Identification Of Hydro-Unit Stiffnesses, Vibrating Masses And Critical Speed(S) Based On Vibration Measurements. A Case Study: Pump-Generating Unit 180 Mw


Ozren Oreskovic, mag. ing. mech

Paper Authored by

Ozren Husnjak
Veski Ltd

Ozren Orešković
Veski Ltd

Enes Zulović
Specialist Hydro Power Mechanical Engineer

Fabian Kaica
Specialist Hydro Power Mechanical Engineer


Hydro unit vibration measurements is, typically, one of the most important diagnostic procedures to evaluate the unit's stability of operation. It can be conducted using either continuous (on-line) condition monitoring systems or using multi-channel portable analyzers with which the vibration data is collected periodically.

Identification of permanent state changes as well as the fault type and its location is of utmost priority. Every change in the unit's vibro-dynamical behaviour is somehow related to the changes in its construction elements such as the bearing, stator, bracket or foundation stiffnesses, vibrating masses of rotating and stationary elements.

It is relatively often a fact that the unit's measured vibrations are significantly different from the design values. An identification procedure was developed which one can use to determine the unit's vibrational characteristics. The procedure is based solely on vibration measurements and not doesn't rely on the documentation / design values.

To perform this identification, it is necessary to measure and analyze both relative shaft and bearing vibrations simoultaneously on all bearings (i.e Upper Guide Bearing, Lower Guide Bearing and Turbine Guide Bearing). The output values of the procedure are the oil-film, bracket, generator stator, shaft stiffnesses as well as the unit's critical speeds.

As an identification example, a reversible pump-storage unit 180 MW was chosen for which the bearing stiffnesses change significantly with its thermal state, that is, with the stator and rotor thermal dilatations.

Beside stiffnesses identification, the first critical speed of a cold vs warm unit was also conducted. The results show that the real vs design vibration-related parameters are significantly different. This is important as very often the real system vibration response is not consistent with design data obtained by OEMs resulting in problems during commissioning and operation that can't be verified and easily identified by the end users. Problems related to vibrations appear in regimes in which the, unexpected, resonance state occurs. State of resonance exists as response to an excitation force with resulting stresses and deflections possibly causing premature equipment detrimental failure.

It is possible to automate the identification procedure and embed it into the multi-channel vibration analyzer unit or continuous diagnostic monitoring system.

Ozren Oreskovic, mag. ing. mech.
Managing Director, Veski Ltd

Education: Master degree at Faculty of Mechanical Engineering and Naval Architecture in Zagreb

Working experience: over 11 years of experience, employed at Veski.

From 2004-2009 Worked as Field Service Engineer commissioning Machine Condition Monitoring systems and troubleshooting vibration problems on rotating machines.

From 2009-2012– Working as Sales and Marketing manager at Veski

From 2012 – Works as Managing director at Veski