Periodic Overhaul Research Articles

Mental Hygiene and Socio‐Environmental Factors

Mental Hygiene and Socio‐Environmental Factors

Statistical Methods for Planning Diesel Engine Overhauls in the U. S. Coast Guard

ABSTRACTThe United States Coast Guard has recently investigated new strategies to maintain cutter propulsion diesel engines. Reliability centered maintenance with statistical methods may allow the time between costly scheduled overhauls to be increased. One indicator of engine aging is the number of failures experienced with increasing operating hours. The purpose of this paper is to investigate the failure‐time relationship of the ALCO 251 marine diesel propulsion engine operated on Reliance class cutters. This analysis used exponential, Weibull, and three‐part composite Weibull failure density functions to model engine casualty data dating back to 1978. The data does not indicate the source of the engine failure, but every failure had a significant operational impact. Results indicate an increasing failure rate as the engine ages to the 24,000 hour overhaul time. The evidence indicates a constant failure useful life region, but the increasing failure rate from the Weibull models suggests that the periodic over‐hauls do not prevent wearout failures. As a result, the Coast Guard should consider refining diesel engine overhaul policy in order to prevent increasing age‐related failures.

An optimal maintenance policy for a system under periodic overhaul

A preventive maintenance policy with minimal repair at failure, periodic overhaul, and replacement is considered. A model describing the effect of overhaul is proposed and the expected cost rate is obtained under negligible or nonnegligible maintenance time. Based on this model, optimal number of overhauls and optimal interval between overhauls for minimizing the expected cost rate over infinite time horizon are determined. Under some mild conditions, a unique optimal maintenance policy exists and a closed-form expression for the optimal interval between overhauls is derived when the lifetime of the system follows Weibull distribution. Numerical studies are made.

4.1.4 Applications of Conditional Inferential Methods for Operational Cost Savings for US Coast Guard C130 Aircraft Maintenance

AbstractMaintenance costs play a significant role in the overall operational cost of the US Coast Guard (USCG) fleet of C130 aircraft. Selection of the preventative maintenance interval for the C130 1500 Series Flight Deck Cooling Turbine provides an opportunity for considerable maintenance cost savings. If this turbine fails in service, it may be damaged beyond repair and cost $30,000 to replace. If overhauled before failure, the cost is typically $500. The USCG currently allows this turbine to fail in service, and is considering a preventative maintenance schedule of periodic overhauls. Failure data for this turbine is relatively scarce however. This paucity of data magnifies the perceived impact of uncertainties in the data, which all but paralyzes USCG decision makers charged with selecting an overhaul interval. The authors process failure and replacement cost data for this turbine using conditional inferential methods. Statistical distributions of Expected Costs Savings per Flight Hour are parameterized as a function of overhaul interval. These distributions and related estimates allow USCG decision makers to confidently select an overhaul interval that minimizes maintenance costs and is commensurate with existing maintenance schedules.

An Automated Robotic System for Jet Engine Overhaul. System Design and Development for Honeycomb Repair

Jet engines run tender very severe conditions and require periodic overhauling. Owing to the high operating temperature and pressure, most service components become significantly distorted from their original shapes. Therefore, the overhaul of a jet engine has been seen as a difficult subject for process automation. This paper reports R&D activities in Gintic/Singapore to achieve the first automated system worldwide for honeycomb repair.

ON THE PREDICTION OF FUTURE FAILURES FOR A REPAIRABLE EQUIPMENT SUBJECT TO OVERHAULS

This paper deals with the prediction, from a Bayes viewpoint, of future failures for a repairable equipment subjected both to minimal repairs and periodic overhauls. The effect of major overhauls on the reliability of the equipment is modeled by a proportional age reduction model, while the failure process between two successive overhaul epochs is modeled by the power law process. Prediction both of the future failure times and of the number of failures in a future time interval are provided on the basis of the observed data and of a number of suitable prior densities, which reflect different degrees of belief on the failure mechanism and overhaul effectiveness. Finally, a numerical application illustrates the proposed prediction procedures and their use in assessing the adequacy of the model to describe the observed data set.

