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WORLD PUMPS February 2009Feature262626
www.worldpumps.com 0262 1762/08 © 2008 Elsevier Ltd. All rights reserved
Despite 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 consump-
tion 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
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
Power generation
Pumping gas from field to application needs large machinesGas 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.
Figure 1. A schematic of a Siemens STC-SR (450) turbocompressor.
WORLD PUMPS February 2009Feature 27
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number of large manufacturers. These are
Rolls-Royce, Siemens Power Generation
(now incorporating Kuhule, Kopp & Kausch),
MAN Turbo AG, GE Oil & Gas (incorpo-
rating 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 compres-
sion trains are needed and gas turbocom-
pressors 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.
High reliability is crucial – GE Oil & Gas,
for example, cites some of its machines
achieving over 100,000 hours reliable opera-
tion 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 explora-
tion, 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 all-
important seals must also cope.
Machinery types
There are various types of gas turbo-
compressor and turbopump. Centrifugal
machines are typically used in oil and gas
applications because of their high pres-
sure ratios. Axial types, designed for lower
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.
Machines for low to medium duty appli-
cations to 50 bar generally use horizon-
tally 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 applica-
tion temperatures are near zero °C to +50 °C
inlet, and 120–204 °C outlet.
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.
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,
Figure 2. A Siemens STC-SR (450) turbocompressor seen being assembled. This was destined for a coal-to-liquids plant.
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.
WORLD PUMPS February 2009Feature282828
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and are ideal for many start/stop cycles and
for required changing load profiles.
MAN Turbo’s MOPICO and HOFIM inte-
gral 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 standard-
ised components are used throughout the
range for differing applications or perform-
ances – 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 compet-
itiveness. 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 compo-
nents – inlet ring, flow path creating
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 double-
flow, according to required compressions
and volume flows. Back-to-back arrange-
ments compensate for high axial loadings.
Impellers are made by milling, welding or
both, plus brazing, but the latter is unsuit-
able 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
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 high-
speed MOPICO electric motor. These are ideal for remote operation.
WORLD PUMPS February 2009Feature 29
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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 tilting-
pad thrust bearings take the axial forces.
Bearings
Most single shaft machines use two tilted
pad bearings, but some pipeline applica-
tions need special bearing supports. Where
oil cannot be used under any circum-
stances, 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 turbo-
compressors 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 tempera-
tures 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, espe-
cially if sufficient natural gas are available.
Yet, there is still a definite trend towards
electric drives.
Control and monitoring
Gas flow can be controlled within
100 – 45%, at constant head and speed,
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.
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:
Oil-lubricated mechanical seals (also
non-running sealing)
Oil-lubricated floating-ring seals
Labyrinth seals
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);
hydro dynamic lift occurs with rotation
using special grooves. Gas leakage is
extremely small. ■
Figure 7. A tandem compressor package for gas storage, by MAN Turbo. The maximum working pressure is 300 bar.
Figure 8. A John Crane type 28AT gas-lubricated
non-contacting dry gas seal that has been designed for
turbocompressors.