Balancing of Gas Turbine

29.5.10

In one of the earlier threads on similar discussion, the following info was posted:

"IPT's Rotating Equipment Training Manual"

Machinery Reliability and Condition Monitoring.

Bruce Basaraba and James Archer. published by IPT Publishing and Training Ltd.

Box 9590, Edmonton, Alberta, Canada T6E 5X2

www.iptbooks.com email iptpub@compusmart.ab.ca

toll free 1-888 808 6763.

You can also try the following link:

https://faculty.uml.edu/pavitabile/22.403/web_downloads/Single_Plane_Balance_091401.PDF

RHABE posted this:

we did build our own dynamic balancing measurement setup.

No encoder but a black or white marking on the shaft to measure by reflected light the phase of the rotor, arbitrarily set to zero where the marking is put.

Two accelerometers (most commercial machines use two voice-coil systems) to catch the vibration signals. These accelerometers to be mounted with some suitable axial distance on the to be measured item. If not complete with housing a suitable bearing assembly and drive has to be added. As this is introducing additional errors we measure preferably in completed spindles. Both possibilities (incomplete with auxiliary bearings and drives or complete spindles with drives) have to be suspended in a flexible support that allows operation of the to be balanced rotor well above the first resonance (this support with the rotor). Isotropic flexibility is best and sometimes mandatory but primitive supports (soft rubber foam) sometimes works well.

Bandwidth of the accelerometers has to be considerably higher (factor of 5 or to be corrected for phase and amplitude) than the maximum speed at which unbalance should be measured.

Accelerometer to be fed to a preamplifier of switchable gain to adopt the various vibrations that are often not synchronous to speed: all signals have to pass the amplifier else there are severe errors! The preamplifier and first analog filter has a subsequent (digital) filter to adopt to the rotor speed.

From the switching signal derived from the marking on the shaft - this is assumed to be derived from a shaft of non changing location and constant speed - a synthetic (in the computer or analog circuitry residing) sine and cosine is generated phase locked to the rotor and the marking on the rotor.

This synthetic sine and cosine is multiplied with the amplified signal from the accelerometers. Then integrated.

This is giving as result the zero time-lag cross-correlation of the accelerometer signal with the synthetic sine and cosine thus filtering out the (often very high) disturbing signals that are not synchronous to speed. This filtering is much better than any ordinary bandpass filter.

The magnitude is giving the amount of unbalance. The relation of sine and cosine derived measurements the phase in relation to the rotor marking.

The amount of unbalance has to be calibrated with a known unbalance as the mass and the inertias of the total system is often not known. And we are doing balancing in the assembled spindles so there is the rotor and the housing contributing to the mass and inertias.

The accelerometers have to be located at some axial distance to measure in two planes (perpendicular to the rotor-axis) as unbalance in a rigid rotor is invariably composed of a radial plus a tilting component - this can be thought as being composed of two radial components in the arbitrarily chosen planes. Calculation is by simple vectorial representation of the unbalance in the respective plane.

Adding or removing mass is used to balance according to these measurements. This can be done in the planes of measurement or in any other two planes.

Caution is recommendable if the two planes of measurement or the two planes of mass removal are on one side of the center of mass.

Sometimes pre-balancing at low speed is necessary for a safe transition through the resonances.

Complete systems available on request.

More detailed information available if questions remain.

RHABE

Good Luck! Nothing is impossible.

Vinay Isloorkar, Pune, India.