Continuing our series on gas turbines, in this issue, we discuss its characteristics and specifications in detail. Gas turbines are based on the Brayton or Joule cycle which consists of four processes: compression with no heat transfer, heating at constant pressure, expansion with no heat transfer, and in a closed cycle system, cooling at constant pressure.
In open cycle gas turbines, the fourth step does not exist since inlet air is taken from the atmosphere and the exhaust is dumped to atmosphere. Due to its higher temperature, there is more energy available from the expansion process than is expended in the compression. The network delivered to drive a generator is the difference between the two. The thermal efficiency of the gas turbine is a function of the pressure ratio of the compressor, the inlet temperature of the power turbine, and any parasitic losses (especially the efficiency of the compressor and power turbine).
Practical limitations on thermal efficiency due to losses and materials technology yield a maximum of about 40 percent at pressure ratios of 30 to 40 and temperatures of approximately 2,500F. These temperatures and pressure ratios are found only in recently developed, large gas turbines. Typically pressure ratios of 5 to 20 and turbine inlet temperatures from 1,400 to 2,000F are common in gas turbines for this application, resulting in efficiencies from 20 to 33 percent. As improved materials and cooling technologies are introduced to smaller units, the efficiencies can be expected to improve if the cost is not prohibitive.
Gas turbine system major components
Gas turbines can be divided into three major components or sections; these are the compressor, the combustor, and the power turbine. Air enters the compressor and is pressurized to a level from 10 to 50 times that of the entering air. The compressed air then passes into the combustor where fuel is introduced and ignited, producing temperatures in the range of 1,400 to 2,000F. The hot gases are then directed to the power turbine where they are expanded to atmospheric pressure and in turn provide power to drive both the compressor and the driven equipment such as a generator. Gas turbine auxiliary systems/components include starting, fuel supply, lubrication, governor/controls, speed reduction gear, inlet air, and engine exhaust.
a. Configuration
Gas turbines are lightweight in comparison to diesel engines, are very compact, and due to their small, well-balanced rotating mass are able to operate at very high speeds (from 10,000 to 25,000 rpm in sizes from 900 to 10,000 kW). Smaller gas turbines are usually single-shaft design that is the compressor and power turbine are mounted on the same shaft. Larger gas turbines are frequently two-shaft machines in which the power turbine is divided into two sections, one of which drives the compressor and the other which drives the generator. The two-shaft design allows the compressor section to be operated at a variable speed (within limits) thus varying the flow to the power turbine section as a function of load.
b. Starting system components
Gas turbines utilize a variety of starting systems based on size of the unit and other considerations. Common starting methods include compressed air, direct current (DC) electric motors with dedicated batteries, or a hydraulic pump driven by an alternating current (AC) motor, small gas turbine, or diesel engine, which in turn drives the hydraulic motor on the gas turbine. Where used, an auxiliary gas turbine or diesel engine also requires a starting system, usually a DC motor and batteries. Regardless of the equipment used, the starting system brings the unit up to a minimum speed at which the burners may be ignited and the turbine is then brought up to operating speed.
c. Fuel system components
Gas turbines are capable of burning either gas or liquid fuels, The following fuel system components are commonly provided as part of the gas turbine package: motor driven booster pump, low-pressure duplex fuel filter, main turbine driven fuel pump, high pressure filter, main fuel control valve (regulated by the governor), fuel manifold and injectors at the combustor and igniter.
d. Lubrication system components
Most gas turbines are provided with complete lubrication systems which include a cooler (air cooled), filter, pre/post lube pumps, engine driven main lube oil pump, alarms, oil storage tank (located in engine skid), and heater. The system is usually packaged with the gas turbine and only the lube oil cooler is remotely located. The lube oil system may supply the speed reduction gear and generator in addition to the gas turbine.
e. Governor/control
The gas turbine speed and fuel flow are controlled by the governor in response to load changes. Typically two types of governors are used on gas turbines driving electric generators:
Self-contained mechanical-hydraulic type or remote electronic governor with separate engine mounted actuator. Electronic governor systems with load sharing capability are the usual choice for multiple engine plants. Plants with multiple engines must have compatible governors to ensure proper operation of engines in parallel.
f. Speed reduction gear
The high operating speeds of most gas turbines require that a speed reduction gear be installed to drive the generator at the appropriate synchronous speed, usually 1,200 to 1,800 rpm. The reduction gear is typically an epicyclical design that permits a straight-through shaft arrangement, thus simplifying alignment. A variety of epicyclical designs are used and depending on the speed of the gas turbine, a two-stage reduction may be required. Two common designs are the standard planetary system and the star compound system. The reduction gear is typically lubricated by the main lube oil system.
g. Inlet and exhaust components
Gas turbines require significantly more combustion air than diesel engines. Flows are typically four to five times as much as that required by a diesel engine of the same capacity. This leads to much larger air filters, intake ducts, and exhaust ducts. Proper air filtration is critical to gas turbine performance. Deposits on compressor and turbine blades can significantly reduce efficiency.
