Steel Spiral Case Research Articles

Hydrodynamic Characteristics of Preloading Spiral Case and Concrete in Turbine Mode with Emphasis on Preloading Clearance

A pump-turbine may generate high-amplitude hydraulic excitations during operation, wherein the flow-induced response of the spiral case and concrete is a key factor affecting the stable and safe operation of the unit. The preloading spiral case can enhance the combined bearing capacity of the entire structure, yet there is still limited research on the impact of the preloading pressure on the hydrodynamic response. In this study, the pressure fluctuation characteristics and dynamic behaviors of preloading a steel spiral case and concrete under different preloading pressures at rated operating conditions are analyzed based on fluid–structure interaction theory and contact model. The results show that the dominant frequency of pressure fluctuations in the spiral case is 15 fn, which is influenced by the rotor–stator interaction with a runner rotation of short and long blades. Under preloading pressures of 0.5, 0.7, and 1 times the maximum static head, higher preloading pressures reduce the contact regions, leading to uneven deformation and stress distributions with a near-positive linear correlation. The maximum deformation of the PSSC can reach 2.6 mm, and the stress is within the allowable range. The preloading pressure has little effect on the dominant frequency of the dynamic behaviors in the spiral case (15 fn), but both the maximum and amplitudes of deformation and stress increase with higher preloading pressure. The high-amplitude regions of deformation and stress along the axial direction are located near the nose vane, with maximum values of 0.003 mm and 0.082 MPa, respectively. The contact of concrete is at risk of stress concentrations and cracking under high preloading pressure. The results can provide references for optimizing the structural design and the selection of preloading pressure, which improves operation reliability.

Analysis of the Joint Bearing Capacity of Composite Cushion-Spiral Case Structures for Hydropower Stations Considering the Damage Mechanisms of Surrounding Concrete

The spiral case structure is an essential part of a hydropower station. To accurately explore the joint load-bearing effect of the cushion-spiral case structure, a cushion-spiral case structure with a high HD value was selected, modeled, and analyzed in this study. The reliability of the model was verified through measured data. Given the contact relation between the spiral case and the cushion, the cushion laying range was used as the control parameter to investigate its impact on the joint bearing capacity of the structure. In addition, the concrete damage theory was introduced to probe the damage mechanism of the structure under assumed extreme working conditions. The steel spiral case bears most of the internal water pressure in the joint bearing system, and the bearing ratio of the surrounding concrete and reinforcement decreases with the increase in the cushion wrap angle. A 1.1–1.2 overload head is the main section that forms penetrating cracks. For the spiral case structure with a high HD value, a reasonable cushion can significantly reduce the damage level of the surrounding concrete and regulate the uneven lifting of the turbine pier and the shear strength of the stay ring. This study can provide reference points for the spiral case arrangement and range and the structural failure response under extreme working conditions.

Embedment of Steel Spiral Cases in Concrete: Lessons from a Structural Deformation Accident in China

A spiral case structure (SCS) plays a significant role in the safe and reliable operation of a hydroelectric power plant (HPP). In an HPP with 700 MW class turbine in China, a structural deformation accident happened in the construction period causing severe loss. Based on in-situ measured data, this study focuses on two major differences of this SCS that might cause the accident: (a) the construction condition, and (b) the shape of steel spiral case (SSC). The accident is reproduced in numerical study, and the simulation results agree reasonably well with in-situ measured data. The results show that the construction condition is a main factor causing the accident, but it is not the only cause of the raising deformation. The findings reveal that the post-accident stresses of steel structures are still at a relatively low level, and it would not be a major concern. The study also shows that the SSC with non-circle sections tends to have larger deformation under internal water pressure, and the deformation of the stay ring needs more attention in the construction period. The major limitation of this study is that this study merely focuses on the construction period. If such SCSs were to be used in a wider range, a follow-up study focusing on the operation period should be considered.

Numerical modeling of preloaded filling spiral case structure

Abstract A spiral case with its steel spiral case (SSC) being embedded in reinforced concrete under a pressurized condition is called a preloaded filling spiral case structure (PFSCS) in a hydropower plant. As a steel-concrete composite structure, a PFSCS is designed to work reliably. The non-uniform gap and contact nonlinearity between the SSC and the surrounding mass concrete have a great effect on the bearing mechanism of the composite structure. However, the description of the gap and contact nonlinearity in a PFSCS is a tough work. With the aim of efficiently describing the evolution process of the non-uniform gap and contact nonlinearity, we performed an experimental investigation and proposed a novel numerical simulation technique for structural finite element analyses (FEA) of PFSCSs. In the technique, the gap and contact nonlinearity as well as the construction process and operation process of a PFSCS are taken into account. A friction-contact model is used to simulate the sliding of the SSC against the concrete. A plasticity damage model is employed to describe the concrete. The development of the gap, contact status between the steel liner and the surrounding concrete, stresses of the steel liner and the steel bars, as well as the concrete cracking time and crack pattern, are presented in this work. The FEA results agree well with the experimental results. The agreement provides evidence that the applicability and competence of the proposed technique are valid and satisfactory.

