2 Software Introduction
2. 1 program flow
The software is mainly divided into four parts: related parameter input module, power calculation data import module, calculation module and result output module (interface display and report generation).
In the relevant parameter input module, input the necessary parameters required for each calculation module, such as selection of crankshaft material, input of basic parameters of the unit, axial dimension of the crankshaft and other parameter settings. Wherein, according to the actual crankshaft is a single turning shaft (one crank pin between adjacent main journal bearings) or a double turning shaft (two crank pins between adjacent main journal bearings), in the axial dimension setting interface of the crankshaft You can make the appropriate selection.
In the power calculation data import module, the result of the power calculation, that is, the tangential force, the normal force, the integrated piston force of each compressor listed at different crank angles, and the total tangential force and average tangential force of the unit are passed. This module is imported.
The calculation module is the core part of the software, including strength calculations, torsional vibration calculations, and motor current fluctuation calculations. The three parts can perform the corresponding functions independently.
In the result output module, the final complete calculation report can be generated, including the basic parameters of the unit, the strength calculation result, the torsional vibration calculation result, and the motor current fluctuation calculation result.
2. 2 main functions
2 .2 .1 strength calculation
The calculation of crankshaft strength mainly includes the calculation of the bearing force of the bearing on both sides of the unit crank, the calculation of the bending moment, the torque and the axial force of each check section, the normal stress and the shear stress of each check section, and the various dangerous sections thus obtained. And the safety factor of the dangerous point. The segmentation method is used to calculate the bearing reaction force. The segmentation method firstly simplifies the stress of the crankshaft as follows: (1) For multi-support crankshafts, consider the section of the main bearing at the midpoint of the main bearing; (2) The force of the connecting rod is concentrated at the midpoint of the crank pin. (3) Slight the inertia force; (4) Slight the weight of the crankshaft. You can check the safety factor of each dangerous section and the corresponding bending moment, torque and axial force calculation results when the unit crank is affected by the maximum piston force or the maximum input torque.
2. 2. 2 Torsional vibration calculation The power of large piston compressors used in the petrochemical industry generally exceeds 150 kW. At this time, the compressor is directly driven by a synchronous motor, and the motor and the compressor are rigidly connected. Therefore, when calculating the torsional vibration of the compressor shaft system, it is necessary to calculate the overall shaft system including the synchronous motor rotor and the flywheel that may exist between the motor and the compressor. If the compressor is indirectly driven by the drive motor through the pulley, Since the belt is an elastomer, only the compressor crankshaft shaft can be considered in the calculation.
Through the torsional vibration calculation, the natural frequencies of the compressor units can be obtained, and the corresponding critical speed is obtained. As a result, it is possible to check whether the compressor speed is in the resonance danger zone and reduce the risk.
The display interface is output for parameter input and calculation result respectively. In the calculation result output display surface, the calculation results of the equivalent inertia and compliance, free torsional vibration, and forced torsional vibration can be separately viewed by clicking different tabs.
2. 2 .3 Motor current fluctuation calculation
The total tangential moment of the compressor consists of the average tangential moment and excess torque (or pulsating torque). The presence of excess torque causes current fluctuations in the synchronous motor that directly drives the compressor, causing the stator winding temperature of the motor to rise, affecting the normal operation of the motor, and causing a shutdown in severe cases. Motor current fluctuations can also cause changes in the voltage in the transmission grid, affecting other users in the same grid. According to the American Petroleum Institute 618 standard, motor current fluctuations must be controlled within 66% of the rated value. Therefore, it is necessary to calculate the current fluctuation value of the synchronous motor during the design of the compressor and the calibration of the existing compressor.
The input parameter interface and the calculation result display interface of the motor current fluctuation calculation are respectively shown.
3 calculation examples
Select a reciprocating compressor for a certain process. The mechanical efficiency of the compressor unit is 0 92, and the crank radius is 0 160 m. Through the calculation of the previous heat and power, the average tangential force of the compressor unit is 490000 N, and the required shaft power is about 1800 kW. The relevant parameters of the motor.
According to the software calculation, under the existing driving conditions, the synchronous motor current fluctuates by about 20%, which is lower than 66%, and conforms to the API 618 standard. Even so, synchronous drive motors may have problems with excessive power fluctuations and high motor temperatures, so the current fluctuations allowed in actual operation need to be considered in conjunction with the motor and grid factors.
4 Conclusion
(1) The calculation software is developed by Visual Basic60, which is user-friendly and easy to operate. (2) Most compressor manufacturers in China generally only perform thermal and dynamic calculations during the design process, and less crankshaft shaft torsional vibration calculations and The calculation of the current fluctuation of the supporting drive motor, the software realized these functions for the first time; (3) The software is based on the Chinese version of Windows, easy to transplant and promote, can replace the early DOS-based related software, will produce certain economic and social benefits. .
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