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Pumping gas from field to application needs large machines
26
Feature WORLD PUMPS
February 2009
Power generation
Pumping gas from field to application needs large machines Gas turbocompressors are essential for gas pumping, transport and other duties in expanding petrochemical, gas and process industry sectors worldwide. James Hunt explains the different uses and demands for the various equipment types and also what is needed by way of materials and construction for optimum results.
D
espite the gloomy economic situation and climate change concerns, and very recent statements that we are on the cusp of peak oil/gas production, there is still a concerted drive to find new oil and gas reserves. There is also a need to extract as much as economically possible from existing reserves. Various large gas turbocompressor machines are used in these applications, where they pump, or otherwise transfer, natural and other gases across a very wide range of pressures and duties. Because natural gas has been, until recently, quite cheap, and because it is relatively clean, it has long been the fastest growing primary energy source, and its share of global energy consumption is expected to increase from 24% in 2003 to 26% by 2030, though the current economic crisis and increasing climate change worries may reduce this. Over half of undiscovered natural gas
www.worldpumps.com
is expected to come from Eurasia, the Middle East, and North Africa; and a quarter is expected to come from North, Central, and South America.
This is the background to use of gas turbocompressor machines. These are large and quite complex, so the tendency has been towards an increasingly small
Figure 1. A schematic of a Siemens STC-SR (450) turbocompressor.
0262 1762/08 © 2008 Elsevier Ltd. All rights reserved
WORLD PUMPS
Feature February 2009
number of large manufacturers. These are Rolls-Royce, Siemens Power Generation (now incorporating Kuhule, Kopp & Kausch), MAN Turbo AG, GE Oil & Gas (incorporating Nuovo Pignone S.p.A), Dresser-Rand Turbomachinery, and Elliott Co.
Applications Gas turbocompressor applications vary widely and include gas lift, gathering, oil/ gas separation, gas transfer, boil-off gas and fuelgas/syngas compression, plus refinery processes. In the latter, flexible gas compression trains are needed and gas turbocompressors are ideal for this. Note that some of these applications are purely compressive, whilst others – such as gas transfer – are essentially pumping duties, albeit with a compression element. For example, natural gas supply and demand often do not match; any resulting supply surplus may mean pumping gas into storage, then feeding it back into the pipeline grid during peak demand.
Figure 2. A Siemens STC-SR (450) turbocompressor seen being assembled. This was destined for a coal-to-liquids plant.
High reliability is crucial – GE Oil & Gas, for example, cites some of its machines achieving over 100,000 hours reliable operation in large liquid natural gas (LNG) plants, which often work at cryogenic temperatures. There has been recent high demand for pumping machinery in oil and gas exploration, where untreated gas is pumped or compressed. In Europe especially, oil/gas field depletion has led to the advent of oil/ gas/water separation processes, so different pumping and compression layouts are needed – therefore, the midstream market has been demanding more gas transport and storage applications. For re-injection into a declining oil deposit to boost supply, gas may be compressed to hundreds of atmospheres. The gas used often derives from crude oil, but nitrogen, CO2, or ‘sour’ gases containing both H2S and CO2 may also be used for re-injection purposes. Indeed, today, increasing amounts of such gas, often also with high moisture content, need to be handled. Using such ‘difficult’ and corrosive gases means finding economically viable modern materials to cope, but materials suitable for corrosion resistance are not usually those suitable for erosion and/or fouling resistance. The allimportant seals must also cope.
Machinery types There are various types of gas turbocompressor and turbopump. Centrifugal machines are typically used in oil and gas applications because of their high pressure ratios. Axial types, designed for lower
Figure 3. A schematic showing a section through a Rolls-Royce pipeline turbocompressor.
Figure 4. A schematic of a Rolls-Royce barrel type machine showing its internals.
pressures with higher volume flows, are highly efficient over a wide operating range. For example, Elliott Co’s A-Line axial machines provide flows to 679,600m3/h at up to 5.2 bar. Combined axial/centrifugal types are mainly used as air compressors for large air separation plant, and so are less relevant here.
