Pump rotor 2 DOF reduced model and unstable onset speed evaluation

Abstract

Because horizontal multi-stage pumps have journal bearings and a balancing piston with narrow gap, it is needed to prevent self-excited vibrations caused by fluid forces in these gaps. For the rotor system, because it is challenging to find the stability limit with a strict model (1 D-FEM), it is better to reduce degree of freedom by fixing nodes of journal bearings and the balancing piston when one is looking to improve the stability limit. This paper suggests a method for finding the stability limit by 2 DOF model, and provides factors affecting the stability limit for horizontal multi-stage pumps. First, the rotor shape and the stiffness and damping of journal bearings and the balancing piston are configured and the rotor system is reduced to 2 DOF by constrained mode synthesis method based on the configurated shape and coefficients. By comparing the solutions calculated with 2 DOF and 1D-FEM, it is confirmed that 2 DOF models are suitable for unstable vibration analysis. Then, the average whirl λeqΩ is calculated by applying Bently-Muszynska model to the coefficients of the journal bearings and the balancing piston and it is concluded that the natural frequency ωn becomes equal to the average whirl λeqΩ at the stability limit. Finally, the influence of the natural frequency ωn and the average whirl λeqΩ on the stability limit is analysed for different cases.

Keywords

• pump
• rotor stability
• stability limit
• 2 dof model
• bently-muszynska model