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Tretiak Oleksii, Candidate of Technical Sciences (Ph. D.), SE "Plant Electrotyazhmash", Deputy Head of Department, Senior Lecturer of Aerospace Thermal Engineering Department, National Aerospace University named after N. Ye. Zhukovsky '"KhAI"
E-mail: alex3tretjak@ukr.net Kobzar Kostyantyn, Candidate of Technical Sciences (Ph. D.), SE "Plant" Electrotyazhmash", Chief Designer on Turbogenerators
E-mail: kk7@ukr.net Kovryga Anton,
SE "Plant" Electrotyazhmash", Head of Department
E-mail: a.kovryga@i.ua Tribushnoi Nickita, Student, Design Engineer, National Aerospace University after N. Ye Zhukovsky, "KhAI", SE Plant Electrotyazhmash
E-mail: leer07770@gmail.com Piatnytska Yevheniia, Student, Design Engineer, National Aerospace University after N. Ye Zhukovsky, "KhAI", SE Plant Electrotyazhmash
E-mail: pyatnitskayaes@bigmir.net
CONTACT TASKS IN ENERGETICS. PRACTICAL AND THEORETICAL RATIONALE FOR USAGE OF NEW FEM METHODS
Abstract. In submitted paper the peculiarities of operation, design and design version of the supporting elements construction of high power electric machines are considered. The possibility of transition from analytical methods of mathematical modeling of the stressed state to three-dimensional is considered in the present paper. A methodology for calculating of rigid supports for high power Hydrogenerators, which includes taking into account the specific features of the contact zone geometry, as well as the influence of the metal structure on the grid parameters of the finite element method was developed.
Keywords: electric machine of high power, the contact stresses, finite element method, metal structure.
Introduction which were in operation since the middle of the last
The history of the electric machines design has century and up to the present moment, are among more than a century. Our modern civilization is the most reliable sources of electricity on the planet. almost entirely based on electrical energy, and the During the last 20 years, a significant increase developed and existing constructions of generators, in the power of computer equipment has allowed
to proceed to calculation of the strain-stress state of the essential parts of the generators in the three-dimensional formulation. At the same time, the main dogmas of analytical methods were shifted to a three-dimensional calculation without rethinking of possibilities of new approaches.
At the present time, in a number of structures, steels of foreign manufacturers without data on their fatigue characteristics are applied. This requires the use of new calculation methods without the possibility of additional laboratory researches.
One of the most stressed elements of Hydro-generator is the thrust bearing support, which takes the load of the entire unit. However, this issue in the open press is not fully covered.
Features of Designing, Development and Operation of Electric Machines
The main stages of the life cycle of any product as per GOST P 53791-2010 are as follows:
- rationale of designing;
- development of Technical Task;
- carrying out of research and development;
- manufacturing and tests;
- modernization;
- usage (operation);
- elimination (with elimination of waste by utilization and / or elimination).
At that it is assumed that the types of used resources namely material, raw materials, fuel and energy shall be used in a similar equivalent.
As a rule, when establishing requirements for resource conservation, the Customer shall proceed from the requirements for ensuring quality, reliability, safety, and protection of life, human health, and the environment.
However, if you look in details at the service life cycle of Turbogenerators and Hydrogenerators, you can see that it comprises 40 years (see GOST 533-2000 for Turbogenerators and 5616-1989 for Hydrogenerators). And, as a rule, according to the work of Sazykin V. G. [1], in the average repair, power units shall be withdrawn from operation when
they reach the operating time between the average repairs in the previous period, or half the time between the overhauls. Routine repairs are carried out in accordance with the need to eliminate the malfunction and to carry out monitoring and testing, the frequency ofwhich is established in technical conditions and other documents for each type of power equipment (PE).
It is necessary to pay a special attention that one of the most perspective methods for repairing of power equipment is the aggregate-unit diagram for replacing of faulty elements with new ones or repaired ones. Transfer to this method of repairing of complex and critical responsible equipment is based on the provision of replaceable repair units, components and parts in equipment, establishment of optimal terms for their replacement, the development of the nomenclature and the necessary margin of replaceable elements. Such an approach to organization of power equipment repair on the technical condition of power equipment is the option associated with the "development and consumption of repair services".
