Specifying Gas Turbines

Amin Almasi WorleyParsons Services Pty Ltd

Gas turbines are widely used throughout the chemical process industries (CPI) — especially in petroleum refineries and petrochemical facilities — to provide both mechanical-drive and power-generation capabilities. Presented below are a variety of recommendations related to the selection and arrangement of gas turbines and auxiliaries, performance testing, and proper operation and maintenance of these systems.

In general, gas turbines have always been tolerant of a wide range of fuels including conventional liquid and gaseous fossil fuels, and high- and low-heating-value fuels (such as gasified coal, wood and biofuels and so on). For gas turbines, the primary performance objectives include the ability to demonstrate optimum fuel consumption, maintain low emissions and ensure reasonable reliability.

Ongoing improvements for gas turbines have been achieved by three main factors:

• Metallurgical advances that have enabled the production of gas turbine components with increased temperature ratings

• The application of the cumulative body of advanced knowledge developed by the aircraft-engine industries has benefited many applications in the chemical process industries (CPI), with favorable results

• Advanced computer technology has been used to optimize the design, simulation and operation of gas turbines

All of these factors have contributed to a vast range of ongoing design improvements for the air-compressor itself (for instance, enabling improved pressure-ratio increases), for the combustion system (producing lower emissions and greater fuel efficiency), and for the turbine (for instance, through the development of single-crystal blades, improved cooling strategies and more).

The design and arrangement of individual gas turbine packages is a complex undertaking. When evaluating competing gas turbine options, the user must weigh the needs and requirements of the application against the specific performance attributes and other defining characteristics of the gas turbines offered. Compromises or tradeoffs are often required to balance the application-specific requirements and constraints against competing turbine options. Condition monitoring and predictive maintenance can help to improve overall operation and reliability.

The ability to maintain control of speed in the face of sudden load changes is also important. Today, thanks to advances in modern control technologies, it is possible to simply and effectively control these highly responsive machines.

Gas turbine types and design

There are two main types of gas turbines — aero-derivative gas turbines and heavy-industrial gas turbines — and each varies in terms of its weight, combustor design, turbine design, bearing design and lube oil system. In general, heavy-frame units are slower in speed than aero-derivative gas turbines, and they have higher air flow and tend to require more time- and labor-intensive maintenance and management of spare parts. Meanwhile, heavy-industrial gas turbines typically use hydrodynamic bearings while aero-derivative units typically use anti-friction bearings.

Ongoing advances in the aircraft engine and space technologies sector have been used to provide more easily maintainable, flexible, lightweight and smaller aero-derivative gas turbines for use in other industrial sectors. The key to easing maintenance is to use a modular concept, which enables the removal and replacement of key components without requiring the entire gas turbine to be removed from its support mounts. In general, heavy-industrial units tend to require greater time and effort than aero-derivative units to remove and replace the combustor parts and more effort to inspect or repair the various sections of the gas turbine.

For both power-generation and mechanical-drive services, the general preference among operators has been to use aero-derivative units in remotely located applications (including offshore applications), and to use heavy-frame industrial units in more easily accessible baseload applications. However, there will always be exceptions.

Heavy-industrial gas turbines tend to consume more fuel and approximately 50% more air than aero-derivative units. Because of this, heavy-industrial gas turbines are exposed to greater amounts of potential contaminants in the air and thus face an increased risk of corrosion (especially sulfur-related corrosion). In particular, the large cross-sectional area of the blades and vanes used in heavy-industrial turbines makes them more susceptible to corrosive attack, but their increased size also enables them to tolerate more corrosion compared to the blades of the aero-derivative gas turbines, which tend to be thinner and have a higher aspect ratio.

Hot-end drives.Gas turbines can be arranged in one of two ways: as a hot-end drive configuration or a cold-end drive configuration. The hot-end drive configuration is more common. In a hot-end drive configuration, the location of the gas turbine output shaft is at the turbine end where exhaust gases can reach high temperatures. This not only affects bearing operation and life, but also makes the turbine more difficult to service, as the train assembly (driven equipment, coupling and so on) must be fitted through the exhaust duct.

In hot-end arrangements, insufficient attention to key design and operational aspects — such as the output shaft length, high temperatures, exhaust duct turbulence, pressure drop and maintenance accessibility — often results in power loss, excessive vibration, shaft or coupling failure, and increased downtime for maintenance.

