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General Articles
Vol.1, No.1 (May, 2009), GA3
 

Virtual Plant and Maintenance Revolution

 
Kenzo MIYA, President of Japan Society of Maintenology
 
 

1. Introduction

      Maintenance work at a nuclear power plant is enormous in many divergences. The role of maintenance business we expect is first to prevent an accident and secondly to increase economic efficiency which is normally accomplished by raising the rate of operation availability. Consequently, that is to bring useful products and services in our society.
      Now the role of the maintenance in view of technology is to prevent the age-related deterioration, i.e. to keep integrity of the facility to be usable for a long period of time. Maintenance is performed within the technology standards at the same time as an inspection is carried out to evaluate the deterioration of a facility. The outcomes from these activities provide useful information and data to help assess effectiveness for extending the equipment lifetime.

      In the meantime, there is one of growing concerns of social issues that a vast amount of industrial waste is built up if we fail to extend equipment lifetime by renewing the facility. Eventually the burden gets exceedingly on top of our society. Conversely if we can extend 50 years of the lifetime to 100 years or what more, 100 years to 150 years, the effectiveness should immeasurably be high. The recent roles and importance of maintenance exists in this perception.?
      One of the effective tools to accomplish this goal is so called “a virtual plant” to be built and used for this purpose.? The conventional way of maintenance is revolutionarily enhanced by building a virtual plant that a chronological pole is set in the computer system, which will increase efficiency of maintenance and safety level of a plant, and facilitate systematization of maintenance planning and assessment of maintenance level. This presumes an advanced maintenance that brings all of that in as result. It would deserve to be called “Maintenance Revolution”.

2. Recovery from Deterioration of Equipment and Facility, and Maintaining a Safety Level

      At a nuclear power plant facility, it is possible for example to maintain the existing nuclear reactor at the same level of the newly-built plant by properly replacing the large facility that refers to safety every 10 years. Why is it possible? Its rationality is laid on the following contexts:

(1) In a case of a nuclear power plant, it is imperative to prevent a radioactive accident in where there is a group of equipment directly involved (safety equipment) and another group of many of equipment not involved or not directly related to safety (ordinary equipment). A conventional maintenance is designed to not only keep functions of the equipment in good condition but also to satisfy the technical standard.

(2) In cope with an age-related deterioration of the facility at a nuclear power plant, it should be achieved to keep the safety level at the same level of newly-built facility by replacing the safekeeping equipment in proper time intervals, or taking deterioration recovery measures. This is performed based on the long-term maintenance plan that is formulated in “periodical safety review”. As for non-safekeeping equipment, the industry has always made the effort to minimize breakdown. In addition, safety inspection by the regulatory authority is an integral part of effective prevention measure.

(3) Not to mention, a safety level of the existing reactor cannot be the same level as a newly-built reactor.? Separating a safety level from a maintenance level, it is conceivable that both safety level of old and new reactors is same when reliability of the existing one can be maintained at the level of the new reactor.

      In brief, that is an essential key to effectiveness of maintenance to maintain an old equipment in the same condition as a new one by taking deterioration recovery measure to safety equipment. The industry is normally responsible for a trouble is not related to safety. Even though the trouble just occurs, it should be bearable as far as their effort toward minimizing accidents continues. It is also socially a common notion that a minor accident is an industry issue. Safety standard referred by this paper is not on a term of “absolute safety” but on socially common idea.?
In approaching to materialization of this idea, a plant safety needs to be quantitatively measured. How we can achieve this is an issue. To clear this, a“virtual plant” is proposed. A virtual plant has potential to drastically help bring about how maintenance should be carried out, which facilitates quantifying the data for safety. Therefore when this idea is materialized, it would be affirmed that “maintenance revolution” is literally realized.