Gas turbine system interfaces
Gas turbines interface with the following supporting systems:
a. Generators
Generators are the primary driven equipment for gas turbines. The gas turbine and the generator must be properly aligned and coupled, either directly or by a flexible coupling. It is critical that the engine and generator are properly matched.
b. Fuel oil systems
The gas turbine is dependent on the fuel oil system to provide fuel to the engine skid. The fuel oil must have the proper characteristics required for the specific engine installation. In general, gas turbines can utilize a wider range of liquid fuels than diesel engines. Most facilities use kerosene, No. 1 fuel oil, or No. 1 diesel, but some use No. 2 fuel (if acceptable to the manufacturer) since it is less expensive than the lighter grades of fuel.
c. Lube oil systems
The proper lubrication of the moving parts inside a gas turbine is critical to obtain satisfactory operation of the engine and maximum life of its components. The lube oil must be approved by the engine manufacturer and analyzed on a regular basis to determine the optimum interval for changing the lube oil. Lube oil change intervals are much longer than those for diesel engines, since the oil does not become contaminated by products of combustion. Lube oil systems cool and filter the lube oil to provide both proper lubrication and cooling of critical components within the engine.
d. Engine air system
The engine intake and exhaust systems provide filtered air to the engine and remove products of combustion from the engine room. These systems may be very simple or relatively complex, incorporating such features as preheating or pre-cooling of the intake air, or hardened design. Restrictions or blockage of either the intake or exhaust systems will severely impact engine performance.
e. Engines starting system
Gas turbines installed in power plants may be started with compressed air, DC motors, or an engine driven hydraulic system. Dedicated compressors typically provide starting air at pressures from 150 to 500 psig, depending on the specific requirements of the gas turbine. The system must provide adequate storage of compressed air to allow multiple attempts to start the engines. DC motors are driven from batteries located at the engine skid, which are charged by a dedicated battery charger. Hydraulic systems are composed of a prime mover, usually a diesel engine or small gas turbine, hydraulic pump, drive motor and accessories, including hydraulic reservoir, air cooled heat exchanger and filter.
f. Engine control systems
The basic control of the engine is maintained by the governor during operation and the control is independent for each engine. The overall control of a multiple engine power plant can be relatively simple or very sophisticated. Possible control options range from local or manual starting and synchronization of each engine to automatic starting, synchronization, and load sharing of the engine generators.
g. Instrumentation
Collection of operating data is critical to planning maintenance and evaluating problems which may occur. In the past (and still the case at most facilities), all data was recorded by operating personnel from instrument panels at each engine. Many newer plants now have automated data logging systems that can also provide warnings for out-of-tolerance conditions and histories of unusual events which can improve the operation of the facility. Regardless of the type of system, data collection provides the basis for trend analysis that can indicate potential problems before they become severe.
h. Ventilation systems
Gas turbines operate at high temperatures and therefore reject large amounts of heat to the surrounding space. Power plants are typically ventilated to remove this heat and to maintain temperatures within acceptable limits for both personnel and equipment. Proper operation of ventilation systems is required to avoid excessive temperatures, reduced equipment capacity, and potential equipment failures.
3.1.2 Rotor
The compressor portion of the gas turbine rotor is an assembly of wheels, a speed ring, ties bolts, the compressor rotor blades, and a forward stub shaft. Each wheel has slots broached around its periphery. The rotor blades and spacers are inserted into these slots and held in axial position by staking at each end of the slot. The wheels are assembled to each other with mating rabbets for concentricity control and are held together with tie bolts. Selective positioning of the wheels is made during assembly to reduce balance correction.
After assembly, the rotor is dynamically balanced. The forward stub shaft is machined to provide the thrust collar which carries the forward and aft thrust loads. The stub shaft also provides the journal for the No. 1 bearing, the sealing surface for the No. 1 bearing oil seals and the compressor low-pressure air seal.
Of the many factors affecting the efficient working of a simple gas turbine, including unbalanced forces, vibrations are the prominent that lead to development of cyclic stresses which in turn results in fatigue failure.