Cold-end drives.By comparison, in the cold-end drive configuration, the gas turbine output shaft connects to the front of the air-compressor. In such a configuration, the driven equipment can be more easily accessed by operators and maintenance technicians. Unlike a gas turbine with a hot-end drive configuration, driven equipment in the cold-end configuration will be exposed to ambient temperatures only.

However, the cold-end configuration has several drawbacks that must be considered. For instance, the air-compressor inlet must be configured to accommodate the gas turbine output shaft to the driven equipment. This will affect the inlet air duct. This inlet duct must be turbulent-free and provide uniform, vortex-free flow throughout the operating speed range. Problems resulting from a poor design can be catastrophic. For example, inlet turbulence can induce surge in the air-compressor, resulting in complete destruction of the unit.

Inlet air-duct turbulence is also a major reliability concern, since air compressors — particularly axial ones — are very sensitive to surge (that is, unstable operation due to low air flow as result of air duct turbulence). Surge can result in machine destruction in several seconds. This is a major reason why hot-end drive configurations are preferred and more widely used.

In cold-end drive configurations, the potential for air duct turbulence can be greatly reduced with the use of turbulence-free ducting designs, but these impose higher pressure drop, which is not acceptable for some applications.

Figure 1 and Figure 2 show examples of heavy-frame industrial and aero-derivative gas turbines, respectively. Figure 3 shows a large power-generation gas turbine.

Figure 1. Shown here is an example of a heavy-frame industrial gas turbine. In recent years, metallurgical advances have helped to increase the temperature ratings of all types of turbines. Reprinted with permission from [1]

FIGURE 2. With an aero-derivative gas turbine, such as the one shown here, the purge period should be designed to displace a minimum of six times the exhaust-system gas volume (including turbine, exhaust duct, waste-recovery device and exhaust stack) before firing the unit. Reprinted with permission from [1]

FIGURE 3. This figure shows the internal structure of high-pressure turbine blades that are equipped with cooling distribution throughout the core of the blade airfoil and root. Reprinted with permission from [1]

Competing designs

Gas turbines can be categorized into two main groups. Single-spool machines and multi-spool machines. In single-spool, integral-output shaft gas turbines, the air-compressor and power turbine are assembled on the same shaft (the gas turbine output is located at the end of this shaft).

Single-spool, integral-output shaft gas turbines — both hot-end drive designs, and cold-end drive designs — are used primarily to drive electric generators (an integral-shaft gas turbine is uncommon for mechanical drive applications). The high torque required to start pumps and compressors under full pressure results in high turbine temperature during the startup cycle (when the flow of cooling air is low or non-existent).

One exception is very large compressors that are driven by gas turbines, as in liquefied natural gas (LNG) refrigeration compressor trains, which typically use a single-spool, integral-output shaft gas turbine (for example, a 40-MW or larger train).

By comparison, a single-spool, split-output-shaft gas turbine is a single-spool gas turbine that drives a free power turbine. Such an arrangement contains an air-compressor and coupled turbine spool, which delivers hot gas to to the turbine, which is coupled to the driven equipment. In this configuration, the air-compressor and its turbine component shaft (the turbine that drives the air-compressor) are not physically connected to the free power- turbine shaft. Rather, these two shafts are coupled aerodynamically. Split-output-shaft gas turbines offer easier startup when used for mechanical-drive applications.

Usually, mechanical drive gas turbines (such as those used to drive a process compressor or large pump) that are referred to as a split-shaft, mechanical-drive gas turbines, are able to attain self-sustaining operation before picking up the load of the driven equipment. Power-generation turbines can be designed to operate at the same speed as the driven equipment, thereby eliminating the need for a gearbox. This can provide an efficiency advantage, because typical gearboxes create losses equivalent to 2–4% of net power generated. However, such a design is limited to hot-end drive configurations, because free-power-turbines should be located next to the air-compressor-turbine hot-gas stream.

Dual-spool gas turbines contain two shafts (each has its own air compressor and turbine section). One shaft passes through the other. In a dual-spool, split-output shaft gas turbine, independent low- and high-pressure compressors and turbines generate the hot gases that drive the free turbine (for higher-power applications, there may be three shafts, each operating at different speeds).

