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How to maximize availability
02-NOV-2005

Maintenance options for gas turbines

The availability of the gas turbine is crucial to the performance of a turbine-based cogeneration plant, and availability is a function of good, well-planned maintenance, writes Simon Raymond. A long-term service agreement, which transfers some financial risk to the service provider, can be the best option for the care of turbines of high capital cost.

Owning and operating a gas turbine is an expensive business. The principle of how a gas turbine works is simplicity itself, but putting that into practice requires mechanically complex pieces of equipment built with high-grade materials and a host of supporting systems that need to work in harmony.

Figure 1. The goals that a gas turbine operator and maintenance provider strive for

The inherent design of a gas turbine is naturally a major factor in how reliably it performs in service, but of at least equal importance is how the equipment is looked after, or maintained, while it is in service. Having invested a large amount of capital in a gas turbine power plant with all its ancillary equipment, the owner will want to maximize the return on that investment by having that power plant running at a high level of reliability while keeping running costs to a minimum.

The power advantages of a gas turbine over, for example, a like-sized diesel engine are offset to some extent by its need for routine maintenance and its relatively high servicing costs. Gas turbines do not forgive poor maintenance. It will cause them to stop functioning soon. The repair costs of a poorly maintained turbine can be frightening, to say nothing of the disruption to the owner’s operation. If a satisfactory balance between maintenance and cost can be found, then extraordinary reliability is achievable while preserving the owner’s profitability, as Figure 1 shows. But how is this maintenance carried out, and what maintenance philosophies exist?

A gas turbine’s power advantages are offset to some extent by its need for routine maintenance and its high servicing costs

The owner of a gas turbine power plant is not generally in the business of gas turbines so has neither the ability nor desire to perform the maintenance themselves. Even though some car owners prefer to service their cars themselves, the majority prefer to leave the job to a specialist. Yet if a car were bought for many millions of dollars, it is extremely unlikely that an owner would be carrying out the maintenance and servicing!

The car analogy falters when considering that driving a car can be a pleasure in itself. Operating an industrial gas turbine is not done for pleasure; it is a means to an end. The power that a gas turbine produces allows the owner-operator to carry on with its core business, be that automobile manufacture, chemical processing or foodstuff extraction.

Table 1. A typical gas turbine maintenance schedule
Interval	Action
Every 4000 running hours (engine remains installed)	Inspection of gas generator inlet
	Removal of low-pressure compressor casings to permit inspection of compressor stator and rotor
	Borescope inspection of high-pressure compressor, combustion section, turbines and integrity features
	Inspection of oil filters and chip detectors
	Inspection of engine’s exterior
Every 8000 running hours (engine remains installed)	As for 4000 hour action plus:
	Replacement (exchange) of high-pressure compressor stator assembly
	Refurbishment of low-pressure compressor vane assembly
Every 25,000 running hours (engine removed)	Replacement (exchange) of hot section module:
	combustion and high-pressure turbine modules
Every 50,000 running hours (engine removed)	Full overhaul and reconditioning of complete engine to return to an ‘as new’ standard

Maintenance, repair and overhaul (MRO) service providers, such as Volvo Aero, take on the challenge of maintaining gas turbines so that customers can focus on their core business. The more risk that a customer passes to its MRO service provider, the greater its peace of mind.

MAINTENANCE REQUIREMENTS

There are intrinsic maintenance requirements for any gas turbine. Dynamic and static parts in any machine do not last forever and will ultimately fail. Being able to predict failures and take the necessary action to prevent them is the basis of any maintenance philosophy. Thermal fatigue, cyclic fatigue, mechanical stress, erosion, corrosion and contamination are some common reasons necessitating an intervention to allow either assessment of damage (and hence remaining-life potential) or correction of defective components or both actions. The design and development testing of a new gas turbine defines initially how often and to what extent these interventions ought to take place. Service experience and observations made during the interventions further modifies the maintenance schedule.

Being able to predict failures and take action to prevent them is the basis of any maintenance philosophy

There are almost as many maintenance schedules as there are types of gas turbine. However, for the purposes of this article, a notional schedule for an aero-derivative gas turbine is shown in Table 1. Note that if any of the scheduled inspections reveal a defect, corrective action will be taken or an assessment made on whether the turbine can continue running safely until the next exposure of the defective part.

Figure 2. Simplified flowchart of a gas turbine overhaul process

In terms of expense, the full overhaul, in this example at 50,000 running hours, is by far the dominating event in an engine’s life cycle. During an overhaul, the turbine is stripped down to piece parts, cleaned and inspected before re-assembly and acceptance test, as Figure 2 shows. A technical decision is made on each inspected part. This leads to its being:

• acceptable for continued use
• not acceptable for continued use but within limits for repair
• not acceptable for continued use and not repairable, in other words ready to be scrapped.

A logistical evaluation is then made on unacceptable parts to:

• replace them with new parts
• repair them (if feasible) and re-use them
• replace them with used parts that have sufficient remaining life.

