technology evolution of lng refrigerant … · ms9001 130 8-10 cascade double train worldwide...

Antonio Pelagotti, Baker Hughes, a GE company

TECHNOLOGY EVOLUTION OF LNG

REFRIGERANT TURBOCOMPRESSORS

Antonio Pelagotti

Baker Hughes, a GE company

The last few years have seen a large variety in capacity for LNG plants. The single LNG

train capacity can change from 0.5 up to 8 MMTPY. This large variability in production

requires flexibility in selecting the drivers, extending the offer from traditional heavy-duty gas

turbine to large electric motor or aero-derivative gas turbine. On the compressor side, the

technology has been driving a continuous development in the train arrangement.

Optimization of casings size, side-stream mixing configuration, new impeller designs and

more accurate performance predictability has allowed the design of compressors for very

high actual flow and power, with efficiency and reliability never seen up to now.

This paper describes how the current driven and driver technology has satisfied the

demands of the LNG business to date and the expectation of the requirements for the near-

term future.

1 Introduction

LNG market has been always in evolution and has implemented new technologies to improve profitability of the

plant itself.

The evolution has impacted both driven and driver equipment always searching an economic way to improve plant

production. In the past years drivers have evolved in power passing from MS5002 to MS9001 including MS6001

and most used MS7001. After this first trend, the aero derivative gas turbine such as LM2500 or LM6000 have

been introduced to improve Gas Turbine (GT) efficiency and reduce days of maintenance.

Fig 1- LNG milestones

© 2019 Baker Hughes, a GE company, LLC - All rights reserved.

Driver ISO Power [MW]

Single Train capacity

[MTPA]

MS5002 40 2-4

MS6001 42 3-5

Processes Layout Location

MS7001 87 4-6

C3MR Single train Tropical

MS9001 130 8-10

Cascade Double train Worldwide

LM2500 23-37 2-6.5

Multi fluid cascade Parallel train On shore

LM6000 43-54,5 4-5

Single Mixed Ref Single compressor Off shore

LM9000 67 6,5-7

Nitrogen Multi compressors

LMS100 105-110 7

Electric Motors

5-85 1-5

Steam Turbine

5-60 1-4

Table 1 – LNG Option

MS5002 MS6001/MS7001 MS7001 MS9001 LM2500 LM6000 LM6000pf+ LM9000 LMS100 EM

GT + CCs GT + CCs + EM GT + CCs + EM GT + CCs + EM GT + CCs GT + CCs GT + CC GT + CCs GT + CCs EM + CC

GT + CCs GT + CCs + EM GT + CCs + EM GT + CCs GT + CC

GT + CCs GT + CCs + EM GT + CCs + EM GT + CCs GT + CC

Table 2 – LNG configuration

These ideas were applied to plants from 4 to 6 MTPA. In parallel to LNG process licensors have kept standard their

processes and train arrangement.

After such a period we are now experiencing a multitude of solution for different plant capacity, involving drivers,

train arrangement, liquefaction processes and plant locations.

Now more than ever experience is becoming of utmost importance to develop safely new LNG concepts and trains

relying on proven solution with just some additional ingredients of novelty.

2 Turbomachinery configurations

The train arrangements have evolved during these years, and they can be applied according to different LNG

liquefaction capacity.

GE MS5002 has been widely used in C3MR or in cascade processes. In both cases the turbine was driving single

or multiple compressors without the need of a starter/helper motor; the gas turbine is a double shaft type with

nozzles between high-pressure and low-pressure turbine and is capable of a full pressure start up. GE MS5002 has

been used for plant up to 4.5 MTPA

GE MS7001 has been widely used for C3MR and just once with cascade. The typical C3MR configuration is with

two turbines driving MR compressors on one side and PR + HPMR on the other. MS7001 has been used for plant

up to 5.5 MTPA while MS9001 up to 7.8 MTPA.

Large heavy-duty Gas Turbine are typical single shaft as they are used for power generation and they need a

starter motor. The motor become a helper to increase further LNG production and need other power generation gas

turbine and variable speed systems. Anyway, the helper motor can inject power in the train during hot weather

condition when the gas turbine is reducing the delivery power keeping flat the LNG production.