BOTDA based water-filling and preloading test of spiral case structure

In the water-filling and preloading test, the sensing cables were installed on the surface of steel spiral case and in the surrounding concrete to monitor the strain distribution of several cross-sections by using Brillouin Optical Time Domain Analysis (BOTDA), a kind of distributed optical fiber sensing (DOFS) technology. The average hoop strain of the spiral case was about 330 u e and 590 u e when the water-filling pressure in the spiral case was 2.6 MPa and 4.1 MPa. The difference between the measured and the calculated strain was only about 50 u e. It was the first time that the stress adjustment of the spiral case was monitored by the sensing cable when the pressure was increased to 1 MPa and the residual strain of 20 u e was obtained after preloading. Meanwhile, the shrinkage of 70 ~ 100 u e of the surrounding concrete was effectively monitored during the depressurization. It is estimated that the width of the gap between the steel spiral case and the surrounding concrete was 0.51 ~ 0.75 mm. BOTDA based distributed optical fiber sensing technology can obtain continuous strain of the structure and it is more reliable than traditional point sensor. The strain distribution obtained by BOTDA provides strong support for the design and optimization of the spiral case structure.

Effect of Cushion Layers on Mechanical Characteristics of Spiral Case Structure in Hydropower Station

The effects of deformation modulus, thickness and range of cushion layer on mechanical characteristics of spiral case in hydropower station has been analyzed using the finite element program ANSYS and nonlinear contact theory. The result shows that when cushion layer scope and poisson ratio is fixed, as the elastic modulus of cushion layer decreases, the deformation and stress of spiral case increases, surrounding concrete load-bearing ratio of water pressure reduces. If the cushion layer deformation modulus is fixed and the range of cushion layer extends to 10 degree site bellow the waist of spiral case, the steel spiral case stress decrease, the radial stress transferred from the spiral case to surrounding concrete increase by about from 0.01MPa to 0.03MPa, and load-bearing ratio of concrete increase, when the cushion layer Poisson ratio increases from 0.25 to 0.35.

Using softened contact relationship describing compressible membrane in FEA of spiral case structure

A steel spiral case surrounded with mass concrete in a hydroelectric power plant is what is called a spiral case structure (SCS). The top exterior surface of a steel spiral case is routinely covered with a thin-layer compressible membrane. Unfortunately, the description of membrane in current structural finite element analyses (FEA) of SCS is a tough work in a conventional element-represented way. With the purpose of both accurately and conveniently describing the membrane, we have made a maiden attempt at modeling it by defining a softened contact relationship between the spiral case liner and the surrounding concrete where the membrane works. The novel simulation concept concentrates on governing the penetration behavior of the spiral case liner into the surrounding concrete, by prescribing an appropriate and straightforward dependence of the contact pressure on the penetration (overclosure). Profiting from that the inconvenient and error-prone FE modeling of a thin-layer membrane can be avoided. The simulation technique's reliability in describing the mechanical property of a membrane material was verified. Its applicability and competence have been demonstrated to be valid and satisfactory. Accepting the simulation concept will help to significantly improve the FE modeling efficiency of a SCS and considerably simplify the material description of a membrane. The possibility of contributing to change the way we describe the membrane in structural FEA of SCS is a very exciting proposition.

Effect of compressible membrane's nonlinear stress-strain behavior on spiral case structure

With an active structural involvement in spiral case structure (SCS) that is always the design and research focus of hydroelectric power plant (HPP), the compressible membrane sandwiched between steel spiral case and surrounding reinforced concrete was often assumed to be linear elastic material in conventional design analysis of SCS. Unfortunately considerable previous studies have proved that the foam material serving as membrane exhibits essentially nonlinear mechanical behavior. In order to clarify the effect of membrane (foam) material's nonlinear stress-strain behavior on SCS, this work performed a case study on SCS with a compressible membrane using the ABAQUS code after a sound calibration of the employed constitutive model describing foam material. In view of the successful capture of fitted stress-strain curve of test by the FEM program, we recommend an application and dissemination of the simulation technique employed in this work for membrane material description to structural designers of SCS. Even more important, the case study argues that taking into account the nonlinear stress-strain response of membrane material in loading process is definitely essential. However, we hold it unnecessary to consider the membrane material's hysteresis and additionally, employment of nonlinear elastic model for membrane material description is adequate to the structural design of SCS. Understanding and accepting these concepts will help to analyze and predict the structural performance of SCS more accurately in design effort.

