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The signature obtained from the turbine-generator bearing vibration instrumentation may reflect a condition of misalignment or unbalance present within the system. Characteristics (frequency, amplitude, and phase) are typically processed by an expert or specialist in vibration diagnostics.

For operators, vibration limits are set to prevent damage caused by exceeding the journal

clearances. The condition assessment of turbine vibration is intended to identify the root cause of problems that may be indirectly reflected in the system vibration signature. When trended, the vibration signatures can also provide a sense of whether the problem is stable or deteriorating over time.

Data Sheet #2 (a) assembles a complete set of turbine-generator vibration amplitude and phase angle data recorded from the unit at both full load and minimum load. Obtaining data on a unit roll-up and roll-down is also strongly recommended. In addition, a frequency scan should be recorded and attached to the respective data sheet at each bearing location. The specialist providing these data should also be the individual who is responsible for tracking, trending, and evaluating any significant data changes in comparison to readings taken at the last outage or to

Turbine-Generator Condition Assessment – In Service

On #2 (b), the data recorded in the audit is then analyzed by the vibration specialist through an interview process. A series of questions are identified. These are designed to assist in soliciting details and opinions that go beyond the data collected and summarized on the initial sheet.

Ultimately, the results from the audit and interview are summarized in the third sheet #2 (c), where both the risk of failure (low, medium, high) and the need for action (immediate, intermediate, long term) are identified relative to the turbine vibration.

To assist in the interpretation of data assembled from both the audit and the interview, typical orbit plots showing symptoms of common problems registered in the vibration signature are provided in Figure 1-4. A more detailed discussion on the symptoms and interpretation of vibration measurements related to common turbine-generator balance and alignment problems can be found in Volume 3 of these Guidelines, specifically the Balance Primer and the

Alignment Primer.

In terms of possible actions relating to turbine vibration, the most common issues likely to be found in an assessment are:

• Repeated balancing attempts are ineffective. This involves a condition where significant changes in unit vibration have required numerous balance shots, but the running speed vibration amplitude and phase angle at the shaft rotation frequency remain high or are still not improved. For example, the failure to relieve the vibration problem may indicate that the problem is instead with a coupling, or that unbalance in the rotor system is highly static (where phase angles are in-line), indicated by the phase angles recorded at each bearing.

When considering recommendations, determine if a coupling has excessive run-out between the coupling halves. If so, this condition will result in high vibration that cannot be improved by balance shots. Coupling disassembly will be required to eliminate excessive run-out. A large static unbalance in the rotor system may require shop balancing of one or more rotors to correct this condition.

• Significant changes are noted at harmonics of shaft speed. Vibration frequency scans should identify any significant changes associated with one-half X, 2X, 3X, 4X, or 5X harmonics of shaft rotational speed (where X represents rotor speed). For example, a frequency that is less than one-half rotor running speed represents an oil whip instability if seen during unit operation. There would also be a significant difference between the filter-out and filter-in vibration amplitudes. A significant change in the 2X frequency response could be noted if a unit has a large peripheral crack (with orientation orthogonal to the shaft centerline). Such a response would be seen in comparing a 2X scan before and after cooling and re-heating a rotor by changing the main or reheat steam temperature by 50–75ºF (10–24ºC). A large difference between filter-in and filter-out vibration typically signifies significant vibration at other frequencies as noted above.

If a vibration problem is identified, it should be considered as serious and treated

immediately in order to prevent a potential catastrophic failure of the rotor during operation.

Vibration technical experts should be immediately contacted to further assess this problem and for guidance as to possible unit shutdown.

Turbine-Generator Condition Assessment – In Service

• A major change is noted when load is added. Major changes in rotor vibration amplitude and phase angle that occur from minimum load to full load can be due to alignment problems on the unit or other reasons (such as partial arc loading issues at minimum load). For example, on some units the static and dynamic vector components may show significant changes in amplitude and phase between minimum load and full load. A subsequent alignment check may reveal significant rim and face misalignment of one rotor to another rotor.

• A major vibration change is noted when passing through shaft critical speeds. Significant vibration amplitude and phase angle changes at rotor critical speeds could be caused by rotor bowing. Such bowing can be due to operation at high temperature over a long time period or due to rubs in the steam packing or elsewhere within the rotor system. This would typically be seen in the HP or IP turbines.