Optimum overrating

Degradation and environmental conditions, such as temperature and humidity, can have considerable impact on gas turbine output power. Discussed below are some guidelines for the sizing of gas turbines based on applicable codes, experiences and lesson learned in various projects.

In general, gas turbines should be designed to provide an average of 12–14% more power than the driven equipment requires (5% as tolerance to meet the driven equipment shaft-brake power), an additional 2% for the gear box (if applicable), an additional 2% for fouling and erosion, and finally an additional 5% for longterm gas turbine deterioration). Care should be taken when selecting the starting device and evaluating its rating.

In general, the preferred starting device is an electro-hydraulic configuration — whereby an electric motor drives a hydraulic pump, which transmits hydraulic power to start the gas turbine. The starting device should be rated to supply minimum 110% of the gas turbine's required starting torque (under worst-case scenarios).

Helper drivers are not recommended except for special cases. A helper driver is a separate rotating machine (based on a motor, engine or similar) that is coupled to the gas turbine train to help the startup train. They may be disconnected after startup or stay in connection during operation. In general, they are not recommended since they decrease reliability and flexibility.

The main exception — very large, compressor trains driven by heavy-frame gas turbines (such as large LNG trains) — often use variable-speed electric motors as helper drivers and power equalizers (for instance, to generate mechanical torque during hot days when gas turbine power is low, and to generate electricity in the winter when gas turbine power is higher than required by the train).

Hot start (that is, startup shortly after gas turbine shutdown, when the machine is still hot) is critical since all complex systems, such as machine cooling systems (particularly the air cooling systems used for the blades) should be ready for this kind of startup (for example to handle hot gas from a still-hot combustion system). The gas turbine should be capable of an immediate hot start at any time after a trip for three consecutive start attempts.

Specification tips

The issues discussed next should be addressed during the specification and purchase of any gas turbine system.

Startup. Cold-start and hot-start restriction details are very important, and such details should be finalized during the bidding stage. Igniters should not remain in the primary combustion zone during operation since extremely high temperature over the long term can degrade them and create reliability and operating problems. The rotating blades and the labyrinths of shrouded rotating blades should be designed to start up without rubbing. Sealing components (such as 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 suitable clearances.This is an ongoing challenge in gas turbines, due to the impact of changing temperatures between cold and accelerating conditions. The most severe conditions, which usually occur after a restart, will determine the minimum clearance that should be required.

For variable-speed, mechanical-drive applications, the speed range for single-shaft gas turbines is recommended to be 25% (80 to 105% of rated speed), and for gas turbines with two or more shafts the speed range is recommended to be 45% (60% to 105%).

For power-generation application, the required speed is usually constant, since generator speed and network frequency are fixed.

Compressors.Two types of air-compressors are available — axial compressors (with up to 19 stages) and centrifugal compressors (with one or two impellers). The air-compressor supplies compressed air to the gas turbine combustor to generate hot gases and drive the turbine section. An increase air-supply pressure to the combustor (to increase the air-compressor ratio) is very important to improve turbine power generation.

An increase in air-compressor ratio is the prime contributor in the overall increase in simple-cycle thermal efficiency (efficiency without either heat recovery or steam generation from the hot exhaust gases) to above 35% (particularly for aero-derivative units). Today, aero-derivative gas turbines are available with simple-cycle thermal efficiency above 44%.

Combustors. Combustor design is a complex task. There are two main designs for combustors: the can-annular combustor design and the annular-section design (including the single combustor). Two types of can-annular combustors are available: more-efficient, straight flow-through designs, and reverse-flow combustors. The advantage of the reverse-flow combustor, as used in many heavy-industrial gas turbines, is the use of a regenerator.

Regenerator. A regenerator has many potential configurations, but in general, it uses hot turbine exhaust gases to increase the heat value of the high–pressure, inlet compressor air feed to the combustor. In general the use of a regenerator helps to improve the overall thermal efficiency of the gas turbine system.

Blades. Aero-derivative units use blades that are relatively long and thin (giving them a relatively high aspect ratio) and incorporate tip shrouds to dampen vibration and improve blade-tip sealing characteristics.

By comparison, heavy-frame industrial gas turbines incorporate blades that are relatively short and thick (that is, they have a low aspect ratio) and have no shroud. Ongoing improvements in metallurgy and casting techniques have allowed turbine manufacturers to eliminate mid-span shrouds and lacing wires in many designs.

