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

Nonlinear system modeling based on constrained Volterra series estimates

A simple nonlinear system modeling algorithm designed to work with limited \emph{a priori }knowledge and short data records, is examined. It creates an empirical Volterra series-based model of a system using an $l_{q}$-constrained least squares algorithm with $q\geq 1$. If the system $m\left( \cdot \right)$ is a continuous and bounded map with a finite memory no longer than some known $\tau$, then (for a $D$ parameter model and for a number of measurements $N$) the difference between the resulting model of the system and the best possible theoretical one is guaranteed to be of order $\sqrt{N^{-1}\ln D}$, even for $D\geq N$. The performance of models obtained for $q=1,1.5$ and $2$ is tested on the Wiener-Hammerstein benchmark system. The results suggest that the models obtained for $q>1$ are better suited to characterize the nature of the system, while the sparse solutions obtained for $q=1$ yield smaller error values in terms of input-output behavior

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

The Perceptions of Teacher Leadership in Elementary Schools According to Female Elementary School Principals

ABSTRACT The purpose of this qualitative research case study was to determine how female elementary school principals’ perceptions of teacher leadership, namely how it is defined, facilitated, and sustained in their schools, were related to their own views on leadership and approaches to leadership theory. Understanding the perspectives of principals with respect to teacher leadership and the opportunities for the development of teacher leadership could help to start the dialogue between principals and teachers toward improved opportunities for teachers to take on leadership roles. Data were gathered from a school district located in the Midwestern region of the United States. Four female elementary principals from the same school district were interviewed two separate times. In addition to the interviews, a demographic questionnaire from each participant was also obtained. Each piece of information provided a deeper look into the experience of the participants in terms of their leadership style and how they facilitate teacher leadership opportunities in their schools. The questionnaire was uploaded and administered as a web based questionnaire, with respondents contacted through email. The qualitative findings of this case study indicated: (a) principals in this study perceive there to be various and numerous roles in which teacher leaders can be involved both formally and informally; (b) principals in this study engage in various methods to facilitate teacher leadership but the most productive method is to identify and then capitalize on individual strengths of teachers; and (c) principals’ leadership styles in this study are not an isolated leadership style or theory; rather they are a combination of various well-known leadership styles and theories. Based on the results from the research, principals should better define the role of teacher leadership and they should refer to their teachers as teacher leaders regularly so that teacher leaders and others view themselves in leadership roles

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

A novel investigation into the application of non-destructive evaluation for vibration assessment and analysis of in-service pipes

Flow induced vibrations that are close to resonance frequencies are a major problem in all oil and gas processing industries, so all piping systems require regular condition monitoring and inspection to assess changes in their dynamic characteristics and structural integrity in order to prevent catastrophic failures. One of the main causes of pipe failure is weak support causing low frequency high amplitude flow-induced vibration. This causes wear and tear, especially near joints due to their dissimilar stiffness resulting in fatigue failure of joints caused by vibration-induced high cyclic stress. Other contributing factors in pipe failure are poor or inadequate design, poor workmanship during installation or maintenance and inadequate or weak and flexible support. These pipes are usually required to work non-stop for 24 hours a day 7 days a week for weeks, months or years at a time. Regular monitoring and in-service dynamic analysis should ensure continuous and safe operation. A novel method of non-destructive testing and evaluation of these pipes, while in service, is proposed in this paper. This technique will enable early detection and identification of the root causes of any impending failure due to excess vibration as a result of cyclic force induced by the flow. The method pinpoints the location of the impending failure prior to condition-based maintenance procedures. The technique relies on the combined application of Operating Deflection Shapes (ODS) analysis and computational mechanics utilizing Finite Element Analysis (FEA), i.e. linear elastic stress analysis. Any structural modification to the pipes and their supports can then be applied virtually and their effects on the system can be analysed. The effect on vibration levels is assessed and verified. The effect of any change in the forces corresponding to changes in the Differential Pressure (DP) at constant flow rate through the pipes can then be estimated. It was concluded that maintaining the differential pressure above some “critical” threshold ensures the pipe operates under the allowable dynamic stress for a theoretically “indefinite” life cycle