WORLD PUMPS February 2009Feature262626

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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.

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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.

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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.

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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.