Making older turbines more competitive

21 February 1999

The pressure to maximise power plant profitability by continuous modernisation throughout plant life has increased in recent years. Options include uprating, changing the fuel, and life extension.

Older gas turbine power plants are facing more competitive market conditions, with increasing pressure to modernize. One driver, of course, has been deregulation of the electric power markets. Another is the dramatic improvement in performance available from new gas turbines. The past 25 years have seen unit capacities rise from 50 MWe to 250 MWe, simple-cycle efficiencies go from 28 per cent to 38 per cent, and unfired combined cycle plant efficiencies reach 58 per cent. With these new machines, which are used by the established power poducers as well as the IPPs, major inspection intervals are scheduled at 25 000 equivalent operating hours (EOH), while stringent emissions targets are met through such technologies as dry low-NOx hybrid burner technology.

The operator of the older gas turbine plant must consider the following kinds of question:

  • Which uprating options utilize the full potential of the gas turbine on the one hand and take account of its limitations on the other hand? What are the consequences for the entire plant in simple cycle or combined cycle operation mode?

  • How can upgrades be integrated into overhaul budgets and overhaul planning?

  • How can upgrades and modern maintenance strategies be used to reduce overhaul downtime and augment power production?

    Among modernization and rehabilitation options available for gas turbines now in service are life extension, including life assessment after an extended operating period, uprating options to boost performance and optimization of operation and maintenance. To meet a changing fuel market, fuel system upgrades are available to avoid dependency on only one fuel and to potentially achieve the benefit of a lower fuel price. These options are key to keeping the contribution made by fuel costs and O&M costs to the overall life cycle costs as low as possible.

    The options for older gas turbine power plants can be illustrated using the Siemens V94.2 gas turbine, but they may also be applicable to other older turbine models.

    Design evolution

    The efficiency and power output of the V94.2 have been improved in several steps by increasing the turbine inlet temperature and by many component upgrading measures. The power output has been increased from 90 MWe to 159 MWe, while the efficiency has been raised from 30.3 to 34.6 per cent. The V94.2 started with a turbine inlet temperature (ISO conditions) of 930°C and has now reached 1060°C for base load operation.

    There have been three main versions of the V94.2. One feature of the version II design was the use of free-standing turbine moving blades in row 4 instead of the damping pins of the original version I design. This resulted in increased component efficiency and improved maintainability. Consequently, parts adjacent to these blades such as disc number 4 and the stage 4 stationary blades have also been modified. With the disc type/central tie rod rotor design used for the V94.2 type, such a disc can be easily replaced on site during overhaul. One additional feature in the design change from version I to version II was the use of variable-pitch compressor inlet guide vanes in combination with an increase in the compressor mass flow instead of the fixed compressor inlet guide vanes used before.

    The design step from Version II to Version III included revising the design of the turbine flow path and the subsequent modernization of the stage 3 and stage 4 turbine blading. The cooling and sealing air system has been modified by the use of air extracted from the 13th compressor stage instead of compressor discharge air for cooling turbine rotor blades in stages two to four. The design step to Version III constitutes a significant improvement in the thermodynamic component performance. The stepwise increase of turbine inlet temperature for the V94.2 was achieved by upgrading hot-gas-path components through the use of stronger high-temperature base materials and coatings and enhanced cooling.

    Uprating options

    Hot gas path and compressor upgrade packages. Turning to units already in service, several hot-gas-path and compressor upgrade packages are available for uprating. For example uprating package U2 for the V94.2 Version I gas turbines:

  • boosts power output of the gas turbine in simple cycle operation by 8.8 per cent or 11.6 MWe and reduces fuel consumption by 1.0 per cent due to efficiency improvement;

  • boosts power output in combined cycle operation by 9.6 per cent or 19 MWe (one gas turbine and steam turbine proportionally) and reduces fuel consumption by 1.4 per cent (unfired combined cycle) due to efficiency improvement. This improvement contributes to the corresponding increase in exhaust energy which is efficiently used in the combined cycle.

    The U3 uprating package includes complete replacement of the four turbine blading stages plus compressor modifications. These measures increase the turbine component efficiency to that of a V94.2 Version III.

    Uprating proposals must make use of the full potential of a turbine, but at the same time take into account limitations such as compressor surge margins or those arising from mechanical components.

    As turbine blade replacements are carried out during overhauls, the additional replacement of turbine blades to meet the higher turbine inlet temperature could mean a modest additional investment for a specific major overhaul. The economic benefit of such a measure would quickly pay for this investment.

    In the combined cycle context, whenever the turbine outlet temperature, and thus the thermal conditions of the gas turbine exhaust, bypass stack and heat recovery steam generator are modified, the effects on the overall plant must be considered if the project is to be successful.

