Damped Unbalance Response

Unfortunately, particularly for the vendors, the undamped analysis opened somewhat of a Pandora's box. While the calculated frequencies were not quite accurate, they were better than any previous analysis, once the actual bearing parameters were input.

It should be mentioned in the course of the rotor analysis development, the art of tilting pad design moved from just carrying a load at speed to one of tailoring the arc of the pad by a design method called preload to achieve the stiffness and damping values needed to control the location of the response (see Figure 9-11). A stiff rotor relative to the bearings

R{. JOURNAL RADIUS Rp PAD MACHINED IORE RADIUS REREADING SET BORI RADIUS

Figure 9-11. Tilting pad bearing schematic with definition of preload. (Courtesy of Turbocare, A Division of Demag Delaval Turbomachinery Corp., Houston facility

R{. JOURNAL RADIUS Rp PAD MACHINED IORE RADIUS REREADING SET BORI RADIUS

Figure 9-11. Tilting pad bearing schematic with definition of preload. (Courtesy of Turbocare, A Division of Demag Delaval Turbomachinery Corp., Houston facility

(where the speeds on the three modes become close to horizontal) isn't influenced by bearing tuning.

The dismay mentioned is that undamped response finds numerous other responses. It also picks up two other modes, referred to as rigid rotor modes. These modes are controlled primarily by the elastic properties of the bearings with little or no rotor bending (rigid). While helicopter and aircraft designers had observed these effects, this came as a shock to quite a few of the compressor design engineers. Besides these classic modes, others would crop up from time to time with little or no apparent physical significance. Seeing these plots, eyebrows were raised and the users decided the vendors were trying to hide something. There is no way of knowing the hours expended on what amounts to phantom criticáis. With the improvement of the damped response analysis and test stand data, it was determined that most of these were indeed unresponsive in real life. Both vendor and user gained with the damped analysis. The criticáis, which are real, can be identified and can be the focus of attention.

Users still like to see the undamped plots because they are easier to interpret, but the vendors are afraid that they may have to design away from an unresponsive value and waste money. This should resolve in time. Figure 9-12 shows the various modes discussed in a classic form.

The damped (unbalance response) plots are not really very impressive, but they address the information every operator would like to see. Amplitude is plotted against speed at various stations such as at the probes (see

Unbalance Response Test

Classical Second Bending

Figure 9-12. Classical mode shapes.

Classical Second Bending

Figure 9-12. Classical mode shapes.

Figure 9-13). A response, of course, is a sudden rise in amplitude at a speed. The analysis is performed using a mathematical model that includes the effects of damping in the equations, making the model much more complex than any previous analysis. The University of Virginia carries out continuous research in which a consortium of users, vendors, and researchers provide funds, and data and interchange ideas to advance the science of rotor dynamics. Other organizations, such as the Bently Rotor Dynamics Research Corporation and Texas A&M University, are also carrying on similar work on a continuing basis.

The value of a damped response is that the areas under seals or other close clearance areas can be investigated on a dynamic basis. This can be done at any selected station on the rotor. If there is encroachment on clearance, the rotor can be tuned to avoid the problem areas. The reliability of the machine can be considered rather than arguing the need for large margins to avoid the unknown.

OUTBOARD BEARING

OUTBOARD BEARING

Figure 9-13. Example of damped response plot.

Figure 9-13. Example of damped response plot.

Renewable Energy 101

Renewable Energy 101

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable. The usage of renewable energy sources is very important when considering the sustainability of the existing energy usage of the world. While there is currently an abundance of non-renewable energy sources, such as nuclear fuels, these energy sources are depleting. In addition to being a non-renewable supply, the non-renewable energy sources release emissions into the air, which has an adverse effect on the environment.

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