IMECE2005-79178 STRESSES DURING IMPACTS ON HORIZONTAL RODS

ABSTRACT The impact of an object striking the tip of a horizontally mounted bar provides some insight into the dynamics of structural impact in general. Modeling a cylindrical bar provides significant simplifications to enable comparison between experiment and theory. In particular, experimental results available in the literature are compared herein to both elastic wave theory and vibration theory. Relating these two theories is the focus of this paper. Vibrations can be directly related to the time of impact, the maximum stress at the tip of the bar, and the frequencies of the struck bar. Once these stresses and frequencies are found, elastic wave theory can then be used to describe the stresses throughout the bar

Application of a Dynamic Stress Theory to Pike Leaks

This report talks about Application of a Dynamic Stress Theory to Pike Leak

IJTC2007-44017 RELATIONSHIP BETWEEN VIBRATIONS AND MECHANICAL SEAL LIFE IN CENTRIFUGAL PUMPS

ABSTRACT A reduction of vibrations in mechanical seals increases the life of the seals in centrifugal pumps by minimizing fatigue damage. Mechanical seals consist of two smooth seal faces. One face is stationary with respect to the pump. The other rotates. Between the faces a fluid film evaporates as the fluid moves radially outward across the seal face. Ideally, the film evaporates as it reaches the outer surface of the seal faces, thereby preventing leakage from the pump and effectively lubricating the two surfaces. Relative vibrations between the two surfaces affect the fluid film and lead to stresses on the seal faces, which lead to fatigue damage. As the fluid film breaks down, impacts between the two seal faces create tensile stresses on the faces, which cycle at the speed of the motor rotation. These cyclic stresses provide the mechanism leading to fatigue crack growth. The magnitude of the stress is directly related to the rate of crack growth and time to failure of a seal. Related to the stress magnitude, vibration data is related to the life of mechanical seals in pumps

CFD [computational fluid dynamics] And Safety Factors. Computer modeling of complex processes needs old-fashioned experiments to stay in touch with reality.

Computational fluid dynamics (CFD) is recognized as a powerful engineering tool. That is, CFD has advanced over the years to the point where it can now give us deep insight into the analysis of very complex processes. There is a danger, though, that an engineer can place too much confidence in a simulation. If a user is not careful, it is easy to believe that if you plug in the numbers, the answer comes out, and you are done. This assumption can lead to significant errors. As we discovered in the course of a study on behalf of the Department of Energy's Savannah River Site in South Carolina, CFD models fail to capture some of the large variations inherent in complex processes. These variations, or scatter, in experimental data emerge from physical tests and are inadequately captured or expressed by calculated mean values for a process. This anomaly between experiment and theory can lead to serious errors in engineering analysis and design unless a correction factor, or safety factor, is experimentally validated. For this study, blending times for the mixing of salt solutions in large storage tanks were the process of concern under investigation. This study focused on the blending processes needed to mix salt solutions to ensure homogeneity within waste tanks, where homogeneity is required to control radioactivity levels during subsequent processing. Two of the requirements for this task were to determine the minimum number of submerged, centrifugal pumps required to blend the salt mixtures in a full-scale tank in half a day or less, and to recommend reasonable blending times to achieve nearly homogeneous salt mixtures. A full-scale, low-flow pump with a total discharge flow rate of 500 to 800 gpm was recommended with two opposing 2.27-inch diameter nozzles. To make this recommendation, both experimental and CFD modeling were performed. Lab researchers found that, although CFD provided good estimates of an average blending time, experimental blending times varied significantly from the average

Experimental Determination of Water Hammer Pressure Transients During Vapor Collapse

ABSTRACT Pressures were measured during water hammer in a steam condensate system. Condensate is formed through condensation of steam upstream of the pump used to pump the condensate. The water hammer mechanism was identified as a vapor collapse in the piping as a pump started, and the mechanism was physically verified by an ultrasonic measurement of water level in one of the pipes. Before the pump started, an overhead pipe was partially full of water. When the pump started, the pipe became full in less than a hundredth of a second. The vapor collapse created audible water hammers and resultant shock waves in the piping. The shock waves were eliminated by controlling the pump start up using a variable frequency drive (VFD) to operate the pump. A slow start prevents the sudden collapse of the vapor space and thus eliminates the water hammer

Proceedings of PVP2007 2007 ASME Pressure Vessels and Piping Division Conference PVP2007-26722 Derivations for Hoop Stresses Due to Shock Waves in a Tube

ABSTRACT Equations describing the hoop stresses in a pipe due to water hammer have been presented in the literature in a series of papers, and this paper discusses the complete derivation of the pertinent equations. The derivation considers the pipe wall response to a water hammer induced shock wave moving along the inner wall of the pipe. Factors such as fluid properties, pipe wall materials, pipe dimensions, and damping are considered. These factors are combined to present a single, albeit rather complicated, equation to describe the pipe wall vibrations and hoop stresses as a function of time. This equation is also compared to another theoretical prediction for hoop stresses, which is also derived herein. Specifically, the two theories predict different maximum stresses, and the differences between these predictions are graphically displayed

Practical Use Of Rotordynamic Analysis To Correct A Vertical Long Shaft Pump's Whirl Problem

Lecturepg. 107The use of long shaft vertical pumps is common practice in the nuclear waste processing industry. Unfortunately, when such pumps employ plain cylindrical journal bearings, they tend to suffer from rotordynamic instability problems due to the inherent lightly-loaded condition that the vertical orientation places on the bearings. This paper describes a case study in which the authors utilized rotordynamic analysis and experimental vibration analysis to diagnose such a problem and designed replacement tilting-pad bearings to solve the problem. The subject pumps are 45 foot long vertical pumps used for mixing nuclear waste in one million gallon tanks. Each pumping system consists of an induction motor driven by a variable frequency drive, a segmented pump shaft with a centrifugal impeller at the bottom, and a pipe column that surrounds the pump shaft and is suspended into the tank. The column is filled with water that serves as the lubricant for the eight plain cylindrical journal bearings that support the pump shaft. Unfortunately, these pumps have been plagued with vibration related problems, such as bearing and seal failures, ever since they were commissioned. In order to evaluate the problem, the pump shaft and column were instrumented with vibration transducers, and vibration testing was performed. This testing revealed that subsynchronous whirling, at approximately one-half the running speed, was the primary culprit responsible for the failures. In an effort to understand the cause of the observed vibrations, a two-level rotordynamic model of the pump shafting and column was constructed. This rotordynamic model, like those for many vertical pumps, was far from routine. The rotordynamic analysis, which is described in detail, then successfully confirmed that the problem was subsynchronous whirling due to rotordynamic instability. This analysis was then used to guide the design of tilting-pad bearings employing offset pivots and geometric preload to replace the original plain cylindrical bearings and, thereby, resolve the instability problem. The plain cylindrical bearings were removed from a pump, tilting-pad bearings were installed, and subsequent testing verified the predicted vibration reduction. Accordingly, the application of tilting-pad bearings, whose design is described in detail, was conclusively shown to be an effective solution for this specific problem