    Water/steam injection. Water or steam injection is another option for power augmentation. Water injection increases the mass flow and heat capacity of the combustion gas. The amount of water which can be injected into the combustor without disturbing the combustion process amounts to 160 per cent of the fuel mass flow for the V94.2 gas turbine. Injecting the maximum amount of water yields a power output increase of around 13 per cent for simple cycle operation on fuel gas. However, efficiency decreases by around 4 per cent because the heat of evaporation required by the water injected cannot be recovered. The water injection upgrade includes installation of water nozzles in the burners, a water injection package and the associated piping as well as a modified fuel/water system logic.

    Inlet cooling. The power output from a gas turbine is always the product of thermal expansion in the turbine and the mass flow. The ingested compressor mass flow decreases with increasing ambient temperature, because the density of air decreases. Therefore by cooling the compressor inlet air, it is possible to augment power output and achieve a small gain in simple cycle efficiency without affecting combined-cycle efficiency. There are two cooling methods available: direct chilling of inlet air by, for example, vapour compression chillers or evaporative cooling. While direct chilling is an expensive technology and requires significant modification of the air intake, evaporative cooling with a spray type nozzle can be more easily retrofitted. Using evaporative cooling of the intake air, the output increase can be high and may reach up to 15 per cent, if the ambient temperature is high and ambient humidity low.

    Fuel preheating. Fuel preheating (FPH) with the waste heat of the exhaust gas is a way to reduce fuel consumption. This is why FPH for natural gas is used with modern combined cycle triple-pressure reheat cycles. A temperature of about 200°C is typically used for natural gas. For older combined cycle plants with V94.2/V84.2 gas turbines, most heat recovery boilers use a double pressure cycle. This configuration allows a fuel preheating temperature of only about 100°C. Even with these restrictions, fuel consumption can be reduced by 0.4 per cent. This makes a fuel preheating retrofit that integrates a small heat exchanger into the fuel gas supply and into the steam–water cycle an attractive option.

    An overview of the available uprating options for gas turbine power plants is given in Table 4. The increase in turbine inlet temperature is the most generally applicable method to improve both power output and efficiency. However, water and steam injection, despite the inherent drawback of decreasing efficiency, are used primarily for meeting peak load demand.

    Changing fuels

    After years of gas turbine operation, the availability or quality of the original fuel source can deteriorate and a change of fuel may be appropriate. A lower fuel price and therefore reduced life cycle cost might sometimes be the incentive for a fuel system retrofit. In such a project it becomes necessary to adapt the fuel supply system together with the fuel control and safety system. The existing system might be completely replaced or expanded up to a dual-fuel system.

    Natural gas with a high calorific value (namely between 30 000 and 50 000 kJ/kg) and also light distillate fuel oil are used as standard fuels. For these fuels, the standard fuel system is the simplest. But in addition, operators may want to be able to deal with a wide range of fuel types, differing in terms of impurities, viscosities, flash points or calorific values.

    Fuel gas with a low calorific value is often a byproduct of chemical plants. Operating experience is available for heavy-duty gas turbines with fuels having calorific values as low as 10 000 kJ/kg. If appropriate provisions are made, these may involve modification of the variable-pitch compressor inlet guide vanes, the fuel supply and the control system, the use of such fuels is feasible.

    If crude oil or residual oil become available in large quantities, it can become worthwhile to modify the gas turbine to facilitate the firing of such fuels in addition to diesel or natural gas. However, the quality of these ash-forming heavy fuel oils normally makes it necessary to provide an additional fuel treatment system to prevent the build-up of deposits on the turbine blading and to prevent hot corrosion and erosion of the hot-gas-path parts. Appropriate fuel treatment is needed to remove salts from the fuel and involves the admixture of additives. Turbine washing may also become necessary as part of day-to-day maintenance and, to prevent glassing of ash deposits on the turbine blading, it is normally essential to derate the turbine inlet temperature relative to the turbine inlet temperature for gas firing. The gas turbine fuel oil supply system has to be modified to meet the viscosity requirements by preheating heavy fuel oil grades and by heating the fuel piping.

    The use of naphtha and certain types of kerosene requires special preventive safety measures. Due to the inherent hazards of accidents involving leakages of such volatile fluids, explosive gas mixtures must be prevented by provision of enclosed and ventilated areas for gas turbine components. But retrofit expenditures can be offset against lower fuel costs.

    In fuel diversification retrofit projects, a choice of low-NOx burner must also be made:

  • Dry low-NOx burners, which combine a diffusion natural gas burner or a diffusion fuel oil burner with a premix burner in a "hybrid burner" assembly. This is mainly of interest for continuous duty plants.

  • Wet low-NOx burners, which combine a diffusion natural gas burner or a diffusion fuel oil burner with a water or steam injection system. This is more applicable for peak load duty with augmented power.