New two-shaft large power GT can get rid of the helper motor for start up but, as delivered power is reduced during

Fig 2 – LNG production without Helper motor


hot days, LNG production will be reduced as well. Partial or limited piping loop depressurization can also be achieved.

FIG 3 – Possible configuration for new LNG trains


Aero Gas Turbine were introduced first in Australia with the cascade process driving compressor without helper

motors. Aero Gas turbines are also used in floating units driving a single compressor. LM2500 has been used for

plant up to 6.4 MTPA (3.2 x two parallel train); LM6000 was recently introduced for a floating unit and for a plant in

Australia; the LM6000PF+ upgraded model was selected more recently for a project in United States.

Most recent developed Gas Turbine LM9000 has been selected for a large LNG project in North Asia.

Two all-electric LNG plants have been built with large electric motor (larger than 65MW each) for 4.5 MTPA LNG

production.

About driven equipment, new trends in the centrifugal compressors are showing the possibility to fit all the three

MR section in one large vertical split casing; the single casing will benefit for reliability and availability of the plant.

Fig 3 – Possible New MR train line up


EPC and end user are attracted by the giant barrel compressor where multiple process stages or the complete

refrigeration loop can be installed in a single compressor reducing the footprint of the plant; in addition, these

equipment are cheaper and easier to maintain then the equivalent horizontal split; the size has arrived to 200 tons

weight with a bundle’s weight of 100 tons. The special tools to handle such heavy components and the logistics of

the maintenance procedure become very critical to mitigate risks during the outages and must be taken care to

design for the shop and for site operation.

2.1 Latest LNG project train configuration

Newer plant in North America will be built with LMS100 as driver and two centrifugal compressors for higher and

lower molecular weight duties for a single liquefaction line. The turbocompressor train will be built on a module

structure.

Another new LNG project is in North Russia. This plant will be equipped with the recently developed LM9000

derived from aviation engine GE90. One train will be done two LM9000 driving respectively one and two

compressors with gearboxes and inlet guide vanes.

3 Compressor Design

3.1 Aerodynamics

Aerodynamics is key in the centrifugal compressor design. A good stage design will grant an optimum efficiency

reducing absorbed power and fuel gas consumption, increasing further LNG production. Aerodynamics is also

responsible for the operating range which is key in the LNG as the operating point will move along the complete

compressor curve according to ambient temperature.

In addition, LNG production has changed through all the years and now is again moving towards larger power gas

turbine which means higher volumetric flows to be handled by the compressor.

This trend is requiring therefore larger flow coefficients with same level of Mach numbers for higher efficiency and

operating range. OEM must stay current with tools and criteria design to satisfy new process licensor operating

condition. OEM must invest in technologies such as Computational Fluid Dynamics (CFD) steady and unsteady,

coupled with Finite Element tools (FE) to assess mechanical behavior in operation; OEM should also invest in

model testing to assess stage performance and correlate with calculation to predict compressor performance well

in advance respect to acceptance test.

Fig 4 – CFD impact on performance prediction


Recent advances in impeller design are impacting the shape of the blade that is not anymore linear but fully 3D to

better fit with the flow inside the impeller: it is possible to reduce separation or low velocity region inside the vane

channel and increase efficiency and operating range.

Such new geometrical shape has been possible thanks to a massive use of CFD and optimization techniques

coupled with manufacturing efforts to produce and control such geometries.

Impeller thickness can also be custom designed to increase inlet flow throat area and reduce for example the Inlet

Relative Mach Number granting again high efficiency and large operating range.

Fig 5 – 3-D Shapes

© 2019 Baker Hughes, a GE company, LLC - All rights reserved

On the contrary the consequences of thickness reduction can impact the mechanical robustness of the impeller

itself. OEM are using finite element to predict mechanical behavior. This tool now need to predict not only the static

but also the dynamic stresses. Unsteady CFD need to be coupled with FE to asses correctly the aeromechanics.

Methods are available, but results must be correlated with prediction to avoid time consuming calculation during job

execution.

