Cogeneration plant needs careful planned maintenance, perhaps more so than single cycle plant, as it is often more complex. What are the possible issues and approaches? James Hunt finds out.

CHP plant maintenance is generally more expensive than for single cycle power generation plant and can cost 30% of the gross cost savings that CHP delivers. Therefore a careful cost comparison and assessment is essential.

There are two main ways of arranging such on-site service and maintenance. One is with the original equipment manufacturer (OEM) or supplier, and the second is with a specialist third party organization. Sometimes, such a third party may be an Energy Service Company (ESCO). This delivers energy services and efficiency programmes in a user’s facility and accepts some technical and financial risk. Payment is based on meeting quality performance standards and/or energy efficiency improvements. An ESCO will also deliver plant repair and maintenance over the contract period.

Rolls-Royce provides maintenance services for its engines and other equipment
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Maintenance warranty contracts require that a monthly fee be paid by the plant owner; that is, in effect, a continuous full protection plan for engine and generation. The purpose is to achieve immediate, fast (typically eight-hour) response times by service personnel in repairing failed plant.

S&M and O&M

Service and maintenance (S&M) agreements are designed to obtain reliable long-term availability and performance from plant equipment using a third party or the OEM. Operation and maintenance (O&M) agreements, on the other hand, may include the day-to-day operation of the entire plant. This leaves the CHP plant’s on-site technical team to do what they do best.

S&M covers CHP equipment rather than the whole plant. Most on-site power generation plant owners contract out S&M simply because they have to – most don’t have the sheer expertise necessary to carry out routine maintenance tasks on reciprocating engine genset, heat exchangers, boilers, etc., let alone major repair or replacement operations. Gas turbines are definitely the province of the professional. In such cases, hosts often contract to buy maintenance services from the OEM or specialist third party contractor for several years at least. Therefore it is usually only a question of whether to stay with the OEM or to use a third party.

The S&M business is becoming highly profitable, often more so than merely supplying equipment, and the role of independent S&M providers in Europe is increasing, especially where the larger independent power producers (IPPs) and utility-sized plants are concerned. Indeed, such operations can generate 40% or more of a engine/genset and plant OEM’s revenue.

S&M costs are generally about equal to the capital cost of the power plant over 10-15 years. Including spares, oil and labour, the costs can be 125%-150% of the capital costs of a cogeneration unit. Contracting out can often be cheaper than a plant operator making the investment in the required skills and equipment.

An O&M process, on the other hand, will help maximize the plant’s overall profitability by ensuring that operational efficiency is at its best while the plant operates only to the degree necessary for the intended function. Maintenance operations, performed systematically, greatly improve plant reliability, reduce equipment degradation and maximize energy efficiency and shareholder value.

Then there are the ESCOs. In Europe, Dalkia, for example, operates and maintains CHP plant on land provided by the host organization, selling its output to the host at a price discounted from market rates. This means that the host company may benefit from obtaining the CHP plant at no capital cost. The supplier recovers the cost over time, by taking a proportion of resulting energy savings in a long-term energy sales agreement. Dalkia can provide operation and maintenance of CHP plant systems on a mobile remote basis. It supplies all the fuel and sets up the electricity export agreement contract to optimize export revenue and reduce electricity import costs.

Inspection of an engine made by Wärtsilä, which believes that conventional scheduled maintenance is no longer sufficient
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Because of increasing plant complexity, the optimal solution for a CHP plant owner may be to contract an experienced and established OEM. If the owner prefers self-operation, it becomes even more important to involve the OEM on a long-term basis because of the complicated solutions installed today. According to plant and engine manufacturer Wärtsilä, it is becoming difficult for third parties to provide proper service of equipment.

The service

Whatever type of maintenance service provider is actually chosen, the service itself typically offers a full after-sales service that provides monitoring and maintenance on all CHP systems supplied – tailored to each customer’s precise needs. Most plant OEMs offer remote 24-hour monitoring operating in the host country and others. Often, automatic phone dialers will contact service personnel for immediate service even before the owner is aware of a problem. Monthly reports show the electrical and heat loads and how they are performing. The customer can, therefore, concentrate on core activities.

