• IE interfaces
• /
• v.16 no.1
• /
• pp.27-33
• /
• 2003

The maintenance cost in K Steelworks has been continuously increased in proportion to the production cost. However, there seems to be a possibility of reducing cost through the optimization of maintenance actions. The failure types of the equipment in steelworks ate various with different failure cost. Thus the failure rate and cost of each type of failures should be considered simultaneously when the optimum maintenance period is to be determined. It is considered that the equipment undergoes periodic replacement and a specified number of incomplete preventive maintenance actions are performed during a replacement period. Assuming that the time to failure follows a Weibull distribution, the parameters of the failure rate are estimated using the maximum likelihood estimation. The optimal replacement period is determined to minimize the average cost per unit time. As the result of analysis it is suggested that the existing maintenance period for a hot-rolling equipment can be extended significantly.

• Journal of KIISE:Computer Systems and Theory
• /
• v.30 no.10
• /
• pp.556-568
• /
• 2003

This paper presents and analyzes the performance of a weighted-window based cache replacement policy for the efficient media caching in streaming media cache servers. The proposed policy makes, for each cached object, use of the reference count, reference time, amount of media delivered to clients and, in particular, the periodic patterns of user requests. Also, by giving weights to the recently referenced media contents, the replacement policy adequately and swiftly reflects the ever-changing characteristics of users preferences. The simulation studies show that the performance of the proposed policy has improved over the conventional policies such as LRU, LFU and SEG - in terms of hit ratio, byte hit ratio, delayed start and cache input.

• KIISE Transactions on Computing Practices
• /
• v.21 no.1
• /
• pp.88-93
• /
• 2015

In modern computer systems, DRAM is commonly used as main memory due to its low read/write latency and high endurance. However, DRAM is volatile memory that requires periodic power supply (i.e., memory refresh) to sustain the data stored in it. On the other hand, PCM is a promising candidate for replacement of DRAM because it is non-volatile memory, which could sustain the stored data without memory refresh. PCM is also available for byte-addressable access and in-place update. However, PCM is unsuitable for using main memory of a computer system because it has two limitations: high read/write latency and low endurance. To take the advantage of both DRAM and PCM, a hybrid main memory, which consists of DRAM and PCM, has been suggested and actively studied. In this paper, we propose a novel page replacement algorithm for hybrid main memory. To cope with the weaknesses of PCM, our scheme focuses on reducing the number of PCM writes in the hybrid main memory. Experimental results shows that our proposed page replacement algorithm reduces the number of PCM writes by up to 80.5% compared with the other page replacement algorithms.

• Journal of Korean Society for Quality Management
• /
• v.25 no.3
• /
• pp.86-95
• /
• 1997

This paper considers a repairable system, which is maintained preventively at periodic times and is minimally repaired at each failure. Most preventive maintenance policies for such repairable systems assume that the cost of minimal repair is constant regardless of its age at failure. However, it is more practical to consider the situations where the cost of minimal repair is dependent not only on its age at failue, but also on the number of preventive maintenance carried out prior to its failure. We consider the preventive maintenance carried out prior to its failure. We consider the preventive maintenance policy with age-dependent minimal repair cost. The optimal policies which minimize the expected cost rate over an infinite time span are discussed. We obtain the optimal period and number of preventive maintenance prior to replacement of the system.

• Proceedings of the Korean Reliability Society Conference
• /
• 2005.06a
• /
• pp.115-120
• /
• 2005

This paper introduces models for preventive maintenance policies and considers periodic preventive maintenance policy with minimal repair when the failure of system occurs. It is assumed that minimal repairs do not change the failure rate of the system. The failure rate under prevention maintenance received an effect by a previously prevention maintenance and the slope of failure rate increases the model where it considered. Also the start point of failure rate under prevention maintenance considers the degradation of system and that it increases quotient, it assumed. Per unit time it bought an expectation cost from under this prevention maintenance policy. We obtain the optimal period time and the number for the periodic preventive maintenance by using Nakagawa's Algorithm, which minimizes the expected cost rate per unit time. Finally, it suppose that the failure time of a system has a Weibull distribution as an example and we obtain an expected cost rate per unit time the optimal period time and the number when cost of replacement and cost of minimal repair change.

• Journal of Applied Reliability
• /
• v.18 no.2
• /
• pp.104-113
• /
• 2018

Purpose: This paper proposes the non-periodic preventive maintenance policy based on the level of cumulative hazard intensity. We aim to construct a cost-effectiveness on the proposed model with relaxing the constraint on reliability. Methods: We use the level of cumulative hazard intensity as a condition variable, instead of reliability. Such a level of cumulative hazard intensity can derive the reliability which decreases as the frequency of preventive maintenance action increases. We also model the imperfect preventive maintenance action using the proportional age setback model. Conclusion: We provide a numerical example to illustrate the proposed model. We also analyze how the parameters of our model affect the optimal preventive maintenance policy. The results show that as long as high reliability is guaranteed, the inefficient preventive maintenance action is performed reducing the system operation time. Moreover, the optimal value of the proposed model is sensitive to changes in preventive maintenance cost and replacement cost.

