Shop Rotordynamic Testing - Options, Objectives, Benefits and Practices

TutorialUnderstanding the lateral rotordynamic behavior is critical in determining the reliability/operability of rotating equipment. Whether examining a centrifugal pump or compressor, steam or gas turbine, motor or generator, rotating machinery share the same need to accurately predict and measure dynamic behavior. Industrial specifications determining fit for purpose rely on the accuracy of rotordynamic predictions where direct measurement is impractical or otherwise impossible in an industrial setting. Testing to confirm rotordynamic prediction and behavior provides both the purchaser and vendor the confidence that the design will meet project expectations. Rotordynamic shop testing has several options available to the project during acceptance tests at the vendor’s shop. These options include mechanical run, string and full load/Type 1 testing as well as verification testing to validate unbalance response and stability predictions. Such testing has numerous advantages; the most important being the avoidance of production disruptions involved with testing at the job site. Each test option has associated costs as well as limitations as to what lateral vibration characteristics are revealed. Understanding these factors is vital to efficiently mitigate the risks associated with the purchased equipment. Applying best practices and an understanding of the industrial (API) test requirements are needed to derive the maximum benefit of each test option. The best practices not only involve the test procedure but also the associated analytical methods used to post process the measurement information. Whether performing a simple mechanical run test or more complex stability verification during ASME Type I testing, ensuring that a logical, repeatable and proven methodology is followed produces reliable evidence to confirm the rotordynamic model and lateral vibration performance. The rationale behind the API test requirements provides an understanding of why that test is being performed and its correct application to the dynamic behavior. Test options can be separated into two categories; tests that reveal portions of the dynamic behavior of the equipment to specific operating conditions and those used to verify the analytical predictions of that behavior. API mechanical, string and Type I (or full load) tests reveal the rotordynamic behavior of the equipment to a given set of conditions. These are used specifically to determine acceptability of the design. Unbalance and stability verification testing is used to confirm (or provide confidence in) the rotordynamic model. Confidence in the model permits extrapolation of the design (vendor) and operation (purchaser) beyond the machine’s asbuilt and specific shop test conditions

Lateral Rotordynamics of Petrochemical Equipment - Review, Examples & Problems

Short Cours

Lateral Rotordynamics of Petrochemical Equipment - Review, Examples and Problems

Short Cours

Retrofit Of Gas Lubricated Face Seals In A Centrifugal Compressor.

LecturePg. 75-84There are significant advantages in using gas lubricated face seals (dry gas seals) in centrifugal compressor service. Foremost among these are the elimination of the seal oil system resulting in lower maintenance, increased safety, and higher operating availability. For these reasons, one of four identical compressors at an installation having severe problems with seal oil contamination was selected for trial conversion to dry gas seals. The rotordynamic engineering portion of this job was done by the compressor manufacturer in conjunction with the gas seal supplier. The seal assembly was designed so that it would be essentially a drop-in conversion from a mechanical standpoint. Rotordynamic studies indicated that the conversion would result in a "better" machine. Unfortunately, sustained operations were not possible, due to excessive vibration levels at startup with the new seals. The shaft vibration exceeded 0.007 in, peak-to-peak, at a subsynchronous frequency of 4900 rpm (the machine rated speed is in excess of 10,000 rpm). These levels were sufficiently high to cause extensive damage to all internal labyrinths. The midspan labyrinths were wiped open in excess of 0.060 in, radial. Analysis of tape recorded data indicated that the vibration was due to a rotor/bearing system dynamic instability. Additional computer simulations of the compressor rotordynamics revealed that the oil seals had provided sufficient damping to the system to bound the instability. This extra damping was not being provided by the gas seals. Bearing redesign to increase stability and realignment of the rotor within the bundle to remove suspected excitation appear to have eliminated the problem

Lateral Rotordynamics of Petrochemical Equipment - Review, Examples, and Problems

