Stress Classification For The Strength Design Of Exposed Steel Penstock In Hydropower Station

Penstock is an important part of water conveyance system in hydropower station. Exposed steel penstock is a commonly used pipe type because of its advantages such as explicit bearing system and easier installation. Meanwhile, exposed steel penstocks are more likely to be damaged without bearing combined with rock or concrete dam, so it is important to use a rational design approach to ensure its security. Strength failure is the main failure mode of exposed steel penstocks, and the main method for strength design is based on stress classification. There are three key issues in this method:structural model and corresponding load conditions, control sections and corresponding stress calculation formula, stress classification and corresponding safety levels. In this paper, research works on the strength design of exposed steel penstocks, which concerned with several representative penstocks codes domestic and abroad. The main contents are as follows:(1)Clarified the calculation model of exposed steel penstocks in hydropower station, and derived additional internal forces formulas caused by support subsidence. According to the force transfer system and load-bearing mechanism, this paper clarified the global and local calculation model for exposed steel penstocks. Additional internal forces formulas caused by support subsidence are derived according to the global calculation model. Studies show that support subsidence has a significant influence on pipe stress, when the value of support subsidence meets 0.05% of the span, an increase of 50% local primary membrane stress may be caused by additional internal forces.(2)Systemized the stress calculation method of the control section for exposed steel penstocks, and gave rational advices to select the control sections. According to the global and local calculation model of exposed steel penstocks, the stress calculation method of the control section for exposed steel penstocks is systemized based on elasticity and structural mechanics. Combined theoretical analysis with numerical analysis, this paper gives the scientific advices for control section selecting. Studies show that if support subsidence can be neglected, the control section for strength design of exposed steel penstocks only need pipe wall of mid-span and supporting ring and its near pipe wall, while support subsidence cannot be neglected, we should also calculate stress in the edge of pipe wall near the supporting ring.(3)A rational stress classification method for exposed steel penstocks is presented, and its corresponding safety levels are determined. According to the causes, inference methods, existing areas and the distribution characteristic along the thickness, a rational stress classification method for exposed steel penstocks is presented. Considering the intrinsic relationship between safety levels for different failure modes, while ensuring no significant plastic deformation and having enough strength safety reserves, the safety level for each stress is determined.