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The design assurance services at Southwest Research Institute (SwRI) help operating companies and their suppliers avoid severe vibrations and other damaging dynamic phenomena in machinery installations.

SwRI uses both commercially available and internally developed software coupled with field experience on past installations to:

• Predict system dynamic characteristics

• Assess the potential for problems

• Identify modifications that will reduce the chance of problems

• Identify operating ranges to be avoided

• Define any areas of risk or uncertainty to be validated during startup

The design assurance services cover:

• Rotor system lateral critical speeds and stability

• Steady-state and transient torsional vibrations

• Bearings, stiffness, damping, and load capacity

• Compressor and turbine skids

• Mechanical integrity for impellers and other components

• Turbomachinery performance

• Control of pulsations in machinery piping systems

• Thermal flexibility and vibration of piping systems

• Management of surge and other system upset conditions

• Modal testing

• Rotordynamic and mechanical design audits

Rotordynamic Modeling

	SwRI engineers use computer models, such as this mass-elastic model, to predict system performance.

SwRI engineers develop mass-elastic models, as this diagram shows, for rotating components (e.g., compressors, turbines, pumps, motors, and gearbox shafts), accounting where appropriate for the lateral stiffening effect of interference fits. The resultant model enables prediction of rotor system dynamic characteristics, such as:

• Lateral critical speeds

• Torsional critical speeds

• Response to unbalance excitation

• Stability

Critical Speed Map

	Critical speed map guides assessment of rotordynamic characteristics.

Critical speed maps, such as this one, help SwRI engineers evaluate a machinery rotor system. Combined with bearing stiffness curves, the critical speed map shows where critical speeds will likely occur. The map indicates the effectiveness of bearing damping in controlling vibration amplitudes. The critical speed map also reveals the likely effectiveness of the changing stiffness of bearings or bearing supports in changing a critical speed.

Bearings

	Prediction of vibration modes for a skid mounted gas turbine boiler

Fluid film, tilting pad bearings influence the dynamics of turbomachinery rotor systems. Plain fluid film bearings act as highly loaded dynamic elements in reciprocating engines. Rolling element bearings carry the high-speed rotors of modern aircraft gas turbine engines and their derivatives in power generating and mechanical drive service. Squeeze film dampers help moderate resonant vibration levels in gas turbine engines, and some manufacturers use them to stabilize high performance centrifugal compressors. For bearings and dampers, SwRI has capabilities that include:

• Modeling

• Application

• Vibration and temperature measurement

• Condition monitoring

• Failure analysis of bearings and dampers

Finite Element Dynamic Analysis

	Stress analysis of centrifugal impeller wheel helps diagnose and avoid failure

SwRI engineers complement their field problem solving and failure analyses services with finite element analysis to diagnose causes and develop design solutions.

For example, a machine may experience a problem resulting from an unexpected resonant frequency or a high local flexibility. A weak element in the system, an unexpectedly high dynamic load, or excessive sensitivity to known dynamic loads may lead to structural failure. Finite element predictions, confirmed by field observations, identified high stress locations that led to stress corrosion cracking failures in this centrifugal compressor wheel. This analysis guided design changes that improved the integrity of the compressor.

Finite element analysis helps find correction factors that make more cost-effective but simplified analyses as accurate as possible. This approach provides flexibility factors for reinforced piping joints, adjustment factors for calculating forces transmitted by reciprocating compressors to their foundations, and the rotor stiffening effect of interference fits in turbomachines.

Structural Integrity Assessment

	Gas turbine driven compressor pipeline service, evaluation for torsional critical speeds

SwRI applies finite element methods to evaluate structural dynamic characteristics of skid-mounted gas turbines in power generation or mechanical drive service.

The illustration (right) shows a gas turbine mode of vibration identified by finite element analysis. Predicting harmonic response to applied dynamic forces helps SwRI engineers assess the severity of resonant vibration. Animation helps identify the elements that contribute most strongly to troublesome modes of vibration and clarifies the most effective locations for bracing or stiffening of the structure.

Modal Testing for Component and System Characteristics

	Facility test to validate rotordynamic predictions and component properties

Modal testing using an instrumented hammer or a shaker helps define important dynamic characteristics of a component or system. Modal testing of this LNG hydraulic turbine-generator, with excitation from a shaker, confirmed the model developed by SwRI for angular flexibility of the gasketed joint between the turbine and the top plate. This flexibility dominates a structural vibration mode in which the entire turbine swings about the gasketed joint. The modal analysis confirmed the prediction that this structural mode would occur at a frequency well below running speed and that the torque on the bolts compressing the gasket would not influence the modal frequency (photo courtesy Ebara International, Cryodynamics Division).

SwRI can offer you a full range of capabilities and experience in gas turbine technology including becoming an extension of your engineering department. For more information about gas turbine mechanical design assurance at SwRI or how you can contract with SwRI, please contact Klaus Brun, Ph.D., or call (210) 522-5449.
 

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Southwest Research Institute® (SwRI®), headquartered in San Antonio, Texas, is a multidisciplinary, independent, nonprofit, applied engineering and physical sciences research and development organization with 11 technical divisions.

July 15, 2008