Modelling of the flow at the rotor disc in a geothermal turbine of 110 MW

To elucidate an excessive erosion damage produced by solid particles in the fourth stage rotor disc of a 110 MW double flow geothermal turbine, a bi-dimensional modelling investigation has been conducted. The study was based on a set of results from a computational model using a Reynolds stress, RSM, turbulence model. The predicted results confirmed characteristic flow conditions that may play a main role in the serious erosion of the fourth stage rotor disc governor side, which has been detected in periodic overhauls. The results show a jet of vapour that hits the disc transition radius surface at velocities around 112 m/s. These conditions are produced by the flow outgoing from the labyrinth seal, which passes through a drastic cross-section reduction in the last seal strip. The flow was then simulated introducing specific changes to the geometry and the grid in order to modify the flow patterns favourably. Actually, the suggested changes have been envisaged indeed to be practically feasible of being implemented. The new results showed that it is possible to reduce the erosion process up to 86% by increasing the distance from the labyrinth seal to the rotor disc, which produces a 38% velocity reduction of the vapour flow in that zone. The design proposed in this work produces a flow pattern of a lower velocity on disc surface together with a modified angle of flow incidence. Furthermore, the proposed design also reduces a recirculating flow at the exit of the last seal strip. Based on these results, an analysis of erosion against velocity demonstrates that the redesigned rotor disc proposed here leads to the duplication of the time period used at present between maintenance repairs.

On the overhaul effect for repairable mechanical units: a Bayes approach

Abstract This paper deals with the statistical analysis, from a Bayes viewpoint, of the failure data of repairable mechanical units subjected to minimal repairs and periodic overhauls. A proportional age reduction model is assumed to model the effect of overhauls on the reliability of the unit, whereas the failure process between two successive overhaul epochs is modeled by the power law process. Point and interval estimation of model parameters, as well as of quantities of large interest, are provided on the basis of a number of suitable prior densities, which reflect different degrees of belief on the failure/repair process. Hypothesis tests on the effectiveness of performed overhauls are developed on the basis of Bayes factor, and some guidelines to perform sensitivity analysis versus the prior information are provided. Finally, a numerical application illustrates the proposed inference and testing procedures.

Modelling of the flow at the last stage blade tenon in a geothermal turbine using renormalization group theory turbulence model

A bi-dimensional modelling investigation of the flow in the last stage of a 110 MW geothermal turbine has been conducted. The study was based upon a Renormalization Group Theory turbulence model. The results confirmed the existence of flow conditions which may play a main role in the erosion of the L-0 stage blade tenon, which had been detected in periodic overhauls. According to predicted results the relationship between erosion and flow patterns might exist due to: (1) a vapour jet hitting directly on tenon surface at velocities around 65 m/s; (2) a low-pressure region identified with recirculating flow, which may be causing cavitation on the damaged surface. Afterwards, the flow was simulated with changes on the geometry and grid. These changes are, indeed, practically feasible of being implemented. The simulations showed that it is possible to reduce the erosion process by enlarging a flat region close to the L-0 rotor stage. Namely, this change of geometry produces a flow pattern that diminishes the strength of recirculation flow making it possible to reduce both the flow rate through tenon region and its velocity on tenon surface. The pressure drop diminishes as well, clearly reducing a risk of cavitation.

Periodic overhaul planning system for electric power plants

Periodic overhaul planning for boilers and turbines in electric power plants has been done based on expert experience in order to satisfy various constraints that are hard to formalize, such as legal regulations, the overhaul interval restriction between units, the adjustment strategy in the case of including special events, the daily electric power requirement, and so forth. Although computer processing using the knowledge base approach has been tried, it does not lead to good results because of excessive hardware requirements, low performance, and no assurance of optimality. A two-step approach is proposed based on the categorization of constraints that are hard to formalize into “strong constraints” and “weak constraints.” It replaces the nonlinear integer programming problem corresponding to “strong constraints” by a combination of integer programming and combinatorial analysis. Consequently, it can lead to optimal solution in terms of fuzzy satisfaction measures within an allowable time with a PC. © 1999 Scripta Technica, Electr Eng Jpn, 127(2): 24–31, 1999

Optimal maintenance models with minimal repair, periodic overhaul and complete renewal

This paper investigates the maintenance decision situation in which three actions, minimal repair, periodic overhaul and complete renewal, may be applied to the system under consideration. A new mathematical model is proposed to describe system improvement due to the maintenance action of an overhaul that differs from the virtual age approach by considering a direct reduction on the system's failure rate. Based on this improvement model, two cost models for determining the optimal overhaul interval and the number of overhauls in a renewal cycle, that minimize the expected unit-time cost or the total discounted cost, are established. Existence conditions of optimal solutions are obtained and special cases of the two cost models are discussed.

Optimal maintenance models with minimal repair, periodic overhaul and complete renewal

This paper investigates the maintenance decision situation in which three actions, minimal repair, periodic overhaul and complete renewal, may be applied to the system under consideration. A new mathematical model is proposed to describe system improvement due to the maintenance action of an overhaul that differs from the virtual age approach by considering a direct reduction on the system's failure rate. Based on this improvement model, two cost models for determining the optimal overhaul interval and the number of overhauls in a renewal cycle, that minimize the expected unit-time cost or the total discounted cost, are established. Existence conditions of optimal solutions are obtained and special cases of the two cost models are discussed.

