Displacement Characteristics of Exposed Steel Pipes Under Solar Temperature Differential

Literature Overview

This study by Du Chao, Wu Hegao, Shi Changzheng, and Su Kai (2017), published in the Journal of Yangtze River Scientific Research Institute (Vol. 34, No. 11, pp. 126–131), investigates the displacement characteristics of exposed steel pipes (明钢管) under solar-induced temperature differentials. The research was supported by the National Natural Science Foundation of China (No. 51409194). The study is classified under TV732 (hydraulic structures and equipment).

Core Technical Content

Exposed steel pipes (also known as open steel penstocks or exposed steel conduits) are commonly used in hydroelectric power stations to convey water from the intake to the turbines. These pipes are exposed to the external environment and are subject to solar radiation, which creates non-uniform temperature distributions across the pipe cross-section. The temperature differential between the sunlit side and the shaded side can cause significant bending and lateral displacement of the pipe, which must be properly accommodated by the support and restraint system.

The authors used ANSYS software to establish a three-dimensional finite element model of a typical exposed steel pipe segment from a hydroelectric power station. The analysis covered two scenarios: (1) displacement and stress during maintenance when the pipe is empty, and (2) displacement and stress during operation after water filling, considering that the initial displacement may not be fully recovered.

Analysis Parameters

Parameter	Description	Typical Value
Solar temperature differential	Temperature difference between sunlit and shaded sides	15–30°C
Pipe diameter	External diameter of exposed steel pipe	3–6 m
Wall thickness	Steel pipe wall thickness	20–50 mm
Span length	Distance between supports	10–20 m
Operating water head	Hydrostatic pressure during operation	50–200 m
Limit stopper gap	Clearance at restraint device	10–50 mm
Support spacing	Distance between pipe supports	15–30 m

Key Findings

Non-uniform temperature causes lateral displacement toward the cold side: When the pipe is empty during maintenance, the sunlit side expands more than the shaded side, causing the pipe to bend toward the cooler (shaded) side. This displacement is significant and can accumulate if not properly managed.
Water filling exacerbates displacement: When water is introduced into the pipe after the initial temperature-induced displacement has occurred, the hydrostatic pressure causes additional deformation. The combined effect results in more pronounced lateral displacement toward the cold side and increased bending.
Higher operating water head amplifies temperature effects: The greater the operating water head, the more significant the impact of non-uniform temperature on the pipe displacement and stress. This is because the hydrostatic pressure increases the overall stress state, making the pipe more susceptible to additional deformation.
Limit stopper gap affects restraint effectiveness: A larger gap at the limit stopper (限位挡板) results in lower forces on the supports and stopper, but also weaker restraint on the pipe segment. This creates a trade-off between structural safety and restraint effectiveness.
Critical locations are near expansion joints: The pipe displacement and support/stopper forces are largest near the expansion joints (伸缩节). This is because the expansion joints provide flexibility but also reduce the constraint on the adjacent pipe segments.

Engineering Recommendations

Based on the analysis results, the following measures are recommended for exposed steel pipe design:

Reduce support spacing near expansion joints: By decreasing the support spacing in the vicinity of expansion joints, the pipe segment constraint can be strengthened, reducing displacement and stress.
Increase limit stopper dimensions: To prevent pipe segments from disengaging from supports under non-uniform temperature conditions, the limit stopper size should be appropriately increased.
Consider thermal expansion in support design: The support system should be designed to accommodate the expected thermal displacement while maintaining adequate restraint.
Monitor temperature-induced displacement: During operation, the actual displacement should be monitored and compared with predicted values to ensure that the structural behavior is within acceptable limits.
Schedule maintenance during cool periods: To minimize the initial temperature-induced displacement, maintenance activities should be scheduled during periods of low solar radiation, such as early morning or cloudy days.

FMEA Analysis of Displacement-Related Failure Modes

Failure Mode	Cause	Effect	Risk Level	Countermeasure
Pipe disengagement from support	Excessive lateral displacement	Loss of structural support	High	Increase limit stopper size
Expansion joint damage	Excessive displacement at joint	Leakage or joint failure	High	Reduce support spacing near joints
Support overload	Combined thermal and hydrostatic loads	Support structural failure	Medium	Increase support capacity
Fatigue cracking	Cyclic thermal loading	Progressive crack initiation	Medium	Periodic inspection and maintenance

Study Insights

This research highlights the importance of considering non-uniform temperature effects in the design of exposed steel pipes for hydroelectric power stations. The finding that water filling exacerbates the temperature-induced displacement is particularly significant, as it means that the worst-case scenario occurs during operation rather than during maintenance. The recommendation to reduce support spacing near expansion joints is a practical and cost-effective measure that can significantly improve the structural performance of the pipe system. The study also demonstrates the value of three-dimensional finite element analysis in capturing the complex interaction between thermal, geometric, and pressure effects. Future work should consider the long-term effects of cyclic thermal loading on the fatigue life of the pipe and its support system, as well as the influence of wind loading combined with temperature effects.