Machinery for gas re-injection typically works at pressures of 350–700 bar, but recently GE Oil & Gas set ‘a milestone’, the company claimed, with the test of the highest pressure centrifugal machine ever built at 820 bar discharge pressure, handling an extremely ‘sour’ gas with 18% H2S. Recent materials developments have allowed such gases to be re-injected. Moreover, MAN Turbo has said that it could build 1000 bar machines if there was the demand.
Machines for low to medium duty applications to 50 bar generally use horizontally split casings. For higher pressures of 500–600 bar or above, or for hydrogen-rich gases, vertically split (barrel type) machines are typically used. Running speeds vary. Barrel types typically run from 7000–13,000 rev/min, while for gas transmission, the speeds may be from 5000–9000 rev/min. Gas temperatures also vary, depending upon the application. Typical natural gas application temperatures are near zero °C to +50 °C inlet, and 120–204 °C outlet.
Pipeline types, such as Rolls-Royce’s RFA36, are also available up to typically 150 bar. Such machines fit into the pipe itself and need to have very strong casings because of high pipe forces, but pressure ratios are low. EU operators have been building up natural gas storage networks that enable quick reaction to market volatility, so more storage/export machines are now required. Many are driven by integral electric motors, www.worldpumps.com
27
28
Feature WORLD PUMPS
February 2009
and are ideal for many start/stop cycles and for required changing load profiles. MAN Turbo’s MOPICO and HOFIM integral electrically-driven centrifugal pipeline machines are claimed to be ideal for rapidly changing gas volumes and varying pressure ratios. The MOPICO’s overhung impeller possesses a high polytropic efficiency and wider turndown thanks to a low impeller mach number and a long flow path. Both designs use hermetic sealing and are truly all electric, making it easier to remotely operate units.
Construction Modularity Modular construction – designing a turbopump or turbocompressor family so that a relatively small number of standardised components are used throughout the range for differing applications or performances – is increasingly common. The result can be significant manufacturing cost savings, reduced lead times and downtime for the customer, plus better product and project quality. Such standardisation also allows the supply chain to be optimised, as well as providing gains on cost competitiveness. However, there does not seem to be a full consensus on the benefits of modularity for gas turbocompressor users. With very large machines, there are often scaling limitations; full modularity may not be possible because very big casings need to be made especially stiff. Also, very large turbocompressors are made in small numbers, with design varying from application to application – large casings are an example, and such ‘custom’ design is the complete opposite of the modular ideal. Moreover, Rolls-Royce’s energy business, while confirming the trend, has also said that modular construction can be actually more costly up front, as more parts and replacement assemblies are needed. The company says, though, that modularity does reduce downtime and, therefore, costs at a later stage. As a good modular example, DATUM machines from Dresser-Rand feature a completely modular bundle assembly for both axially- and radially-split units. This helps address ease of maintenance issues.
Nozzles and moving parts Good nozzle design is crucial. Nozzles may be cast or forged, then welded onto the casing. Internal flow-conducting components – inlet ring, flow path creating www.worldpumps.com
Figure 5. A gas storage barrel type turbo machine by MAN Turbo.
Figure 6. This MAN Turbo gas transport turbo machine is compact and hermetically sealed, and is driven by an integral highspeed MOPICO electric motor. These are ideal for remote operation.
diaphragms (often fully-machined), discharge volute and adjustable inlet guide vanes – are usually in nodular cast iron or steel. Computational Fluid Dynamics (CFD) techniques are used to make accurate predictions of performance in terms of gas flows. CFD takes into account the simultaneous flows of heat, mass transfer, phase change, and chemical reactions, plus the mechanical movement of machinery concerned, and any related stresses. Using the technique, maximum machine efficiency can be designed in. GE, using its own CFD expertise, may be able to cut centrifugal machine diffuser sizes by up to 25%, the company says. Impellers are supplied single or doubleflow, according to required compressions
and volume flows. Back-to-back arrangements compensate for high axial loadings. Impellers are made by milling, welding or both, plus brazing, but the latter is unsuitable for ‘sour’ gases. Impellers can also be cast; more recently, spark erosion has been used. The latest single-piece impellers can reduce cycle time, increase productivity and improve efficiency. The use of 3D impeller blading for efficiency is increasing. 3D blades are more efficient and provide higher volume flows. As with 2D blades, they can be used in both axial and radial machines. 2D blades tend to be used for higher pressures, and feature a radial section, while 3D blades’ inlet sections are in the axial direction, changing to a curvature across the blade to the outlet. 2D
WORLD PUMPS
Feature February 2009
using movable diffuser vanes – giving a power draw almost proportional to the flow. Such vanes were first introduced by KK&K. Different control regimes are available, including inlet or diffuser guide vane control, or a combination of the two, and variable speed control. KK&K’s Variable Diffuser Vane system matches performance curves to the plant resistance curve using pneumatic, electric or hydraulic actuation.