Study on Joint Load Bearing of Steel Spiral Case and Surrounding Concrete

Spiral case are important diversion parts of reaction type wheel, this paper adopts finite element method to carry out simulation analysis for spiral case structure of Shuikou hydropower station. Considering five calculation schemes in analysis, under different thickness and materials of cushion, researching stress and deformation distribution law of steel spiral case and surrounding concrete. Analyzing bearing ratio of steel spiral case and surrounding concrete, Research results provides reliable reference for design and construction of steel spiral case.

Study on Friction Coefficient of Spiral Case with Cushion Layer in Large-Scale Hydropower Plant

Elastic cushion layer between the steel spiral case and surrounding concrete is often used as a structural type in large-scale hydropower plant in China. In view of the uncertainty of the optimal elastic coefficient of friction and the transfer mechanism of internal water pressure to the surrounding concrete, Spiral case with cushion layer of large-scale hydropower plant is simulated with ANSYS nonlinear finite element program, considering the slip contact characteristics between steel spiral case and the surrounding concrete, the more reasonable coefficient of friction between the steel spiral case and concrete is studied. This provides reliable theoretical analysis for the design of large-scale hydropower plants.

Research on Optimizing Precast Water Head on Preloading Filling Spiral Case of a Large-Scale Hydropower Station

In combination with the practice of a pumped storage hydropower station, a 3-D nonlinear finite element method on the preloading filling spiral case is carried out to compare and analyze stresses of the steel spiral case and hoop reinforcement, the stress and bearing ratio of the surrounding concrete, the contact state between the spiral case and concrete in different precast water head. The results indicate that the stress of the surrounding concrete will get better with the increasing of precast water head. But there is a distance between the spiral case and the surrounding concrete when the water head is excessive, which is unhelpful for operating of the unit. When the precast water head of the spiral case is 439m or so, it can play a greater degree of mechanical characteristics of steel and concrete under the premise of operating stably of the unit.

Research on Nonlinear Simulation of Spiral Case in High Head Hydropower Station

Based on applying nonlinear simulation with 3D FEM and elastic and plastic damage model of concrete, considering contact property between steel spiral case and peripheral reinforced concrete, calculating the stress levels of reinforced concrete and steel liner computed between directly-embedded spiral case (scheme I) and keeping constant internal pressure spiral case (scheme II), as well as examining the reinforced concrete crack and damage produced by water pressure, the result clearly indicated that scheme II was more excellent than scheme I in high head hydropower station on having the best performance of steel, limiting crack of the concrete, reducing the damage region and decreasing damage index of the concrete.

Effect of concrete cracks on dynamic characteristics of powerhouse for giant-scale hydrostation

With the increase of capacity and size of the hydro-generator unit, the spiral case becomes a more super-giant hydraulic structure with very high HD value, where H and D denote water head and maximum intake diameter of spiral case, respectively. Due to the induced lower stiffness by the more giant size and adverse operation conditions, dynamic performances of the powerhouse and the supporting structure for the giant units have become more important and attracted much attention. If the manner of steel spiral case embedded directly in concrete is adopted, on some locations of the concrete surrounding the spiral case, distributed and concentrated cracks will emerge due to high tensile stress. Although the concrete is reinforced well to control the maximum crack width, definitely these cracks will reduce the local and entire stiffness of the powerhouse. Under dynamic loads such as hydraulic forces including water pressure pulsation in flow passage acting on the structure, effect of the cracks on the dynamic characteristics of the local members and entire structure needs to be evaluated. However, research on this subject is few in hydroelectric engineering. In this paper, Three-Gorge Project was taken as an example to evaluate effect of such cracks on natural frequencies and the vibration responses of the powerhouse under hydraulic and earthquake forces in detail. Results show that cracks only reduce the local structural stiffness greatly but have little effect on the entire powerhouse especially the superstructure; vibrations of powerhouse with cracks in concrete surrounding the spiral case are still under the design limits. Results in this paper have been verified by practice of Three-Gorge Project.

Symposium on Welded Spiral Cases From the Manufacturer’s Viewpoint: A Five-Part Symposium

An important but unspectacular part of hydroelectric power developments is the steel spiral case that guides the water from the penstock into the hydraulic turbine. The increasing acceptance of welding and the use of high-strength steels in the construction of spiral cases have permitted and required the development of new construction methods. This report is a condensation of the five prepared statements by representatives of the U. S. and Canadian manufacturers presented at the 1957 Annual Meeting of The American Society of Mechanical Engineers.