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Performance and vibration monitoring should be properly interfaced to achieve a level of operation free from excessive maintenance and downtime and to maximize operating efficiency at every possible point in the system.
.ompressor and turbine sections can be analyzed effectively by combining vibration spectra with changes in performance data. Ma.or problem areas in each of these components can be identified with proper monitoring and analysis.
Subsynchronous Vibration Analysis Using.TA
High-speed， flexible-shaft rotor systems， especially those that operate at more than twice the first criticalspeed， are prone to subsynchronous instabilities. These instabilities can be induced by various elements in the rotor system from fluid-filmbearings， bushing and labyrinthseals， toaerodynamic components such as impellers， shrinkfits， and shaft hyster-esis.認ith vibration instability， the rotor"s rotation provides the energy and source of rotation. In high-speed rotor systems subsynchronous instabilities are a ma.or cause of catastrophic failures of rotor and bearing systems. The application of high-pressure rein.ection in recent years has resulted in a very high incidence of problems and failures due to subsynchronous vibration. The causes of many of these problems were not identified because the conventional analog-tuned filter vibration analyzer was incap-able of analyzing the problem-except when catastrophic levels of subsyn-chronous vibrations were reached.At this condition， machine failure was very rapid.
In the early-to-marginal stages of subsynchronous vibration the phenom-enon is highlyintermittent， and requires the rapid analysis and high-resolution capability of the real-time analyzer for its identification.
This study shows the analysis and identification of subsynchronous instability on a high-pressure centrifugal compressor operating at more than the first critical speed of the unit. The test plots given in Figures 16-5 through 16-8 show the vibration spectra. The bearing .ournal displacementin peak-to-peak mils， on the .axis， is on a logarithmic scale. This scale enables identification of the low levels of subsynchronous vibrations which occur during the marginal conditions of subsynchronous instability.
Figure 16-5 shows the vibration spectrum with the machine operatingat卻 ，   rpm， 5  psig (34.5 Bar) suction pressure， and1卻  psig(8卻.7 Bar) discharge pressure. Here a synchronous peak of .5 mil ( . 1卻7 mm) at卻 ， rpm due to rotor system unbalance is the only component that shows up on the spectrum plot. Figure 16-6 shows the vibration spectrum with themachine operating at卻 ，   rpm and suction pressure of 5  psig (34.5 Bar)while the discharge pressure has been raised to1卻5 (86.卻 Bar) psig. Observe on the plot the .卻 mil ( . 5 8 mm) subsynchronous component at 9 rpm. Using the analyzer in the continuous real-timemode， this

Figure 16-5.Vibration spectrum (rpm =20，000， Pd = 1200 psig)

Figure 16-6.Vibration spectrum (rpm =20，000， Pd = 1250 psig)

Figure 16-7.Vibration spectrum (rpm =20，000， Pd = 12.0 psig)
rpm component was very intermittent and was captured by setting the realtime analyzer controls to the ""peak hold"" mode.
Figure 16-7 shows the vibration spectrum with the speed and suctionpressure kept constant but with a small卻 psig increase in discharge pres-sure. Notice the large increase in the 9   rpm component from .卻 to 1.5 mil( . 1卻7- . 381 mm). A further small increase in discharge pressure would have increased the subsynchronous vibrations to more than 1. mil( . 卻54 mm) and wrecked the unit.
認hen the suction pressure was raised by some 5 psig (3.45 Bar) whilemaintaining the same dischargepressure， the unit regained its stability with the elimination of the subsynchronous component as shown in Figure 16-8. The subsynchronous instability in this machine was the result of aero-dynamic excitation of the rotor systems occurring at a critical pressure rise across the machine of 77 psi differential (5  -1卻7 psig).
　
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