Figure 7. A tandem compressor package for gas storage, by MAN Turbo. The maximum working pressure is 300 bar.
blades have a better operational stability and are less likely to stall. Rotors comprise impellers and shaft sleeves mounted on a low-alloy steel shaft. An axial bearing collar takes the thrust. Rotors usually run on oil-lubricated radial tilting pad bearings – pivot points allow both rotational directions. These can be easily changed without shaft removal. Double-acting tiltingpad thrust bearings take the axial forces.
Bearings Most single shaft machines use two tilted pad bearings, but some pipeline applications need special bearing supports. Where oil cannot be used under any circumstances, magnetic bearings are specified. ‘Active’ types monitor bearing and shaft behaviour and change the magnetic fields so that the shaft always rotates under optimum conditions.
Materials Good materials are essential. Pipeline turbocompressors generally use cast or welded steel casings, with gas path components in nodular cast iron or forged steel. Barrel type machines for high-pressure gas re-injection are mostly of rolled, forged or low-carbon steels, with outer casings of cast-iron. For low temperatures, or for gas with a high H2S content, high nickel/chromium content steels avoid H2S corrosion. Rotor shafts are usually of a low-alloy steel. Where the H2S content is high, or temperatures low (such as found with boil-off gas compressors, which typically run a −160°C inlet temperature), shafts may be made from the more ductile 17.4% nickel steel.
Drivers In terms of the driver, both electrical and gas turbine drives are used. Electric drives are typically installed where new sets augment existing pipeline systems. Electric drives demand a reliable electricity source, so will typically be used where the electrical grid and supporting infrastructure is proven. However, most pipeline development occurs where infrastructure is developing and the benefits of a ‘stand-alone’, highly reliable gas turbine-driven machine are significant, especially if sufficient natural gas are available. Yet, there is still a definite trend towards electric drives.
Control and monitoring Figure 8. A John Crane type 28AT gas-lubricated non-contacting dry gas seal that has been designed for turbocompressors.
Gas flow can be controlled within 100 – 45%, at constant head and speed,
Condition monitoring increases reliability and life between overhauls, especially for bearings. Monitoring bearing vibration and temperature can establish trends so that maintenance work can be planned in. Some customers link their control and monitoring systems to the overall plant control via the Internet or using fibre optic-based communication systems. There is also a strong trend towards using control and monitoring equipment to send machine/plant information to remote control centers, via control bus systems or the Internet.
Seals Sealing excellence is essential for the safe running of these machines. Natural gas is combustible and some handled gases are also toxic, damaging to the environment or explosive, so must not be allowed to escape. Types of seals available for these applications include: t Oil-lubricated mechanical seals (also non-running sealing) t Oil-lubricated floating-ring seals t Labyrinth seals t Dry-running gas seals (DGS) Labyrinth seals are used internally, but the main shaft seals require something better because of the higher differential pressure. Either oil film seals or DGS are used. Oil-lubricated seals often leak oil into the process and require a substantial degassing/support system. Power losses and heat loads are also said to be higher with oil-lubricated seals, and both speed and pressure is limited. Such seals can withstand 100m/s rotational speeds and a maximum of 100 bar, but this is not enough for gas re-injection duties. Consequently, DGS is now the industry standard. With DGS, the balance of opening and closing forces governs hydrostatic lift so that lift off occurs when pressurised (no rotation); hydrodynamic lift occurs with rotation using special grooves. Gas leakage is extremely small. ■ www.worldpumps.com
29
Feature WORLD PUMPS
February 2009
Power generation
Pumping gas from field to application needs large machines Gas turbocompressors are essential for gas pumping, transport and other duties in expanding petrochemical, gas and process industry sectors worldwide. James Hunt explains the different uses and demands for the various equipment types and also what is needed by way of materials and construction for optimum results.