In all types of gas turbines, the turbine blades are subject to stresses resulting from high temperature, high centrifugal forces and thermal cycling. Most designs rely on various cooling systems, and these cooling mechanisms decrease the effects of the extremely high temperature of the gases delivered from the combustor. But high temperatues are still experienced by the blades.

Shaft.As a rule-of-thumb for power-generation gas turbine packages, the generator shaft diameter should be equal to or greater than the gas turbine shaft diameter because the gas turbine shaft is usually fabricated from higher-grade alloy materials. For mechanical-drive applications, both shafts should have approximately the same diameters (in case of the same operating speed).

For all gas turbines, the shaft materials should be high-strength, suitable grade steel. Proper weld procedures and material compatibility must be considered. Fabrication details should consider anticipated loads that could result from vibration.

Performance curves.The gas turbine manufacturer should supply the following performance curves: net output, net heat rate, exhaust temperature, and exhaust flow versus ambient temperature for the specified fuels at site conditions.

Couplings. Due to their extreme operating conditions, all gas turbines have the potential for blade failures resulting from torsional, lateral or resonance forces or fatigue. The proper selection of couplings (that connect the gas turbine to the driven equipment) is the best way to tune the torsional character of the train and avoid the coincidence of system dynamic natural frequencies and train excitation frequencies that can lead to blade failures.

A variety of coupling options are available:

1. High-torsional-stiffness couplings (preferably a dry, flexible- diaphragm type) or direct-forged, flanged, rigid connections. These are optimum selections for all types of gas turbines if proper coupling or connections with suitable load-carrying capacity and misalignment capability are required, if there are no interferences between the system’s natural frequencies and the train’s excitation frequencies and if transient situations do not impose any specific problems.

2. Flexible couplings using soft elements like rubber. These provide greater elasticity and damping, but also tend to require more maintenance since rubber elements may become degraded and require replacement.

Managing excessive excitation

The blades’ natural frequencies must not coincide with any source of excitation within a range that spans from 10% below the minimum governed speed to 10% above the maximum continuous speed. Stress analysis should be performed if the torsional, lateral or blade excitation falls close to the train natural frequencies, to ensure that the resonance will not be harmful for the system.

The main potential sources of excitation in a gas turbine train include:

• Unbalance in the rotor system

• First-harmonic passing frequencies of various rotating and stationary components in the train

• The first ten rotor-speed harmonics

• Frequencies generated by gas-passage splitters

• Irregularities in vane and nozzle pitch

• Periodic impulses caused by the combustor arrangement

• Oil-film instabilities (whirl)

• Internal rubbing points

• Diffuser passing frequencies

• Gear-tooth meshing and side bands

• Gear problems

• Coupling misalignment

• Loose rotor-system components

• Whirl resulting from hysteresis and friction

• Boundary-layer flow separation

• Acoustic and aerodynamic cross-coupling forces

• Asynchronous whirl

• Startup or shutdown conditions

• Governor control-loop resonances

• Fuel pressure pulsation

• Rolling element/race frequencies of anti-friction bearings for aero-derivative gas turbines (Note: Anti-friction bearings rely on rolling elements to carry loads, so various frequencies known as rolling element/race frequencies are generated by them)

Auxiliaries. Auxiliaries and accessories (such as a filter inlet system, exhaust system and so on) must be installed with proper supports since they are in the vicinity of a gas turbine, and are thus subjected to vibrations.

Similarly, corrosion protection must be provided for the filter, ducting, and silencer. For instance, the filter house (mounted on top of the gas turbine enclosure) and silencers (including the inlet-silencer perforated-plate element) exhaust plenum and exhaust silencer must be fabricated from suitable grades of stainless steel. Silencers should have a rigid structure and be suitably designed to prevent damage from anticipated acoustical or mechanical resonances or differential thermal expansion.

Inlet and exhaust filters.The inlet and exhaust systems should be designed for a minimum practical pressure drop. A filter with 100% removal efficiency for particle sizes of 3 microns or larger (and minimum 99% removal of particles of 0.5 to 3 microns) is typically used. The filter system requires an entrance screen to prevent debris from entering the system, and the design should include a downward-oriented air inlet or a louver or cowling to keep rain and snow out.

