Prevention and cure

    In the cutthroat energy market, onerous demands are being made on plant in order for power utility companies to survive. Tactics and techniques to reduce costs and minimize downtime are ever more essential.

    Clive Sanders, RWE Power International, UK

    With its roots as part of the UK state owned Central Electricity Generating Board (CEGB), RWE npower has a history of plant engineering expertise to draw upon. This knowledge is now marketed under the banner of RWE Power International and is supplemented through collaboration with fellow RWE Group company, RWE Power. This combination of dedicated maintenance engineers and specialists from across the power industry, enables RWE to provide services to diagnose specific problems within a plant, devise solutions and implement them successfully with minimum impact on plant operation.

    The objective of RWE npower is to maximize the life and reliability of its plant. To achieve this a number of methods have been established for use within the company’s fleet, ranging from management level reviews to the use of sophisticated diagnostic and condition monitoring tools and enhanced repair techniques. By implementing effective maintenance programmes to optimise reliability both the cost and duration of maintenance activities can be minimized while ensuring high standards of plant safety. The company’s approach is to examine what is happening to its plant, why it is happening and to develop the most rewarding way forward.

    Figure 1. A plant’s ‘cradle to grave’ curve. Early detection of deterioration can result in large savings compared with realisation and repair later in the plant’s life
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    The outcome can be a significant saving on replacement costs, as well as a reduction in downtime or major improvements in efficiency and availability. An overall strategy that extends the life and adequately maintains a safe, flexible and reliable plant is the overall target. Routine maintenance will always be required and the use of enhanced test and repair techniques help manage major service overhauls within programme.

    From an asset management point of view, the main requirement is to maximize a plant’s profitability over its residual life. There are clear commercial drivers to avoid excessive elective maintenance and the associated cost in both its execution and downtime or the similar consequences of under maintenance. The ideal situation is to manage the overall condition of the plant with optimal maintenance, refurbishment and investment.

    Management tools

    The path that allows adequate but not excessive through life maintenance of a generating asset is not an easy one to navigate. PLUS (Plant Life Usage System) is a successful management tool developed by RWE npower, which looks at the power plant asset degradation that arises from normal operation and maintenance. The prime objective is to evaluate residual life or asset value, which are closely related, and also the rate at which that life or asset value is being used. Engineering costs which comprise maintenance, overhaul and investment spend are evaluated along with the reduction in asset value due to plant usage.

    Figure 2. The OMSP model
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    A key driver is availability, so the next link that can be put in place is to establish a relationship between plant condition, plant reliability and availability. In nearly all cases plant condition depends on the historic expenditure on the plant and the past usage regime as evaluated by the PLUS process. RWE npower’s asset model OMSP (Optimum Maintenance Spend Process) then performs an analysis on these interactions to provide an indication of the future investment required in order to achieve life expectancy and operational availability targets.

    Information systems

    Both PLUS and OMSP are used within the RWE npower fleet and also by a number of international clients and while they do not necessarily deliver immediate maintenance savings, the life-long opportunity of the plant to generate income is maximized.

    To further assist in the day to day operating and maintenance of plant, RWE npower has developed information systems and operating procedures that improve the management of reliability, taking in the overall picture of revenues and costs of operation. Information systems installed provide both operations and trading teams with the information they need to make day-to-day decisions that best suit the business.

    Operational plant methods

    Effective reporting of impending damage or failure of operational plant makes it possible to mitigate a forced overhaul. Analysis can be carried out through plant data being observed by specialist staff. The latest approach is for the analysis of such data to be undertaken using PlantDoctor software. This is contained within plant data acquisition systems and is capable of drawing the attention of corporate specialists within headquarters to complex interactive events or slow moving changes that might normally go unnoticed. Traditionally, condition based monitoring of power plants was generally performed at the site, although this allows centralized monitoring by specialist engineers. The Internet has also ensured there are no boundaries for remote support and RWE npower currently monitors over 70 turbines around the world in this way.

    Figure 3. In-situ end ring inspection overview
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    The PlantDoctor technique prevented a major failure on a 520 MW generator within the RWE npower fleet. Failures on a sister machine were identified with potential cracks developing on the generator rotor. Due to commercial constraints, the plant remained in operation although ideally the repair would have required a planned outage. RWE npower therefore took an informed decision to continue operation of the unit in the knowledge that the crack would be detected by means of specialist remote monitoring, prior to the crack growth reaching a critical size. The machine continued to run safely for a number of months without a forced outage, this resulted in considerable cost savings.

    A slow vibration rise that would normally go unnoticed was detected on a 9F gas turbine compressor. The machine was taken out of service immediately and investigation revealed a cracked compressor tie bolt. This action avoided what would have most certainly been a catastrophic failure.

    Both of these impending problems were detected by the PlantDoctor event engine, which prompted specialists to investigate further.