Inclusive five-year (sometimes up to 20-year) service/maintenance contracts provide full service for scheduled and un-scheduled outages. The required engineering and spare parts are usually included, as may be guarantees for plant availability and performance. Penalty schemes mean that the maintenance contractor pays the bill if it fails to respond to an emergency within the agreed-on time period.

Such contracts are generally tailored to larger plants, but for smaller packaged plant, OEMs often suggest that customers opt for a largely ‘fit and forget’ CHP unit in the boiler house. This is monitored remotely by the OEM’s own engineers back at HQ.

A complete O&M contract, including site-wide facilities, provides many benefits. Dalkia, for example, can provide plant operation and maintenance services; mechanical/electrical maintenance; computerized maintenance management systems; central plant operations, statutory inspections; forward maintenance planning; a reactive response service; and much more besides. The company’s 1500 multi-skilled engineers control and maintain vital services for thousands of customers right across the UK, 24-7-365.

Once a maintenance contractor has been selected, it is essential to establish clear lines of communication, feedback and protocols regarding emergency service, after-hours service and regular planned maintenance visits. For short notice unplanned maintenance, a maintenance staff rota system and 24-hour helpline guarantee site attendance to customers within a few hours of a call. Maintenance scheduling can be problematic, so careful planning is crucial. The contract should also clearly state grounds for cancellation.

A maintenance service plan should require that the contractor carry out work based on the plant owner’s needs and the plant design documentation. It should contain an equipment list and the operating and maintenance tasks for all equipment, along with the service schedule. A list of important data to be tracked over time should be drawn up – performance, fuel consumption (a measure of engine thermal efficiency), exhaust emissions, all temperatures and pressures, electrical and thermal parameters, and chiller performance (where trigeneration plant is involved).

Maintenance contractors should ensure that periodic instrument calibration checks are carried out by independent test establishments or government agencies. For the all-important diagnostics, contractors must be expert in measuring the various parameters that indicate correct plant operation.

In addition, a customer should ask to see the contractor’s safety records. At least minimum liability insurance and proof of coverage should be provided.

Remote monitoring

CHP plants, with their many systems, need careful condition monitoring to get the best out of them. Predictive and periodic maintenance techniques are essential to obtain best service and life. An excellent way of achieving this is to use remote monitoring techniques. These are highly cost-effective for organizations wanting to re-deploy resources and interrogate unmanned facilities by downloading data to a central site.

Also, power plant is increasingly sited remotely and is, therefore, helping to bring the concept of remote monitoring into its own. It is not cost-effective to have engineers stay for hours in-plant to observe operating strategies. Today, equipment can be checked from the comfort of a cosy office, sometimes hundreds of kilometres away.

Remote monitoring systems typically employ:

  • simple hard-wired fieldbus systems for plant-wide applications
  • wireless systems such as radio frequency or GSM telemetry for communication that is more remote
  • the internet for global communication.

Such remote monitoring covers many layers, including supervisory control and data acquisition (SCADA), condition, environmental and equipment monitoring, communication/ networking, industrial control and automation, telemetry and device equipment management. The data obtained can be interfaced to other systems so that, for example, a desktop personal computer in a Paris office could control a CHP plant in Africa.

Very sophisticated software-oriented, non-intrusive, on-line signal processing and analysis techniques are available so that plant operation can continue even while investigation is taking place. These rely on state-of-the-art signal processing and analytical techniques, and they alleviate the need for costly purpose-built monitoring systems.

However, to obtain maximum benefit requires both the operations and maintenance teams to agree on the information to be gathered and the shared budget to achieve this. Remote monitoring has mainly been used for control and monitoring purposes in the past. Asset management functions require additional data to drive methodologies based on optimization and condition, and have often not been included in the initial budgets in the past. This may be slowly changing.

Maintenance of reciprocating engines

Today’s reciprocating engines are highly reliable, but faults will occur, and they need to be diagnosed and repaired quickly. Any engine coolant temperature rise, worsening oil or fuel consumption, or increasing exhaust emissions are all pretty safe indicators of engine problems in the making. Regular periodic and predictive maintenance is, therefore, key to reliable running. Such work can be carried out by S&M contractors, but engine OEMs usually offer their own maintenance services. MTU (a core brand of Tognum AG), for example, has a ‘flexible maintenance concept’. This matches maintenance intervals optimally to the application and improves availability while reducing operating and maintenance costs. It also allows accurate estimates of maintenance intervals.