• Sudden step increases in vibration are measured at the journals. Large, sudden step increases in journal vibration often indicate a loss of rotating blade material. Such changes generally require immediate inspection to assess damage and determine other corrective actions that may be required.

As a rule, progressive changes in measured vibration over extended periods of time reflect degradation due to wear of the bearings, settling of the foundation, permanent rotor bowing, or the cumulative effect of individual section overhauls that eventually require a major correction.

A condition assessment should seek to determine the point in time when a major unit realignment would be worthwhile to restore the unit to its originally aligned condition. An interview of the operators can aid in determining whether unit vibration problems have been chronic, are getting worse, or have only recently started.

After multiple rotor overhauls, possible settling of the bearing foundation, and/or thermal

distortion of stationary components, the unit will require extensive alignment, This will establish the radial position of the rotor with respect to stationary components and to re-establish the cantenary position of the bearings to the initial cantenary line. If a system becomes badly misaligned, it may become impossible to find a reasonable alignment solution without a complete disassembly of the unit to perform a tops-on and tops-off alignment to correct the problem.

Pronounced step changes in vibration typically signify a situation that requires immediate concern and attention. These steps changes reflect a loss of noticeable rotating mass, often caused when portions of a rotating blade such as a tip or cover are lost. This indication is significant in that it may represent an early warning of the progressive deterioration of the structural system as a prelude to a more catastrophic failure. For example, if the blade is

designed to have its natural frequencies fall within certain prescribed operating bands; the loss of part of a cover band may shift one of the fundamental frequencies into a condition of resonance.

If a large blade fails at the platform or root, the loss of mass can be sufficient to cause an

unbalance that will cause extensive damage to the entire machine. Such cases have been recently documented.

Turbine-Generator Condition Assessment – In Service

Figure A shows the orbit of a shaft with several concurrent whirls at different frequencies taken from unfiltered vibration. Figure B shows the same plot at synchronous speed where

nonsynchronous frequencies have been filtered out, showing the unbalance whirl orbit.

Stiffness affects the shape of the orbit as noted by Figures C–E. Identical bearing stiffness gives a circular orbit, dissimilar stiffness gives an elliptical orbit, and cross coupling of stiffness in vertical and horizontal direction gives a rotated elliptical orbit.

Misalignment can be indicated as shown in Figures F–H. Note that the orbit is highly elliptical in F, indicating poor alignment. In Figure G the orbit for two bearings on each side of a coupling are banana-shaped,

indicating severe misalignment. In Figure H the misalignment is also severe and suggests backward precession.

Fluid whirl, a subsynchronous fluid instability, occurs within the range of 30–48%

of machine operating speed. Figure I shows precession of vibration in the same direction as shaft rotation, displaying a circular orbit and two key-phasor dots. If the dots slowly rotate against shaft rotation, the subsynchronous frequency is less than 50% of shaft speed; if the two dots remain stationary, the frequency is exactly 50% of shaft speed.

Fluid whip is a subsynchronous excitation at the first critical speed of a rotor that operates far above twice first critical speed and has vibration amplitudes

equivalent to bearing clearance. The orbit for this is shown in Figure J. Note the multiple key-phasor dots that this excitation produces.

Rubs are a typical problem seen on steam turbines and are generally at 1X frequency for units that operate at less than twice first critical speed. If a rotor operates well above first critical speed, frequencies generated would be at 1X and 1/2X frequencies. Figure K shows a rub in a unit whose speed is well above first critical speed frequency and has a predominant 1/2X frequency.

Figure 1-3

Typical Orbits Showing Different Problems

Turbine-Generator Condition Assessment – In Service

In addition to the information referenced in Volume 3 of these guidelines, previous research published by EPRI related to turbine-generator vibration is as follows (listed by year of publication):

Periodic Vibration Monitoring: Utility Experience, EPRI, Palo Alto CA: 1987. CS-5517 Symposium Proceedings: Rotating Machinery Dynamics, Bearings and Seals, EPRI, Palo Alto, CA: 1988. CS-5858.

Applying Vibration Monitoring, EPRI, Palo Alto CA: 1991. NP-6340.

Shaft Alignment Guide, EPRI, Palo Alto CA: 1999. TR-112449.

Technology Development for Shaft Crack Detection in Rotating Equipment Using Torsional Vibration, EPRI, Palo Alto CA: 2003. 1009060.