Research On Reverse Rotation During The Multi-Stage Centrifugal Compressor Failure Shutdown Process

Centrifugal compressor plays an important role in every branch of the national economy. Comparing with the positive displacement compressor, the centrifugal compressor has the characteristic of large mass flow, better processing property and simple structure. Therefore, it has been widely used in the field of chemical industry, power industry and metallurgy.In recent years, with the development of design, manufacturing and control technology, the large-scale Ethylene plant was built in petrochemical enterprise in succession. The cracked gas centrifugal compressor which is called the pivotal equipment of ethylene plant is developed to the tendency of becoming to large scale under the guide of market. During the production operation, the quantity of high-pressure process gas in the compressor system increases significantly. The high-pressure process gas can not exhaust in time in the event of failure shut-down. Therefore, the pressure between discharge and suction in every section of the compressor could not come to balance. The process gas goes from the outlet to the inlet through the inside of the centrifugal compressor. The expansion of the gas will produce driving force that might make the compressor rotate reversely during the failure shut-down process. The reverse rotation will result in destruction of the dry gas seal and other components. Thus, the stability and high efficiency operation of the ethylene plant will be seriously affected.Firstly, a set of cracked gas compressor of large-scale ethylene plant is analyzed as an example. The production process of ethylene plant and function of the cracked gas compressor are briefly introduced. Meanwhile, the multi-stage centrifugal compressor normal shut-down process and reverse rotation during the failure shutdown process are studied on the basis of motion law and rotating mechanics. Have a comprehensive analysis of the moment of resistance and pressure distribution in every stage and acquire the commonness of the motion and pressure balance during the failure shut-down process. As for the destruction of compressor reverse rotation, the structure and working principle of the dry gas seal and tilting pad bearing are analyzed to find out the destruction reason and some corrective actions are propounded.The normal shut-down process and failure shut-down reverse rotation process model for the simplified system are established on the basis of above-mentioned analysis and summary. At the same time, establish the pressure balance model of every stage during the failure shut-down reverse rotation process. And then, have a simulation of the normal shut-down process and failure shut-down process that reverse rotation turns up using the program. The actual recorded data is set for reference to make the verification of the model. The computational results show that the reverse rotation during the failure shut-down process is due to the unbalanced pressure of every stage at the end of the idle process. The reverse moment of thrust makes the centrifugal compressor rotate reversely. The high pressure process gas goes from the outlet to the inlet through the inside of the centrifugal compressor. It will produce driving force that might make the compressor rotate reversely. The computational results of the theoretical analysis are in agreement with the actual recorded data to a certain extent. As for the characteristic of the complicated flow in the centrifugal compressor during the shut-down process, numerical calculation is made on the shut-down and reverse rotation process. The calculation stimulates the flow in the centrifugal compressor at different speed and acquires the pressure distribution. The relationship between axial thrust, torque and the speed is analyzed. Furthermore, the reliability of the rotating mechanics model is validated once again.The validation of actual recorded data and CFD calculation show that the theoretical model can simulate the reverse rotation process efficiently. It could provide reference for the equipment modification and adjustment of control strategy. That will eliminate the risk of reverse rotation during the production. Therefore, it indicates that the analysis and model established in this research possess certain practical significance and engineering value.