MTU’s (Tognum AG) ‘flexible maintenance concept’ matches maintenance intervals optimally to the application
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But what needs to be examined and why? Lean burn gas, diesel and multi-fuel engines all have the same basic mechanical parts and construction, so maintenance will be broadly similar for all, except for their fuel/air and control systems.

Annual checks must be made on all major engine components following installation. Once trends have been established, maintenance intervals can be extended but inspections should be frequent. Permanent maintenance records must be kept and all work should be tested to the required regulations.

Conventional maintenance duties will cover:

  • diesel engines – inspection/repair of fuel injection equipment (nozzles, pump governors, valves and drives)
  • gas engines – cleaning, repair and adjustment of gas admission, injection and metering valves; calibration of pneumatic proportioning equipment, flow regulators and transmitters
  • checking, replacement and fitting of mechanicals – bearings, timing gears, cylinders/pistons and valve gear;
  • replacement, cleaning and overhaul of engine subsystems – lube oil and water pumps, and turbocharger lubrication systems
  • checking shaft alignments
  • checking all fluid levels, engine safety systems, oil changes, no-load and service tests, plus consumables supply (usually excluding fuel).

However, some OEMs, such as Wärtsilä, believe that conventional scheduled maintenance is no longer sufficient in today’s very competitive energy markets, and that condition-based maintenance (CBM) is crucial in the drive to minimize rising fuel costs. Such programmes can simultaneously lengthen component life and reduce the number of unplanned stops and failures. While scheduled maintenance is safe, it is expensive. CBM, however, offers continuous and detailed monitoring of each installation’s real performance.

Wärtsilä’s system is based on automatic performance data collection, followed by remote analysis. The measured performance data is compared with calculated ‘ideal’ values, taking into account unique application conditions. Continuous plant performance follow-up, including regular feedback with predictions for the future and recommendations once a month, allows the right maintenance at the right time. Cost savings are typically 10%-20% over 30,000 operating hours, with fuel savings of up to 5%.

CBM includes inspections and operating data trending, plus engine signature analyses of abnormal hot spots, increasing vibration and noise or a change in frequency. These are all warning signs. A hot spot can be caused by a fast wearing bearing or reduced lube oil supply. Thermography, using infrared imaging cameras, can detect certain hot spots and is a very cost-effective diagnostics tool.

A thermographic image taken by a Flir infrared camera shows that components inside an electrical control cabinet are overheating
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Spectrometric analysis of engine lube oil samples may show iron, which indicates that wear debris is present from cylinder liners, connecting rods, piston rings, rocker arms or push rods. Chromium readings may suggest that the cylinder is wearing too fast as liners and piston rings are often chrome plated. Large amounts of tin or copper may indicate bearing wear. Silicon is usually contaminated with oil by airborne sand or dirt. In this way, a picture can be built up on how the engine is faring. Rectification can then be made in good time to prevent failure and also to preserve top performance, saving potentially large amounts of time and money.

Excess carbon monoxide (CO) is a product of incomplete combustion. The usual causes are fuel injection equipment problems or restricted air intake. Excess nitrogen oxides (NOx) are usually produced by excessive combustion temperatures and can be caused by incorrect timing or valve settings. Sulphur dioxide (SO2) derives from the sulphur content in certain diesel fuels. Excessive SO2 can be caused by poor fuel or by some of the above conditions. Diesel particulate matter (DPM) – a suspected human carcinogen – is also made worse by incomplete combustion, which may also cause a fall in power and a rise in fuel consumption. Carbon dioxide emissions are difficult to avoid with carbon-based fuels, but inefficient combustion will make them worse.

Perfect combustion is, therefore, crucial for good performance with low emissions. Modern analysis techniques allow good combustion to be maintained. For example, cylinder pressure waveform, compression ratio change detection (to less than 5% in a single cylinder), detection of small fuel injection pressure drops, plus any change in valve and combustion timing can all be accurately measured and analysed.

