Curvature-Aware Derivative-Free Optimization

The paper discusses derivative-free optimization (DFO), which involves minimizing a function without access to gradients or directional derivatives, only function evaluations. Classical DFO methods, which mimic gradient-based methods, such as Nelder-Mead and direct search have limited scalability for high-dimensional problems. Zeroth-order methods have been gaining popularity due to the demands of large-scale machine learning applications, and the paper focuses on the selection of the step size $\alpha_k$ in these methods. The proposed approach, called Curvature-Aware Random Search (CARS), uses first- and second-order finite difference approximations to compute a candidate $\alpha_{+}$. We prove that for strongly convex objective functions, CARS converges linearly provided that the search direction is drawn from a distribution satisfying very mild conditions. We also present a Cubic Regularized variant of CARS, named CARS-CR, which converges in a rate of $\mathcal{O}(k^{-1})$ without the assumption of strong convexity. Numerical experiments show that CARS and CARS-CR match or exceed the state-of-the-arts on benchmark problem sets.Comment: 31 pages, 9 figure

Analysis and Countermeasures for Sideband of Gear Mesh Frequency(GMF) Induced by Shaft Fretting Corrosion in a Gearbox

Case StudyCS deals with trouble shooting and solving a chronic fretting corrosion damage b/w a gear and shaft (shrink fit). The damage on the geared shaft always accompanied by an increase in the amplitude and frequency side bands at the gear mesh frequency (GMF). The troubleshooting process and analysis revealed that the distance between the two gears is a critical design parameter for preventing fretting corrosion

Reliability Improvement Of Magnetic Drive Pump For Special Services

Case StudyWe, SKI, had used successfully the sealless pumps such as magnetic drive pump and canned motor pump in order to meet the environmental regulation for clean service. Based on successful experiences for many years, magnetic drive pumps have been aggressively applied to improve the seal life for special services such as sulfur, caustic, ammonia, etc since 2005. The magnetic drive pump has been preferred because of various advantages (easy maintenance, robust design, leak free, etc). However, we have faced important and didactic issues due to inappropriate operating procedures, protection systems and design. Unless the application and operation are taken into a consideration for special services like as molten sulfur and ammonia, the consequences of pump failure could result in devastating damage to pump parts and even fire. This case study will show how the bad actors were improved, lessons learned and guidelines for reliable operation. Users are reluctant to apply protection systems fully compliant with API685 or vendor recommendations for non-hazardous services based on economic considerations. But they should consider providing minimum protection systems for special services, if not for all pumps

Reliability Improvement for Booster Reciprocating Compressor In CCR Reformer

Case Stud

Analysis and on-stream countermeasures of High Thrust Bearing Temperature of A Centrifugal Compressor

Case StudyThe problem of high bearing temperature is a frequent concern for most rotating machinery. In case of critical equipment such as critical compressors or turbines, its consequences could lead to total plant shut down and huge losses. Furthermore it is very hard to find its cause, and solutions. On-stream remedies are very limited because the machines aren’t stopped. For the investigation and analysis of high thrust bearing problem of centrifugal compressor, it is required to understand the mechanism of bearing temperature increase as well as thrust load balance. In addition, it is also needed to review the quality and quantity of lube oil supply, and maintenance and operation history in various aspects. Based on actual experience of high thrust bearing temperature rise in recycle gas compressor of Residue Hydrogen Desulfied (RHDS) plant, this case study will show how to analyze it and take on-stream countermeasures, as well as lessons-learned for maintenance and design

Analysis and Countermeasures for Sideband of Gear Mesh Frequency(GMF) Induced by Shaft Fretting Corrosion in a Gearbox

Case StudyCS deals with trouble shooting and solving a chronic fretting corrosion damage b/w a gear and shaft (shrink fit). The damage on the geared shaft always accompanied by an increase in the amplitude and frequency side bands at the gear mesh frequency (GMF). The troubleshooting process and analysis revealed that the distance between the two gears is a critical design parameter for preventing fretting corrosion

Reliability Improvement for Booster Reciprocating Compressor In CCR Reformer

Case Stud

Vacancy driven surface disorder catalyzes anisotropic evaporation of ZnO (0001) polar surface

The evaporation and crystal growth rates of ZnO are highly anisotropic and are fastest on the Zn-terminated ZnO (0001) polar surface. Herein, we study this behavior by direct atomic-scale observations and simulations of the dynamic processes of the ZnO (0001) polar surface during evaporation. The evaporation of the (0001) polar surface is accelerated dramatically at around 300 °C with the spontaneous formation of a few nanometer-thick quasi-liquid layer. This structurally disordered and chemically Zn-deficient quasi-liquid is derived from the formation and inward diffusion of Zn vacancies that stabilize the (0001) polar surface. The quasi-liquid controls the dissociative evaporation of ZnO with establishing steady state reactions with Zn and O$_{2}$ vapors and the underlying ZnO crystal; while the quasi-liquid catalyzes the disordering of ZnO lattice by injecting Zn vacancies, it facilitates the desorption of O$_{2}$ molecules. This study reveals that the polarity-driven surface disorder is the key structural feature driving the fast anisotropic evaporation and crystal growth of ZnO nanostructures along the [0001] direction