    Figure 4. Photomacrograph of ‘cold’ weld repair on a 57 mm thick boiler shell
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    Over time, RWE npower has also developed extensive knowledge of creep and fatigue mechanisms allowing the life of boiler headers to be extended. Through this knowledge, appropriate management policies have been put in place with due regard to statutory requirements. These allow for boiler header life extension and mitigate the need for replacement at the end of original design life. A key to the success of this approach is again the use of sophisticated monitoring methods and in this case a method was developed using the same PlantDoctor system to monitor positive steam temperature excursions or any quenching (rapid decrease in steam temperature) that might occur while also recording the time spent within defined temperature bands. Display data is returned to the plant operator in the form of a ‘traffic light’ display, allowing easy visual assimilation of plant condition.

    Plant overhaul

    Elective plant maintenance is carried out during agreed timeframes and it is important to minimize the loss of revenue by overrun and expensive unexpected repairs. Unexpected repairs following an overhaul usually have an adverse effect on either or both the cost and duration.

    During an overhaul period it is important that inspection and test activities are managed in a way that allows the maximum amount of work to be carried out within the minimum of time and with the least possible disruption. Non-destructive test methods that are used for carrying out assessments must be optimal. RWE npower has developed techniques that allow rapid automated testing to be carried out, such as in-situ inspection of generator rotor end rings. End ring inspection is traditionally carried out by removing the generator rotor, a process itself that is not without risk and best avoided if it is at all possible. By carrying out an in-situ inspection, a timesaving of over a week can be achieved.

    Physical inspection of boiler internals are benefiting from advanced non-destructive test techniques, the thickness assessment of boiler tubing is now being carried out using electromagnetic acoustic transducers (EMAT). The EMAT transducers allow for the rapid and accurate assessment of true tube wall thickness without the need for extensive external surface preparation prior to testing; this saves a considerable amount of pre test preparation time and also allows for the inspection to be carried out far quicker than by conventional means.

    Figure 5. Orbital temper-bead weld on an 80 mm thick A508 Cl 2 nozzle
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    In the event of repairs being required, cold weld repair techniques (avoiding extensive post weld heat treatment) have been used successfully. This type of repair has generally been applied when a conventional repair involving post-weld heat treatment was impractical or the penalties of time and cost for conventional repair were sufficiently high. A typical cold weld repair has historically involved low alloy ferritic steel components welded with nickel based MMA consumables. Until recently an alternative cold weld repair method using matching ferritic weld metal, although available, has been used less frequently. However the increasing use of flux-cored arc welding for conventional power plant repair, and the development of cored wires providing suitable mechanical properties, has led to renewed interest in the ferritic option. In the last four years RWE npower, in collaboration with Mitsui Babcock, has carried out an increasing number of ferritic flux-cored cold welds on a variety of applications, including the full repair of main steam butt welds.

    Christmas bonus

    Innovative on-site machining techniques can deliver significant savings. An example of this followed an event that occurred on Christmas Eve 2003 when one of the four 500 MW main steam turbines at the Didcot A plant was shut down due to high vibration levels. RWE npower used its site machining and repair techniques to great effect, saving the company over seven weeks of lost generation. On inspection, the tip of an LP blade was found to have failed. The type of repair normally required would mean the LP rotors would have to be removed from site and returned to the OEM for further inspections. To replace the blades would have meant taking the machine out of service for over 12 weeks in what was the peak winter demand period.

    In order to return the unit to service in the shortest possible time, a team of turbine and materials specialists carried out inspections to assess the failure and also the extent of the damage. It was determined that a repair could be made on site without needing to remove the turbine LP rotor. RWE npower used its site machining and repair techniques to great effect, saving the company over seven weeks of lost generation. Close co-operation by our technical services staff and station engineers resulted in a workshop repair technique being adapted for the in-situ repair condition.

    The repair process involved removing the erosion damaged area by cutting off the blade tip and welding in place a new laser hardened tip under closely controlled conditions. The repair was fully validated for LP turbine operating conditions and was specifically designed to avoid the lengthy fitting process associated with pinned root blades. The repair requirements for the blade replacement were met and resulted in significant time and cost savings, with the whole repair process being completed within five weeks.

    Future plant maintenance

    In future, the use of innovative monitoring, assessment and repair techniques are going to become the accepted norm within power plants globally, due to the widespread liberalization of the energy market. “An integrated approach to safe operation, monitoring, inspection, repair and routine preventative maintenance, along with management review process, all play their part in the survival of a utility in the energy market,” says Mark Thomas, international business director, RWE Power International. Many of the large coal fired plants within the UK have operated outside of their original design conditions due to UK energy market conditions. However, the requirement to continue to operate safely, reliably and profitably remains key to survival, as do the commercial constraints placed upon generators. Something as simple as a properly conducted boiler air ingress survey can yield savings of as much as €700 000 in boiler losses on a large coal fired plant over a year.

    Figure 6. Innovative on-site machining techniques can deliver significant savings
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    Now more than ever, we should not be just looking at the instantaneous data presented to us from our plant systems but it is now of vital importance that this data is converted to useful information that can give a reliable view to the long term implications, relating to the longevity and availability of our generating assets. “It’s having the right systems in place and leveraging the best information from them that realises the ability for plant and personnel to respond rapidly and reliably to the varying levels of demand and price that keep us in the market,” adds Thomas.

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