The design should include proper access to facilitate maintenance, a differential-pressure alarm for each stage of filtration, and should use modular construction (via fully factory assembled modules).

Some of the worst effects of turbine hot-section corrosion are experienced in offshore applications or facilities that operate near the sea coast. Sulfidation corrosion instigated by sea salt exposure can be minimized through the design of the inlet-air filtration system and selection of suitable turbine materials and material coatings.

The duct system.The optimum duct system has a minimum number of direction changes, and includes proper turning vanes (to assure uniform flow distribution and to avoid resonance). The system should be designed to ensure a velocity limit of 20 m/s and 30 m/s for the inlet and exhaust, respectively.

The ducts should be sufficiently rigid to minimize vibration (a plate that is 5 to 10 mm thick is generally used), and the access points required for cleaning and inspection should be considered. The ducting and casing design must permit field balancing in the end planes of the rotors without requiring the removal of major casing components (in other words, the machine and ducting arrangement should allow proper access for various rotor rebalancing in situ).

Inlet air and exhaust system. The layout of the inlet and exhaust system must be designed with great care. For instance, the air inlet must be upstream of the exhaust stack during prevailing wind conditions, and its relative position must avoid any recirculation of exhaust gases that could result from any conceivable potential wind conditions. (As a general rule of thumb, the minimum horizontal separation is typically on the order of 7.5 meters). The air inlet should also be elevated a minimum of 5 m from the ground, and the gas turbine exhaust must also be outside of the specified, three-dimensional fire-hazard zone (this is the zone with the greatest potential for flammable gas release, as determined by the site’s safety team) and outside any classified electrical areas.

Thermal analysis is also necessary, and great care must be executed, especially for extremely cold ambient temperature, or packages are likely to operate over a wide range of conditions.

The lubrication system.In any gas turbine system, the lubrication system is often a source of trouble. Necessary lubrication points and lubrication spare points should be provided. The lubrication oil system for any gas turbine should include two pumps, each of which is sized for at least 20% greater flow than the train oil demand. The oil supply line to critical components should be monitored (mainly with regard to oil pressure).

As a rough indication, the inlet oil temperature and oil temperature rise through the bearing should be maintained at les than 50°C and 30°C, respectively.

Dual removable bundle shell-and-tube oil coolers, in a parallel arrangement, and double filters with removable element and stainless steel piping and valves are typically used. For oil reservoir volume, a retention time of more than eight minutes is recommended. For aero-derivative gas turbines, which typically have anti-friction bearings and use synthetic lubrication oil, the turbine lubricating-oil system is usually separate from the driven equipment lubricating-oil system. And when the gas turbine is equipped with antifriction-type bearings, the use of an instrumented, metal chip-detection system — an online system to monitor for metallic debris — is strongly recommended.

With heavy industrial-type gas turbines, hydrodynamic bearings are typically preferred, and these tend to require mineral-based lubricating oils. Such systems tend to require one integral lubricating-oil system per train. In a common oil system, the lubricant is a typically a hydrocarbon oil corresponding to ISO Grade 32 or similar.

The fuel system.The fuel system is also a critical component of any gas turbine system and needs special attention. A fuel strainer (typically a Y-type strainer with stainless steel internals) and a blowdown system with a manual valve is typically included for purging and warming up the fuel system for approximately 20 minutes prior to startup. To prevent condensate mist carryover or hydrate formation (if required), a fuel gas super-heater designed to deliver 40C fuel gas should be included. If fuel gas compression is required, a screw compressor is recommended.

Evaporative coolers are not recommended due to the possibility of damage on gas turbine internals and decreasing reliability from water carryover or poor water quality. Liquid-to-air heat exchangers to cool the inlet air (for performance enhancement) and steam- or water-injected exchangers for emissions-control purposes are not recommended, because they tend to engender significant maintenance requirements.

Mechanical design issues

The most critical areas for mechanical design are the sealing system, bearing system, number of stages and staging arrangement, casing size and design, casing joint design, rotor dynamics, rotor and blade structural design and performance, gas turbine component material selection, and design and arrangement of power-transmission components.

