Dry Gas Seal Material Investigation for Enhanced Slow Roll & Wind-Milling Reliability

LectureDry gas seals, while ideally non-contacting, must reliably perform under all operating conditions encountered by a compressor, including those conditions which induce contact such as slow roll, turning gear, ratcheting, and windmilling. Windmilling is of concern particularly for compressors driven by aero-derivative turbines, in which the shaft may rotate freely at a relatively low speed, presumably lower than the lift off speed of the gas seal. The critical sealing interface in a dry gas seal must therefore withstand contact without experiencing excessive wear and without degrading the performance of the seal. In an effort to improve the friction and wear-resistance of this interface while in contact, a new material combination was experimentally evaluated: a silicon carbide rotating ring against a stationary ring made of a solid lubricant interspersed within a carbon graphite matrix. The tribological performance of this material combination was assessed via ball-on-disk and full-scale seal tests. The full-scale seal tests were designed to mimic the operating conditions encountered over the life of the seal, comprised of maximum pressures and speeds, numerous start-ups and shut-downs, 20 hours of windmilling, 1000 hours of slow roll, and 37 hours at turbine wash speed. The windmilling speed was run at 90% of the lift off speed of the seal, established through acoustic emission measurements during seal start-ups. This speed presents a challenging scenario for the gas seal as it is running at a relatively high speed while still in contact. After testing, the seal faces exhibited minor wear, with the solid lubricant-doped carbon graphite rings exhibiting the majority of the wear while the harder silicon carbide mating rings remained in excellent condition. Residue matching the carbon graphite material composition was observed on the surface of the silicon carbide ring surface. This material transfer was likely providing a lubricious, protective layer during contacting conditions. Seal leakages were also tracked throughout the test program and found to remain consistent from the beginning to the end of the test program. Dynamic leakages even decreased from the first to the last performance test, indicating that the seal is capable of continued service despite the challenging test program designed to create an “end-of-life” seal condition. Actual leakage values were compared to predicted leakages from a combined CFD and FE model; correlation between the two was quite good for dynamic testing but was off by as much as a factor of ten for static testing. The static leakage discrepancy between the model and measurements is likely caused by manufacturing deviations, whose effect is magnified by the very small film thickness and consequently low leakage during static conditions

Introduction to Dry Gas Seals and Systems

TutorialDry gas seals are used as low-leakage shaft end seals for many centrifugal compressors and other turbomachines. This short course provides a comprehensive overview of sealing system and dry gas seals in various turbomachinery applications, addressing multiple topics ranging from fundamentals to detailed design considerations for reliable operation. A course attendee can expect a greater understanding of technologies, failure modes, and requirements for components in dry gas seals and seal supply/vent systems, with perspectives from an end user, a seal manufacturer, and a research organization.This short course will give listeners a thorough understanding of dry gas seals, including design, operation, and maintenance. Starting with the background of how dry gas seals were developed as a response to issues with wet seals, the course will then move into a detailed discussion on seal design. The instructors will explain how each component of the seal contributes to its operation and issues that can arise if parts are selected incorrectly. Next, seal selection for various applications (pipeline, process, advanced applications) will be discussed. Methods for seal testing to ensure that design conditions are met will be described, including test rigs studying off-design conditions, such as transients or contaminant injection.The gas conditioning process can be critical to successful seal operation, so seal gas panels and their components will be discussed in great detail. Operation during transients can be particularly challenging, so panel considerations specific to transient operation will be discussed. The recently-released API 692 will be discussed as it pertains to dry gas seal panel design, seal requirements, and seal testing.Understanding common failure modes is an important step to improving dry gas seal reliability. Recent research on dry gas seal failures will be presented, including failure statistics and failure modes. Insight on failure modes specific to heat generation from liquid contamination will be discussed, and recommendations will be provided to reduce failures.Copyright© 2020 by Turbomachinery Laboratory, Texas A&M Engineering Experiment StationThis short course is aimed primarily at end users, but the multifaceted approach (end user, OEM, research) will provide a valuable perspective on dry gas seals to anyone in the rotating equipment industry. By the end of the course, attendees will have a detailed understanding of dry gas seals and their associated systems

