This example shows the rotordynamic analysis performed to a centrifugal compressor of seven stages handling hydrogen for refinery service. The study helps in the decision-taking process for two important design aspects; 1) modernization of the shaft sealing system, and 2) redesign of the hydrodynamic journal bearings. The equipment compression ratio is 1.5, with a capacity of 4,445 cfm, and with a temperature and suction pressures of 100 F and 315 psia, respectively. The modernization of the sealing system consisted in the substitution of the original floating-ring seals (lubricated with oil) by dry face seals (lubricated with process gas). The selection of the dry seals was based on a better control of emissions to ambient, further they do not require lube consoles, and therefore lead to savings in operation and maintenance activities and expenses. Measurements of the vibration characteristics of the compressor in normal operating conditions showed the presence of a strong subsynchronous activity at approximately 0.5X, and almost as high as the synchronous vibration. Furthermore, the equipment was uncapable of operating at the original design speed of 9,850 rpm. The high amplitude of subsynchronous vibration was limiting the operating speed at a maximum of 5,760 rpm. Any attempt of running the equipment at a higher speed, simply led to very unacceptable vibration levels for the integrity of the machine and the operators.
The redesign process rendered several types of three-lobe bearings, each one offered advantages and disadvantages. Careful analysis delivered a preload of 0.5 for a journal eccentricity of 0.8 (minimum clearance) as adequate for the compressor stable operation up to a speed of 8,750 rpm. In the full model, the stations belonging to the oil floating ring seals were represented as connections that provide rotordynamic coefficients. Later, they were replaced by mass and moments of inertia (polar and transversal) of the dry gas seals. The rotor weight and static loads at the bearings varied slightly.
The compressor operated satisfactorily over the next five years with no stability issues. At the end of this period, a thorough machine revamping was effectuated. The end-user requirement to operate at a higher speed led to consider changing the three-lobe journal bearings by tinting pad journal bearings to eliminate the possibility of hydrodynamic instability. After the corresponding rotordynamic analysis, the final tinting pad journal bearing configuration was a 4-pad, load between pad, and preload of 0.19. This new bearing minimized the cross-coupled rotordynamic forces responsible of the original compressor instability. The rotor remained stable in the full operating speed range up to the maximum continuous speed of 11,000 rm, with damping factors above 0.19 for the cylindric-elastic vibration mode excited at the natural frequency of 4,200 rpm. The rotor synchronous response to unbalance showed a maximum peak (critical speed) at the speed of 4,200 rpm, which is coincident with the natural frequency of the compressor.
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