LNG plants are equipped with several auxiliary services that accompany compressor trains. These services are needed to increase plant efficiency (boil-off gas [BOG], end flash) or are necessary to keep the gas flowing (feed gas, domestic gas, stabilizer) or for fuel gas.
BOG compressor: Among all the auxiliaries services, the BOG compressor is one of the most critical. The BOG service recovers gas from the tank, avoiding flaring. A typical BOG train consists of a fixed-speed electric motor, a gearbox and a couple of centrifugal compressors to pressurize gas from almost ambient pressure to fuel-gas pressure for a gas turbine. The critical aspect of a BOG compressor is the temperature.
The minimum design metal temperature is typically -256°F (-160°C), which is also the suction temperature of the compressor; moreover, the compressor must work in an alternate case at around -184°F (-120°C) and at other conditions at ambient temperature (during recycle or at startup). This multiple-temperature operating condition makes the compressor difficult to design from a mechanical point of view, from the IGV movement in a cryogenic environment to transient operation to dry-gas seal insulation. The only material suitable at such low temperatures is 9% nickel. A qualification test campaign was developed and performed to verify the behavior of the material at different temperatures, avoiding issues of different relative displacement among components.
An OEM built a dedicated test model for IGV and insulation systems at low ambient temperatures (Figure 9). A scale 1:1 model was built and inserted in a tank that can be filled with liquid nitrogen at -310°F (-190°C). A detailed test sequence then verified the proper functioning of the system at different temperatures with aerodynamic load simulations on the IGV blades. The same tank has also been used to test an insulation system for the DGS with synthetic oil.
For other components, such as the rotor and diaphragm, where testing was not possible, an intensive finite element method campaign was performed to simulate transient behavior of the rotor and stator, checking relative movement and clearances.
All these studies were later applied to a BOG service for Qatar. In this case, three compressors were built, each one equipped with an IGV to compress gas up to Frame 9 fuel-gas pressure, and these have been in operation since 2014.
CO2 compressor: More recent protocols regulation will lead oil and gas companies to reduce CO2 emissions. Many of them have decided to re-inject CO2 in geological formations (Figure 10). In LNG, CO2 can be in the extracted gas and must be separated from the other components before liquefaction. For this reason, the CO2 is not pure as in a petrochemical plant, but it is mixed with water or hydrogen sulfide (H2S) that makes it an acid gas application. Even if there is still a large debate on which technology should be applied in CO2 applications, in case of high-pressure acid gas, the best choice for the moment is still a beam compressor versus an integrally geared compressor.
An example of a CO2 re-injection service is in operation in Australia where the service will be accomplished by two barrel compressors as described in detail in the paper “CO2 Compression at World’s Largest Carbon Dioxide Injection Plant,” presented at Turbo Symposium 2012 .
Other compressors: All the other compressors are more or less standard compressors. There are services that deal with clean natural gas, such as fuel gas, feed gas or booster gas, that usually use midsize barrel compressors driven by gas turbines at 33,000 to 40,000 hp (25 to 30 MW).
Dynamic simulation is relatively new for the oil and gas business. It has increased its importance for multiple applications, such as startup; emergency shutdown; anti-surge valve verification; and process control software validation, debug and tuning (anti-surge, load sharing, performance controller) (Figure 11).
The startup simulation is extremely important for helper motor sizing and to increase the startup pressure of the compressor. Instead of depressurizing the loop completely, a higher pressure can be determined with good savings for refrigerator gas.
As several LNG liquefaction plants are being built in remote areas where labor is very scarce and expensive, there is a trend towards modularization where a majority of the work can be done by the compressor OEM or in a separate module yard. The extent of modularization has to be an economic decision depending on the labor availability and other economic factors. The extent of modularization ranges from partially assembled units to full large-scale modules.
A fully modularized LNG plant can also be remotely controlled and implement new digital solutions that can optimize various equipment and boost LNG production.
Many innovative considerations and solutions have been presented, from new aerodynamics for impellers to new mechanical arrangements for impellers and casings; robust solutions for new projects and for rebundle have been also presented in addition to innovative solutions for auxiliary compressors. All considerations may lead to laying the foundation for the next generation of centrifugal compressors in the LNG market.
About the authors:
Antonio Pelagotti is manager of new product development within the Compressor Design Department for Baker Hughes, a GE company in Florence, Italy. Contact him at: firstname.lastname@example.org. Leonardo Baldassarre is the engineering leader for compressors, expanders, transmissions and electrical systems within Baker Hughes, a GE company in Florence, Italy. Contact him at: email@example.com.