RotorDynamics

Over the years, the ØDS Machinery Dynamics Group has gained extensive practical as well as theoretical knowledge of turbomachinery rotor dynamics.

Today, we are one of the most respected independent consulting companies in the world with state of the art tools to make rotor dynamic studies.

• Centrifugal Compressors
• Centrifugal Pumps
• Axial Compressors
• Gas or Steam Turbines
• Motors and Generators
• Turbo-Expanders
• Free-Free Modes
• Critical Speed Map
• Natural Frequencies and Mode Shapes
• Unbalance Response
• Asynchronous Forcing
• Rotor Dynamic Stability

Rotor Dynamics Analysis

Why?
Predicting the dynamics of rotating machinery can be difficult, even for experienced engineers. Turbomachinery is often custom-built, and experience limits are continuously being pushed. For standard applications, new combinations of known technologies may also have a detrimental impact on rotor dynamics.

Independent rotor dynamic verification studies address these concerns by offering:

• Identification of human design errors
• High-level technical dialogue about the latest advances in rotor dynamic technology
  

A rotor dynamic verification study will help to uncover design weaknesses. In this way, appropriate changes can be implemented before the equipment leaves the shop and costly downtime of working machinery can be minimized.

When?
The decision to make a rotor dynamic verification study should be based on an evaluation of the machine type, the selected machine elements, the intended duty and the manufacturer’s experience with similar applications. ØDS has solid experience in performing such reviews.

• Typically, a rotor dynamic analysis will be relevant in the following contexts:
• Development of new prototypes
• Verification of key turbomachinery in
  new production facilities
• Retrofit and revamp projects
• Troubleshooting of running machines

Shaft Model

The most common and also most relevant dynamic analysis of turbomachinery deals with lateral shaft deflections.

A finite element model of the shaft is based on drawings or measurements. Ideally, the model is verified by comparing calculated free-free natural frequencies with impact test results.

ØDS deals with complex structural issues, such as:

• Multi-layered shafts with internal degrees of freedom
• Internal friction in shrink fits and material hysteresis
• Arbitrary flexible elements (impellers, thrust disk, blades, etc.)

Bearings and Seals
The dynamic properties of bearings and other interaction forces between rotor and stator are equally important to predict critical speeds, damped natural frequencies and rotor dynamic stability margin (logarithmic decrement) correctly. ØDS’s expertise therefore includes:

• Journal bearings (sleeve, tilting pad, rolling element, magnetic, active lubrication, etc.)
• Squeeze film damper bearings
• Thrust bearings
• Seals (labyrinth, honeycomb, wet, etc.) and other narrow internal clearances
• Impeller/diffuser interaction
• Pedestal and foundation flexibility

A machine could have a critical speed in the operating speed range and run successfully for years before trouble is encountered.

A rotor dynamic study that considers the entire range of possible values for bearing and seal parameters, shaft unbalance, process conditions, etc. will usually show the cause and remedy of the problem.

Excitation Mechanisms
ØDS can calculate the forced response for
excitation mechanisms such as:

• Unbalance
• Rotating stall
• Rub and other parametric excitations
• Bearing pedestal movement or base excitation

Experience and Resources
The ØDS Machinery Dynamics Group has gained considerable experience, especially from our work in the oil and gas industry with a variety of high performance turbomachinery.

ØDS has in-house, state of the art software for rotor dynamic and bearing dynamic calculations. This enables us to tailor-make the software to solve nontrivial problems. For example, statistical information on clearances can be related to stability margin, thereby allowing a cost-benefit analysis of recommended bearing modifications.

ØDS has also recognized commercial programs to predict fluid dynamic forces in seals and other clearances.

ØDS has well-established contacts with leading universities and laboratories around the world. This implies that we can assist in organizing both component testing and advanced research at the most qualified institutes.