D
espite the gloomy economic situation and climate change concerns, and very recent statements that we are on the cusp of peak oil/gas production, there is still a concerted drive to find new oil and gas reserves. There is also a need to extract as much as economically possible from existing reserves. Various large gas turbocompressor machines are used in these applications, where they pump, or otherwise transfer, natural and other gases across a very wide range of pressures and duties. Because natural gas has been, until recently, quite cheap, and because it is relatively clean, it has long been the fastest growing primary energy source, and its share of global energy consumption is expected to increase from 24% in 2003 to 26% by 2030, though the current economic crisis and increasing climate change worries may reduce this. Over half of undiscovered natural gas
www.worldpumps.com
is expected to come from Eurasia, the Middle East, and North Africa; and a quarter is expected to come from North, Central, and South America.
This is the background to use of gas turbocompressor machines. These are large and quite complex, so the tendency has been towards an increasingly small
Figure 1. A schematic of a Siemens STC-SR (450) turbocompressor.
0262 1762/08 © 2008 Elsevier Ltd. All rights reserved
WORLD PUMPS
Feature February 2009
number of large manufacturers. These are Rolls-Royce, Siemens Power Generation (now incorporating Kuhule, Kopp & Kausch), MAN Turbo AG, GE Oil & Gas (incorporating Nuovo Pignone S.p.A), Dresser-Rand Turbomachinery, and Elliott Co.
Applications Gas turbocompressor applications vary widely and include gas lift, gathering, oil/ gas separation, gas transfer, boil-off gas and fuelgas/syngas compression, plus refinery processes. In the latter, flexible gas compression trains are needed and gas turbocompressors are ideal for this. Note that some of these applications are purely compressive, whilst others – such as gas transfer – are essentially pumping duties, albeit with a compression element. For example, natural gas supply and demand often do not match; any resulting supply surplus may mean pumping gas into storage, then feeding it back into the pipeline grid during peak demand.
Figure 2. A Siemens STC-SR (450) turbocompressor seen being assembled. This was destined for a coal-to-liquids plant.
High reliability is crucial – GE Oil & Gas, for example, cites some of its machines achieving over 100,000 hours reliable operation in large liquid natural gas (LNG) plants, which often work at cryogenic temperatures. There has been recent high demand for pumping machinery in oil and gas exploration, where untreated gas is pumped or compressed. In Europe especially, oil/gas field depletion has led to the advent of oil/ gas/water separation processes, so different pumping and compression layouts are needed – therefore, the midstream market has been demanding more gas transport and storage applications. For re-injection into a declining oil deposit to boost supply, gas may be compressed to hundreds of atmospheres. The gas used often derives from crude oil, but nitrogen, CO2, or ‘sour’ gases containing both H2S and CO2 may also be used for re-injection purposes. Indeed, today, increasing amounts of such gas, often also with high moisture content, need to be handled. Using such ‘difficult’ and corrosive gases means finding economically viable modern materials to cope, but materials suitable for corrosion resistance are not usually those suitable for erosion and/or fouling resistance. The allimportant seals must also cope.
Machinery types There are various types of gas turbocompressor and turbopump. Centrifugal machines are typically used in oil and gas applications because of their high pressure ratios. Axial types, designed for lower
Figure 3. A schematic showing a section through a Rolls-Royce pipeline turbocompressor.
Figure 4. A schematic of a Rolls-Royce barrel type machine showing its internals.
pressures with higher volume flows, are highly efficient over a wide operating range. For example, Elliott Co’s A-Line axial machines provide flows to 679,600m3/h at up to 5.2 bar. Combined axial/centrifugal types are mainly used as air compressors for large air separation plant, and so are less relevant here.