    Upgrading operation and maintenance

    Progress has been made in optimization of operation and maintenance by the use of state-of-the-art diagnostic systems as well as new information and communication technologies. One example is the WIN_TS diagnostic system, which can be linked to the Siemens Teleperm system and can be retrofitted for continuous condition monitoring of the gas turbine. This system makes it possible to transfer operating data between the customer and manufacturer via communication networks for systematic performance improvement assistance or for trouble-shooting by experts.

    The outage time for major overhauls can be drastically reduced if repair work can be eliminated by adopting the philosophy "replace first/repair later". Examples of graded replacement package strategies available to achieve this goal include:

  • Coated turbine rotor and stator blades to bridge the refurbishment time;

  • Spare inner casings and mixing chambers to eliminate weld repair/heat treatment outage time;

  • Prebladed turbine stator blade carrier;

  • Spare rotor;

  • Spare rotor plus complete stationary blade assembly;

  • Auxiliary spare components, eg spare valves.

    These options can be used in various combinations to reduce outage time during major overhauls. In some cases, spare parts can be pooled with other operators.

    Intervals between major overhauls have been increased. The V94.2 hot-gas-path component upgrades described above not only increase the turbine inlet temperature significantly (from 930°C (or, in the case of a V94.0, even lower) to today's 1060°C) but in addition the major-inspection interval has been increased, from 16 000 equivalent operating hours (EOH) to 25 000 EOH and then to 33 000 EOH. This last step eliminates two major overhauls within a plant's life (which typically might be 200 000 EOH) and significantly reduces power production costs.

    A major factor in achieving these longer intervals has been use of M-CrAlY vacuum plasma spray coating, SICOAT 2231, as part of the upgrade measures. This has seen 39 574 EOH (168 starts, 36 179 hours at base load). While at the leading edge, the place with the highest thermal and mechanical loading, the coating has been largely consumed, a significant coating thickness

    (b phase), around 120 microns, is still available on the rest of the blade.

    Further upgrades with even more durable coatings including thermal barrier coatings and upgraded hot-gas-path component design will allow further extension of major overhaul intervals in the near future.

    Life extension

    While the useful life of the power plant may extend beyond 30 years, hot-gas-path components have a shorter life due to creep and low cycle fatigue. For example for the majority of hot-gas-path components in V93 and V94.2 gas turbines, life expectancy is 100 000 EOH, which may be accumulated over 10 or 12 years of continuous operation.

    The replacement of hot-gas-path components would be the main element of a life extension programme that might be scheduled at, say 100 000 EOH as part of a major overhaul. The life extension programme encompasses a life assessment of the basic gas turbine as well as of the auxiliaries and the balance of plant. The life assessment is prepared with the help of the previous major overhaul results and those of the most recent minor inspection. Within the life extension programme, specific checklists based on the design know-how and on the experience of the overall gas turbine fleet are provided by the turbine manufacturer.

    Table 5 gives an overview of the design criteria and life consumption effects for the basic gas turbine components.

    Life consumption of the turbine blading as well as of hot casings is due to erosion, high temperature oxidation and hot corrosion, by internal creep damage, low cycle fatigue and also high cycle fatigue.

    Gas turbine components not in the hot-gas-path such as compressor blading, rotor parts and various outer casings are affected by creep only very slightly or not at all. Provided that time-dependent degradation due to oxidation, corrosion or erosion and also cyclic life consumption is below the design limit, these gas turbine components are expected to remain usable throughout the entire service life of the power plant.

    During the assessment, rotor parts, casings and piping are visually inspected and some NDE (non-destructive examination) is done in regions with high stress concentrations. A disc type rotor design facilitates examination of discs and hollow shafts, as the rotor can easily be disassembled and reassembled in the field, while state-of-art NDE techniques are now capable of detecting smaller indications than was possible during the fabrication of these parts years ago.

    Life assessment and overhaul of auxiliaries, I&C and the balance of plant round out the life extension programme rendering the overall plant fit for further operation.

    In a life extension programme of the kind outlined above utilization of the original spares is envisaged, with restoration of the plant to its original condition, in terms of output, efficiency and life expectancy – ie no upgrades.

    However such a programme involves considerable investment due to replacement of hot-gas-path components, so it is worth considering uprating packages when replacing components which have reached the end of their service life, thereby raising turbine inlet temperature and thus output and efficiency. Because the additional investment will often pay for itself within a short time, most life extension projects also include turbine uprating as well as other modernization measures to improve plant profitability in an ever more competitive marketplace.

    Finally it should be emphasised that gas turbine life extension programmes, with or without upgrades, require extremely thorough preparation and a joint project management approach involving both customer and manufacturer.

    Table 1 Developments in V94.2 hot-gas-path components, over 20 years
    Table 2 Uprating packages for the V94.2 gas turbine
    Table 3 Gas turbine uprating and assessment of its effect on the overall combined cycle power plant
    Table 4 Evaluation of uprating options for gas turbine power plants
    Table 5 Design criteria and life consumption effects for basic turbine components

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