Review of Centrifugal Compressors High Pressure Testing for Offshore Applications

TutorialThere are many doubts about the risk / reward relation when it comes to ordering tests involving high pressure centrifugal compression systems. There are different tests that can be classified in this category, namely Full Load, Full Pressure, Full Density, Full Speed or a combination of those along with the ASME PTC 10 Type 1 tests. Normally these tests are carried out as a Complete Unit Test (String Test). This paper aims to discuss the advantages and disadvantages of the various types of tests. The codes from API and ASME do not detail the requirements of full pressure tests. The installation, the procedure, and the acceptance criteria are subject of agreement between vendor and purchaser. Therefore the importance of a meticulous test description during the bid phase is also discussed. In this paper, the various types of full load tests will be presented and along with a discussion on their capabilities to detect a variety of problems. Some issues which occurred during full load tests and at site will be described. Based on test characteristics and at the related cases, recommendations are made when ordering full pressure tests along with considerations regarding the return of investment of these tests

Operational modal analysis application for the measure of logarithm decrement in centrifugal compressor

LectureThe results of a stability test on a centrifugal compressor during the full load test of the entire compression train are presented. A Magnetic Exciter was installed at the shaft end in order to excite the first lateral mode of the rotor at running conditions, the frequency and logarithmic decrement of the first mode was identified by analyzing the Frequency Response Functions with classic techniques (D.J. Ewins, 1984). A new method for turbomachinery applications based on Operational Modal Analysis (OMA) is presented for the identification of modal parameters. Thanks to the natural excitation exerted by the gas flow on the rotor it is possible to avoid the use of any external device to perform the identification. The vibration signals have been recorded over proper measurement time windows with the compressor at steady state. The Stochastic Subspace Identification (a statespace algorithm in time domain) has been used to post-process the data. Modal parameters of the first lateral mode (natural frequency, damping ratio and mode shape) have been identified. A comparison between the two identification techniques on the identified frequency and logarithmic decrement of the first lateral mode was performed showing the potential of the OMA methodology. In addition four numerical cases with different stability levels were studied to confirm the reliability of the methodology for rotating machinery. A transient analysis was performed by applying a random excitation at mid-span and vibration responses have been evaluated at bearing locations. For each case the modal parameters were identified using OMA and compared with the exact theoretical values

Early Detection Of Rotating Stall Phenomenon In Centrifugal Compressors By Means Of ASME PTC 10 Type 2 Test

LectureA centrifugal compressor presenting rotating stall can exhibit many problems, as limitation on its operational range, high level of subsynchronous vibrations, reduced efficiency and premature bearing wear. There are many design criteria and standard practices for aerodynamic design of impellers and diffusers of centrifugal compressors. Nonetheless, the inception of rotating stall in new projects is still a reality. Therefore, an early detection of this phenomenon in a compressor is highly desirable. This paper proposes a way to detect the rotating stall problem during an ASME PTC10 Type 2 Test and how to scale these results (amplitude of vibration and inception point) as an estimative to site conditions. A comparison between the results of the proposed extrapolation from ASME PTC10 Type 2 Tests to the full load/density (almost an ASME PTC-10 Type 1 test) conditions is also presented. Scaling up guidelines for vibrations from similarity test to full density conditions are presented. Finally, an acceptance criterion for the amplitude of subsynchronous vibrations is proposed