Journal Bearing Design Types And Their Applications To Turbomachinery.

Tutorialpg. 179-190A review of several different types of hydrodynamic journal bearings that are commonly found in turbomachinery is presented. Emphasis is placed on the key geometric design parameters of each type. The discussion covers plain journal, axial groove, pressure dam, offset split, lemon bore, multilobe and titling-pad bearings. The application of the critical speed map and some basic non-dimensional bearing parameters as tools for preliminary bearing selection and comparison are discussed. These tools are applied to two case studies, which demonstrate the proper application of different bearing designs to industrial turbomachinery

Rotor Dynamic Analysis And Bearing Optimization Study Of A 3800 Hp Steam Turbine

PaperPg. 19-27.This paper presents the highlights of a complete rotor dynamics analysis and bearing optimization study that was performed on a 3800 HP multistage steam turbine. This unit is coupled to a three stage, poster-type, centrifugal air compressor with two high speed pinion shafts engaging a common bull gear. The turbine drives the lower speed pinion at a maximum speed of 10,500 rpm. Located at the other end of this same pinion is the first stage impeller wheel. The second and third stages are mounted on the higher speed pinion, rotating at a maximum speed of 13,600 rpm. The maximum speed of the bull gear is 1, 940 rpm. All the existing compressor bearings are the four-lobe type. The original turbine rotor consists of three stages and has a 41.34 inch bearing span. The bearings are lemon-bore type with journal diameters of 3.0 and 4.0 inches on the inlet and exhaust ends, respectively. This turbine has exhibited subsynchronous vibration, at approximately half rotational speed, during field testing. The half frequency vibration prevented operation above 9,400 rpm because a shaft resonance was excited and vibration levels became prohibitively high. The analysis of the existing rotor-bearing system predicts that the turbine operates close to the second critical speed and that the rotor is stable under normal load conditions. However, the rotor becomes unstable when the lemon-bore bearings are unloaded. There are two major sources of bearing load in addition to gravity. These loads are misalignment and gear mesh loading. Misalignment can result from differential thermal growth of the turbine and compressor cases and supports. If the coupling does not act as an ideal joint, this differential growth can impose external loads at the bearings. The bearing load resulting from the gear mesh is a reaction to the torque transmission at the gear interface. For example, the turbine driver rotates in the counter-clockwise direction. Therefore, the low speed pinion gear tends to climb up the bull gear, which causes an upward external load on the pinion bearings. As in the case of misalignment, if the coupling is not ideal, the turbine bearings can also be affected by these gear mesh reaction loads. This analysis is primarily concerned with the effect of these external loads that oppose gravity and cause degrees of net bearing unloading because the unloading causes rotor instability. The rotor is unstable with 75 pounds unloading at each bearing or 110 pounds unloading at the exhaust end bearing alone (with a normal gravity load at the inlet end bearing). As a solution to the problem, a new set of bearings was designed to stabilize the rotor and move the peak response speeds away from the operating speed range. The new bearings are five-shoe-tilt-pad bearings with load-between-pad orientation. The inlet end bearing has a 3.0 inch diameter bore with length-to-diameter ratio of 1.0; and preload of 0.8. The exhaust end bearing has a 3. 995 inch diameter bore with a length-to-diameter ratio of 0.4 and preload of 0.0. With the new tilt-pad bearing design there are no response peaks in the normal operating speed range and the system log decrement is 2.8 times higher than with the existing system, under normal load conditions. The log decrement for the tilt-pad bearings is +0.52 for an unloading of 100 pounds. Therefore, the tilt-pads provide a stable rotor support that is not sensitive to unloading

Dynamic Analysis Of An 8000 HP Steam Turbine Operating Near Its Second Critical Speed.