Machinery for gas re-injection typically works at pressures of 350–700 bar, but recently GE Oil & Gas set ‘a milestone’, the company claimed, with the test of the highest pressure centrifugal machine ever built at 820 bar discharge pressure, handling an extremely ‘sour’ gas with 18% H2S. Recent materials developments have allowed such gases to be re-injected. Moreover, MAN Turbo has said that it could build 1000 bar machines if there was the demand.
Machines for low to medium duty applications to 50 bar generally use horizontally split casings. For higher pressures of 500–600 bar or above, or for hydrogen-rich gases, vertically split (barrel type) machines are typically used. Running speeds vary. Barrel types typically run from 7000–13,000 rev/min, while for gas transmission, the speeds may be from 5000–9000 rev/min. Gas temperatures also vary, depending upon the application. Typical natural gas application temperatures are near zero °C to +50 °C inlet, and 120–204 °C outlet.
Pipeline types, such as Rolls-Royce’s RFA36, are also available up to typically 150 bar. Such machines fit into the pipe itself and need to have very strong casings because of high pipe forces, but pressure ratios are low. EU operators have been building up natural gas storage networks that enable quick reaction to market volatility, so more storage/export machines are now required. Many are driven by integral electric motors, www.worldpumps.com
27
28
Feature WORLD PUMPS
February 2009
and are ideal for many start/stop cycles and for required changing load profiles. MAN Turbo’s MOPICO and HOFIM integral electrically-driven centrifugal pipeline machines are claimed to be ideal for rapidly changing gas volumes and varying pressure ratios. The MOPICO’s overhung impeller possesses a high polytropic efficiency and wider turndown thanks to a low impeller mach number and a long flow path. Both designs use hermetic sealing and are truly all electric, making it easier to remotely operate units.
Construction Modularity Modular construction – designing a turbopump or turbocompressor family so that a relatively small number of standardised components are used throughout the range for differing applications or performances – is increasingly common. The result can be significant manufacturing cost savings, reduced lead times and downtime for the customer, plus better product and project quality. Such standardisation also allows the supply chain to be optimised, as well as providing gains on cost competitiveness. However, there does not seem to be a full consensus on the benefits of modularity for gas turbocompressor users. With very large machines, there are often scaling limitations; full modularity may not be possible because very big casings need to be made especially stiff. Also, very large turbocompressors are made in small numbers, with design varying from application to application – large casings are an example, and such ‘custom’ design is the complete opposite of the modular ideal. Moreover, Rolls-Royce’s energy business, while confirming the trend, has also said that modular construction can be actually more costly up front, as more parts and replacement assemblies are needed. The company says, though, that modularity does reduce downtime and, therefore, costs at a later stage. As a good modular example, DATUM machines from Dresser-Rand feature a completely modular bundle assembly for both axially- and radially-split units. This helps address ease of maintenance issues.
Nozzles and moving parts Good nozzle design is crucial. Nozzles may be cast or forged, then welded onto the casing. Internal flow-conducting components – inlet ring, flow path creating www.worldpumps.com
Figure 5. A gas storage barrel type turbo machine by MAN Turbo.
Figure 6. This MAN Turbo gas transport turbo machine is compact and hermetically sealed, and is driven by an integral highspeed MOPICO electric motor. These are ideal for remote operation.
diaphragms (often fully-machined), discharge volute and adjustable inlet guide vanes – are usually in nodular cast iron or steel. Computational Fluid Dynamics (CFD) techniques are used to make accurate predictions of performance in terms of gas flows. CFD takes into account the simultaneous flows of heat, mass transfer, phase change, and chemical reactions, plus the mechanical movement of machinery concerned, and any related stresses. Using the technique, maximum machine efficiency can be designed in. GE, using its own CFD expertise, may be able to cut centrifugal machine diffuser sizes by up to 25%, the company says. Impellers are supplied single or doubleflow, according to required compressions
and volume flows. Back-to-back arrangements compensate for high axial loadings. Impellers are made by milling, welding or both, plus brazing, but the latter is unsuitable for ‘sour’ gases. Impellers can also be cast; more recently, spark erosion has been used. The latest single-piece impellers can reduce cycle time, increase productivity and improve efficiency. The use of 3D impeller blading for efficiency is increasing. 3D blades are more efficient and provide higher volume flows. As with 2D blades, they can be used in both axial and radial machines. 2D blades tend to be used for higher pressures, and feature a radial section, while 3D blades’ inlet sections are in the axial direction, changing to a curvature across the blade to the outlet. 2D
WORLD PUMPS
Feature February 2009
using movable diffuser vanes – giving a power draw almost proportional to the flow. Such vanes were first introduced by KK&K. Different control regimes are available, including inlet or diffuser guide vane control, or a combination of the two, and variable speed control. KK&K’s Variable Diffuser Vane system matches performance curves to the plant resistance curve using pneumatic, electric or hydraulic actuation.