The dynamic analysis can improve not only the impeller itself but can assess the impact of some vaned part of the

flow path that can increase efficiency such as inlet guide vanes, vaned diffusers or return channel blades.

3.2 High Fidelity CFD

Some OEM has developed proprietary CFD methodology with remarkable performance prediction accuracy.

High geometrical fidelity can include cavities in the computational domain, as well as blade fillets, and increased

grid resolution. Additional and improved numerical models has improved a lot the accuracy of static performance

prediction.

OEMs have now a very good tool to assess compressor stage performance during both research and development

activities (to build performance database to select best compressor performances) or during job execution to

anticipate and mitigate any compressor performance well in advance respect to acceptance test.

Fig 5 – High Fidelity CFD output


Fig 6 – Model test results


3.3 Side Stream

Side stream are typical for refrigerator compressors. In the side stream cold gas is injected with considerable

impact on the following stage: the side stream portion is at the tip inlet of the blade and therefore is responsible for

inlet relative Mach number (M1rs) of the downstream blade. The two flows (main and side stream) are not mixing in

temperature and the tip of the blade is receiving the colder gas increasing the Mach number and impacting

efficiency and operating range.

Designer can change relative velocity reducing the M1rs with positive impact both on efficiency and operating

range looking at the velocity triangle and designing properly the volute and maybe adding some blades. In case of

vaned sidestream an aeromechanics analysis should be performed to avoid dangerous crossing of modes.

Fig 7 – Side stream old and new configuration and performances comparison


3.4 Thermoplastic Seals

Thermoplastic seals are divided in two categories:

- rotating teeth (teeth on rotor ... TOR)

- statoric teeth (teeth on stator … TOS)

Rotating teeth are machined directly on the impellers and the statoric part is done by a material that can be eroded

by teeth. This material can be alluminium derived (for example Metco) or plastic derived such as

polytetrafluoroethylene (PTFE)

Statoric teeth seals have typically same construction design of standard aluminum seals but are done by new

materials such as PTFE.

PTFE raw materials can be provided by various supplier and for major application they’re similar.

More fancy materials must be selected once temperatures grow above 180°C or humid gas.

Fig 8 – Thermoplastic seals


As both type of design seals is supposed to have contact between stator and rotor passing through critical speed

and reduced clearance during operating, very detailed calculation must be performed to proper design the bundle

and be confident of reach target efficiency during test.

Typical these calculations are done with FE analysis considering stator and rotor thermal transient impeller during

start up and steady state condition plus impeller mechanical deformation.

Fig 9 – Finite element model


Fig 10 – Clearance variation during start up and steady state


Calculated clearance value is set to avoid rubbing. Each impeller seal is assembled with such designed clearance.

New thermoplastic seals can increase stage efficiency, by reducing internal leakage, from 0.5 to 2 points

depending from flow coefficient. Standard material can reach 150°C maintaining mechanical properties, while to

reach higher temperature (>180°C) more expensive material must be selected.

3.5 Performance Predictability

Compressor performance is the most important objective together with rotordynamic stability. A big improvement

has been done in the last ten years with the introduction of the CFD. This tool has helped design engineer to

predict stage performances during job execution and, in case, to modify impeller geometry to avoid issue during

testing. CFD is not applied to all impellers in production but is typically limited to the most critical ones (like high

Mach stages); CFD is widely used during aerodynamics research and developments to develop new geometries for

high efficiency and operating range. CFD results are then compared with test results and compressor performance

database is updated accordingly.

The other resource to improve performance predictability is the performance test both type 2 or full-scale test.

Every single test should be recorded and compared with prediction tool; correction to test results must be applied to

consider different design parameters or operating condition such as:

- back-to-back with two balance drums

- straight through machine

- scale effect due to machine size (diameter can vary from 200mm to 2000mm)

- operating pressure & Reynolds effect; high pressure machines are typically tested at low pressure

condition without Reynolds effect and should be compared with prediction at same level of pressure

- pressure and temperature probes

- test uncertainty

All the above consideration should be considered during the performance prediction and be included in the

prediction tool.

Implementing all these statistical results we have seen the improvement below as en example.

