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Analysis and Structural Design of Large Support Span Distance Centric Compressor

At present, my country's petroleum, chemical, refrigeration, fertilizer and other enterprises are developing in the direction of large-scale, centralized, high-efficiency, and low-cost, so there are higher requirements for the core equipment of chemical processes, centrifugal compressors. In recent years, our company has designed many large-scale and high-pressure compressors in order to meet the user's requirements for compressors such as large flow, high pressure ratio, and interstage aeration. For example, the current air compressor flow is basically in the range of 36000 ~ 52000 Nm³/h, and the impeller diameter is basically between 900mm ~ 1200mm. In 2006, methanol compressors were basically projects with an annual output of 100,000 tons of methanol, but since 2007, they are basically projects with an annual output of 20 tons of methanol, and the annual output of the device has doubled. Recently, it has also designed and manufactured three ethylene machines, namely propylene compressor, cracked gas compressor, and ethylene compressor, completely breaking the monopoly of foreign developed countries and realizing the localization of three ethylene machines. This is of great significance to the high-yield and localization of petrochemical industries, but it is also more and more difficult to design and calculate the body structure of the compressor.


1. Application of large support span compressor in practical work


The large support span refers to the ratio of the distance between the drive end support bearing and the non-drive end support bearing of the compressor to the z*large inner hole diameter of the impeller, abbreviated as L/D. This is an important indicator to measure whether a compressor is stable, and an important content of rotor dynamics analysis of centrifugal compressors. With the development of large-scale and multi-stage centrifugal compressors, this indicator has been paid more and more attention by people. It is mainly caused by the excessive length of the compressor rotor due to the excessive number of stages of the compressor.


1.1 Relationship between gas molecular weight and compressor stage


The smaller the molecular weight of the gas, the more difficult it is to compress. For example, in a hydrogen compressor, the pressure ratio of the first-stage impeller is only about 1.0 to 1.12. For example, the 200,000-ton methanol compressor of Yunnan Yunwei Group has a hydrogen content of 68% and an average molecular weight. is 11.55, the total pressure ratio of gas in the compressor is 2.6, the compressor uses 9 stages of compression, there are two interstage coolers and an anti-surge cooler, the model is 3BCL529. In a compressor with a larger molecular weight, the gas is easier to compress, but since the Mach number cannot be greater than 1, the peripheral speed of the impeller cannot be too high.


1.2 The speed and work of the impeller


The working force of the same type of impeller is related to the power and speed provided by the driving function. The function of sufficient power is to enable the main shaft to transmit enough torque to ensure the normal operation of the compressor; the higher the speed of the impeller, the more the impeller does work on the gas The more, that is, the greater the pressure rise of the gas, but it is limited by the driver, the peripheral speed of the impeller, the strength of the material, the Mach number, etc. The ability of the compressor to provide work by increasing the speed is limited.


1.3 Impellers commonly used in centrifugal compressors


The working force of the impeller on the gas is related to the working efficiency of the impeller, and the efficiency of the impeller mainly depends on the aerodynamic profile of the impeller, that is, the blade profile of the impeller. At present, the commonly used impellers are large ternary impeller, high-efficiency binary impeller, and binary impeller. The large ternary impeller adopts ternary blades, and the outlet angle of the impeller is large, which is the impeller with the highest efficiency, and its efficiency is between 83% and 90%, but the working range of the large ternary impeller is small. The blades of the binary impeller are binary blades, the outlet angle of the impeller is smaller, and the working range of the flow is larger. The high-efficiency binary wheel is between the large ternary impeller and the binary wheel. These 3 impellers are often used in combination. In order to improve the design work efficiency, the basic stage of the impeller has been standardized, the average efficiency has reached more than 80%, and the room for improvement is very limited.


Increasing the working speed of the impeller by increasing the number of stages of the compressor has become the most effective and direct method to improve the working pressure ratio of the compressor. As a result, super-long units such as 3BCL529, 2MCL609, and BCL6010 of Yunnan Yunwei's 200,000-ton methanol project appeared.


2. Analysis of technical difficulties and introduction to the design process


Product 2MCL609 is a typical large support span compressor. Combining theory and practice, the technical difficulties and calculation process of this product are briefly introduced as follows.


According to the introduction of the literature [1], the number of stages of centrifugal compressors is generally less than 9 stages, but designing the stage number of the compressor to 9 stages or even 10 stages is the basis for increasing the compressor speed and improving the working efficiency of the impeller. It is a method adopted after z* when the parameters required by the user cannot be met. The more stages of the compressor, the longer the rotor of the compressor, and the greater the distance between the support bearing at the drive end and the support bearing at the non-drive end, which is called L/D here. Once the value of L/D exceeds the specified value, the strength analysis of the compressor rotor becomes the content that must be checked. Unqualified strength analysis results will lead to changes in the structural scheme and even the aerodynamic scheme.


The strength analysis mainly includes the strength analysis of a single impeller and the dynamic analysis of the rotor shafting. Strength analysis of a single impeller includes impeller stress calculation, disc natural vibration frequency analysis of semi-open impeller, and blade natural vibration frequency analysis of semi-open impeller; the dynamic analysis of rotor shafting is mainly rotor stability analysis, including gas Vibration analysis Shaft and bond strength calculations. The mass of the rotating rigid body, the center of gravity, the calculation of the moment of inertia, the calculation of the axial thrust, and the determination of the size of the balance plate are also included.


Gas excitation is the key content of rotordynamic analysis. Gas excitation refers to the gas excitation of the machine due to the rotational separation inside the impeller in the compressor. For centrifugal compressors with large molecular weight and high pressure, such as CO 2 compressors and syngas compressors in fertilizer plants, such issues need to be considered in the project design. Self-excited vibration means that gas with high pressure and high molecular weight may reach the speed of sound due to the high pressure ratio when it is sealed by the balance disc, thereby inducing gas excitation to the rotor.


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