Short CourseThis course is aimed at machinery and project engineers and technicians who need a basic understanding of critical speeds, response to unbalance, and stability of rotating machinery. The course is intended for the layman’s approach to rotordynamics and its application to Turbomachinery. The focus of the course will be the rotordynamic behavior of petrochemical gas handling rotating equipment; centrifugal/axial compressors and steam/gas turbines. However, the concepts presented can be applied to virtually any class of rotating equipment

Lateral Rotordynamics of Petrochemical Equipment - Review, Examples, and Problem

Short Cours

Impact Of Electrical Noise On The Torsional Response Of VFD Compressor Trains

LectureThe popularity of VFD motors in compressor trains has increased in recent years. Increased process flexibility is a primary consideration for implementation of VFD systems. However, this comes at a cost of increased mechanical and electrical complexity. Problems rarely experienced with single speed motors have become more likely with VFD's. These include impacts on the torsional behavior leading to component failure and extended downtimes. VFD's can excite the torsional system in several ways. This paper examines the impact of white noise on the torsional response of VFD compressor trains and contrasts it against the response to single frequency harmonic excitation. The characteristics of white noise, noise modeling characteristics for torsional response, and noise generation techniques are examined. Generation of a frequency banded noise signal is presented using a commonly used math modeling code. Two case studies are presented representing VFD compressor trains found in the LNG industry. Both experienced torsional problems related to the VFD and, in one case, led to a coupling failure and release of parts from the train. In the examination of these trains, a single frequency harmonic (SFH) sweep and banded Gaussian white noise (GWN) encompassing the dynamic response envelop of the 1st TNF, are used as inputs to the torsional analysis

Case Study 08: Torsional Oscillation Trouble on VFD Motor Driven Recip Compressor

Case StudyHigh vibration problems including failed coupling parts on a VFD motor driven reciprocating compressor are analyzed and the root causes and solutions are discussed in this costly field issue

Subtracting Residual Unbalance For Improved Test Stand Vibration Correlation.

LecturePg. 7-18The importance of correlating rotordynamic analytical results to actual test data has become extremely important in recent years. Many user specifications require the verification of vendor generated analytical results in conjunction with shop unbalance testing. This correlation generally includes critical speed, amplification factor, and vibration magnitude. Several problems arise during shop unbalance testing. First, it is not always feasible to place an unbalance test weight in a location where it will logically excite the mode in question. This can lead to test runs where, despite a large unbalance weight, the residual unbalance is more influential than the test weight. Thus, the resulting rotor vibration is controlled by the residual unbalance and not the test unbalance. Consequently, any type of analytical correlation for this situation is extremely difficult. Another problem with test stand data is that it usually comes from four different vibration probe locations. This results in four vibration plots and, in many cases, four different critical speeds, amplification factors, and vibration magnitudes. Again, the analytical correlation for this situation is extremely difficult, since determining the actual values for these parameters is somewhat nebulous. These problems are addressed herein and a solution is offered. Several examples of analytical and test stand results are presented for steam turbines and centrifugal compressors. These examples highlight and discuss the problem areas identified. A method is presented for subtracting the vibration due to residual unbalance from that of the verification test. The resulting vibration with residual subtraction isolates the effect of the unbalance weight, and leads to improved correlation with analytical predictio

Subsynchronous Vibration Problem And Solution In Multistage Centrifugal Compressor.

LecturePg. 65-74The investigation of a subsynchronous vibration problem encountered in a six stage centrifugal compressor is discussed. At a running speed of approximately 9000 rpm, a subsynchronous vibration (at 4200 rpm) of nearly two times the synchronous vibration level was encountered. A systematic program was undertaken to identify the problem and correct it. A detailed analysis of the floating ring annular oil seals, balance piston labyrinth seals and impeller aerodynamic cross coupling was conducted. The oil seals were identified as the primary cause of the subsynchronous vibration due to lock up, and a modified seal design incorporating circumferential grooves was developed. This radically reduced the seal cross coupled stiffness. Further, a modified bearing design was investigated to increase the rotor logarithmic decrement. Changes were implemented in the compressor with the result of no subsynchronous vibrations for the operating conditions of the compressor thus far