The role of NHPP models in the practical analysis of maintenance failure data

The analysis of failure data is an important facet in the development of maintenance strategy for equipment. Only by properly understanding the mechanism of failure, through the modelling of failure data, can a proper maintenance plan be developed. This is normally done by means of probabilistic analysis of the failure data. From this, conclusions can be reached regarding the effectiveness and efficiency of preventive replacement (and overhaul) as well as that of predictive maintenance. The optimal frequency of maintenance can also be established by using well developed optimisation models. These optimise outputs, such as profit, cost and availability. The problem with this approach is that it assumes that all repairable systems are repaired to the ‘good-as-new’ condition at each repair occasion. Maintenance practice has learnt, however, that in many cases equipment slowly degrades even while being properly maintained (including part replacement and periodic overhaul). The result of this is that failure data sets often display degradation. This renders conventional probabilistic analysis useless. During the last two decades, a few researches applied themselves to the solution of this problem. This paper briefly examines the present state of the theoretical foundation of repairable systems analysis techniques and then develops two formats of the Non-Homogeneous Poisson Process model (NHPP model) for practical use by the maintenance analyst. This includes an identification framework, goodness-of-fit tests and optimisation modelling. The model is tested on two failure data sets from literature and one from industry.

Scheduling of maintenance activities in a sugar industry using simulation

This study has been carried out in a sugar industry located in Tamil Nadu, India. During the production period, the plant is shut down for a couple of shifts for periodic overhauling, corrective and minor preventive maintenance activities, and inspections. This periodic shutdown is necessary because in the event of breakdown of machines, the plant is forced to crush the same amount of cane (that would have been crushed during the breakdown period) towards the end of the season when the recovery factor is low. The problem is to determine the frequency of this periodic shutdown maintenance in order to minimize the downtime losses that are time-dependent. A simulation model is developed towards this endeavor.

Optimal inventories for repairable redundant systems with aging components

AbstractComplex systems that are required to perform very reliably are often designed to be “fault‐tolerant,” so that they can function even though some component parts have failed. Often fault‐tolerance is achieved through redundancy, involving the use of extra components. One prevalent redundant component configuration is the m‐out‐of‐n system, where at least m of n identical and independent components must function for the system to function adequately.Often machines containing m‐out‐of‐n systems are scheduled for periodic overhauls, during which all failed components are replaced, in order to renew the machine's reliability. Periodic overhauls are appropriate when repair of component failures as they occur is impossible or very costly. This will often be the case for machines which are sent on “missions” during which they are unavailable for repair. Examples of such machines include computerized control systems on space vehicles, military and commercial aircraft, and submarines.An interesting inventory problem arises when periodic overhauls are scheduled. How many spare parts should be stocked at the maintenance center in order to meet demands? Complex electronic equipment is rarely scrapped when it fails. Instead, it is sent to a repair shop, from which it eventually returns to the maintenance center to be used as a spare. A Markov model of spares availability at such a maintenance center is developed in this article. Steady‐state probabilities are used to determine the initial spares inventory that minimizes total shortage cost and inventory holding cost. The optimal initial spares inventory will depend upon many factors, including the values of m and n, component failure rate, repair rate, time between overhauls, and the shortage and holding costs.In a recent paper, Lawrence and Schaefer [4] determined the optimal maintenance center inventories for fault‐tolerant repairable systems. They found optimal maintenance center inventories for machines containing several sets of redundant systems under a budget constraint on total inventory investment. This article extends that work in several important ways. First, we relax the assumption that the parts have constant failure rates. In this model, component failure rates increase as the parts age. Second, we determine the optimal preventive maintenance policy, calculating the optimal age at which a part should be replaced even if it has not failed because the probability of subsequent failure has become unacceptably high. Third, we relax the earlier assumption that component repair times are independent, identically distributed random variables. In this article we allow congestion to develop at the repair shop, making repair times longer when there are many items requiring repair. Fourth, we introduce a more efficient solution method, marginal analysis, as an alternative to dynamic programming, which was used in the earlier paper. Fifth, we modify the model in order to deal with an alternative objective of maximizing the job‐completion rate.In this article, the notation and assumptions of the earlier model are reviewed. The requisite changes in the model development and solution in order to extend the model are described. Several illustrative examples are included.

