Assembly of Alstom’s GT26 gas turbine Source: Alstom

Amin Alamsi, lead rotating equipment engineer for WorleyParsons Services, explains the design considerations and manufacturing lessons that lead to the optimal power generation configuration for gas turbines.

Amin Alamsi, WorleyParsons, Australia

One of the greatest strengths of gas turbines is their versatility, being tolerant to a wide range of fuels, including liquids and gases with high and low heating values, as well as gasified coal, wood and biofuels. Furthermore, the latest technology has simplified the control of this highly responsive machine to govern their minute-to-minute operation, allow rapid starts and stops, while also reporting on their current and future health through diagnostic and prognostic techniques. Despite these advances, the gas turbine package and arrangement is often complex and compromises must be made in the design to allow for the most appropriate balance between operation, reliability, commerce and maintenance.

Growing sensitivity to environmental issues, higher fuel prices and expectations of high reliability present challenges to the power generation industry that mean finding the optimum gas turbine arrangement for a given situation is vitally important.

Heavy-frame vs Aeroderivative

The most significant differences between heavy industrial gas turbines and those that are based on aircraft technology, known as aeroderivative turbines, include weight, design of the combustor, turbine and bearing design, and the lubrication system. Heavy-frame industrial turbines operate at slower speeds, have higher air flow and require more space and time for maintenance.They also use hydrodynamic bearings, whereas anti-friction bearings are fitted in aeroderivative turbines, which are easier to maintain, more flexible, lighter weight and smaller. The latter’s modular design allows components to be removed and replaced without removing the gas turbine from its support mounts. The heavy industrial units, by contrast, require more effort to remove and replace components, especially combustor parts, or to inspect or repair the sections.

The heavy industrial turbines also consume more fuel and around 50 per cent more air than aeroderivative units, which means they are exposed to a greater volume of contaminants in the air that cause corrosion, especially sulfidation. While the more substantial area of blades and vanes of heavier turbines aggravates the threat of corrosion and contamination, they can also tolerate it more than the thin and high aspect ratio turbine blades of the aeroderivative turbines.

The requirements of a particular situation will dictate the choice of gas turbine. So far the preference has been to place aeroderivative units in remote applications, such as offshore, and to place heavy frame industrial units in easily accessible baseload applications.

Gas Turbine Configuration

In hot-end drive configuration, the output shaft is located at the end of the turbine where exhaust gas can reach high temperatures, which affects bearing operation and life. It is also a difficult part to service, as the assembly must be fitted through the exhaust duct. There are several constraints to this design. Insufficient attention to output shaft length, high temperature, exhaust duct turbulence, pressure drop and maintenance accessibility often results in power loss, vibration, shaft or coupling failure, and increased downtime for maintenance.

In the cold-end drive configuration, the output shaft extends out the front of the compressor. Here the electrical power generator is accessible, relatively easy to service, and only exposed to ambient temperatures. The drawback to this design is that the compressor inlet must be configured to accommodate the output shaft and the generator, or a gear unit if applicable.

This inlet duct must be turbulent free and provide uniform, vortex free flow throughout the operating speed range. A poor design can cause catastrophic problems. For example, inlet turbulence can induce surges in the compressor, resulting in complete destruction of the unit. Inlet duct turbulence is a major concern, however, it can often be eliminated at the expense of pressure drop.

Single spool integral output shaft gas turbines in both hot-end and cold-end drive configurations can be used to drive electric generators. These units can also use a split output shaft, which drives a free power turbine. The compressor and turbine component shaft in this set-up is not physically connected to the power turbine shaft, but is coupled aerodynamically, which makes it easier to start in fast response units. However, the single spool split output shaft turbine design is usually limited to hot-end drive configurations.

In dual spool split output shaft gas turbines, independent low and high-pressure compressors and turbines generate the hot gases that drive the free turbine. There are three shafts, each operating at different speeds, for higher power applications.

The total optimum over-rating is around 10–12 per cent, which takes into account a 4 per cent tolerance to meet the required power for the generator, 2 per cent loss for the gear box (if applicable), 2 per cent for fouling and erosion, and 4 per cent for the long-term deterioration of the gas turbine.

Rolls-Royce’s aeroderivative RB211-H63 gas engine Source: Rolls-Royce

Special care should be taken when selecting the starting device and rating. In this application, the preferred starting device is electro-hydraulic – an electric motor driving a hydraulic pump, which in turn transmits hydraulic power to start the gas turbine – rated to supply a minimum of 110 per cent of the starting and acceleration torque in the worst case scenario. Helper drivers, in contrast, are not recommended for this application. The gas turbine should be capable of an immediate hot start at any time after being tripped for three consecutive start attempts.

Cold-start and hot-start restrictions are very important and the igniters must not remain in the primary combustion zone during operation.

The blades must be able to start-up without rubbing, while renewable sealing components such as the labyrinths, honeycombs or abradable surfaces are required at all internal close-clearance points between the rotating and stationary parts, and all external points where shafts pass through the casings.

