Maintenance, planned

Main steamline section and weld of the same Unit 4 f600 MW3 as above the weld connects 2 steamline sections of different materials (stainless steel and low-alloy steel) through a transition material section, ahead of the Y-piece where branching of the turbine inlet lines takes place (255 mm internal diameter, 44.5 mm thickness). Ultrasonic inspection pointed out potential integrity problems in the weld. The requirement was again that AE could support safe operation of the weld until the next incoming planned maintenance shutdown. 

This header has undergone repair after through-thickness cracking of an assembly weld after 108.000 hours of service, it is presently in operation and periodically submitted to ultrasonic inspection during planned maintenance shutdown. Continuous on-line AE monitoring of the integrity of critical welds is expected to contribute supporting continued and safe operation of the header until its replacement, platmed to take place in 1999. 

By analysing the scans, different types of tube characteristics, wastage patterns or profiles can be documented and effectively used to make immediate decisions during shut downs and in planning maintenance programs. 

J. W. CrisweU, Planned Maintenance for Productivity and Energy Conservation, Fairmont Press, LUbum, Ga., 1989. 

Example 3. A centrifugal pump moving a corrosive Hquid is known to have a time-to-failure that is well approximated by a normal distribution with a mean of 1400 h and a standard deviation of 120 h. A particular pump has been in operation for 1080 h. In order to plan maintenance activities the chances of the pump surviving the next 48 h must be deterrnined. 

Effective and well planned maintenance can reduce the operating cost of your pumps and other ec]uipment as this example demonstrates. With differential pressure gauges on the pump, an amp meter and flow meter you can determine if strict tolerance parts arc worn. This indicates the need to take the pump into the shop for corrective procedures. If you don t do it, you are wasting your annual operating budget. As we mentioned in Chapter 6, the Wear Rings should be called Efficiency Rings. Now you know why. 

The benefits of total performance based planned maintenance not only ensure the best and lowest cost maintenance program but also that the plant is operated at its most efficient point. An important supplementary effect is that the plant will be operating consistently within its environmental constraints. 

Maintenance costs can be minimized by, and are directly related to, good operation likewise, better operating results can be obtained when the equipment is under the control of a planned maintenance program. Improper operation of mechanical equipment can be as much or more the cause of its deterioration and failure as is actual, normal mechanical wear. Thus, operation and maintenance go together. 

S = Scheduled outage hours for planned maintenance F= Forced outage hours or unplanned outage due to repair... 

Figure 21-7 shows the time savings one may obtain by the proper use of borescopic inspection for planned maintenance. 

Figure 21-7. Effect on planned maintenance with usage of horoscope.

Procedures governing the decommissioning of plant prior to planned maintenance... 

Planned maintenance is maintenance carried out with forethought as to what is to be checked, adjusted, replaced, etc. 

Most combustion equipment is not controlled by means of a feedback from flue gas analysis but is preset at the time of commissioning and preferably checked and reset at intervals as part of a planned maintenance schedule. It is difficult to set the burner for optimum efficiency at all firing rates and some compromise is necessary, depending on the control valves used and the control mode (e.g. on/off, fully modulating, etc.). 

Planned maintenance programs are an essential weapon in a department s armory to ensure that the services it is called on in meeting its responsibilities are fully met. The traditional method of working from pieces of paper or individuals own notebooks as to when maintenance is to be carried out or when the insurance representative is due to visit to carry out an inspection are no longer satisfactory. This is especially the case when the skilled resources necessary to carry out the work are more difficult to obtain. 

It is therefore essential that a planned maintenance program be established, which can encompass all (or elements of) the different maintenance methods of establishing the frequency and/or work to be carried out. This programming requires skills that, in most instances, can have only been gained by experience in the field of maintenance and operation. Operation must play an important part in the programming. If the planned maintenance program is prepared without due consideration of the demands placed on the operation element, the program would probably collapse when the plant s equipment could not be released (i.e. switched off) when the maintenance technician arrived at the plant to carry out his duties indicated on his work docket. 