Figure 7. A tandem compressor package for gas storage, by MAN Turbo. The maximum working pressure is 300 bar.
blades have a better operational stability and are less likely to stall. Rotors comprise impellers and shaft sleeves mounted on a low-alloy steel shaft. An axial bearing collar takes the thrust. Rotors usually run on oil-lubricated radial tilting pad bearings – pivot points allow both rotational directions. These can be easily changed without shaft removal. Double-acting tiltingpad thrust bearings take the axial forces.
Bearings Most single shaft machines use two tilted pad bearings, but some pipeline applications need special bearing supports. Where oil cannot be used under any circumstances, magnetic bearings are specified. ‘Active’ types monitor bearing and shaft behaviour and change the magnetic fields so that the shaft always rotates under optimum conditions.
Materials Good materials are essential. Pipeline turbocompressors generally use cast or welded steel casings, with gas path components in nodular cast iron or forged steel. Barrel type machines for high-pressure gas re-injection are mostly of rolled, forged or low-carbon steels, with outer casings of cast-iron. For low temperatures, or for gas with a high H2S content, high nickel/chromium content steels avoid H2S corrosion. Rotor shafts are usually of a low-alloy steel. Where the H2S content is high, or temperatures low (such as found with boil-off gas compressors, which typically run a −160°C inlet temperature), shafts may be made from the more ductile 17.4% nickel steel.
Drivers In terms of the driver, both electrical and gas turbine drives are used. Electric drives are typically installed where new sets augment existing pipeline systems. Electric drives demand a reliable electricity source, so will typically be used where the electrical grid and supporting infrastructure is proven. However, most pipeline development occurs where infrastructure is developing and the benefits of a ‘stand-alone’, highly reliable gas turbine-driven machine are significant, especially if sufficient natural gas are available. Yet, there is still a definite trend towards electric drives.
Control and monitoring Figure 8. A John Crane type 28AT gas-lubricated non-contacting dry gas seal that has been designed for turbocompressors.
Gas flow can be controlled within 100 – 45%, at constant head and speed,
Condition monitoring increases reliability and life between overhauls, especially for bearings. Monitoring bearing vibration and temperature can establish trends so that maintenance work can be planned in. Some customers link their control and monitoring systems to the overall plant control via the Internet or using fibre optic-based communication systems. There is also a strong trend towards using control and monitoring equipment to send machine/plant information to remote control centers, via control bus systems or the Internet.
Seals Sealing excellence is essential for the safe running of these machines. Natural gas is combustible and some handled gases are also toxic, damaging to the environment or explosive, so must not be allowed to escape. Types of seals available for these applications include: t Oil-lubricated mechanical seals (also non-running sealing) t Oil-lubricated floating-ring seals t Labyrinth seals t Dry-running gas seals (DGS) Labyrinth seals are used internally, but the main shaft seals require something better because of the higher differential pressure. Either oil film seals or DGS are used. Oil-lubricated seals often leak oil into the process and require a substantial degassing/support system. Power losses and heat loads are also said to be higher with oil-lubricated seals, and both speed and pressure is limited. Such seals can withstand 100m/s rotational speeds and a maximum of 100 bar, but this is not enough for gas re-injection duties. Consequently, DGS is now the industry standard. With DGS, the balance of opening and closing forces governs hydrostatic lift so that lift off occurs when pressurised (no rotation); hydrodynamic lift occurs with rotation using special grooves. Gas leakage is extremely small. ■ www.worldpumps.com
29