LecturePg. 79-92A rotordynamics analysis of an 8000 hp steam turbine which drives an ethylene compressor is presented. This machine characteristically exhibited increasing vibration levels with increasing speed, in the upper portion of the speed range. It was suspected that this ramping vibration was due to operation near a critical speed. The purpose of this analysis was to identify the source of this high vibration with the present rotor and hearings and then to determine the best solution to the problem. The computer analysis determined that the original rotor/bearing system was operating near the second critical speed. The predicted peaks were within nine percent to thirteen percent of miming speed (8400 cpm). The optimum solution was to shorten the shaft at the governor end and replace the bearings. The shaft modification consisted of cutting off the governor worm gear assembly and replacing it with an electronic governor disk. This new bearing design was also a four-shoe tilting pad journal bearing, but it had longer pads and higher preload than the existing design. Because there was concern about coupling gear tooth lock-up in the original design, a dry, flexible-element coupling design was evaluated. The coupling change was also of interest because of its potential effect on moving the second critical speed. However, in this ease, the analysis determined that the rotor vibration characteristics were very close with either the existing gear coupling or the dry coupling. Therefore, the justification fo r changing couplings would be that the dry coupling would not be susceptible to drive tooth lockup and unbalance eccentricity due to tooth wear

Rotor Dynamics Modification Of An Eight Stage Compressor For Safety/Reliability Improvement.

LecturePg. 81-96The catastrophic failure of a high pressure centrifugal air compressor utilizing air-pressurized bearing housings dictated the redesign of its sister compressor. The modification of this early 1950's compressor involved elements of both practical and analytical design. The project included the design of new bearings and seals and the alteration of various support systems to permit operation with non-pressurized bearing housings. The project was undertaken as risk minimization, with all performance and reliability improvements being additional benefits. The matching of the computer model with the field data of the existing system was the key to its successful evaluation. It was found that a case seal was acting as an additional bearing during certain operating conditions. This third bearing caused the compressor to behave differently than had been predicted with the initial two-bearing models. With the system properly modeled, changes were proposed to optimize the rotor dynamics. The main objectives were to achieve reliable rotor dynamic characteristics and to maintain the integrity of the process labyrinth seals. The changes were made in April, 1982, with the normal hardware fitting considerations. The subsequent start-up and operation were without incident. The modified compressor has performed well through a number of emergency trips and subsequent restarts, exhibiting exceptional rotor stability and overall reliability

Rotor Bearing And Shaft Dynamics Redesign Of A Double-Overhung Turboexpander For Reliability Improvement.

LecturePg. 35-56A solution is presented for a lateral rotor and bearing dynamics problem on a high pressure, double-overhung turboexpander with a design operating speed of 57,300 rpm. The rotor has an expander impeller mounted at one end and a compressor impeller mounted at the other end. This unit had a long history of failures at the expander-end radial bearing. It was suspected that the frequent expander upsets were caused by unbalance degradation due to solid particle erosion on the aluminum expander-end impeller. The rotordynamics analysis of the expander with the original three lobe journal bearings predicted a high unbalance sensitivity at the expander-end of the rotor and a low logarithmic decrement for the third backward whirl mode near the design operating speed. The concern was that this mode might be excited by an unbalance initiated rub. The redesign analysis consisted of two phases. The purpose of Phase One was to determine the best bearing redesign that could be "directly substituted" for the original bearings without modifying the shaft or the bearing housing fit diameters and fit lengths. As a backup contingency plan, the Phase Two analysis was pursued while the Phase One retrofit was in operation. The purpose of Phase Two was to find the optimal redesign configuration that would even further improve upon the predicted Phase One retrofit achievements, without the "direct bearing substitution constraint." This option would include the possibility of a new rotor, new bearings, and possibly a new bearing housing. All objectives were met. The Phase One retrofit ran successfully for over four and one-half years without incident

Effects Of High Pressure Oil Seals On The Rotordynamics Response Of Centrifugal Compressores.

LecturePg. 35-52The influence of high pressure bushing-type oil seals on the rotordynamic characteristics of a centrifugal gas compressor, which compresses hydrogen from 1700 psi to 1800 psi is discussed. The compressor rotor is driven through a gear coupling by a 10000 hp steam turbine, which has a governor speed range of 7900 cpm to 8850 cpm. This machine has had a long history of exhibiting high synchronous vibration under certain operating conditions. A computer simulation identified that this rotor system can operate as two entirely different systems, which correlates well with field observations. The first system corresponds to the "normal" operating condition, where the bearings support the rotor and the oil seal bushings operate near the "centered" position. The second system corresponds to the condition where the seals carry the rotor weight and unload the bearings. This allows a level of flexibility to be introduced under the bearings, which reduces the effective damping. These rotor support changes shift the second critical speed into the operating speed range, which produces the high synchronous vibrations. In addition, the analysis indicates that the rotor is unstable in the condition where the seals support the rotor and where flexibility is introduced under the unloaded bearings

