• Journal of Korean Institute of Industrial Engineers
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• v.21 no.4
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• pp.555-570
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• 1995

We consider a preventive replacement policy for a cold-standby system with N components, in which only one component is in operation at a time. If the component in operation fails, a standby component is immediately switched into operation. If all components fail, the system fails. The system is inspected at random poins in time to determine whether it is to be replaced or not. If the number of failed components at the time of inspection exceeds a threshold value r, the system is replaced. Otherwise the decision is put off until the next inspection point arrives. Under the cost structure which includes a replacement cost, a system down-time cost and a holding cost of the components, we develop an efficient procedure to find the optimal control values N and r, which minimize the expected cost per unit time.

• The Transactions of The Korean Institute of Electrical Engineers
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• v.66 no.8
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• pp.1278-1284
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• 2017

As of December 2016, there are 608,828 elevators in operation in Korea and 179,790 elevators in more than 15 years. 30.4% of all elevator are aging. Improved maintenance of the elevator and proactive replacement of the parts of the elevator can extend the lifetime of the elevator and ensure safety. An unclean environment reduces the lifetime of elevator parts. If you do not clean the environment and prevent preventive parts replacement, eventually shortening the lifetime of the parts connected to the failed part or causing more damage will result in greater economic loss. Also, the risk of elevator safety accidents due to failures of elevator parts will be increased accordingly. The study of optimum replacement time of elevator parts will contribute to prevention of safety accident of elevator and prolongation of lifetime of elevator through preventive replacement of elevator parts.

• Journal of Korea Society of Industrial Information Systems
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• v.20 no.5
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• pp.45-52
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• 2015

The read performance of hybrid hard disk is better than the legacy hard disk and power consumption is also considerably low. As blocks with enough localities may be located in the non-volatile cache whose size is generally limited, an effective block replacement scheme is required. As this replacement is inevitably affected by various parameters, we define this issue as a kind of multiple criteria decision model. To solve this problem, this paper suggests a new block replacement algorithm based on the analytic hierarchy process. Through simulation for our model, we confirmed that the proposed model could be used as a replacement algorithm of the hybrid hard disk as it may improve boot time as well as response time of general applications.

• Journal of Chest Surgery
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• v.23 no.3
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• pp.488-500
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• 1990

Author compared the effect of surgical methods between 40 patients who received mitral valve replacement with complete excision of the mitral valve[resected group] and 41 patients who received mitral valve replacement with preservation of posterior chorda tendineae and posterior mitral leaflet[preserved group] from 1985. 2. to 1989. 4. at cardiothoracic department of Pusan National University Hospital.v 1. There was no significant difference between the preserved group and resected group in cardiopulmonary bypass time and aortic cross clamping time and NYHA classification. 2. In preserved group of Mitral stenosis and Mitral regurgitation, the left ventricular functions were much improved after mitral valve replacement than resected group, but there was not so difference between the preserved group and reserved group in Mitral steno-regurgitation. 3. There were remarkable decrease in complication rate in preserved group compared to resected group. And also the death rates were remarkably decreased in preserved group which was 4.9% compared to resected group which was 17.5%. As the preservation of the posterior mitral leaflet and chorda tendineae during mitral valve replacement in mitral valve disease showed significantly improved effects in the maintaining of left ventricular function and reducing the postoperative complication, I assume the preservation of posterior mitral leaflet and chordae during mitral valve replacement will bring better result.

• Journal of Chest Surgery
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• v.18 no.3
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• pp.446-455
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• 1985