The essence of the method is in the fact that after a certain period of time as wearing out of the main parts of power equipment increases, it becomes necessary to replace them with new parts or previously repaired parts. This is power equipment demand for a repair service. If the demand is not satisfied, then the worn-out part shall break down after a while with shutdown of the functioning of power equipment unit in the production process and emergency demand for the repair service shall arise. At that, other parts and assemblies may be damaged, up to complete destruction, and power equipment units shall be rejected.
According to the results of the papers of Kob-zar K. A. [2] during operation, the following damage pattern is available, submitted in Figure 1. In general, the nature of damages is similar also for Hydrogenerators.
Figure 1. Diagram of Data on Emerge of Typical Emergency Situations
of Electric Generators
As per their physical nature structural elements with respect to the mechanical strength calculation methods are distributed as follows:
- shielding;
- large bodies with preload fit;
- plates;
- contact tasks.
For the existing calculation methods in the classical formulation, temperature accounting is almost completely absent.
At that the issue of resource design remains open even with the usage of new methods of calculation and design.
At the turn of the 20th century, there was a tendency to transfer from two-dimensional analytical calculations to three-dimensional ones. At the same time, the classical performances submitted in [3; 4] were repeated and improved.
Studied Design
Studied design of Hydrogenerator is carried out in vertical design version, of suspended type with two guide bearings (1) and the thrust bearing (4), arranged above the rotor (3) with the support on to the spider (2). The excitation of Hydrogenerator shall be carried out from the system of thyristor independent excitation.
The rotor flange is connected to the turbine shaft by means of the flange connection. The stator (6) is
installed on the foundation inside the access-shaft of the Hydrogenerator and is attached to the foundation using anchor studs (5). The spider (2) rests on the upper shelf of the stator casing. The overlap of the turbine access-shaft, installed on the beams under the rotor of Hydrogenerator, serves as a platform for maintenance of the brakes (7).
Ventilation of Hydrogenerator is carried out in a closed cycle with partial intake of hot air for heating of the machine room. Air coolers (8) are arranged around the stator casing of Hydrogenerator. The zones of cold and hot air are separated by upper and lower air-separation shields. The general arrangement of Hydrogenerator is shown in Figure 2.
The Reasons of Destruction of the Thrust Bearing Design
In the literature, great attention is paid to the "active parts", to their fastening elements and the rotors of Hydrogenerators. However, attention is not paid to the supporting elements of the aggregates.
The main causes of damage of the thrust bearings is macroroughness. It comprises a separate protrusions and cavities with a distance between them of hundreds of millimeters, located on the mirror surface of the disk, usually in the direction of rotation. When the rotor rotates, these protrusions and cavities create periodic changes in the load on each segment.
Figure 2. General Arrangement of Hydrogenerator
The reasons of macroroughnesses emerge are as follows namely structural defects or peculiarities, such as a thin bottom of the thrust bearing bush, resulting in it bending under the load between the ribs of the bush, causing emerging of the protrusions and cavities on the thrust bearing disk fastened to it; residual deformations of the thrust bearing bush as a result of its hot fit on the shaft, which, after fastening of the disk to it, causes unevenness on its mirror surface; residual deformations of the disk, which emerged during operation, at installation or before it; destruction of the gaskets installed between the disk and the bush [5].
The increased macroroughness irregularity of the mirror surface of the disk leads primarily to wors-
ened working conditions of the thrust bearing during starts and stops. During start-up, the formation of an oil film separating the friction surface is impeded and slows down. When stopping earlier, i.e. at higher speeds, the oil film is broken. As a result, the process of direct contact of friction surfaces is lengthened, rubbing on the segments emerges, and then the geometry of the fluoroplastic surface of the segments changes, and the thrust bearing loses its serviceability.
Formulation of the Task
Development of the method of calculating of the supporting elements of rigid thrust bearings in a three-dimensional setting with the possibility of taking into account the use of new materials.
Calculation Diagram of the Analytical Method
In Figure 3 the Diagram of calculating of the thrust bearing in a two-dimensional setting is shown. In this case, a significant disadvantage is the assumption of a uniform distribution of the load, which does not reflect reality.