Turbine section shafts experience high temperature changes during transient operations such as startup, and shutdown. Turning equipment (typically a turning gear or ratchet device) should be furnished where the turbine shaft requires rotation to avoid thermal distortion of the shaft during startup or immediately following a shutdown. The turning equipment should be automatically engaged and preferably driven by an electric motor.

Degradation of each stage or section of gas turbine has a cumulative effect. For instance, a degraded stage or section will create different exit conditions compared to a new stage, and each subsequent stage will end up operating further away from its design point. The main causes of degradation are increased tip clearances, changes in airfoil geometry, and changes in the surface quality of the components. Such degradation is caused by a variety of mechanisms:

• Fouling caused by the adherence of particles to foils and annulus surfaces

• Hot corrosion that results in the loss or deterioration of material from components as they are exposed to hot gases (typically by chemical reactions)

• Abrasion-related erosion resulting from hard or incompressible particles in the gas streams impinging on flow surfaces

• Abrasion resulting from rotating surfaces rubbing on a stationary surface or damage caused by foreign objects striking the flow-path components (the use of an inlet filtration system can help to reduce some of these issues)

Case studies for degradation

In a study on a mechanical-drive gas turbine, the clearance was increased from around 3% (design value) to around 4.5%, and this led to the following changes:

• A 20% increase in the surge flow coefficient (the surge flow coefficient identifies the minimum flow that results in surge — a very dangerous instability that can result in machine destruction even in several seconds)

• A 12% reduction in pressure coefficient (that is, a 12% reduction in the discharge pressure of the air-compressor under constant suction conditions)

• A 2.5% efficiency loss for the entire gas turbine system

Extensive studies showed the performance reduction of the air-compressor section of the gas turbine deteriorated as a result of spraying salt water in the inlet. The resulting deposits caused increased surface roughness on the compressor foils (the majority of the deposits occurred at the first stage and had become insignificant after the fourth stage). This buildup shifted the compressor operating line to both a lower flowrate and a lower pressure ratio.

Several gas turbines being overhauled after three to four years in service showed major degradation issues, mainly related to reduced flow in the air-compressor section and reduced efficiency in the turbine section.

The effect of individual component degradation is also influenced by the control system and the control modes of the gas turbine. Additionally, the method and location of measuring the control parameter (such as temperature and pressure-measuring sensors, which are used to control gas turbines) will determine the behavior of the machine in a degraded state.

It is commonly accepted that gas turbine degradation cannot be entirely avoided, but certain precautions (such as careful selection and maintenance of the inlet filtration system) can clearly reduce the rate of its occurrence and impact. The site-specific conditions that dictate contaminants, their size, concentration and composition, need to be carefully considered during the selection of the inlet filtration system.

Similarly, the rate of deterioration can be slowed by regular cleaning. However, online cleaning (some washing methods without disassembling gas turbine) will usually only clean the first few stages of the air-compressor (because the increase in temperature at later stages will evaporate the washing or cleaning fluid). If the gas turbine internals (especially the blades) can be accessed with moderate effort (for example, when the machine casing is horizontally split) additional cleaning by hand can be effective.

Any degradation of the components will always lead to observable changes in parameters. Because different types of degradation on different components will alter the gas turbine in different ways, this finding can also be used for diagnostic purposes. For example, monitoring of pressure (using discharge pressure against a reference) is the optimum way to monitor degradation (also for the practical reason, flow is usually not as easily monitored compared to pressure).

Condition monitoring

Because degradation or deterioration in a gas turbine system often creates an unbalanced situation, vibration monitoring is an excellent way to monitor gas turbine systems for signs of ongoing degradation. Vibration monitoring of the casings (using a minimum of two sets for the compressor and turbine casings) is always recommended (using both velocity measurements for low-speed vibrations up to 2k Hz, and accelerometers for higher-speed vibrations and for hot sections). Non-contacting probes are typically used for axial and radial vibration monitoring. For journal bearings, non-contacting X–Y probes mounted at a 45-deg angle from the vertical centerline are typically used, in addition to velocity seismic transducers for bearing housings and two sensor probes for axial- position thrust bearings.

Temperature monitoring at strategic locations — for instance, to track temperatures related to the gas turbine rotating system, oil temperature and hot-air flow path — is also important. Thermocouples mounted at the lubricating oil outlets of the bearings can provide for alarms (and sometimes emergency shutdown). Hydrodynamic thrust and radial bearings are often equipped with replaceable resistance temperature detectors (RTDs).