Maintaining the clearances is a particular problem in gas turbines because of changes in temperature between cold and accelerating conditions – stationary components expand at a different rate than rotating components. Some new designs use abradable seals to minimize these clearances, however the most severe case, which is usually after a re-start, will determine the minimum clearance.

The optimum gas turbine configuration has at least three successful references and a satisfactory record of continuous operation for a minimum of 30 000 fired hours without major damage to the compressor or turbine rotors (and their main components) or stationary parts in the combustion hot gas path.

The other main components, including the rotors, casings, bearing housings, supports and base-frame, should all have a minimum life expectancy of 160 000 operating hours or 20 years, where the time between starts is assumed to average 100 hours. Planned time between major overhauls, hot gas path inspection and borescope inspection are ideally around 40 000 fired hours or five years, 16 000 fired hours or two years, and 8000 fired hours or one year respectively.

Compressor and Combustor

Compressors are conventionally either an axial design of up to 19 stages or a centrifugal design that includes one or two impellers. In the axial configuration a beam and cantilever (when stage loading is light) style stator is used. An increase in compressor ratio is the prime contributor to the overall increase in simple cycle thermal efficiency to above 35 per cent, particularly in aeroderivative turbines.

The combustor is one of the most complex components to design. It assumed two distinct configurations fairly early on in the evolution of the gas turbine: the can-annular combustor and the annular section, including the single combustor. There are two types of can-annular combustors: straight flow through, which is the most efficient, and reverse-flow.

Heavy industrial gas turbine applications tend to use reverse flow combustors, which also allows for the inclusion of regenerators, which improve overall thermal efficiency. Can-annular designs can use either a single fuel nozzle or multiple nozzles per combustion chamber.

Gas turbine air compressor Source: Alstom

The current crop of gas turbines take advantage of impulse reactions. Aeroderivative units use long, thin blades (high aspect ratio) incorporating tip shrouds to dampen vibration and improve sealing characteristics. Heavy-frame industrial gas turbines incorporate short and thick, low aspect ratio blades with no shroud.

Turbine blades are subject to stresses resulting from high temperature, high centrifugal forces and thermal cycling. The potential usually exists for torsional, lateral or blade resonance and fatigue failure. Coupling is the best available option to tune the torsional character.

These options include high torsional stiffness coupling (preferably of a dry flexible diaphragm type) or direct forged flanged rigid connection, which is the best choice if the torsional analysis allows. Flexible coupling is also possible and provides more elasticity and damping, but also requires more maintenance.

The blades’ natural frequencies cannot coincide with any source of excitation from 10 per cent below minimum governed speed to 10 per cent above maximum continuous speed. When the torsional, lateral or blade excitation falls close to the natural frequency, stress analysis is required to ensure that the resonance will not be harmful to the system.

The vibration analysis reports should include data used in mass-elastic systems, critical speeds (rotor lateral, train torsional and blading modes), mode shape diagrams, worst-case-design, and how the input data variance will affect the results (sensitivity analysis).

As a rule of thumb for power generation gas turbine packages, the diameter of the generator shaft should be equal to or greater than that of the gas turbine shaft. This is because the shaft is manufactured from stronger alloys and its design results in lower stress-concentration factors. Connection methods also require careful selection and analysis, since numerous torsional vibration problems continue to occur in gas turbine power generation trains. The main reasons are usually a lack of comprehensive torsional analysis, improper application and maintenance of coupling, and insufficient monitoring.

Coupling torque is usually chosen on the basis of mean requirements for full load. It must have a sufficient service factor to handle likely overload, such as electrical faults in power generation and transient conditions in mechanical drive applications. That means the calculation method for each rotating part’s inertia and stiffness is important.

A detailed finite element analysis (FEA) has been shown to reduce the margin of error by 25 per cent when calculating shaft stiffness and inertia, compared to simplified methods. Rotor torsional data from simplified methods could lead to missing (or shift of) torsional critical speed(s) and other torsional problems.

Other considerations include the performance curves in respect of net output, net heat rate, exhaust temperature and exhaust flow versus ambient temperature for the specified fuels at site conditions. Pulsation on generator load (current pulsation) is another important issue, especially where the turbine is providing power to weak electrical grids and networks.

The degree of irregularity in gas turbine power generation units should be limited to below acceptable levels based on electrical requirements, while ensuring sufficient inertial capacity for the train.

Auxiliary equipment and accessories

Since they are in the vicinity of the gas turbine and subjected to vibrations and shaking forces the auxiliaries and accessories (i.e. filter system inlet, exhaust system, etc) also require secure supports. Selecting the right materials and designing the details, attachments and supports, nozzles and their orientation, and internal piping and connections all require careful review.

The filter, ducting and silencer each require adequate corrosion protection. The filter house (mounted on top of the gas turbine enclosure) and silencers, including the perforated plate element on the inlet, exhaust plenum and exhaust silencer, are usually fabricated from suitable grades of stainless steel. The silencers need a rigid structure that is designed to prevent damage from acoustical or mechanical resonances, or differential thermal expansion.

