Rotordynamic instability field problems

Vibration data obtained during several rotor instability investigations is presented to illustrate the effect of changes in system parameters on overall rotor stability. The data includes the effects of bearing and seal changes as well as those due to variations in speed and pressure ratio. Field Problems indicate that the stability of rotors is often highly sensitive to fairly minor variations in bearing and seal parameters. Measured field data is valuable in normalizing analytical computer models so that effective solutions can be obtained

Field verification of lateral-torsional coupling effects on rotor instabilities in centrifugal compressors

Lateral and torsional vibration data obtained on a centrifugal compressor train which had shaft instabilities and gear failures is examined. The field data verifies that the stability of centrifugal compressors can be adversely affected by coincidence of torsional natural frequencies with lateral instability frequencies. The data also indicates that excitation energy from gear boxes can reduce stability margins if energy is transmitted either laterally or torsionally to the compressors. The lateral and torsional coupling mechanisms of shaft systems is discussed. The coupling mechanisms in a large industrial compressor train are documented and the potential effect on rotor stability is demonstrated. Guidelines are set forth to eliminate these potential problems by minimizing the interaction of torsional and lateral responses and their effect on rotor stability

Experiences with nonsynchronous forced vibration in centrifugal compressors

The high subsynchronous vibrations which are often forced vibrations caused by flow instabilities, such as stage stall were examined. Modifications to improve the rotor stability by changing the bearings or seals have little effects on the subsynchronous vibrations. Understanding of the differences between forced vibrations and self excited vibrations to properly diagnose the problem and to correct it, is recommended. A list of characteristics of the two types of subsynchronous vibration is presented

Case Histories Of Specialized Turbomachinery Problems

PaperPg. 33-50.Vibration problems in turbomachinery occur due to many factors. Some of the problems could have been prevented if more detailed design analyses were performed. Other times the problems occur due to design extrapolations which are pushing the state of the art. The case histories of excessive vibrations and failures that will be discussed in this presentation are examples of those where additional design analyses would not necessarily have predicted or anticipated the problem that occurred because the analytical models are not sufficient to take into account all the variables

Design Audits.

Tutorialpg. 153-168Dynamic design audits of machinery can identify potential problems before the machine is manufactured, thus preventing costly project delays and downtime. The types of audits that should be performed are discussed and typical analysis results are presented along with guidelines as to their interpretation

Vibrations In Reciprocating Machinery And Piping Systems.

Tutorialpg. 243-272A wide variety of vibration and failure problems occur in reciprocating machinery and piping systems. Excessive piping vibration problems usually occur when a mechanical natural frequency of the piping system or compressor manifold system is excited by a pulsation or mechanical excitation source. Since reciprocating compressors and pumps generate high pulsation levels at numerous harmonics, which in turn produce shaking forces, vibration and failure problems in these systems are common. Other problems, not associated with the piping, can be encountered with the compressor/engine frame foundation and anchoring systems. These can lead to failures of the bearings and crankshaft. In addition, special problems can occur due to the torsional natural frequencies and the high harmonic torques, due to the compressor loading. Whenever high vibrations are encountered in reciprocating compressors, pumps and/or piping, it is necessary to determine if the vibrations and dynamic stresses are acceptable. Criteria to judge the acceptability of the vibrations are presented in this paper, along with troubleshooting methods to determine if the problems are caused by pulsation or mechanical resonances. The basic principles of pulsation generation and control are presented. The key to designing and operating safe piping systems is to control the pulsation levels and separate the mechanical natural frequencies from the pulsation excitation frequencies

Pulsations In Liquid Pumps And Piping Systems

PaperPg. 55-62.The existence of high intensity, low frequency pulsation problems in centrifugal compressor and pump piping systems has now been well documented throughout industry. Strong pulsations have been observed at frequencies ranging from two to several hundred Hz which typically are not harmonically related to (and do not vary with) pump rotor speed. Investigation has revealed that in most cases, the problem is not caused simply by pump or compressor characteristics, but instead is the result of dynamic interaction of the passive response of the piping, the head curve characteristics of the pump, dynamic flow damping, and location of the pump in the piping geometry. Since pulsation problems are usually vibration fatigue problems of the piping or machine internals, problems can often be mitigated by effective vibration control. A more fundamental and often more economical approach, however, lies in controlling the pulsation levels and frequencies, either by controlling the generation sources or pulsation response of the piping. Unfortunately, these two phenomena cannot be effectively separated, and the problem must be solved with a more comprehensive system dynamic analysis. It has been shown, for example, that relative low level vortex formation at piping discontinuities can be amplified by the pump or compressor, but such amplification must be at one of the responsive acoustic length resonant modes of the piping system, and then only if the pump or compressor is situated at or near a velocity maximum in the standing wave field. Additionally, however, the compressor itself may influence the acoustic response (distributed impedance) of the piping as well as serve as an amplifying element. Flow or pressure drop is also an important factor in sustaining such pulsations as it affects modal damping. Techniques have now been developed whereby many of the phenomena associated with low frequency pulsations in centrifugal pump and compressor systems can be effectively simulated either for design or problem solving. The paper will briefly describe the phenomena associated with low frequency pump pulsations and will illustrate use of the technology in several case histories

Analysis Of Torsional Vibrations In Rotating Machinery.

Tutorialpg. 127-15

Piping Vibration Analysis.

Tutorialpg. 119-134Excessive piping vibrations are a major cause of machinery downtime, leaks, fatigue failures, high noise, fires, and explosions in refineries and petrochemical plants. Excessive vibration levels usually occur when a mechanical natural frequency of the piping system is excited by some pulsation or mechanical source. The vibration mode shapes usually involve lateral vibrations and/or shell wall radial vibrations. Simplified methods are presented for analyzing lateral and shell wall piping vibrations and judging their severity. The methods are thought to be conservative and are intended to be used as screening criteria to determine if more sophisticated analyses, such as computer stress modelling or strain gage testing are necessary. Frequency factors for calculating the mechanical natural frequencies for the classical piping configurations (uniform straight beams) and various piping bend configurations are presented. Factors are presented to compensate the natural frequency calculations for concentrated and distributed weight effects. The relationships between piping vibration displacement, velocity and stress are presented and criteria for judging the severity of piping vibration in terms of the endurance stress limit are shown. The mechanisms that can excite piping vibrations will be discussed, as well as methods for controlling their severity

Improved Reliability Through The Use Of Design Audits.

LecturePg. 203-220Rotordynamic design audits of machinery and systems can be used to identify potential problems before manufacture, thus preventing costly project delays and downtime. Machinery engineers need to understand the types of analyses that can be performed to evaluate proposed designs. Understanding the analysis types can also help the engineer to determine if the audit is necessary and/or economically desirable. The types of audits that should be performed are described. Typical analysis results are presented along with guidelines for their interpretation