersection.
Boundary conditions: The boundary conditions must accurately represent the support conditions and the connection to adjacent piping.
Material model: The material model must be appropriate for the loading conditions (elastic, elastic-plastic, or creep).
Load definition: All relevant loads (internal pressure, external forces, thermal loads) must be included.

Integration with Engineering Practice

For engineers involved in pressure vessel design, the findings of this study offer several practical insights:

FEA as a design tool: When conventional design methods are not applicable, FEA provides a viable alternative. However, the results must be carefully processed using the linearization method to ensure compliance with design standards.
Wall thickness optimization: The FEA results can guide wall thickness selection, allowing for optimization that balances structural integrity with material cost. In this case, the elbow wall thickness was increased to 28 mm and the nozzle wall thickness to 40 mm, which may represent a more efficient design than a uniform thickening approach.
Stress concentration management: The nozzle-to-elbow intersection is a stress concentration region, and the FEA results highlight the importance of proper reinforcement design. The nozzle extension length of 858 mm provides adequate support and reduces stress at the intersection.
Design verification: The FEA-based design should be verified through additional checks, such as fatigue analysis, buckling analysis, and plastic collapse analysis, depending on the operating conditions and service life requirements.
Documentation and traceability: The FEA model, loading conditions, boundary conditions, and stress results should be documented and made available for review and audit. This is particularly important for pressure vessels subject to regulatory oversight.

Key Questions and Reflections

One question that arises is the validation of the FEA model against experimental or analytical results. The paper does not describe any validation procedure, which is a concern for engineering practice. In the absence of validation, the FEA results should be treated with appropriate caution, and the design should incorporate adequate safety margins.

Another consideration is the effect of manufacturing tolerances on the stress state. The actual geometry of the elbow and nozzle may differ from the idealized FEA model due to manufacturing tolerances, weld geometry, and post-weld distortion. These differences can affect the stress distribution and should be considered in the design.

The study focuses on static loading conditions, but in practice, the Venturi scrubber may be subjected to dynamic loads such as pressure surges, vibration, and thermal cycling. These dynamic loads may require additional analysis, such as fatigue analysis or dynamic analysis, to ensure long-term structural integrity.

Study Insights and Implications

This paper demonstrates the effective application of FEA to a non-standard pressure vessel design problem, which is a common challenge in the chemical and petrochemical industry. The use of the linearization method to process FEA results and evaluate compliance with design standards is a critical step that ensures the results are meaningful and applicable. For engineers involved in pressure vessel design, the key takeaway is that FEA is a powerful tool for addressing non-standard geometries, but the results must be carefully processed and validated. The study also highlights the importance of proper reinforcement design at stress concentration regions, such as nozzle-to-vessel intersections. As the industry continues to develop more complex and efficient equipment designs, the role of FEA in supporting design decisions will continue to grow, and engineers must develop the skills to apply FEA effectively and interpret results accurately.