Maintenance procedures that should be considered when preparing the planned maintenance program include ... 

The benefits to be accrued from the implementation of a program of planned maintenance can be found in the efficient and economical operation of the plant and equipment and the utilization of resources (i.e. plant and equipment and manpower) while also maintaining a sound standard of safe working and environmental conditions for operators, other occupants and employees within the workplace. Maintenance systems vary, depending on the location of the plant and equipment and/or company policy. Systems can range from the complete maintenance of plant and equipment using all available methods to their replacement on failure. To meet the company s requirements it is then necessary to decide on the maintenance system that provides the most satisfactory benefits overall. 

Planned maintenance is work having benefited from information issued by manufacturers and suppliers, the experience and knowledge of the service department staff, and reports and records from previous service visits. 

As planned maintenance encompasses all types covered within the preventive or scheduled systems, this can be examined in more detail. In preparing a planned maintenance system, all the available sources of information should be used. These include manufacturers and suppliers literature, trade associations, professional institutions, knowledge and experience from within the department and history and feedback from previous work for the specific type of equipment. 

Condition monitoring, life cycle costing and predictive maintenance procedures should all be considered during the preparation of the planned maintenance system. 

In preparing a planned maintenance system the opportunity should be taken to involve the whole department. This can be achieved by using the operatives who will subsequently action the work to carry out the initial survey of plant and equipment. 

The planned maintenance program forms the basis of a system whereby an in-house department may prepare and implement its own maintenance program or introduce a trial system along similar lines. This would be prior to seeking the assistance of a consultant to provide guidance on the system that would satisfy the demands placed on the department by others and yet remain under the department s control. Irrespective of whether it is a manual or a computer system that is introduced, the elements of its... 

To assist engineers to implement a manual planned maintenance system that can be of benefit to the department it is important that a program be set with respect to the system s implementation. Items that should be considered are ... 

It should be noted that initial interest in the preparation and implementation of planned maintenance systems can gradually decrease if only one person (preferably an experienced engineer) is given the responsibility to ensure that the proposal is carried out to its satisfactory conclusion. It is essential that the nominated person is given adequate support when necessary, to ensure that the planned maintenance system s introduction into the working operation of the department meets with success. 

A suitable planned maintenance system, irrespective of the location or type of business, is complied from a number of standard elements (see the appendix at the end of this chapter) ... 

Each item of plant and equipment is allocated a specific asset number. This number can be either for a complete boiler (with associated equipment) or a specific asset number for the boiler and individual asset numbers for the associated equipment. It is advisable to restrict this numbering sequence to a minimum while ensuring that it meets the specific needs of the company and location. Care must be exercised in determining the asset numbering during this manual phase if it is envisaged that, on completing a satisfactory trial period, the planned maintenance system will be transferred onto a computer. 

To reduce the amount of information entered in the record, predetermined work of a planned maintenance nature can be entered in code form, while additional work or breakdowns may be more fully detailed. 

From this method of entry, the engineer can observe at a glance if the planned maintenance program is effective... 

Plant and equipment that require a service on completion of a certain specified period of hours may, through experience, be catered for on a fixed frequency basis. If employed, this method avoids the need to record running hours (or, in the case of transport, mileage) on a daily or weekly basis to schedule the relevant planned maintenance. If it is essential that the maintenance of the plant and equipment be carried out on the completion of a certain number of operating hours or mileage, then this must be allowed for in the allocated work hours of the relevant trade group. 

Peaks and troughs in the man-hours allocated weekly for planned maintenance can be avoided if hours entered are added up for each week after a number of assets have been scheduled. Treating each plant room or department in this way provides the number of man-hours required for the respective plant room and/or department. All assets to be covered by the planned maintenance system are scheduled in the same format, thereby providing the engineer responsible for allocating the work tasks with total manhours for each trade group. 

The actual hours necessary to carry out the planned maintenance work task ... 

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