Material costs make up the largest part of the total cost for an overhaul, so these logistical decisions have a great bearing on offer price and profitability.

There are various maintenance agreements that can exist between a customer and an MRO service provider:

• Time and material (or call-out) agreement. Here, the customer pays for exactly the amount of time and material used for a specific maintenance action at a separately agreed rate per hour and parts price list.
• Event-based agreement. Here, fixed prices are agreed in advance for specific maintenance activities: 4000-hour inspection, 8000-hour inspection, even full overhaul. The cost for replacement material can be included or excluded.
• Long-term service agreement (LTSA). This is a form of partnership between the customer and supplier in which a fixed fee per running hour or calendar period is paid throughout a complete life cycle. No separate charges are made for specific maintenance activities.

 

There are three main levels of LTSA:

• Level 1: All scheduled maintenance is included in a fee per running hour or month (typically). In the schedule shown in Table 1, all the work described could be included in a fixed fee per month. Any unscheduled maintenance would be charged separately.
• Level 2: As for level 1 but also including all unscheduled maintenance.
• Level 3: As for level 1 but also including all unscheduled maintenance and guaranteeing a minimum level of availability. This is often achieved by the provision of one or more reserve engines that can be used by the customer while their own engine is undergoing unplanned maintenance back at the supplier’s workshop. Availability is defined by the actual running hours divided by the potential running hours that could have been achieved over a given duration. If a gas turbine plant operates all of its potential hours for a given period, then availability is 100%.

Figure 3. Risk share between owner and maintenance provider for differing levels of maintenance contract

Figure 3 shows that much risk is transferred to the service provider when level 2 or level 3 LTSAs are in place. These are an industrial engine equivalent of the all-inclusive rate-perflying- hour aero engine agreements popular with airlines that are looking for a stable cash flow. These LTSAs let the customer know precisely what their financial outgoings will be during an engine’s life cycle, irrespective of any breakdowns and unplanned maintenance. The service provider uses their experience and forecasting to assess and provide for a likely amount of unscheduled maintenance during an engine’s life cycle. This provides an incentive to maximize reliability so that both the supplier and customer share a common goal.

PRACTICAL SOLUTIONS

There is an increasing trend for gas turbine users to seek a single service provider for maintenance of not just their gas turbine but also the related gearboxes, alternators, control systems, valves, fire and gas protection equipment, heat exchangers and so on. Having a single point of contact for customers to turn to, whatever the problem, reduces administration time and expense for the customer. This has led to the emergence of the so-called multi-service provider, which bundles support options into packages. Volvo Aero, for example, is a multi-service provider although its core business is design, manufacture and maintenance of aerospace and industrial engines. For related services, a network of specialist subsuppliers is used. Volvo Aero then has the responsibility to call on these sub-suppliers as required. Many gas turbine operators are running their power plants around the clock and depend wholly on the power and heat produced for their own production lines. Therefore, having a maintenance contract with a guaranteed level of availability is often an attractive option. Living up to that availability guarantee is something the maintenance provider has to plan carefully for.

Having a field service engineering team close to the customer’s site is one of the most important success factors in achieving a high level of availability. The ability to have an engineer on-site to diagnose and rectify technical problems at short notice is crucial when the difference between achieving and failing an availability guarantee is a matter of a few hours. An infrastructure must exist to ensure that the maintenance provider learns quickly of any problems that arise and has the means to act on them.

The conventional 24-hour hotline between the customer and supplier can be enhanced by remote monitoring equipment that allows the maintenance provider to check in real time on a number of power plant parameters from a remote terminal or PC. Self-diagnosis software and the ability to modify control systems from afar is another time-saving step. The more accurate the fault-diagnosis, the more chance a field service engineer has of being able to rectify problems quickly. Tooling and spare parts also need to be available off-the-shelf. This ties up an amount of capital. If technical problems arise that are not possible or too time consuming to rectify on-site, the entire gas turbine is replaced to allow the customer’s production to continue while those problems are dealt with off-site.

Small gas turbines in CHP applications Figure A. Layout of a small cogen gas turbine facility Small gas turbines are used around the world to provide continuous heat and power in many applications. Figure A shows the typical layout of a cogen package, in this case Volvo Aero’s VT4400DLE, which produces around 4.4 MW of electricity and 10 tonnes of steam per hour. Small gas turbines are normally used to provide heat and power to an industrial facility. However, like their larger cousins, electricity produced can also be fed into a national grid or sold on. Öresundskraft’s 600 kW cogen plant in Sweden An industrial user with its own cogeneration unit enjoys the advantages of high power availability (dual supply sources), few power losses due to the short length of transmission and the reduced transformation. Emissions and environmental impact are low, more so with a dry low emission configuration as shown. There are also a number of fuel options, from natural gas to liquid fuels and gas from processed waste and sewage. A reference 600 kW cogen plant operated by Öresundskraft in Helsingborg, Sweden, produces power for 1200 households and heat for 600 households using biogas from recycled household waste. Daniel Thwaites Brewery in Blackburn, UK, uses a similar power plant to provide its brewing process with power, as do UK hospitals and local authority communal housing estates. Thames Valley Power’s 15 MW cogen plant at Heathrow Airport near London produces steam to heat the Terminal 4 cargo building and electricity for the airport as a whole. Power not used by the airport is sold to the national grid.

