TPS 2023: Turbomachinery in LNG Production Plants

On day two of Turbomachinery & Pump Symposia (TPS) 2023, industry professionals presented on Turbomachinery in LNG Production Plants, offering attendees a review of major turbomachinery in LNG plants, focusing on how compressors, pumps, and gas turbines fit into the various LNG plant cycles, including their integration, operation, and application limits. The tutorial also discussed how to best match turbomachinery for optimal service in LNG plants based on the power, speed, and utility needs of the most common LNG cycles.

The session featured folks from the Elliott Group: Derrick Bauer (Manager), Klaus Brun (Director), Brian Hantz (Senior Engineer), Enver Karakas (Manager), and Brian Pettinato (Manager). They were joined by Rainer Kurz, Manager of Gas Compressor Engineering at Solar Turbines.

The Rise in Demand for LNG

The worldwide demand for LNG has risen over the last 20 years, and due to geopolitical instability, this trend is expected to continue in the foreseeable future. LNG plants often use a wide range of turbomachines—including centrifugal compressors, gas turbines, steam turbines, cryogenic pumps, and expanders.

Brun led the session with a preliminary explanation of LNG, defining it as the large-scale refrigeration of natural gas to achieve liquid form. It is typically liquefied at temperatures ranging from -260°F to -280°F, which varies depending on the type of gas chosen for liquefaction. Originally derived from small-scale refrigeration compressors, modern LNG liquefaction converts a feed gas into a finished fuel product for power generation, heavy-duty transport, alternative fuels, etc.

Klaus Burn of Elliott Group kicked off the discussion at TPS 2023 on Turbomachinery in LNG Plants. Credit: Turbomachinery International

Brun discussed the ever-growing popularity of LNG production through an analysis of plant scale and output over multiple decades: the largest plants in the 60s and 70s produced a maximum of 2,000,000 metric tons per annum (MTPA), while modern, larger LNG facilities push the boundaries upwards of 7,000,000 MTPA. Despite the rapid growth in average LNG production, the output range of modern LNG plants can still range from as low as 5,000 MTPA to the aforementioned 7,000,000 MTPA.

Processing LNG

A series of turbomachinery equipment and processes are required to complete the transfer of the raw material into a viable fuel source, namely compressors, expanders, pumps, a variety of seals, and differing refrigerant methods.

Brun introduced an extensive list of compressors used in LNG applications, as well as the purpose of a compressor in an LNG train. Compressors pressurize and refrigerate the gas to prepare it for storage and transportation as a finished product, but the sheer variety of available compressors allows for unique focuses in LNG production. Examples of compressor models commonly used in modern LNG production include:

• end flash gas compressors
• boil-off gas compressors
• CO2 injection compressors
• refrigeration compressors

This machinery, in addition to the rest of the LNG train, processes natural gas so that it is safe for large-scale industrial applications.

Compressors 101

Pettinato gave a complete component breakdown, organized by rotating and stationary parts. The rotating parts of a centrifugal compressor include impellers, sleeves, shafts, coupling hubs, and thrust disks. Stationary parts include end walls, diaphragms, casings, nozzles, labyrinth seals, bearings, end seals, and piston lines. Determining the size of a compressor in an LNG train is vital for optimal efficiency.

“The sizing is determined by the flow and available technology: The larger the amount of flow you have to get through, the more you tend to scale up the compressor,” Pettinato said.

Pettinato said the various degrees of testing that a compressor arrangement is exposed to largely attempt to identify excessive vibration and assess the performance efficiency of the equipment. Operators may use mechanical running tests, performance tests, or full-load full-pressure inert gas string tests—a costly option—to determine inconsistencies and improve the compressor operation guided by speed, inlet vanes, and suction throttling.

Cryogenic Pumps and Expanders

Karakas introduced the role of cryogenic pumps and expanders within an LNG application.

“To use a pump, you have to liquefy the gas,” he said, “So while pumps may be more efficient in terms of transporting fuel, there are a lot of processes that need to first take place to be able to use the pump.”

Pumps for LNG processes transfer and pressurize the fluid while it’s in liquid form, while cryogenic liquid expanders disperse the fluid through a Joule-Thomson (JT) valve. Karakas stressed the importance of proper cooling in an LNG train to avoid issues such as high flammability and to improve the overall quality of the gas, citing a submerged motor-driven pump design as favorable for this process because “A submerged motor design has a submerged motor inside the LNG so that the OEM doesn’t need to worry about certification for hazardous areas. Putting the motor in fluids means I don’t have to worry about classifications for hazardous areas. Another benefit is the cooling by the LNG fluid,” he said.

Turbine and Motor Components

Kurz offered a simple overview of gas turbines, electric motors, and steam turbines. A gas turbine is comprised of a compressor, a combustor, and a turbine section, which provides the power to drive the engine compressor, and whatever power is left over is used to drive a compressor, pump, or generator.

The two primary methods of combustion in a gas turbine are conventional combustion and low NOx combustion. The available driver power and speed have to be matched to the LNG compressors. For the best efficiency and economy, the gas turbines should run at maximum power.

Materials

The types of materials chosen to construct LNG components must be properly heat treated and field tested to ensure operational safety, according to Bauer, as a poorly tempered metal could cause a catastrophic failure when subjected to high pressures, corrosive gas, or extreme temperatures. Metals with high nickel content, low carbon content, cleanliness, proper heat treatment, and small grain size are optimal for inclusion in LNG equipment.

Bauer delved into atomic structure to explain the advantages of a ductile material for compressor casing design and internal components, “The ability of the planes of atom cubes to slide over each other enables a material to be ductile; it allows the planes to move no matter where the temperature is at,” he said.

Having established the optimal atomic structure of an LNG metal and provided a list of sought-after material requirements, Bauer advised the audience to avoid metals with two specific qualities: high carbon content and low alloy content. Carbon steels and low alloy steels are susceptible to the “ductile-to-brittle” transition, in which a material exposed to LNG processes over time begins to weaken, crack, erode, and eventually fail. Testing of the material’s toughness is required prior to its integration into components, so OEMs often conduct Charpy v-notch impact testing to ensure the material can endure massive amounts of stress. The test applies an immediate downward force to the material in an effort to shatter it, then data is collected to determine the resistance of the material in an LNG application. Conducting material testing is perhaps the foremost step in preventing catastrophic failure at LNG plants.