Servicing gas turbines

Gas turbines, found in many large CHP plants, are expensive, complex and need many support systems. These machines, and particularly their hot path components, have relatively high maintenance cost and short in-service inspection cycles compared with most other prime movers. Their on-site S&M needs are, therefore, more exacting, and gas turbine-based plants are usually more costly to maintain than their counterparts that employ reciprocating engines.

All gas turbine bladed components, such as this compressor from Siemens, need very careful inspection
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A maintenance agreement typically involves the provision of packaged services, including the supply of high quality parts, advanced technology component repair, field and engineering services. Volvo Aero Corporation is just one such company. It maintains a diverse range of gas turbines in CHP plant rated up to 15 MW right across the world. This means that it has gained much knowledge that saves its customers money.

Gas turbine parts that need regular inspection or continuous monitoring include all blades, including those in the compressor section, inlet guide vane stators, combustion and other hot gas path components, casings, fuel systems, interconnect shafts, turbine disc/rotor assemblies, and skid ancillaries.

In-service inspections are the most demanding, because the available access and inspection methods can be limited. Most such inspections use visual and surface techniques, but non-destructive testing can determine the depth and type of certain material flaws. Such techniques (dye penetrant, eddy current and ultrasonic testing) are especially valuable with inspections of hot-end components, such as blades and vanes. Typical faults include cracks, discontinuities, wear, corrosion, erosion, high strain and cooling channel blocking.

Special coatings protect modern gas turbine critical hot path components. Any growing defect here could easily result in the shutdown of the plant. However, such coated systems are not easily inspected, especially when ceramic thermal barrier coatings are used, so maintenance contractors in this sector have to be especially experienced.

In many other respects, servicing and maintaining gas turbines uses similar tools and techniques as reciprocating engines, and for similar reasons. For example, if a rise occurs in gas turbine temperatures or emissions, if power falls off, or if fuel consumption rises, operators can be sure that there is a fault somewhere.

Electrical equipment maintenance

Maintenance contractors must carry out all maintenance work necessary to ensure that a plant’s generator runs correctly over a long period, whatever its manufacturer and rating. Preventative maintenance is just as important here. Electrical inspections include the alternator, its windings (commutator, brushes and field windings if DC), rotor and laminations, bearings, lubrication and cooling systems, casing, framework/end-brackets/mountings, plus all auxiliaries such as the power distribution unit, any batteries and chargers.

Electrical equipment, from generators to control panels, such as this Rolls-Royce example, needs servicing too
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Genset audits, covering power calculation, number of generators, automatic system process, measurement parameters, etc should be carried out, load bank tests too. The contractor may also be required to maintain power distribution units and all electrical cabinets with or without their controllers.

Other equipment

Similar maintenance procedures are carried out on CHP plant package items, steam turbogenerators, control systems, heat exchangers/boilers and compressors, etc.

Companies, such as TJ Foster & Co, can carry out the following, either informally or as part of a maintenance contract: renewal of the heat exchanger/boiler tubes; welding repairs to steel pressure vessels, refractory repairs and renewals; descaling and the repair or renewal of boiler mountings and fittings. Hydraulic testing and certification must be carried out; ultrasonics tests should find material flaws. Also essential is the servicing and repair of oil or gas burners (where used), followed by a combustion analysis. All pipework must be inspected and renewed where necessary.

Note that with CHP plant running on certain biomass fuels, heat exchangers can foul very easily, badly reducing heat transfer. The optimal cleaning interval must, therefore, be especially carefully considered. The time taken is a function of the size and number of plates/tubes.


The jury is probably still out on whether S&M or O&M are the best methods of providing planned maintenance, and whether OEMs or third party contractors are most suitable. The answer depends on the needs of individual CHP plant, which vary widely. Although the trend towards outsourcing of services has not been universal – German power plant operators still apparently prefer in-house operations – it is now fairly certain that with CHP plant becoming more sophisticated and complex, the best plan for most plant owners is to contract out.

James Hunt writes on energy issues from the UK e-mail:

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