Fig 11 – Performance Predictability impact on test results before and after.


3.2 Mechanics

3.2.1 Casings

In the recent years there was the need to reduce the size of the casing to reduce weight and installation cost. In

addition, the design pressure (Max Allowable Working pressure MAWP) has increased to reduce gas waste to

flare.

Moreover, different process section has been coupled in just one casing increasing the MAWP.

This is for example the case of a compressor installed in Australia, where LP and MP MR section were combined in

one casing with the following results:

- Large inlet Flange 72inches

- Largest Helper Motor

- Large Casing Design Pressure 42.5 barg

- Largest power for single casing 100MW compressor

Fig 12 – Largest horizontally split compressor casing (hydro test and string test)


Large barrel compressors have been used first for large LNG project in Middle East and now are coming back to fit

with single refrigerant LNG process; they fit well with turbocompressor train with just one compressor coupled with

a large gas turbine (LM6000/LM9000) or large EM (> 50MW).

Barrel type compressor are very robust, simpler and cheaper than horizontally split compressor. On the contrary

they have O-rings that have a limited life and should be changed. Dedicated design, procedures, special tools and

cranes to assembly and disassembly the bundle from the casing must be carefully studied to handle very heavy

pieces.

Compressor assembled weight can be larger than 260 tons, the bundle can overcome 100 tons and the heaviest

part for maintenance can reach 20tons, complicating the logistic of an outage.

All references for horizontal and vertical split compressor can be represented in scatter plot diagram (fig 13).

Fig 13 – Casing references


3.2.2 Impellers

A series of scatter plot are presented relevant to all impeller used as high Mach Impeller (above 0.75).

In the first two figures are presented three types of impeller developed since 2000 when CFD was at the beginning.

Two evolutions of the same family were studied and deployed during the recent years when Mach number has

always been high as peripheral and as inlet relative.

Impellers have been designed to accommodate higher flow coefficient increasing efficiency at high flow. New CFD

approach like High Fidelity has allowed more precise calculation and more efficient aerodynamic profiles.

Mechanical limits of the peripheral speed have remained unchanged while less power for impellers has been

required due to a switch to aero-derivative drivers or reduced size plant.

Fig 14 – High Mach impeller. Peripheral and Inlet Relative


Fig 15 – High Mach impeller. External Diameter and Peripheral Speed


Fig 16 – High Mach impeller. Work coefficient and Efficiency


3.5 Rotordynamics

Lateral and torsional behavior are still very important and critical during the compressor design. In particular lateral

stability of the compressor should be well assessed during design to avoid delays during train comminsiong and

eventually rework of the compressor itself.

New methodologies have been applied in the recent years such as Operational Modal Analysis (OMA). This

technique is coming from the civil engineer and it’s used to measure the stability of bridges or skyscrapers;

technicians install accelerometers, wait for winds or cars and measure the response to all these excitation modes;

then with this mathematical technique it’s possible to measure exactly mode shapes, critical frequencies and

stability.

In the rotating machinery the excitation is coming from the aerodynamic noise and the probes are the vibration

probes calibrated for this purpose. Results have been excellent even compared with traditional methods such as

magnetic exciter.

With such new techniques it's possible to minimize the safety factor historically used to calculate compressor

logarithmic decrement and design more modern compressor.

Fig 17 – High Mach impeller. Power per impeller


Fig 18 – Exciter assembled on LNG String Test


Fig 19 – Stability results comparison btw Exciter and OMA


Conclusion

A new era is approaching in the LNG industry. Driver type has been doubled introducing new large power high

efficiency gas turbine, LNG liquefaction processes have more than double, and location are now worldwide, from

tropical to cold winter and hot summer.

Compressor designer have developed new technologies to keep updated the driven equipment. Some novelties

are impacting all compressor components such as impeller, diffuser, volutes and seals aiming to increase

efficiencies or operating range; some other are aiming to reduce weight and footprint such as new casing design or

new rotordynamic methodologies. At the end compressor is slowly evolving introducing little changes to follow the

LNG trend.

References

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technology evolution of lng refrigerant … · ms9001 130 8-10 cascade double train worldwide...

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