Antimicrobial resistance among migrants in Europe: a systematic review and meta-analysis

BACKGROUND: Rates of antimicrobial resistance (AMR) are rising globally and there is concern that increased migration is contributing to the burden of antibiotic resistance in Europe. However, the effect of migration on the burden of AMR in Europe has not yet been comprehensively examined. Therefore, we did a systematic review and meta-analysis to identify and synthesise data for AMR carriage or infection in migrants to Europe to examine differences in patterns of AMR across migrant groups and in different settings. METHODS: For this systematic review and meta-analysis, we searched MEDLINE, Embase, PubMed, and Scopus with no language restrictions from Jan 1, 2000, to Jan 18, 2017, for primary data from observational studies reporting antibacterial resistance in common bacterial pathogens among migrants to 21 European Union-15 and European Economic Area countries. To be eligible for inclusion, studies had to report data on carriage or infection with laboratory-confirmed antibiotic-resistant organisms in migrant populations. We extracted data from eligible studies and assessed quality using piloted, standardised forms. We did not examine drug resistance in tuberculosis and excluded articles solely reporting on this parameter. We also excluded articles in which migrant status was determined by ethnicity, country of birth of participants' parents, or was not defined, and articles in which data were not disaggregated by migrant status. Outcomes were carriage of or infection with antibiotic-resistant organisms. We used random-effects models to calculate the pooled prevalence of each outcome. The study protocol is registered with PROSPERO, number CRD42016043681. FINDINGS: We identified 2274 articles, of which 23 observational studies reporting on antibiotic resistance in 2319 migrants were included. The pooled prevalence of any AMR carriage or AMR infection in migrants was 25·4% (95% CI 19·1-31·8; I2 =98%), including meticillin-resistant Staphylococcus aureus (7·8%, 4·8-10·7; I2 =92%) and antibiotic-resistant Gram-negative bacteria (27·2%, 17·6-36·8; I2 =94%). The pooled prevalence of any AMR carriage or infection was higher in refugees and asylum seekers (33·0%, 18·3-47·6; I2 =98%) than in other migrant groups (6·6%, 1·8-11·3; I2 =92%). The pooled prevalence of antibiotic-resistant organisms was slightly higher in high-migrant community settings (33·1%, 11·1-55·1; I2 =96%) than in migrants in hospitals (24·3%, 16·1-32·6; I2 =98%). We did not find evidence of high rates of transmission of AMR from migrant to host populations. INTERPRETATION: Migrants are exposed to conditions favouring the emergence of drug resistance during transit and in host countries in Europe. Increased antibiotic resistance among refugees and asylum seekers and in high-migrant community settings (such as refugee camps and detention facilities) highlights the need for improved living conditions, access to health care, and initiatives to facilitate detection of and appropriate high-quality treatment for antibiotic-resistant infections during transit and in host countries. Protocols for the prevention and control of infection and for antibiotic surveillance need to be integrated in all aspects of health care, which should be accessible for all migrant groups, and should target determinants of AMR before, during, and after migration. FUNDING: UK National Institute for Health Research Imperial Biomedical Research Centre, Imperial College Healthcare Charity, the Wellcome Trust, and UK National Institute for Health Research Health Protection Research Unit in Healthcare-associated Infections and Antimictobial Resistance at Imperial College London

Surgical site infection after gastrointestinal surgery in high-income, middle-income, and low-income countries: a prospective, international, multicentre cohort study