• Transactions of the KSME C: Technology and Education
• /
• v.2 no.2
• /
• pp.89-97
• /
• 2014

The diesel driven power generation facility provides most of electricity of island region where is isolated from the main land power supply system. It is possible to extend the life of diesel driven power generation facility by periodic maintenance. However, the long-term use of diesel driven power generation facility increases the operating cost due to aging hardware and reduced plant efficiency. In other words, on-time replacement of aging diesel driven power generation facility reduces plant operation cost. The purpose of this study presents major indicators to represent the characteristics of the diesel generator and conducts sensitivity analysis using Monte Carlo analysis. In addition, status core which indicates quantitatively the state of the diesel driven power generation facility is defined and prepare the replacement criteria based on the state score.

• Smart Structures and Systems
• /
• v.22 no.2
• /
• pp.151-159
• /
• 2018

Maintenance activities are regarded as a key part of the repairable deteriorating system because they maintain the equipment in good condition. In practice, many maintenance policies are used in engineering fields to reduce unexpected failures and slow down the deterioration of the system. However, in traditional maintenance policies, maintenance activities have often been assumed to be performed at the same time interval, which may result in higher operational costs and more system failures. Thus, this study presents two non-periodic preventive maintenance (PM) policies for repairable deteriorating systems, employing the failure rate of the system as a conditional variable. In the proposed PM models, the failure rate of the system was restored via the failure rate reduction factors after imperfect PM activities. Operational costs were also considered, which increased along with the operating time of the system and the frequency of PM activities to reflect the deterioration process of the system. A numerical example was provided to illustrate the proposed PM policy. The results showed that PM activities performed at a low failure rate threshold slowed down the degradation of the system and thus extended the system lifetime. Moreover, when the operational cost was considered in the proposed maintenance scheme, the system replacement was more cost-effective than frequent PM activities in the severely degraded system.

• Journal of Chest Surgery
• /
• v.17 no.3
• /
• pp.346-355
• /
• 1984

77 cases of Aortic Valve Replacement, which were composed of 64 rheumatic valvular heart disease and 13 combined congenital heart disease, were operated at Seoul National University Hospital for Aortic valvular disease during the period from June 1968 to December 1983. Among these 64 rheumatic aortic valvular heart disease cases, 8 patients were expired during and immediate after operation and overall mortality rate was 12.5%. For more precise remarks, these patients were divided into two periodic groups, 1st period [from 1968 to 1976] and 2nd [from 1977 to 1983] when annual open heart surgery were over 100 cases, and in 1st period three of four patients were died and in 2nd period five of sixty patients were died and its mortality rate was 8.3%. There were 12 cases of postoperative complication, which were 3 cases of remaining other valvular heart disease required MVR, 2 paravalvular leaks [one of them got Redo AVR], 4 thromboembolism or problem of anticoagulant therapy, 2 late death due to SBE with replaced valve failure and one functional AS with small sized valve. Operative death was affected by pump-time and aortic cross-clamping time, heart size, Ejection Fraction, LVEDP and symptom duration, and other many factors may influence the survival rate. Improved operative technique and myocardial protection and meticulous evaluation of the preoperative patient status will make the AVR safer.

• Pediatric Infection and Vaccine
• /
• v.2 no.2
• /
• pp.194-199
• /
• 1995

Most boys afflicted with panhypogammaglobulinemia, also known as X-linked agammaglobulinemia, remain healthy during the first 6 to 12 months of life because of protection by maternally transmitted IgG antibodies. Thereafter, they repeatedly acquire infections with high-grade pathogens, such as pneumococci, streptococci, and hemophilus unless given antibiotics or immunoglobulin replacement therapy. We experienced a case of panhypogammaglobulinemia in a 4 years old boy. He had been suffered from recurrent upper respiratory tract infection, otitis media and pneumonia since late infancy. He was admitted due to right pleural effusion with pneumonia, and streptococcus pneumoniae was isolated from pleural fluid and blood cultures. His immune status revealed panhypogammaglobulinemia and deficiency in mature B lymphocyte. He was treated with appropriate antibiotics therapy, but showed poor responses. He was transferred to department of thoracic surgery, and received minithoracotomy (decortication) operation. He was successfully treated with operation, antibiotics, and IV gammaglobulin infusions. Now he is being followed with periodic IV gammaglobulin replacement therapy.