Does UV Light Curing Pipe Rehabilitation Reduce Pipe Flow Diameter?

In municipal stormwater and sewer network trenchless rehabilitation, UV-CIPP (UV light-cured in-place pipe) technology often raises a common concern: does the liner reduce the internal diameter of the pipe and affect drainage capacity, especially during peak rainfall events? Based on the technical specification Technical Specification for CIPP Rehabilitation Engineering (T/CECS 559) and field measured data, this article briefly explains the reality of diameter reduction, its actual impact, and control methods to address this industry-wide question.

Conclusion: Yes, but the reduction is minimal

UV-CIPP is a typical “pipe-in-pipe” rehabilitation method. It involves inserting a resin-impregnated fiberglass liner into the existing pipe, which is then cured to form a new structural pipe inside the host pipe. Therefore, some reduction in internal diameter is inevitable.

The reduction can be calculated as:

Actual internal diameter reduction = 2 × liner wall thickness

Based on common municipal construction parameters, the typical reduction ranges are:

• DN150–DN400 laterals: structural wall thickness 4–6 mm → diameter reduction 8–12 mm

• DN500–DN800 main lines: structural wall thickness 6–8 mm → diameter reduction 12–16 mm

• DN800 and above large-diameter pipes: diameter reduction generally ≤ 20 mm

From a numerical standpoint, this level of reduction falls within acceptable standards defined by relevant codes and is considered negligible in most engineering applications.

Core misconception: diameter reduction ≠ lower flow capacity

Most concerns focus only on internal diameter, while ignoring hydraulic roughness — the key factor affecting flow capacity.

Significant improvement in internal surface smoothness

Old pipelines (concrete, corrugated pipes, or aging systems) often suffer from scaling, corrosion, and joint misalignment. Their Manning roughness coefficient is typically around 0.014.

UV-cured fiberglass liners form a smooth, seamless internal surface with a coefficient of approximately 0.010, reducing flow resistance by nearly 40%, which significantly improves flow velocity.

Elimination of local hydraulic bottlenecks

Damaged pipelines often contain root intrusion, pipe collapse, offset joints, or internal protrusions. These defects create localized blockages and are the real cause of drainage inefficiency.

The liner restores a continuous and smooth flow path, effectively eliminating these bottlenecks and improving overall hydraulic performance.

Field data shows that even with slight diameter reduction, the improved smoothness and removal of internal defects often result in equal or even higher flow capacity compared to the original damaged pipe.

How to minimize unnecessary diameter reduction

Excessive or uneven diameter loss is usually caused by poor inspection and improper installation. It can be controlled through standardized procedures:

1. Precise pre-rehabilitation pipe preparation

Using CCTV inspection robots, internal pipe conditions are evaluated in advance. High spots, burrs, and offsets are ground and cleaned to ensure proper liner contact and prevent wrinkles or voids that may increase thickness locally.

2. Two-stage inflation and calibration process

• Low-pressure pre-inflation: removes wrinkles and initial deformation

• High-pressure stabilization: ensures uniform expansion and proper fit against the pipe wall

This prevents over-stretching or localized material buildup, ensuring consistent wall thickness and avoiding bulging.

Summary

In conclusion, UV light-cured pipe rehabilitation does cause a slight reduction in internal diameter, but it does not negatively affect drainage performance under normal conditions:

• Diameter reduction is typically within 20 mm and complies with engineering standards

• Improved internal smoothness significantly reduces hydraulic resistance

• The overall flow capacity after rehabilitation is often better than that of the original damaged pipe

With proper CCTV inspection, optimized liner thickness design, and standardized installation procedures, abnormal or excessive diameter reduction can be effectively avoided.