Zhuojin Pipe Fitting Co., LtdInfluence of Drawn Tee C Value on Stress Distribution and Burst Pressure
Literature Overview
The paper authored by Zhao Shubing et al. from China Petroleum Natural Gas Pipeline Engineering Co., Ltd. and the Pipeline Research Institute of PetroChina was published in Petrochemical Equipment (Vol. 44, No. 1, 2015, pp. 23-26). This study addresses a critical design parameter for drawn (isothermal forming) tees — the C value, which governs the wall thickness at the branch-to-run intersection. Using finite element analysis (FEA) on a DN1200×1000 drawn reducing tee, the authors investigate how variations in the C value influence stress concentrations at critical locations and the resulting burst pressure of the fitting.
Core Technical Content
The C value in drawn tee design is defined as the ratio of the actual wall thickness at the branch intersection to the nominal wall thickness of the branch pipe. This parameter is specified in standards such as ASME B16.9 and SY/T 5257. The study systematically varies the C value beyond and below the standard-specified minimum to evaluate structural performance.
Key Findings
The FEA results reveal three distinct behavioral regimes:
- When the C value is below the standard-specified minimum, increasing the C value produces a pronounced reduction in stress at the shoulder (branch intersection) and belly (outer surface at the branch run) regions. The burst pressure also increases significantly in this range.
- When the C value exceeds the standard-specified value, further increases yield diminishing returns — stress reductions become marginal and burst pressure improvements plateau.
- Beyond the current standard-specified C value, additional increases cannot effectively enhance the structural strength of the drawn tee.
| Parameter | Below Standard C Value | At Standard C Value | Above Standard C Value |
|---|---|---|---|
| Shoulder stress | High, decreases with C increase | Moderate | Low, minimal further reduction |
| Belly stress | Elevated | Acceptable | Slight improvement |
| Burst pressure | Increases notably with C | Baseline | Plateau, no significant gain |
| Material utilization | Inefficient | Optimized | Over-designed |
| Manufacturing cost | Lower | Moderate | Higher with no benefit |
Process and Standards Analysis
Drawn tees are manufactured by piercing a blank and then isothermally drawing the branch through a heated die. The C value is inherently determined by the forming process — specifically by the die geometry, forming temperature, and the number of drawing passes. For large-diameter tees (DN1200 and above), achieving a uniform C value across the entire intersection is challenging due to material flow inconsistencies.
The standard-specified C value represents a balance between structural adequacy and manufacturing feasibility. The study confirms that the existing standards (ASME B16.9, SY/T 5257, GB/T 12459) have already captured the optimal C value range where the marginal benefit of additional thickness diminishes. This has important implications for procurement and design:
- Designers should not specify C values exceeding the standard minimum in expectation of improved performance.
- Manufacturers should focus on achieving consistent C values at or above the standard minimum rather than attempting to exceed it.
- Quality control should verify the C value through wall thickness measurements at the intersection, typically using ultrasonic thickness gauging at multiple clock positions.
Engineering Practice Integration
In practice, large-diameter drawn tees for oil and gas pipelines (typically DN600 and above) are subject to strict wall thickness verification. The shoulder region is the most critical for stress concentration, particularly under internal pressure combined with bending moments. For a DN1200×1000 tee in X70 or X80 grade steel, the nominal wall thickness may be 21.4 mm for the run and 17.5 mm for the branch, with the C value determining the actual thickness at the intersection.
A practical concern is that drawn tees often exhibit wall thinning at the belly region during forming. If the C value is too low, the belly thickness may fall below the minimum required by ASME B16.9, leading to rejection. The study's findings support the view that designers should not over-specify C values to compensate for anticipated thinning — instead, the forming process should be optimized to achieve the standard-specified C value with appropriate tolerance.
Key Reflections and Insights
This study provides valuable confirmation that the C value specified in current standards is near-optimal. The diminishing returns beyond the standard value suggest that any attempt to increase C values for "extra safety" is economically unjustified. From a manufacturing perspective, this means that process development efforts should focus on consistency and reproducibility of the C value rather than pushing for higher values. For quality assurance, the study underscores the importance of measuring wall thickness at the shoulder, belly, and heel positions during incoming inspection of drawn tees.
The work also implicitly highlights a broader principle in pressure vessel and piping component design: the standard-specified minimum thicknesses are not arbitrary but represent a carefully calibrated balance of structural performance, manufacturing feasibility, and cost. Engineers who understand this balance can make more informed decisions when evaluating non-conformances or proposing design deviations.