Maximum stresses shall emerge in the center of the support. However, based on this diagram, they emerge at the edges, in the place of availability of concentrators.
2a
Figure 3. The Design Diagram of Thrust Bearing in Two-Dimensional Setting
Implementation of the Method of Three-Dimensional Calculation
In paper [6], the method of calculating and designing of elastic structural elements was considered, but the calculation of support bolts and plates was not considered there. The support bolt and plates perceive an alternating load at all stages of operation, and failure often results in the need for replacement with disassembly of the entire Hydrogenerator.
Considering the above, it is necessary to develop the method of calculating the thrust bearings of Hy-drogenerators in a three-dimensional setting, taking into account the characteristics of the chosen material and supporting elements (bolts and plates).
As calculation material Steel 40XH or Steel 34CrMo4 are used. Their mechanical characteristics are determined as per GOST 8479-70 gr. 5 KP (killed) 590, and availability of internal defects is regulated at supersonic diagnostic as per GOST 24507-80 group 4P.
The macrostructure of steel is sorbitol. It is necessary to pay a special attention to the fact that the availability of lowercase non-metallic inclusions in the steel structure can negatively affect the mechanical strength of the samples.
Controlling the dimension of a grid of solid body includes specifying finite element values in various areas of the model. The smaller dimension of the element in the area improves the accuracy of the results in this area.
At that special attention shall be paid to the type of grid. It is proposed to use the grid at the basis of curvature. SolidWork Simulation currently includes solid continuum elements, curved surface shell elements (thin and thick) and truss and frame line elements [7]. Solid elements have only displacement degrees of freedom. Solid and membrane shell elements use linear and quadratic interpolation for the solution based on whether they have two (see Figure 4.) or three units on an edge. Solid elements have their stresses and strains recovered at a number of tabulated locations inside the elements. Stress or strain results from adjacent solids are averaged at their common nodes.
Figure 4. The Grid in General Coordinates At that the order of the dimension of the minimum element corresponds to the maximum grain dimension of the crystal lattice in the contact zone (see Figure 5).
Figure 5. The Grain Dimension of the Crystal Lattice
In Figure 6 the loads acting on the thrust bearing and the boundary conditions are shown. The calcula-
tion results of the mechanical stresses acting on the support of the thrust bearing, obtained using Solid-Work Simulation, are shown in Figure 7.
The calculated axial force P on the thrust bearing unit, caused by the weight of the Hydroaggre-gate unit rotor group and the hydraulic force, may comprise about 500 tons and is distributed evenly between the supports.
As the basis of the choice of safety margin for the finite element method is proposed to use the criterion of Cramer-von Mises, which is typical for isotropic materials with a viscous nature of fracture. However, it is necessary to take in to consideration that under pure compression/tension, the both of criteria are identical. And with a pure bend, the strength as per Cramer-von Mises is about 15% more.
Figure 6. Loads, acting on to the thrust bearing and limiting conditions
However, if we consider the results in details, then the actual strength margin shall be supplemented by a contact stress coefficient.
For a ball with radius Rball and plane Rn => « at condition of a touch provided in the form of an el- ticity modulus of steel.
lipse equation — the greatest stress comprises 2R
0.3883 PE2 —j— (see Figure 8), where E - is the elas-
R
Figure 7. The Results of Calculation
Figure 8. The Diagram of Contact Stress Got stresses comprise of 590 MPa. At that the average ones do not exceed the admissible ones in accordance with the requirements of GOST 56161989, and the maximum ones satisfy the following formulae:
0.3883 PE2
R2
Conclusion
The features of operation, design and design versions of the supporting elements construction of electric machines of high power are considered in given paper. It is shown that the most loaded elements that perceive contact loads are rigid thrust bearings, namely plates and support bolts. The method of mathematical modeling of the stressed state in a three-dimensional formulation was implemented. The permissible stresses in the contact zone are specified taking into account the geometry of the zone of contact between the plate and the bolt, the influence of the metal structure on the grid parameters of the finite element method.
References:
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2.
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