Tribological Interfaces and Fluid Flows Containing Particles and Chemically Designed Additives

This work investigates bi-phase flows in a series of experimental and computational studies with a tribological focus. Three silver-based complexes are chemically designed, through collaboration with the Marks group, for use as additives in high-temperature lubrication. Mixtures of engine oil and various concentrations of these nanoparticle complexes lubricate sliding steel surfaces in ball-on-disk tests. Friction and wear measurements demonstrate that the silver complexes provide beneficial wear reduction over a range of temperatures and loads. The silver pyrazole-pyridine complex in particular provides excellent friction performance at high temperatures (> 200°C) and high concentration (20 wt%). In a similar set of experiments, hexyltrimethoxy silane is added to polyalphaolefin (PAO) oil contaminated with sand. As debris, dirt, and dust are commonly present in lubricants, it is desirable to find additives which lessen the abrasive damage of these hard contaminant particles. Chemical reaction between the sand and organosilane is found to be low, which results in minimal improvements in friction and wear. A tin catalyst is recommended to improve the reactivity of the sand and organosilane for future testing. Finally, a computational model of fluid-solid flow is developed to examine the characteristics of particle-laden viscous flows. Existing computational techniques utilizing a distributed Lagrange multiplier (DLM) method and a mechanistic collision model are extended for multi-particle collisions and elastohydrodynamic lubrication, accounting for surface deformations through the solution of elasticity equations. This deterministic numerical model allows for various sizes and shapes of particles, as well as dilute to dense suspensions in viscous, laminar flow. Furthermore, the model is computationally fast and applicable to a wide range of thin-film flows. The numerical model is utilized in a study of particle-fluid flow through narrow channels with wall features of increasing size. Particles are found to have different equilibrium positions based on the flow Reynolds number, with minimal effects from the wall features. In addition, results for rigid particle motion entrapped in a deformable channel are presented as a preliminary investigation. The location of maximum surface pressure and deformation is predicted to shift slightly from the point of contact in the direction of motion of the moving particle

INTRODUCTION TO DRY GAS SEALS AND SYSTEMS

TutorialThis tutorial presents an introduction to dry gas seals (DGSs) and their associated systems. DGSs are used as a shaft end seals for compressors and turbines, and they are installed on virtually all centrifugal compressors produced today. A DGS is able to provide very low leakage flow rates due to the small running clearance between the rotating and stationary components. This tutorial discusses the basic principles of DGS operation, terminology, applications, and API Standard 692 design requirements for DGS systems. The last section covers causes of DGS failures and practices to prevent them

Synthesis and Characterization of Silver(I) Pyrazolyl­methyl­pyridine Complexes and Their Implementation as Metallic Silver Thin Film Precursors

