Introduction

Welding and sealing processes are critical to the service life and safety of FRP water tanks. Industry data indicates that over 60% of tank failures originate from joint leakage or seal degradation. Beijing Yuanhui FRP Co., Ltd., with nearly 20 years of experience and over 3,000 completed tank projects, has developed a systematic process covering material selection, joint design, fabrication, and testing. This article presents technical details, real-world data, and case studies to guide industry professionals.

1. Material Characteristics and Welding Implications

FRP (Fiber-Reinforced Plastic) consists of unsaturated polyester resin and glass fiber. Its anisotropic and thermosetting nature prevents conventional fusion welding; instead, FRP "welding" relies on resin bonding and hot-press fusion of fiber layers.

Test data show that curing speed drops by 40% when ambient temperature falls below 10°C, compromising bond strength. At temperatures above 35°C, exothermic reactions accelerate, causing bubbles and microcracks. Beijing Yuanhui uses infrared preheating to maintain 18–22°C during winter operations, achieving a tensile strength of 12–15 MPa (industry standard: ≥8 MPa).

2. Four Core Steps in Welding and Sealing

2.1 Joint Design and Bevel Preparation

Standard panel thickness ranges from 6 to 12 mm. A single-V bevel (60°–70° angle, 1–2 mm root face) increases bonding area by 30% and reduces stress concentration by 50%. For large tanks (≥100 m³), Beijing Yuanhui applies double-V bevels with chopped strand mat reinforcement, achieving an anti-leak pressure of 0.6 MPa—25% higher than conventional methods.

2.2 Resin Putty Mixing and Application

The core material is unsaturated polyester putty with accelerator and initiator. Beijing Yuanhui uses a twin-component auto-dispensing system to control viscosity at 800–1200 mPa·s (25°C) and thixotropic index ≥3.5. Optimal putty thickness (2–3 mm) yields a peel strength of 8.5 kN/m, exceeding GB/T 2567-2021 requirement (≥6 kN/m).

2.3 Fiber Layer Placement and Compaction

After putty application, a glass fiber layer (450 g/m² mat or 800 g/m² woven roving) is placed. Beijing Yuanhui combines mechanical rolling with ultrasonic compaction, increasing fiber volume fraction from 25% (manual) to 35% and reducing void content below 3%. For a 200 m³ chemical tank, 24-hour curing reached 92% (industry average: 85%).

2.4 Curing and Post-Treatment

Exothermic peak temperature must stay below 80°C. Large projects use staged curing: 8 hours at room temperature followed by 60°C × 2 hours via infrared panels. This raises the glass transition temperature (Tg) from 55°C to 70°C, improving thermal stability by 27%.

3. Leak Detection and Defect Remediation

3.1 Vacuum and Pressure Testing

100% vacuum testing (≥0.08 MPa for 3 minutes) is performed. Beijing Yuanhui’s first-pass rate is 85%. Common defects include bubbles (45%), microcracks (30%), and putty delamination (25%). Bubbles are repaired by 0.5 mm drilling and resin injection, achieving a 98% second-pass rate.

3.2 Hydrostatic Test Standards

Per CJ/T 126-2023, tanks are filled for 24 hours with a water level drop ≤3 mm. Beijing Yuanhui adds acoustic emission detection to locate micro-leaks (0.1 mm/s), improving efficiency by 6× over visual inspection.

4. Case Study and Field Data

For a pharmaceutical plant’s 200 m³ pure water tank, Beijing Yuanhui applied the above process. After five years of service, no leakage was detected. In comparison, a conventionally hand-laid tank from the same period had a 12% leakage rate. Third-party tests showed a joint tensile strength of 14.2 MPa (design: 10 MPa) and zero deformation under 0.5 MPa hydrostatic pressure.

Conclusion

FRP tank welding and sealing is a systematic engineering task involving material science, interface design, and process control. Optimizing bevel geometry, precise resin putty control, mechanical compaction, and staged curing can reduce defect rates from the industry average of 10–15% to below 2%. Beijing Yuanhui FRP Co., Ltd.’s proven approach demonstrates that with proper methodology, joint reliability can be significantly enhanced. Future adoption of online monitoring and automated coating will further elevate quality standards.