A total and consecutive 156 patients have undergone cardiac valve surgery including 13 closed mitral commissurotomy, 13 open mitral commissurotomy, one mitral annuloplasty, 75 mitral valve replacement, one aortic annuloplasty, 24 aortic valve replacement, 3 tricuspid valve replacement, 25 double valve replacement and one triple valve replacement. 155 prosthetic valves were replaced in a period between September 1976 and August 1985. There were 68 males and 88 females with age range from 8 to 69 yrs [mean 36.5 yr]. Out of replaced valves, 61 was tissue valve including 54 Carpentier-Edwards, and 4 was mechanical valves including 74 St. Jude Medical, and the position replaced was 101 valves for mitral, 46 for aortic and 8 for tricuspid. Single valve replacement in 102 cases, double valve replacement in 25 cases [17 for AVR+MVR, and 8 for MVR+TVR], and only one case was noted in the triple valve replacement. Early mortality within 30 days after operation was noted in 11 cases [7%]; 7 after MVR, 2 after DVR, and each one after open mitral commissurotomy and mitral annuloplasty. Cause of death was valve thrombus, cerebral air embolism, low output syndrome, uncontrollable arrhythmia, parapneumonic sepsis, acute cardiac tamponade and left atrial rupture. 7 late deaths were noted during the follow-up period from 1 to 104 months [average 48 month]; three due to valve and left atrial thrombus formation, two due to CVA from overdose of warfarin, and each one due to congestive heart failure and chronic constrictive pericarditis, Anticoagulants after prosthetic valve replacement were maintained with warfarin, dipyridamole and aspirin to the level of around 50% of normal prothrombin time in 79 cases, and Ticlopidine with aspirin in 47 cases to compare the result of each group. There were 11 major thromboembolic episodes including 3 deaths in the warfarin group. Two cases of CVA due to overdose of warfarin was noted in the warfarin group. In the ticlopidine group, there was only one left atrial thrombus confirmed at the time of autopsy. Among the survived 138 cases, nearly all cases[136 cases] were included in NYHA functional class I and II during the follow-up period. In conclusion, surgical treatment of the cardiac valve disease in 156 clinical cases revealed excellent result with acceptable operative risk and late mortality. Prevention of thrombus formation with anti-platelet aggregator Ticlopidine has better result than warfarin group presently with no specific side effect such as bleeding or gastrointestinal trouble.

• Journal of Chest Surgery
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• v.27 no.10
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• pp.838-845
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• 1994

From April, 1982 to December, 1992, multiple valve replacement was performed in 100 patients. Mitral and aortic valve replacement were done in 86 patients, 9 underwent mitral and tricuspid valve replacement, 4 patients underwent triple valve replacement and 1 patient underwent aortic and tricuspid valve replacement. Of the valve implanted, 100 were St. Jude, 64 Duromedics, 19 Carpentier-Edwards, 13 Bj rk-Shiley, 6 Ionescu-Shiley, and 2 Medronics.The hospital mortality rate was 15%[15 patients] and the late mortality rate was 7%[7 patients], the mortality rate was high in early operative period but decreased with time[20% at 1986, 18.2% at 1987, 9.5% at 1988, 11.1% at 1989, 12.5% at 1990, 11.8% at 1991, 0% at 1992]. The causes of death were low cardiac output in 8, sudden death in 3, CHF in 3, bleeding in 2, cerebral thromboembolism in 1, leukemia in 1, multiorgan failure in 1 and so on. The actuarial survival rate excluding operative death was 73% at 10 years.

• International Journal of Reliability and Applications
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• v.12 no.1
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• pp.41-48
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• 2011

This paper considers the maintenance model suggested by Jung and Park (2010) to adopt the Bayesian approach and obtain an optimal replacement policy following the expiration of NFRRW. As the criteria to determine the optimal maintenance period, we use the expected cost during the life cycle of the system. When the failure times are assumed to follow a Weibull distribution with unknown parameters, we propose an optimal maintenance policy based on the Bayesian approach. Also, we describe the revision of uncertainty about parameters in the light of data observed. Some numerical examples are presented for illustrative purpose.

• Journal of the military operations research society of Korea
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• v.22 no.2
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• pp.215-226
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• 1996

The estimation of lifetimes are examined when the distribution of lifetimes are Weibull. It is assumed that, due to physical restrictions and/or economic requirements, the lifetimes are investigated only at certain time intervals during the test period with 'mixed replacement' experiment, even though it is well known that 'with replacement' experiment produces better accuracy than 'without replacement' one. The maximum likelihood estimators are derived through the iterative method like as Lawless(1982). Also Cramer-Rao lower bounds are found as the asymptotic variances of the estimates.

• Proceedings of the Korea Concrete Institute Conference
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• 1996.10a
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• pp.387-393
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• 1996

The purpose of this study is analyze the properties of undispersed concrete according to replacement of the ratio of fly-ash. The test results show that as the ratio of flyash replacement which increasing fluidity but the amounts of air content, suspended solid and pH values dicreased and setting time is delayed. The ten persent replacment of fly-ash has less water pollution and high compressive strength value than other ratio of fly-ash replacement.

• Journal of Korean Society of Industrial and Systems Engineering
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• v.17 no.30
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• pp.51-57
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• 1994

This paper considers the problem of equipment replacement A equipment replacement problem is treated in a thchnological development environment. Our model assumes that the costs associated with the presently available thchnology and future technology is known, but the appearance times of future technology is uncertain. A cost model is presented, and method is suggested for finding the optimal operating time for replacement A numerical example is shown.