Strength Calculation of Tee Structures in Pressure Vessels

Literature Overview

Chen Yongqiang from Foshan Chemical Machinery Equipment Engineering Co., Ltd. addresses a practical and frequently encountered problem in pressure vessel design: the structural assessment of tee configurations where a branch pipe of equal diameter is opened on the vessel shell. The paper highlights a significant gap in Chinese national standards, noting that GB 150.1-150.4-2011 and the widely used calculation software SW6 do not provide provisions for this specific configuration. The author turns to HG/T 20582-2011, which offers two calculation methods: the large opening reinforcement calculation and the welded tee calculation.

Standards Gap Analysis

This paper identifies a critical deficiency in the current standards framework. Engineers routinely encounter tee structures in chemical process equipment, where branch connections of the same diameter as the main vessel shell are common in reactor systems, heat exchanger manifolds, and pipeline manifolds. The absence of explicit provisions in GB 150 means that designers must either rely on alternative standards or develop proprietary assessment methods, introducing variability and potential non-compliance risks.

The paper discusses two calculation approaches under HG/T 20582-2011: the large opening reinforcement method, which treats the tee opening as a large hole requiring local reinforcement through added material or compensating areas, and the welded tee method, which considers the tee as a fabricated fitting with specific welding and structural requirements. The distinction between these two approaches is significant because the reinforcement method focuses on compensating the loss of shell material, while the welded tee method accounts for the geometric discontinuity and stress concentration inherent in the fabricated configuration.

Engineering Practice Implications

In engineering practice, the selection between these two methods depends on several factors: the operating conditions (pressure, temperature, cyclic loading), the material grade, the inspection and maintenance requirements, and the regulatory jurisdiction. For design-basis conditions involving high pressure or temperature cycling, the welded tee method is generally more conservative because it explicitly considers the stress concentration at the branch-to-run intersection. The large opening reinforcement method may be more economical for low-pressure applications where the primary concern is maintaining the overall shell integrity.

The practical recommendation emerging from this study is that engineers should not rely solely on any single standard or software tool for tee structure assessment. Instead, a multi-standard approach combining the provisions of GB 150 for general shell design, HG/T 20582 for tee-specific calculations, and ASME BPV VIII for reference verification provides the most robust design basis. Additionally, for critical applications, finite element analysis should supplement the code-based calculations to verify stress levels at the intersection, particularly at the saddle and corner regions where stress concentrations are highest.

This paper serves as an important reminder that standards are not exhaustive and that engineers must develop the competence to identify gaps and apply appropriate alternative methods. The ability to navigate between multiple standards and select the most appropriate one for a given configuration is a hathe writing systemark of experienced engineering practice.