A series of light- and air-stable silver­(I) pyrazolyl­methyl­pyridine complexes [Ag­(L^R)]_<i>n</i>(BF₄)<i>_n</i> (L = pyrazolylmethylpyridine; R = H, <b>1</b>; R = Me, <b>2</b>; R = i-Pr, <b>3</b>) and [Ag­(L^R)­(NO₃)]₂ (L = pyrazolylmethylpyridine; R = H, <b>4</b>; R = Me, <b>5</b>; R = i-Pr, <b>6</b>) has been synthesized and structurally and spectroscopically characterized. In all of the molecular structures, the pyrazolylmethylpyridine ligands bridge two metal centers, thus giving rise to dinuclear (<b>2</b>, <b>4</b>, <b>5</b>, and <b>6</b>) or polynuclear structures (<b>1</b> and <b>3</b>). The role played by the counteranions is also of relevance, because dimeric structures are invariably obtained with NO₃^– (<b>4</b>, <b>5</b>, and <b>6</b>), whereas the less-coordinating BF₄^– counteranion affords polymeric structures (<b>1</b> and <b>3</b>). Also, through atoms-in-molecules (AIM) analysis of the electron density, an argentophilic Ag···Ag interaction is found in complexes <b>2</b> and <b>4</b>. Thermogravimetric analysis (TGA) shows that the thermolytic properties of the present complexes can be significantly modified by altering the ligand structure and counteranion. These complexes were further investigated as thin silver film precursors by spin-coating solutions, followed by annealing at 310 °C on 52100 steel substrates. The resulting polycrystalline cubic-phase Ag films of ∼55 nm thickness exhibit low levels of extraneous element contamination by X-ray photoelectron spectroscopy (XPS). Atomic force microscopy (AFM) and scanning electron microscopy (SEM) indicate that film growth proceeds primarily via an island growth (Volmer–Weber) mechanism. Complex <b>4</b> was also evaluated as a lubricant additive in ball-on-disk tribological tests. The results of the friction evaluation and wear measurements indicate a significant reduction in wear (<i>∼ </i>88%) at optimized Ag complex concentrations with little change in friction. The enhanced wear performance is attributed to facile shearing of Ag metal in the contact region, resulting from thermolysis of the silver complexes, and is confirmed by energy-dispersive X-ray analysis of the resulting wear scars

Synthesis and Characterization of Silver(I) Pyrazolylmethylpyridine Complexes and Their Implementation as Metallic Silver Thin Film Precursors

A series of light- and air-stable silver(I) pyrazolylmethylpyridine complexes [Ag(LR)]n(BF4)n (L = pyrazolylmethylpyridine; R = H, 1; R = Me, 2; R = i-Pr, 3) and [Ag(LR)(NO3)]2 (L = pyrazolylmethylpyridine; R = H, 4; R = Me, 5; R = i-Pr, 6) has been synthesized and structurally and spectroscopically characterized. In all of the molecular structures, the pyrazolylmethylpyridine ligands bridge two metal centers, thus giving rise to dinuclear (2, 4, 5, and 6) or polynuclear structures (1 and 3). The role played by the counteranions is also of relevance, because dimeric structures are invariably obtained with NO3– (4, 5, and 6), whereas the less-coordinating BF4– counteranion affords polymeric structures (1 and 3). Also, through atoms-in-molecules (AIM) analysis of the electron density, an argentophilic Ag···Ag interaction is found in complexes 2 and 4. Thermogravimetric analysis (TGA) shows that the thermolytic properties of the present complexes can be significantly modified by altering the ligand structure and counteranion. These complexes were further investigated as thin silver film precursors by spin-coating solutions, followed by annealing at 310 °C on 52100 steel substrates. The resulting polycrystalline cubic-phase Ag films of ∼55 nm thickness exhibit low levels of extraneous element contamination by X-ray photoelectron spectroscopy (XPS). Atomic force microscopy (AFM) and scanning electron microscopy (SEM) indicate that film growth proceeds primarily via an island growth (Volmer–Weber) mechanism. Complex 4 was also evaluated as a lubricant additive in ball-on-disk tribological tests. The results of the friction evaluation and wear measurements indicate a significant reduction in wear (∼ 88%) at optimized Ag complex concentrations with little change in friction. The enhanced wear performance is attributed to facile shearing of Ag metal in the contact region, resulting from thermolysis of the silver complexes, and is confirmed by energy-dispersive X-ray analysis of the resulting wear scars

Motion of rigid aggregates under different flow conditions

The response of rigid aggregates to different flow fields was investigated theoretically using model clusters with realistic three-dimensional structure composed of identical spherical primary particles. The aim is to relate the main fluid dynamic properties of the system with the geometry and morphology of the aggregates. Our simulations were based on Stokesian dynamics. The dilute limit of a colloidal aggregate system was studied, where aggregates are very far from each other and hence mutual interaggregate interactions are negligible. The motion of aggregates was characterized in terms of translational mobility and angular velocity, and the ability of simple models, based on either simplified aggregate geometry or the concept of permeability, to capture the main features of the motion was examined