In a typical installation, six thermocouples may be placed around the turbine exhaust-gas frame to measure the exhaust gas temperatures for alarm and trip capabilities. Heavy-duty industrial turbines usually have two sets of thermocouples, which can monitor and generate an alarm for the maximum-allowable turbine-space temperature.

For aero-derivative gas turbines, two wheel-space thermocouples should be located downstream of the last turbine wheel (using thermocouples and conduits that are as small as possible). Electronic governors should be provided with triple-input sensors and triple processor redundancy (using two-out-of-three voting logic). It should prevent the turbine speed from increasing beyond the specified over-speed limit in any case of loss of rated load (resulting from, for instance, coupling failure or process upset).

In the case of multiple shafts, each shaft should have its own overspeed trip-protection system, which allows for online testing without overspeeding the turbine (that is, the overspeed trip system should be independent of the governor).

Meanwhile, the following automatic-shutdown systems should be considered: Overspeed, low fuel supply, combustor flame out, low lube oil pressure, radial and axial shaft vibration in addition to driven equipment necessary shutdown(s).

When it comes to ongoing operations, it is difficult to identify the rate of degradation. However, as a rule of thumb, the performance degradation during the first 24,000 hours (around three years) of operation can be expected to be around 2 to 6%. This assumes degraded parts are not replaced (if parts are replaced and the machine is properly revamped, the expected performance degradation can be assumed to be around 1 to 1.5%).

Before the gas turbine leaves the manufacturer, it must be tested for performance. Due to difficulties in commissioning new gas turbines, a comprehensive shop performance test should be conducted; this is especially important for units destined for remote areas and offshore applications.

Specific settings for various control, alarm and shutdown thresholds are often the subject of considerable debate. Although certain guidelines can be set up to predict what level is acceptable, analysis of shop performance test results can provide an excellent source of data for this purpose.

Any shop performance test (carried out in accordance with ASME PTC 22) should include measurement and verification of the following important system attributes:

• Inlet system pressure drop

• Exhaust back pressure

• Barometric pressure

• Emissions (specified levels of NOx, CO, CO₂ and unburned hydrocarbons)

• Oil system performance

• Bearing and seal performance

• Starting device

• Fuel system

• Vibration condition and control corrections

It should also include six satisfactory starts and stops in the automatic mode, and should demonstrate satisfactory performance up to a minimum load (typically 50 to 100% load, with a maximum load variation of 5%) and fuel crossover (if applicable). Performance data conducted at the existing shop conditions should be corrected to compare with the guaranteed performance data, using reviewed and accepted performance correction curves (since the manufacturer’s shop conditions, such as ambient temperatures and altitude, are usually different with jobsite conditions, these correction curves are necessary to identify optimal site conditions based on available shop performance data).

The purge period (to purge the whole gas turbine system and make it ready for startup) should displace a minimum of six times of the exhaust system volume (including turbine, exhaust duct, waste recovery device and exhaust stack) before firing the unit.

Meanwhile, the ignition temperature of the gas should be higher than the surface temperatures of gas turbine. Generally high temperatures and pressures can lead to damages and safety issues. To avoid this, an independent pressure relief valve (PRV) should be provided for protection.

On the other hand, temperatures and pressures that are too low may also lead to low efficiency. When a noise enclosure is required, a ventilation system that is used to generate a negative pressure within the enclosure (when located within a safe area) or a positive pressure (when located within a hazardous area) that has two 100% ventilation fans — one operating and one standby fan, each sized for 100% capacity required for the enclosure — with controls including automatic start, should be considered.

Closing thoughts

This article provides recommendations for the optimization of gas turbine arrangements for a large variety of power and compression applications. It is intended to support users during the specification and purchase of gas turbine packages. The impact of degradation on gas turbine performance underscores the importance of condition-monitoring systems. Proper design and selection of inlet filtration and treatment systems, together with proper maintenance and operating practices, can significantly affect the level of performance degradation, environmental impacts and thus time between repairs or overhauls.n

Edited by Suzanne Shelley

Reference

1. Giampaolo, T., “Gas Turbine Handbook Principles and Practices,” 3^rd Ed., The Fairmont Press, Inc., U.S., 2006.

Posted in: Chemical Engineering Processing

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