Thankfully, it is rare for gas turbines to fail spontaneously. There are nearly always some warning signs that enable an turbine’s ultimate failure to function to be predicted. Trend monitoring is hence an important part of a maintenance provider’s work. A number of parameters are monitored over time: oil consumption, oil-borne contaminants, temperature and speed for a given power rating, specific fuel consumption, exhaust emissions, vibration levels and the amounts of wear identified during scheduled borescope inspections, among others. These are all indicators of a gas turbine’s health at any given point in time. Being able to interpret and assess these parameters and take the appropriate action, if deemed necessary, is a key factor in how reliable (and thus available) an individual gas turbine will be.

It is rare for gas turbines to fail spontaneously. There are nearly always some warning signs

Planning maintenance interventions to coincide with scheduled plant shutdowns is a simple way of improving availability. If a plant is not in use at weekends or overnight, for example, it is logical to perform maintenance actions during these periods rather than interrupting production with a requirement for maintenance. This requires a close liaison between customer and maintenance provider to schedule work to the best of both parties’ interests.

LEARNING FROM EXPERIENCE

To take on the financial risk of unplanned failures in a gas turbine power plant, or even offer fixed prices for specific events, requires a high level of confidence and technical competence.

A mature engine with a proven track record of reliability and consistent costs is an ideal candidate for an all-inclusive level 3 type of contract. A new engine model with less predictable maintenance costs may be run at a lower level of contract for a number of years to gain the experience to allow unplanned arisings to be included in the maintenance price, if that price is to be realistic. Furthermore, an experienced maintenance provider has the flexibility to adjust the work content during scheduled maintenance to increase engine reliability and prolong engine life. If the timing of the full overhaul at the end of an engine’s life is not stipulated as a finite limit by the original equipment manufacturer, then there is scope for the overhaul to be delayed by up to several years. Cost drivers, like the expense of an overhaul, can then be spread over a longer period. Again, a focus area for a company like Volvo Aero is to optimize maintenance costs while preserving engine integrity and reliability.

Naturally, the more LTSAs that a maintenance provider has, the more the risk of unplanned maintenance expense can be amortized over several contracts, thereby reducing risk and price.

REDUCING COSTS

We have seen that, with level 2 and 3 LTSAs, there is a financial incentive for the maintenance provider to attain as high a level of availability as possible for the gas turbine in question. Therefore, the extra costs of performing some work beyond a minimum work scope during maintenance activities can be justified. In the notional maintenance schedule in Table 1, the refurbishment of the lowpressure compressor assembly is not an obligatory action and the gas turbine would surely continue to run for a period were this refurbishment not to be performed. However, the risk of a subsequent unplanned maintenance intervention to rectify a worn compressor assembly, with the possibility of consequential damage elsewhere, is deemed as high enough to warrant this non-obligatory additional work to be done, thus ensuring dependable service.

With the costs of replacement material making up a high proportion of the total cost of operating a gas turbine, the ability to repair worn and damaged parts rather than replacing them with new is a key factor in reducing costs. Successful maintenance providers focus on the development of repair schemes to salvage parts within the bounds of technical and economic viability. Many companies, large and small, offer specialized repair services such as:

• shot peening to improve wear resistance
• nickel and chrome plating to restore worn surfaces
• plasma spray to restore sealing features
• welding and brazing of fabricated components
• heat treatment to restore material properties
• thermal barrier coatings to improve heat resistance.

Many of the above principles have been applied at a 15 MW gas turbine cogen plant at Heathrow Airport near London, UK, operated by Thames Valley Power, which signed an LTSA with the Volvo Aero Corporation in 2003. Before the implementation of an LTSA, an annual availability of some 84% was being achieved at the plant. During the year to July 2005, the plant availability was 99.4%. This is living proof that a smart maintenance philosophy with close liaison between customer and supplier gives results.

99.4% availability provides living proof that close liaison between customer and supplier gives results

Volvo Aero is succeeding similarly on several maintenance agreements elsewhere while remembering that a maintenance philosophy is never complete but forever being fine-tuned in the quest to balance cost, risk and availability.

Even today, the gas turbine is not a completely mature product. Its future potential to produce more power by running faster and hotter – while running quieter, more fuel efficiently and with less impact on the environment – remains large as material and technological advances continue to be made. In parallel, maintenance philosophies will also have to advance to keep up with the demands for reducing operating costs and improving reliability.

Simon Raymond is Marketing and Sales Manager with Engine Services at the Volvo Aero Corporation, Trollhättan, Sweden, a maintenance provider for industrial gas turbines rated at up to 15 MW.
e-mail: simon.raymond@volvo.com

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