Dynamic simulation and monitoring of a non-contacting flexibly mounted rotor mechanical face seal

Journal of Engineering for Gas Turbines and Power

2012

An increase in the power-to-weight ratio demand on rotordynamic systems causes increased susceptibility to transverse fatigue cracking of the shaft. The ability to detect cracks at an early stage of progression is imperative for minimizing off-Jine repair time and cost. The vibration monitoring system initially proposed in Part I is employed herein, using the 2X harmonic response component of the rotor tilt as a signature indicating a transverse shaft crack. In addition, the analytic work presented in Part I is expanded to include a new notch crack model to better approximate experimental results. To effectively capture the 2X response, the crack model must include the local nature of the crack, the depth of the crack, and the stiffness asymmetiy inducing the gravity-forced 2X harmonic response. The transfer matrix technique is well suited to incorporate these crack attributes due to its modular nature. Two transfer matrix models are proposed to predict the 2X harmonic response. The first model applies local crack flexibility coefficients determined using the strain energy release rate, while the second incorporates the crack as a rectangular notch to emulate a manufactured crack used in the experiments. Analytic results are compared to experimental measurement of the rotor tilt gleaned from an overhung rotor test rig originally designed to monitor seal face dynamics. The test rig is discussed, and experimental atigular response orbits and 2X harmonic amplitudes of the rotor tilt are provided for shafis containing manufactured cracks of depths between 0% and 40%. Feasibility of simultaneous multiple-fault detection of transverse shaft cracks and seal face contact is discussed.

Section: Tilt Orbit Monitoring and Orbit Shapementioning

confidence: 99%

Crack Detection in a Rotor Dynamic System by Vibration Monitoring—Part II: Extended Analysis and Experimental Results

Varney

Journal of Engineering for Gas Turbines and Power

2012

“…The maximum relative misalignment can be calculated according to the closed form solutions of the Green dynamic model [5,8]. Based on this solution, parametric and sensitivity studies [6,7,9] were performed to investigate the effect of basic seal parameters (including shaft speed, sealed fluid pressure, coning angle, and clearance) on the maximum relative misalignment for a noncontacting FMR seal test rig (shown in Fig. 2).…”

Section: Relative Misalignmentmentioning

confidence: 99%

“…However, other factors, such as kinematics of the flexible support and its rotordynamic coefficients uncertainties, machine deterioration, transients in sealed pressure or shaft speed, or unexpected shaft vibration, affect the dynamic behavior of the seal and, hence, the relative position and misalignment between the rotor and the stator. The parametric studies ( [6,7,9]) explore the effects of various seal parameters on this γ max and provide valuable information concerning seal design and performance prediction. These studies also provide guidelines for contact elimination strategy.…”

Section: B Contact Detection and Contact Elimination Strategymentioning

confidence: 99%

See 1 more Smart Citation

Contact elimination in mechanical face seals using active control

Dayan

Zou

IEEE Trans. Contr. Syst. Technol.

2002

Wear and failure of mechanical seals may be critical in certain applications and should be avoided. Large relative misalignment between the seal faces is the most likely cause for intermittent contact and the increased friction that eventually brings failure. Adjustment of the seal clearance is probably the most readily implemented method of reducing the relative misalignment and eliminating seal face contact during operation. This method is demonstrated with the aid of a noncontacting flexibly mounted rotor (FMR) mechanical face seal test rig employing a cascade control scheme. Eddy current proximity probes measure the seal clearance directly. The inner loop controls the clearance, maintaining a desired gap by adjusting the air pressure in the rotor chamber of the seal. When contact is detected the outer loop adjusts the clearance set point according to variance differences in the probes' signals. These differences in variance have been found to be a reliable quantitative indication for such contacts. They are complimentary to other more qualitative phenomenological indications, and provide the controlled variable data for the outer loop. Experiments are conducted to test and verify this active control scheme and strategy. Analysis and results both show that contrary to intuition for the seal under investigation, reducing seal clearance can eliminate contact, and the outer cascade loop indeed drives the control toward this solution.

“…Mitigating undesired face contact requires seal redesign and real-time condition monitoring to detect the onset of face contact. Most previous studies focus on detecting contact experimentally using methods such as vibration monitoring [15,16,18], ultrasonic techniques [19][20][21], acoustic emission [22,23] or a combination of methods [24]. Others have used these same experimental measurement techniques to heuristically identify contact signatures and apply these signatures to an activelycontrolled seal in an attempt to eliminate contact [25][26][27][28].…”

Section: Introductionmentioning

confidence: 99%

Impact Phenomena in a Noncontacting Mechanical Face Seal

Varney

2016

Journal of Tribology

Noncontacting mechanical face seals are often described as unpredictable machine elements, gaining this moniker from numerous instances of premature and unexpected failure. Machine faults such as misalignment or imbalance exacerbate seal vibration, leading to undesirable and unforeseen contact between the seal faces. A hypothesis explaining the high probability of failure in noncontacting mechanical face seals is this undesired seal face contact. However, research supporting this hypothesis is heuristic and experiential and lacks the rigor provided by robust simulation incorporating contact into the seal dynamics. Here, recent developments in modeling rotor–stator rub using rough surface contact are employed to simulate impact phenomena in a flexibly mounted stator (FMS) mechanical face seal designed to operate in a noncontacting regime. Specifically, the elastoplastic Jackson–Green rough surface contact model is used to quantify the contact forces using real and measurable surface and material parameters. This method also ensures that the seal face clearance remains positive, thus allowing one to calculate fluid-film forces. The seal equations of motion are simulated to indicate several modes of contacting operation, where contact is identified using waveforms, frequency spectra, and contact force calculations. Interestingly, and for the first time, certain parameters generating contact are shown to induce aperiodic mechanical face seal vibration, which is a useful machine vibration monitoring symptom. Also for the first time, this work analytically shows a mechanism where severe contact precipitates seal failure, which was previously known only through intuition and/or experience. The utility of seal face contact diagnostics is discussed along with directions for future work.