Rotortest 

Publication authors: Jonas Valani Stein and Thales Freitas Peixoto  

Rotortest is a software for computational studies about the dynamic behavior of rotors. It is also possible to evaluate different rotor components. Based on data from the components and their operating parameters, a model is generated in the program through which it is possible to simulate the rotor behavior and the influence of these components on the dynamics of the rotating system.

Bearings

Bearings are components of rotating machines that support the rotor shaft in order to allow the rotating movement without excessive losses due to the contact between the shaft and the structure.

• Hydrodynamic bearings

Used mainly in turbomachinery as they allow work at high speeds and with high loads. They guarantee a long operational life of the equipment under normal conditions, because the friction between the rotor and the bearing is reduced to nearly zero. A lubricating oil film prevents contact between the shaft and the bearing walls, in addition to ensuring shaft support through hydrodynamic forces.

• Rolling Bearing

Rolling bearings are applied in a wide range of rotating machines, from small, medium, and even large sizes. A fixed structure is used to support the rolling bearing, interacting with the shaft, enabling rotation (Paulo, 2023). 

• Mechanical seals

Seals are essential components for the efficient operation of a wide range of rotating machines. Due to this relevance, there are several models of seals, suitable for each operation. Along with this variety, there are also several parameters that govern its operation, demanding many theoretical, computational and experimental studies. 

Rotating systems can be modeled and analyzed in Rotortest

• Hydraulic pumps 

Flow machines that perform the displacement of fluids, widely used in industries. They have a shaft that rotates at high speeds, creating a pressure gradient responsible for the displacement of the fluid. This shaft is supported by bearings, and, to avoid leakages, it uses mechanical seals.

• Reducers

Reducers are mechanical components whose purpose is to convert rotating speed into torque. The transmission shafts must be supported by bearings in order to operate. The gears operate submerged in lubricating oil, so, most reducers require mechanical seals to avoid leakage and contaminations byimpurities.

• Compressors

Compressors are machines whose function is to increase the pressure of a gaseous fluid. They have many applications, such as supplying compressed air for pressure valve activation or compressing gas for refrigeration cycles. There are many models and methods for achieving compression, but the majority of them use rotors and their components.  

• Turbines

Turbines operate at high temperatures and velocities. The bearings, sustaining the rotor shaft, must operate in a way to minimize efficiency losses in the turbine, due to friction. Turbines with multiple compression stages, besides conventional seals in the shaft, also have seals in the compression stages, which reduces the pressure losses.

• Case study: unbalance response of a multi-stage turbine 

A multi-stage turbine of approximately 2.5 m of length and 1840 kg is shown in Figure 11. The finite element model of the main shaft of the turbine is composed of 26 beam elements and 4 rigid discs, representing the equivalent inertia of the turbine blades. 

The turbine is supported by tilting pad journal bearings and contains flow seals to prevent leakage of the working fluid in each turbine stage. The figure below represents the model of each component of the rotating system.

The analysis procedure can estimate the load in the bearings, in order to calculate their equivalent coefficients, to analyze the stability of the rotating system.

The Campbell diagram below illustrates the variation of damped natural frequencies of the turbine analyzed as a function of the spin speed. It is also possible to estimate the vibration modes of the system.

Figure 15: Vibrating modes at 10,000 rpm

It is also possible to investigate the unbalance response. The chart below shows the oscillation amplitude in the vertical direction of some selected nodes in the rotor. It is seen that the oscillation amplitude is maximum at the rotation speed of 9600 rpm. It is desirable to operate the turbine at a rotation speed different than this critical velocity.

Figure 16: Unbalance response: amplitude of vertical displacement (Z) as a function of spin speed.

Case study: aerodynamic stability analysis of a compressor 

A compressor with a length of 2.3 m and weight of 1400 kg is shown in the Figure 17. The finite element model of the shaft is composed of 23 beam elements. The image also shows two hydrodynamic bearings (in blue) that support the compressor and the location of the flow seals (in green).

The aerodynamic stability analysis investigates the dynamic response of the system due to the centrifugal flow of the working fluid in the compressor stages (wherein the flow seals are located). The stability analysis also investigates the responses for the nominal clearance (nom) of the bearing and for the allowed tolerance limits, considering the maximum (max) and minimum (min) clearances. The figure below shows the logarithmic decrement of the first mode of the rotor, depending on the (aerodynamic) stiffness. The three curves show the logarithmic decrement in the nominal clearance condition of the bearings, as well as the conditions of minimum and maximum clearance.

The system can be considered stable (or not) according to the API standard, evaluating the logarithmic decrement. The output window displays the result of the stability analysis in a simple way, as shown below.

Figure 19: Output window - level 1 stability criterion (in Portuguese).

References

CWS, A. (2023). Manufacture of Din Bearings. Source: White Metal:  

https://www.whitemetalmancais.com.br/fabricacao-de-mancais-din/ 

Industrial, F. (2023). PLS e SPLS Labyrinth seals. Source: Fieza Industrial Solutions:  https://fieza.ind.br/product/pls-e-spls-selo-labirinto-solido-ou-partido/ 

Industrial, M. (2023). Types of hydraulic pumps. Source: Industrial Mechanics:  https://www.mecanicaindustrial.com.br/135-tipos-de-bombas-hidraulicas/ 

Monferrato. (2023). What are bearings and their types?    Source: Monferrato :  https://monferrato.com.br/o-que-sao-mancais-e-os-seus-tipos/ 

Niza, E. M. (31 de 08 de 2021). 6 Tips on how to choose the correct bearing for your bearing.  Source: ABECOM: abecom.com.br/como-escolher-mancal-para-rolamento/

Paulo, E. P. (2023). PMR3320 – Introduction to Machine Elements - Class 6. In P. Ronaldo.  São  Paulo - SP. 

Rio, P. (s.d.). Hydrodynamic Bearings. In Dynamics of Rotating Machines on Hydrodynamic Bearings. Rio de Janeiro - RJ. 

Seal, U. (2023). Mechanical Seals. Source: ULTRASEAL Mechanical Seals:   https://www.ultraseal.com.br/produtos/selos-mecanicos/ 

Turbos, B. (2019). Learn about the differentials of the Heavy Duty Diesel Line turbochargers.      Sources: Biagio  Turbos  Diesel: https://www.biagioturbos.com/br/linha_diesel_pesada.asp