Numerical Simulation and Parameter Optimization of GIS Shell Tee Opening Forming

Literature Overview

This paper by Ma Yongjie, Wang Hongwei, and Hou Yueling, published in Forging & Stamping Technology (2008, Vol. 33, No. 6), investigates the forming process of tee openings in GIS (Gas-Insulated Switchgear) shells using the Dynaform finite element software. The study explores the influence of blank holding force, pre-cut elliptical hole major-to-minor axis ratio, die corner radius, and die clearance on the flanging forming of GIS shell tee openings. Orthogonal experimental design is employed for parameter optimization, and the optimized numerical simulation results are compared with experimental results.

Core Technical Methodology

Forming Process Characteristics

The GIS shell tee opening forming process involves several critical operations:

Pre-cutting of elliptical hole: An elliptical hole is cut in the sheet metal blank to facilitate subsequent flanging operations.
Flanging forming: The pre-cut elliptical hole is flanged to form the tee opening geometry.
Drawing and trimming: Final shaping operations to achieve the required dimensional accuracy.

The pre-cut elliptical hole is a critical process feature. The major-to-minor axis ratio of this ellipse directly affects the material flow during flanging and consequently influences the forming quality, particularly the thickness distribution and risk of cracking.

Parameter Influence Analysis

Parameter	Influence on Forming	Optimization Direction
Blank holding force	Controls material flow and wrinkling	Moderate force to prevent wrinkles without excessive thinning
Elliptical hole axis ratio	Affects material flow uniformity	Ratio matched to tee opening geometry
Die corner radius	Influences material deformation and stress concentration	Larger radius for reduced stress concentration
Die clearance	Controls material flow and thickness distribution	Optimized for minimum thickness reduction

Orthogonal Experimental Design

The orthogonal experimental method was employed to efficiently explore the multi-parameter design space with a reduced number of simulation runs. This approach is particularly valuable for finite element simulation studies, where each simulation run can be computationally expensive.

Experimental Factor	Level 1	Level 2	Level 3
Blank holding force	Low	Medium	High
Elliptical axis ratio	1.0	1.5	2.0
Die corner radius	Small	Medium	Large
Die clearance	Tight	Medium	Loose

Key Findings

Thickness Distribution and Forming Quality

The numerical simulation results reveal that the minimum thickness after forming is a critical quality indicator. With the optimized parameters, the minimum thickness after forming is improved by 3.2% compared to the baseline condition. This improvement is achieved through:

Optimized blank holding force that prevents excessive material thinning at the die corner
Appropriate elliptical hole geometry that promotes uniform material flow
Adequate die corner radius that reduces stress concentration and localized thinning
Proper die clearance that allows controlled material flow without excessive stretching

Simulation-Experiment Correlation

The comparison between optimized numerical simulation results and experimental results shows good agreement in the thickness variation patterns. This validation is important for establishing confidence in the simulation methodology for future process development and optimization.

Engineering Practice Implications

Application to Pipe Fitting Manufacturing

Although this study specifically addresses GIS shell tee opening forming, the principles and methodology are directly applicable to the manufacturing of butt-weld pipe fittings, particularly tees and reducers. The pre-cut hole and flanging approach is analogous to the pre-forming operations used in some tee manufacturing processes.

Application Area	GIS Shell Tee	Pipe Fitting Tee	Common Principle
Pre-forming geometry	Elliptical hole	Pre-cut or pre-formed opening	Material flow control
Forming operation	Flanging	Flanging or stretch forming	Thickness distribution optimization
Quality indicator	Minimum thickness	Minimum thickness	Uniform deformation
Parameter optimization	Orthogonal design	Orthogonal design	Multi-parameter interaction analysis

FMEA for Tee Opening Forming Process

Failure Mode	Potential Cause	Effect	Prevention
Cracking at hole edge	Excessive strain concentration	Component rejection	Optimize elliptical hole geometry
Excessive thinning	Insufficient blank holding force	Reduced pressure capacity	Optimize blank holding force
Wrinkling	Excessive blank holding force	Surface quality degradation	Optimize blank holding force
Dimensional inaccuracy	Inadequate die clearance	Assembly issues	Optimize die clearance
Springback	Residual stress release	Dimensional deviation	Stress-relief heat treatment

Integration with Standards and Codes

The forming of tee openings in pipe fittings must comply with relevant standards:

ASME B16.9: Specifies dimensional requirements for butt-weld fittings, including tee branch and run dimensions, wall thickness requirements, and pressure-temperature ratings.
ASTM A403: Specifies material requirements for forged carbon steel and alloy steel butt-weld fittings, including mechanical properties and chemical composition.
ASME B31.3/B31.4: Specifies piping design and fabrication requirements, including thickness requirements, inspection requirements, and hydrostatic test requirements.

The numerical simulation approach described in this paper can be used to verify that the forming process produces fittings that meet these dimensional and mechanical property requirements before committing to production tooling.

Study Reflections

This paper demonstrates the value of finite element simulation combined with orthogonal experimental design for optimizing metal forming processes. The 3.2% improvement in minimum thickness, while seemingly modest, represents a significant engineering achievement in terms of process optimization. In pipe fitting manufacturing, minimum thickness directly affects pressure containment capability and fatigue life.

The use of orthogonal experimental design is particularly noteworthy. In my experience, many engineers attempt to optimize forming processes through trial-and-error approaches, which are time-consuming and may not identify the optimal parameter combination. The orthogonal design approach systematically explores the parameter space and identifies significant factors and optimal levels with a manageable number of experiments.

The simulation-experiment correlation is a critical aspect of this work. Finite element simulation results are only valuable if they accurately predict real-world forming behavior. The agreement between simulation and experiment establishes confidence in the simulation methodology and enables its application to new forming problems without requiring extensive experimental validation for each case.

I would recommend that engineers in the pipe fitting manufacturing industry adopt this simulation-based optimization approach for developing new fitting geometries or modifying existing processes. The investment in simulation capability and expertise is typically recovered through reduced tooling iterations, improved first-pass yield, and enhanced process understanding.