The inlet and exhaust systems should be designed to induce a minimal practical pressure drop. The filter must remove 100 per cent of particles of three microns and larger (and minimum 99 per cent removal of 0.5–3 microns particles). It also requires a screen to prevent debris entering the inlet, which must be properly orientated, or provided with a louver or cowling, to minimize entry of driving rain, snow or sand.

Aeroderivative gas turbine Source: GE

The system also must allow for proper access to facilitate maintenance, have a differential pressure alarm for each stage of filtration, modular construction comprised of fully factory assembled modules, and the clean air side must be completely free of objects that can become loose during operation. Some of the worst effects of turbine hot section corrosion are experienced in offshore applications or power generation facilities near the coast. Prevention or reduction of seasalt instigated corrosion can be addressed in the inlet air filter system design and selection of turbine material and coatings.

The optimum duct system involves a minimum number of direction changes, including proper turning vanes, to assure uniform flow distribution and avoid resonance, a velocity limit of 20 m/s and 30 m/s for the inlet and exhaust respectively. Ducts must be sufficiently rigid to avoid vibration, generally requiring plate of between 5 mm and 10 mm thick to be used, while considering required access for cleaning and inspection.

The ducting and casing design must also permit field balancing in the end planes of the rotors, without requiring the removal of major casing components. The inlet must be upstream of the exhaust stack during prevailing wind conditions and relative position to avoid recirculating exhaust gases under any potential wind conditions.

This calls for a minimum horizontal separation of 7.5 metres and the air inlet must be elevated by at least 5 metres. Meanwhile, the gas turbine exhaust must also be outside a 3D fire hazardous zone and outside any classified electrical area.

The thermal piping and ducting design often requires flexibility in the system that is counter to the requirement for support and stiffness to meet vibration damping needs. This means both of these analyses should be conducted by the same party to optimize the overall system.

Thermal analysis calls for great care, especially when the recovery heat exchangers are off-skid, multiple turbines are on a common header where there is extremely cold ambient temperature, or when a turbine must operate over a wide range of conditions. The ducting and piping configuration and spacing should ensure that expanding parts cannot touch adjacent components.

A local panel should be installed around 50 cm to the gas turbine skid, but on a separate frame to avoid vibration damage. Support, ducting and piping all need to be designed with respect to operation and maintenance, as well as to facilitate piping spool and ducting module removal and avoid support removal.

The lubrication oil system calls for two pumps, each sized for at least 20 per cent greater flow than oil demand, and care must be taken with the operating curve and proper slope. The pump size should not be significantly over-designed, but it should be optimized, with the oil inlet temperature and oil temperature rise through the bearing less than 50 °C and 33 °C respectively. Dual removable bundle shell and tube oil coolers in a parallel arrangement, double filters with removable elements and stainless steel piping and valves shall also be provided.

The lubrication system is often one of the major sources of trouble, so the necessary lubrication points and spare points should be provided. The oil supply line to critical components requires monitoring, particularly the oil pressure. It is advisable for the oil reservoir volume retention time to be greater than eight minutes.

For aeroderivative gas turbines, which have anti-friction bearings and use synthetic oil, the lubricating system is usually separate from that of the generator and gear unit, if applicable. An online system to detect metal debris and chips is strongly recommended for turbines equipped with anti-friction type bearings. Industrial type gas turbines that use hydrodynamic bearings, usually with mineral based lubricating oil, generally use one integral lubricating oil system per train. This is often in a common oil system where the lubricant is a hydrocarbon oil corresponding to the ISO grade 32 or similar.

The fuel system is critical and needs special attention. It requires a Y-type fuel strainer with stainless steel internals and a blow-down system (manual valve) for purging and warming up the fuel system for approximately 20 minutes prior to starting. The manual valve is closed around two minutes after starting. It also should have a safety shutdown valve.

The system also calls for a high liquid level limit valve (fail safe) for the primary gas knockout drum, preceded by a high level alarm that is also capable of being manually tripped. To prevent condensate mist being carried over or forming a hydrate, a fuel gas super-heater designed to deliver 40 °C fuel gas can be included when required. When the design calls for fuel gas to be compressed, a screw compressor is a good choice.

The design must also allow proper spaces and areas to withdraw components and remove cooler bundles, as well as appropriate lay down areas. An overhead crane should be sized to reflect the maximum weights for routine maintenance of gas turbine train components such as the rotors, gear units and upper casings. Mobile and overhead cranes that meet specifications for this maximum installation weight will be required for routine maintenance. Lifting lugs, details and a lifting study should be provided for all skids, modules and equipment.

One final point is the importance of ensuring the performace of the gas turbine is throughly tested prior to it leaving the manufacturer’s shop. Site repair and downtime costs that have been estimated to be four to five times higher than most shop-based corrections. Gas turbines can be optimized for a large variety of power generating operations with the optimal configuration described above offering advantages integrating all different aspects of the turbine and allowing for gas turbine packages to be correctly specified and purchased.

The degradation on measurable performance over time is indicated by the changing parameters often used in the condition monitoring, but these can be mitigated to some extent by proper initial design and selection of inlet filtration and treatment systems, together with appropriate maintenance and operating practices. That can significantly affect the time between repairs or overhauls.

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