Machines in the best of operating condition will have some vibration because of small, minor defects. Therefore, each machine will have a level of vibration that may be regarded as normal or inherent. However, when machinery vibration increases or becomes excessive, some mechanical trouble is usually the reason. Vibration does not increase or become excessive for no reason at all. Something causes it - unbalance, misalignment, worn gears or bearings, looseness, etc.
6B Heavy Duty Gas Turbine
The 6B gas turbine features low-cost installation and maintenance requirements. It offers high availability and reliability in simple cycle and cogeneration applications and from barge-mounted to industrial installations. The 6B gas turbine can handle multiple start-ups required for peak load, accommodate a wide variety of gaseous and liquid fuels, including heavy fuel oils, drive a compressor, and be installed quickly for fast track projects.
Features & Benefits
· Fuel flexibility to burn wide range of alternative fuels from low calorific and synthetic gas to heavy fuel oil and bio-ethanol for lower operating costs, better efficiency and lower emissions than other technologies.
· Flexible choice for cogeneration applications capable of producing a thermal output ranging from 200 to 400 GJ/hr (190 to380MBtu/hr) with supplementary firing with steam up to 110 bar (1600 psi).
· High reliability and low emissions with a Dry Low NOx combustion system upgrade capable of achieving 5 ppm NOx on natural gas.
· Advanced technology to extend the life of existing 6B gas turbine with higher availability, output, and efficiency.
· Excellent fit for selective island grids, mechanical applications and barge mounted applications due to fuel flexibility, low cost per horsepower, and high horsepower per square meter.
Application
The 6B gas turbine can be utilized in a wide range of power generation applications with a wide range of gas and liquid fuels. With its lengthy industrial oil and gas experience, fuel flexibility, high exhaust energy to power ratio, and high reliability, the 6B gas turbine is an excellent fit for industrial and refinery cogenerations, providing electricity, and process steam and for decentralized power generation.
· Local power generation involves remote areas and barge mounted or peaking applications. The 6B gas turbine has a compact simple-cycle arrangement with short delivery time and can be started quickly.
· Industrial cogeneration is the simultaneous production of electricity and thermal energy from a common fuel source. Generated steam can be used for industrial processes or to power a steam turbine to generate electricity.
· Mechanical drive involves upstream and midstream oil and gas, where the 6B gas turbine can deliver horsepower to drive a compressor in aggressive environments.
· For remote areas with no access to natural gas, the 6B gas turbine is a robust solution to provide electricity with heavy fuel oil, naphtha, and bio-fuels.
In open cycle gas turbines, the fourth step does not exist since inlet air is taken from the atmosphere and the exhaust is dumped to atmosphere. Due to its higher temperature, there is more energy available from the expansion process than is expended in the compression. The network delivered to drive a generator is the difference between the two. The thermal efficiency of the gas turbine is a function of the pressure ratio of the compressor, the inlet temperature of the power turbine, and any parasitic losses (especially the efficiency of the compressor and power turbine).
Practical limitations on thermal efficiency due to losses and materials technology yield a maximum of about 40 percent at pressure ratios of 30 to 40 and temperatures of approximately 2,500F. These temperatures and pressure ratios are found only in recently developed, large gas turbines. Typically pressure ratios of 5 to 20 and turbine inlet temperatures from 1,400 to 2,000F are common in gas turbines for this application, resulting in efficiencies from 20 to 33 percent. As improved materials and cooling technologies are introduced to smaller units, the efficiencies can be expected to improve if the cost is not prohibitive.
Gas turbine system major components
Gas turbines can be divided into three major components or sections; these are the compressor, the combustor, and the power turbine. Air enters the compressor and is pressurized to a level from 10 to 50 times that of the entering air. The compressed air then passes into the combustor where fuel is introduced and ignited, producing temperatures in the range of 1,400 to 2,000F. The hot gases are then directed to the power turbine where they are expanded to atmospheric pressure and in turn provide power to drive both the compressor and the driven equipment such as a generator. Gas turbine auxiliary systems/components include starting, fuel supply, lubrication, governor/controls, speed reduction gear, inlet air, and engine exhaust.
a. Configuration
Gas turbines are lightweight in comparison to diesel engines, are very compact, and due to their small, well-balanced rotating mass are able to operate at very high speeds (from 10,000 to 25,000 rpm in sizes from 900 to 10,000 kW). Smaller gas turbines are usually single-shaft design that is the compressor and power turbine are mounted on the same shaft. Larger gas turbines are frequently two-shaft machines in which the power turbine is divided into two sections, one of which drives the compressor and the other which drives the generator. The two-shaft design allows the compressor section to be operated at a variable speed (within limits) thus varying the flow to the power turbine section as a function of load.
