Maximum .verall Efficiency二 27.04 Maximum .verall Efficiency二 25.02 Power Demands二 MW (Maximum demand二 87.5)
selects the units that should be operated to provide the powerload demand at the maximum overall efficiency of the combination of units.
.echanical .roblem .iagnostics
The advent ofnew， more reliable， and sensitive vibration instrumentation such as the eddy-current sensor and the accelerometer coupled with modern technology analysis equipment (the real-time vibration spectrum analyzer and low-cost computers) gives the mechanical engineer very powerful aids in achieving machinery diagnostics.
A chart for vibration diagnosis is presented in Table 19-9. While this is ageneral criterion or rough guideline for diagnosis of mechanical problems， it can be developed into a very powerful diagnostic system when specific problems and their associated frequency domain vibration spectra are
Table 19-9
.ibration .iagnosis
Usual .redominant Fre.uency. Cause of .ibration
.unning frequency at 0-40%.oose assembly of bearingliner，bearingcasing， or casing and support
.oose rotor shrink fits Friction-induced whirl Thrust bearing damage
.unning frequency at 40-50% Bearing-support excitation.oose assembly of bearingliner，bearingcase， or casing and support
.il whirl
.esonant whirl Clearance induced vibration
.unning frequency Initial unbalance
.otor bow
.ost rotor parts Casing distortion Foundation distortion Misalignment Piping forces .ournal . bearing eccentricity Bearing damage
.otor-bearing system critical Coupling critical Structural resonances Thrust-bearing damage
.dd frequency .oose casing and support Pressure pulsations
.ibration transmission Gear inaccuracy
.alve vibration
.ery high frequency Dry whirl Blade passage
..ccurs in most cases predominantly at this frequency; harmonics may or may not exist.
logged and correlated in a computerized system. With the extensive memorycapability of the computer system， case histories can be recalled and efficient diagnostics achieved.
.ata .etrieval
In addition to being valuable as a diagnostic and analysistool， a data retrieval program also provides an extremely flexible method of datastorage and recovery. By careful design of a health monitoringsystem， an engineer or technician can compare the present operation of a unit withthe operation of the same machine， or of another machine， under similar conditions in the past. This can be done by selecting one or several limit-ing parameters and defining the other parameters that are to be displayed when the limiting parameters are met. This eliminates the necessity of sifting through large amounts of data. A few examples of how this system is used are:
1. .etrie.al by time.In thismode， the computer retrieves data takenduring a specified time period， thus enabling the user to evaluate the period of interest.
2. .etrie.al by ambient temperature. The failure of a gas turbine mayoccur during an unusually hot or coldperiod， and the operator may wish to determine how his unit functioned at this temperature in the past.
3. .etrie.al by turbine e.haust temperature. The exhaust temperature can be an important parameter in failure investigations. An analysis of this parameter in failure investigations. An analysis of this parameter can verify the existence of a problem with either the combustor or turbine.
4. .etrie.al by .ibration le.els. Inspection of data provided by this modecan be useful in determining compressor fouling， compressor or tur-bine blade failure， nozzle bowing， uneven combustion， and bearing problems. 中国航空网 www.aero.cn 航空翻译 www.aviation.cn 本文链接地址：燃气涡轮工程手册 Gas Turbine Engineering Handbook 3(58)