Background: Surgical site infection (SSI) is one of the most common infections associated with health care, but its importance as a global health priority is not fully understood. We quantified the burden of SSI after gastrointestinal surgery in countries in all parts of the world. Methods: This international, prospective, multicentre cohort study included consecutive patients undergoing elective or emergency gastrointestinal resection within 2-week time periods at any health-care facility in any country. Countries with participating centres were stratified into high-income, middle-income, and low-income groups according to the UN's Human Development Index (HDI). Data variables from the GlobalSurg 1 study and other studies that have been found to affect the likelihood of SSI were entered into risk adjustment models. The primary outcome measure was the 30-day SSI incidence (defined by US Centers for Disease Control and Prevention criteria for superficial and deep incisional SSI). Relationships with explanatory variables were examined using Bayesian multilevel logistic regression models. This trial is registered with ClinicalTrials.gov, number NCT02662231. Findings: Between Jan 4, 2016, and July 31, 2016, 13 265 records were submitted for analysis. 12 539 patients from 343 hospitals in 66 countries were included. 7339 (58·5%) patient were from high-HDI countries (193 hospitals in 30 countries), 3918 (31·2%) patients were from middle-HDI countries (82 hospitals in 18 countries), and 1282 (10·2%) patients were from low-HDI countries (68 hospitals in 18 countries). In total, 1538 (12·3%) patients had SSI within 30 days of surgery. The incidence of SSI varied between countries with high (691 [9·4%] of 7339 patients), middle (549 [14·0%] of 3918 patients), and low (298 [23·2%] of 1282) HDI (p < 0·001). The highest SSI incidence in each HDI group was after dirty surgery (102 [17·8%] of 574 patients in high-HDI countries; 74 [31·4%] of 236 patients in middle-HDI countries; 72 [39·8%] of 181 patients in low-HDI countries). Following risk factor adjustment, patients in low-HDI countries were at greatest risk of SSI (adjusted odds ratio 1·60, 95% credible interval 1·05–2·37; p=0·030). 132 (21·6%) of 610 patients with an SSI and a microbiology culture result had an infection that was resistant to the prophylactic antibiotic used. Resistant infections were detected in 49 (16·6%) of 295 patients in high-HDI countries, in 37 (19·8%) of 187 patients in middle-HDI countries, and in 46 (35·9%) of 128 patients in low-HDI countries (p < 0·001). Interpretation: Countries with a low HDI carry a disproportionately greater burden of SSI than countries with a middle or high HDI and might have higher rates of antibiotic resistance. In view of WHO recommendations on SSI prevention that highlight the absence of high-quality interventional research, urgent, pragmatic, randomised trials based in LMICs are needed to assess measures aiming to reduce this preventable complication

On Steam Solution Of A Prototype 8000 HP Motor Instability Problem.

LecturePg. 53-72This paper presents a typical field problem that most maintenance engineers in any petrochemical plant could encounter. Two 4000 hp through-drive electric motors were totally destroyed when the inboard coupling failed. Market conditions dictated that the compressor be put back into service as expeditiously as possible. A 3600 rpm prototype 8000 hp motor which could be fitted on the existing foundation was acquired from a power company. The existing half shell journal bearings had to be converted to a full shell design in order to solve a vibration problem. After start-up, the motor was plagued with a random vibration problem which occurred during a load change. Vibration data taken during one of these excursions indicated sub-synchronous frequencies at halfspeed, which were equal in amplitude to the horizontal readings at synchronous speed. A computer model simulating the rotor and bearing systems indicated a whirl instability problem at 1800 rpm. The computer was used to design and optimize a "between the pad," four shoe tilting pad bearing which was installed during the December 1982 turnaround. The paper goes into details on the new bearing design and installation, motor mechanical and hot optical alignment, and start-up data. All of the design and fabrication of the bearings was done while the unit was running, and the bearings were installed during a normal scheduled downtime. The result was a "first try fix." This example illustrates that the "trial and error" method of problem solving should be done on paper and not with hardware, particularly when the stakes are high and the time short. Equipment and techniques used to solve this problem are well established and used daily in most petrochemical plants

The influence of three genes on whether adolescents use contraception, USA 1994–2002

In a further contribution to recent investigations of the relevance of genetic processes for demographic outcomes, we investigate genetic associations with whether adolescents use contraception. Using data from the National Longitudinal Study of Adolescent Health, we find that variants in the dopamine transporter gene DAT1, the dopamine receptor gene DRD2, and the monoamine oxidase gene MAOA are associated with unprotected sexual intercourse. Consistent with previous analyses of these data, the genotypes DRD2*A1/A2, DRD2*A2/A2, DAT1*9R/10R, and MAOA*2R/ are associated with higher odds of unprotected sexual intercourse than other genotypes at these loci. The DRD2 associations apply to both men and women, whereas the other associations apply to women only. These results are robust to controls for population stratification by continental ancestry, do not vary by contraceptive type, and are consistent with previous research showing that these genetic variants are associated with higher rates of impulsivity