b. Starting system components
Gas turbines utilize a variety of starting systems based on size of the unit and other considerations. Common starting methods include compressed air, direct current (DC) electric motors with dedicated batteries, or a hydraulic pump driven by an alternating current (AC) motor, small gas turbine, or diesel engine, which in turn drives the hydraulic motor on the gas turbine. Where used, an auxiliary gas turbine or diesel engine also requires a starting system, usually a DC motor and batteries. Regardless of the equipment used, the starting system brings the unit up to a minimum speed at which the burners may be ignited and the turbine is then brought up to operating speed.
c. Fuel system components
Gas turbines are capable of burning either gas or liquid fuels, The following fuel system components are commonly provided as part of the gas turbine package: motor driven booster pump, low-pressure duplex fuel filter, main turbine driven fuel pump, high pressure filter, main fuel control valve (regulated by the governor), fuel manifold and injectors at the combustor and igniter.
d. Lubrication system components
Most gas turbines are provided with complete lubrication systems which include a cooler (air cooled), filter, pre/post lube pumps, engine driven main lube oil pump, alarms, oil storage tank (located in engine skid), and heater. The system is usually packaged with the gas turbine and only the lube oil cooler is remotely located. The lube oil system may supply the speed reduction gear and generator in addition to the gas turbine.
e. Governor/control
The gas turbine speed and fuel flow are controlled by the governor in response to load changes. Typically two types of governors are used on gas turbines driving electric generators:
Self-contained mechanical-hydraulic type or remote electronic governor with separate engine mounted actuator. Electronic governor systems with load sharing capability are the usual choice for multiple engine plants. Plants with multiple engines must have compatible governors to ensure proper operation of engines in parallel.
f. Speed reduction gear
The high operating speeds of most gas turbines require that a speed reduction gear be installed to drive the generator at the appropriate synchronous speed, usually 1,200 to 1,800 rpm. The reduction gear is typically an epicyclical design that permits a straight-through shaft arrangement, thus simplifying alignment. A variety of epicyclical designs are used and depending on the speed of the gas turbine, a two-stage reduction may be required. Two common designs are the standard planetary system and the star compound system. The reduction gear is typically lubricated by the main lube oil system.
g. Inlet and exhaust components
Gas turbines require significantly more combustion air than diesel engines. Flows are typically four to five times as much as that required by a diesel engine of the same capacity. This leads to much larger air filters, intake ducts, and exhaust ducts. Proper air filtration is critical to gas turbine performance. Deposits on compressor and turbine blades can significantly reduce efficiency.
Gas turbine system interfaces
Gas turbines interface with the following supporting systems:
a. Generators
Generators are the primary driven equipment for gas turbines. The gas turbine and the generator must be properly aligned and coupled, either directly or by a flexible coupling. It is critical that the engine and generator are properly matched.
b. Fuel oil systems
The gas turbine is dependent on the fuel oil system to provide fuel to the engine skid. The fuel oil must have the proper characteristics required for the specific engine installation. In general, gas turbines can utilize a wider range of liquid fuels than diesel engines. Most facilities use kerosene, No. 1 fuel oil, or No. 1 diesel, but some use No. 2 fuel (if acceptable to the manufacturer) since it is less expensive than the lighter grades of fuel.
c. Lube oil systems
The proper lubrication of the moving parts inside a gas turbine is critical to obtain satisfactory operation of the engine and maximum life of its components. The lube oil must be approved by the engine manufacturer and analyzed on a regular basis to determine the optimum interval for changing the lube oil. Lube oil change intervals are much longer than those for diesel engines, since the oil does not become contaminated by products of combustion. Lube oil systems cool and filter the lube oil to provide both proper lubrication and cooling of critical components within the engine.
d. Engine air system
The engine intake and exhaust systems provide filtered air to the engine and remove products of combustion from the engine room. These systems may be very simple or relatively complex, incorporating such features as preheating or pre-cooling of the intake air, or hardened design. Restrictions or blockage of either the intake or exhaust systems will severely impact engine performance.
e. Engines starting system
Gas turbines installed in power plants may be started with compressed air, DC motors, or an engine driven hydraulic system. Dedicated compressors typically provide starting air at pressures from 150 to 500 psig, depending on the specific requirements of the gas turbine. The system must provide adequate storage of compressed air to allow multiple attempts to start the engines. DC motors are driven from batteries located at the engine skid, which are charged by a dedicated battery charger. Hydraulic systems are composed of a prime mover, usually a diesel engine or small gas turbine, hydraulic pump, drive motor and accessories, including hydraulic reservoir, air cooled heat exchanger and filter.