3. What Can a Virtual Plant Do


      Briefly speaking, a virtual plant is a copy of actual plant. A static virtual plant can be built in a capacity of a computer. As an example of what it is, CAD (Computer Aided Design) seems to be a kind of a virtual plant but it is not, because its functions are limited in it. CAD provides only part of functions for the virtual plant and does not include a concept of time, i.e. the point at which an event occurs. For the reason, CAD cannot be equal to that of an actual reactor. How can we adapt the concept of time to the system? The way to do is to measure deterioration data changing every moment in the actual reactor and to link the data properly to each component concerned in the virtual plant. In this way, a chronological pole is ideally set in the virtual plant. Considering the fact that an equipment lifetime is just a few decades in case of a nuclear plant, it would be reliably enough to measure and update the data annually. The more the actual data is linked to the virtual plant, the closer the virtual plant comes to the actual reactor. This is a considerable advantage. Details of what a virtual plant can do is specifically outlined as follows:

(1) Grasp of Tendency of Deterioration
      The running behavior of an actual reactor can be simulated using a virtual plant.
It can be fed with all deterioration data measured and is capable of catching a tendency to deterioration instantly. The more data is fed in, the closer to the actual reactor and the virtual plant improves accuracy of functions as described later. The behavior of an actual reactor is simulated in this way to grasp tendency of deterioration in the virtual plant??

(2) Assessment and Projection of Deterioration
      The running behavior of the equipment can be projected for a long period of time from present to future by solving the governing equation related to distribution of temperature, stress, pressure, and speed of flow that are based on plant parameters, initial conditions and boundary conditions. A degree of deterioration can be quantitatively evaluated using the value obtained, and accuracy of a projected value can be examined in comparison with the actual value. The effort to improve the accuracy is made by repeating PDCA cycle. As a result, it makes a highly accurate projection of deterioration possible.

(3) Likelihood of Weak Components
      In this way, the system can find out likelihood of weak components by searching and assessing information related to deterioration at each part and also by referring to the result from the load condition and the governing equation. How far can we project a degree of functional loss in complex parts. To find it, we must examine instances of the past troubles, because loss of equipment functions is not necessarily caused by only stress and speed of flow. For example, the problem of a stuck valve is an issue in another category.

(4) Implementation of a Virtual Maintenance     
      Examining the past instances of troubles makes accuracy of “likelihood of weak parts” to improve. The integrity of applicable parts is periodically assessed using the results from projection models for deterioration and the governing equations. Weak parts are checked out by the virtual plant, and then the result provides at least a valuable suggestion for integrity of equipment functions. These processes are that of virtual maintenance. At time when an actual plant is maintained in the way that takes account of the result of assessment from a virtual plant, a concept of Reliability Centered Maintenance (RCM) would be realized mechanically.

(5) Determination of Maintenance Plan
      The maintenance plan mentioned here is, needless to say, meant to be the one in the conditions where the business institution is called for submission of a maintenance plan based on the results of a virtual maintenance to the regulatory authority. In light of the safety priority of equipment as well as the maintenance priority, the plan is determined efficiently by evaluating each part, such as weak components, tendency and the degree of deterioration and accident ratio.

(6) Assessment of Safety Level and Maintenance Level
      A virtual plant itself can evaluate CDF (core damage frequency) and maintenance level during the time of evaluating its effectiveness, because the equipment breakdown ratio is updated at each time of the incident. Now, the way to assess? a safety level of a plant? is a methodological issue and yet not simple. A breakdown ratio is the past data. With that, it is not sure to see how well the future breakdown ratio is projected. There are two main factors to determine the future’s breakdown ratio. One is the current condition of deterioration of equipment and the system, and the other is a human factor. If a degree of deterioration of equipment is known in capacity of the virtual plant, the integrity of equipment is clearly found out. Hence, a safety level of a plant becomes quantifiable. How about human fault     ??

(7) Human Factor and Virtual Maintenance
      The facility maintenance is a crucial subject as soon as the operations department takes over the facility after the construction of a facility is complete. Integrity of a facility largely relies on a quality of equipment. Actually the operation and management of the facility is dominantly controlled by human being. Figuratively speaking, as an example of cookie making factory, a quality of the product is checked by a human eyes seeing if it is good or bad, but it would be impractical as the number of cookies get countless. What would be the solution? ?An answer to this is to make sure that the facility is manufactured and installed as is designed and also to make sure that an organization to manage the production functions as is expected.

      As explained in the above a concept of a virtual plant and its effectives, it would drastically changes the way of maintenance. The new way of maintenance will bring benefits such as ability to detect weakness of equipment/facility in advance, and so forth. We can remark that there appears the proper situation where maintenance revolution will be taken up as we perceive the concept of a virtual plant that would be adapted to the actual plant maintenance.