A Reassessment of the Work of the Peckham Health Centre, 1926-1951

George Scott Williamson, a pathologist, and Innes Pearse, a general practitioner, who met while working at the London Hospital during World War I. Williamson and Pearse set up the Centre in the South London suburb of Peckham-hence, its more common designation as the Peckham Centre-as an independent research experiment to test their ideas regarding the nature of and the conditions necessary for its maintenance. The Centre was open between 1926 and 1930, 1935 and 1939 (in new premises built for the purpose), and 1945 and 1950. It functioned as a family club with two conditions for membership: each family paid a weekly subscription, and all family members subjected themselves to periodic health overhauls. The Centre provided a broad range of services for the whole family: creche facilities; fresh produce for sale from a farm in Kent which was a part of the enterprise; and, towards the end of the experiment, an elementary school for the children. It was this combination of activities, together with Williamson and Pearse's tireless propaganda efforts, that attracted attention to the Centre. The Centre worked at a number of different levels. To the exmembers we interviewed, it combined a good primary care

Aircraft fault isolation based on pattern of cockpit indications: a human factors approach

It is commonly recognised among operators of modern commercial aircraft that maintenance is a significant and costly part of their operation. In fact, it is estimated that 28% of all aircraft operating costs can be attributed to maintenance-related activities. A major portion of this activity can be classified as unscheduled, that is, maintenance other than regular servicing and periodic overhauls. Much of the cost of unscheduled maintenance derives from the actual man-hours and materials involved in isolating and replacing faulty components. However, additional consideration must be given to the fact that an aircraft which is detained beyond its scheduled departure time is neither producing revenue for the operating airline during that period, nor promoting goodwill among passengers inconvenienced by delays or cancellations.

Edificio para Cristalería Española

In the course of a series of lectures on the subject of curtain walls, held at the Higher Technical College of Architecture, in Madrid, the author described his own experience whilst constructing a building (designed by him and by Mr. Manuel Barbero) for the firm Cristaleria Espanola, at San Andres del Palomar, Barcelona. Mr. De la Joya summarised the advantages that foreign experts attribute to curtain walls. These advantages are: less weight of the materials of the enclosing walls; maximum use of the available building surface; cheaper foundations; better building time control; superior quality and accuracy; and greater maintenance facility. The lecturer then explained why, and in what aspects, he was in disagreement with the above supposed advantages. He dealt with these as follows: 1. In terms of present day construction, with concrete blocks and lightweight flooring structures, it is not easy to save much weight, owing to the number of special fittings that are necessary to support and keep the curtain wall in place.—2. Although in theory a. curtain wall may be only 4 or 5 cm thick, the truth is that unless heating or air conditioning is provided on a much larger scale, a special internal protection layer is necessary.—3. It is difficult to reduce the cost of the foundations by fitting curtain walls, and any calculated saving in cost in this aspect is more theoretical than real.—4. Building time estimates usually fail to work out in practice, and in addition to this, workmanship quality is not yet very satisfactory.—5. Curtain walls require periodic overhauls, so that they do not age too rapidly, and in this sense they are no better than traditional walls. The lecturer also made a number of additional points, one of them being that he did not advise the use of curtain walls in Spain (because of the cheapness of brickwalls, the climate, the high cost of air conditioning, etc. He also emphasized how important it is to make a very full and careful study of cost when preparing a project involving curtain walls, in order to evaluate correctly their advantages.

The Peckham Experiment

THERE was a time I wished I lived in Peckham, for in 1926 the Pioneer Health Centre was opened there by a group of young people who set put to study health and to foster conditions in which it could be maintained. They offered to families the facilities of a well-provided social club, and a periodic health overhaul. It would have been nice to be one of their 'guinea pigs'; the social experiment alone was commendable, and the possibilities of a new kind of medical research seemed enormous. The first two books published by the Peckham 'biologists', "The Case for Action"(1931) and "Biologists in Search of Material"(1938), were disappointing, but it was perhaps too early to decide whether they were a little 'cranky', or merely picturesque. To social biologists, however, "The Peckham Experiment" is even more disappointing. It gives a lengthy exposition of the experimenters' philosophy and descriptions of life at the Centre, but it reports very little advance on the results of research published in its predecessors. These results were certainly interesting and sometimes unexpected. For example, only about 10 per cent of the Centre members had no physiological defects, and about 68 per cent had some disorder but were seemingly well. The high incidence of worm infestation, reaching so much as 41 per cent in boys of 6–10 years of age, is surprising. Two further interesting findings published earlier but not unfortunately expanded in this book are the frequent occurrence of a state of mild chronic debility and the profound physiological effects of night work. The Peckham Experiment A Study in the Living Structure of Society. By Dr. Innes H. Pearse and Lucy H. Crocker. (Published for the Sir Halley Stewart Trust.) Pp. 334 + 16 plates. (London: George Allen and Unwin, Ltd., 1943.) 12s. 6d. net.

Airplane Servicing From the Operator’s Point of View

Abstract The necessity for periodic overhauls of equipment and the large financial outlay necessary to do this work properly form one of the operator’s greatest problems. The author tells why centrally located shops equipped to do this work at lowest possible prices constitute the answer to the problem. This is an able and well-informed paper on airplane servicing, drawn from actual experience, and should be helpful to airplane operators.