f. Engine control systems
The basic control of the engine is maintained by the governor during operation and the control is independent for each engine. The overall control of a multiple engine power plant can be relatively simple or very sophisticated. Possible control options range from local or manual starting and synchronization of each engine to automatic starting, synchronization, and load sharing of the engine generators.
g. Instrumentation
Collection of operating data is critical to planning maintenance and evaluating problems which may occur. In the past (and still the case at most facilities), all data was recorded by operating personnel from instrument panels at each engine. Many newer plants now have automated data logging systems that can also provide warnings for out-of-tolerance conditions and histories of unusual events which can improve the operation of the facility. Regardless of the type of system, data collection provides the basis for trend analysis that can indicate potential problems before they become severe.
h. Ventilation systems
Gas turbines operate at high temperatures and therefore reject large amounts of heat to the surrounding space. Power plants are typically ventilated to remove this heat and to maintain temperatures within acceptable limits for both personnel and equipment. Proper operation of ventilation systems is required to avoid excessive temperatures, reduced equipment capacity, and potential equipment failures.
3.1.2 Rotor
The compressor portion of the gas turbine rotor is an assembly of wheels, a speed ring, ties bolts, the compressor rotor blades, and a forward stub shaft. Each wheel has slots broached around its periphery. The rotor blades and spacers are inserted into these slots and held in axial position by staking at each end of the slot. The wheels are assembled to each other with mating rabbets for concentricity control and are held together with tie bolts. Selective positioning of the wheels is made during assembly to reduce balance correction.
After assembly, the rotor is dynamically balanced. The forward stub shaft is machined to provide the thrust collar which carries the forward and aft thrust loads. The stub shaft also provides the journal for the No. 1 bearing, the sealing surface for the No. 1 bearing oil seals and the compressor low-pressure air seal.
Of the many factors affecting the efficient working of a simple gas turbine, including unbalanced forces, vibrations are the prominent that lead to development of cyclic stresses which in turn results in fatigue failure.
Machines in the best of operating condition will have some vibration because of small, minor defects. Therefore, each machine will have a level of vibration that may be regarded as normal or inherent. However, when machinery vibration increases or becomes excessive, some mechanical trouble is usually the reason. Vibration does not increase or become excessive for no reason at all. Something causes it - unbalance, misalignment, worn gears or bearings, looseness, etc.
6B Heavy Duty Gas Turbine
The 6B gas turbine features low-cost installation and maintenance requirements. It offers high availability and reliability in simple cycle and cogeneration applications and from barge-mounted to industrial installations. The 6B gas turbine can handle multiple start-ups required for peak load, accommodate a wide variety of gaseous and liquid fuels, including heavy fuel oils, drive a compressor, and be installed quickly for fast track projects.
Features & Benefits
· Fuel flexibility to burn wide range of alternative fuels from low calorific and synthetic gas to heavy fuel oil and bio-ethanol for lower operating costs, better efficiency and lower emissions than other technologies.
· Flexible choice for cogeneration applications capable of producing a thermal output ranging from 200 to 400 GJ/hr (190 to380MBtu/hr) with supplementary firing with steam up to 110 bar (1600 psi).
· High reliability and low emissions with a Dry Low NOx combustion system upgrade capable of achieving 5 ppm NOx on natural gas.
· Advanced technology to extend the life of existing 6B gas turbine with higher availability, output, and efficiency.
· Excellent fit for selective island grids, mechanical applications and barge mounted applications due to fuel flexibility, low cost per horsepower, and high horsepower per square meter.
Application
The 6B gas turbine can be utilized in a wide range of power generation applications with a wide range of gas and liquid fuels. With its lengthy industrial oil and gas experience, fuel flexibility, high exhaust energy to power ratio, and high reliability, the 6B gas turbine is an excellent fit for industrial and refinery cogenerations, providing electricity, and process steam and for decentralized power generation.
· Local power generation involves remote areas and barge mounted or peaking applications. The 6B gas turbine has a compact simple-cycle arrangement with short delivery time and can be started quickly.
· Industrial cogeneration is the simultaneous production of electricity and thermal energy from a common fuel source. Generated steam can be used for industrial processes or to power a steam turbine to generate electricity.
· Mechanical drive involves upstream and midstream oil and gas, where the 6B gas turbine can deliver horsepower to drive a compressor in aggressive environments.
· For remote areas with no access to natural gas, the 6B gas turbine is a robust solution to provide electricity with heavy fuel oil, naphtha, and bio-fuels.
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