The new LMS100 (Figure 3) is a driver that can be used in LNG thanks to its high flexibility and efficiency. The train can consist of the driver plus just two compressors to produce 4 mtpa of LNG.
It is worth noting that the longest refrigeration train in the world is installed in Angola and is configured as a Frame 7 and three centrifugal compressors plus an electric motor (Figure 4). This configuration has a second train (Frame 6 and three centrifugal compressors plus an electric motor) to complete the refrigeration loop. The entire liquefaction train comprises two trains using Frame 7s and two with Frame 6s for a production total of 5.2 mtpa.
The configuration of the Frame 7 with three compressors and an electric motor is very attractive because it can accommodate all the services (mixed refrigerant and propane) on the same shaft for a single liquefaction train. This can then be copied and the liquefaction plant can have a parallel train configuration, granting 365 days of production including maintenance of one driver. The helper motor can be designed to compensate for the gas turbine’s lack of power during the hottest days, granting full production during the year. A helper can also be used as a starter, reducing the gas flared during startup. This adds the disadvantage of complicating plant layout and complexity, but it is easily managed thanks to large engineering, procurement and construction (EPC) and oil company experience.
The most recent trend in LNG plant layout is to have multiple modules of equal capacity, with each module having one driver and one compressor with a single production variable from 0.6 to 1.5 mtpa, depending on driver power.
E-LNG has recently been used for a 5 mtpa plant in the U.S. The motor size is rather large at around 107,000 hp (80 MW). New, high-power gas turbines such as the LM6000PF+ and LM9000 (Figure 5) have been studied and deployed to maximize module capability. In these projects, there is only one process gas, typically a hydrocarbon mixture.
FLNG compressors
Several projects have been developed for floating liquefied natural gas (FLNG) applications with different liquefaction technologies. In FLNG applications, due to size and weight constraints, single-casing compressors are used, and these are often barrel designs for ease of installation and maintenance. Typical arrangements include a barrel compressor with an upward nozzle and optimized casing design to reduce weight and dimensions. Special tools must be developed to take care of bundle maintenance in offshore situations and must also take into consideration barge motion.
High-pressure-ratio compressor development
Increasing financial pressures are causing a push for a reduction of the capital expenditures in the LNG market. A good way to limit costs is to reduce the volume of equipment installed and/or its size. Some original equipment manufacturers (OEMs) have recently introduced a high-pressure-ratio compressor (HPRC) to fulfill such a need. The main advantage of this equipment is related to the rotor construction, which is not made with a solid shaft piece with impellers assembled by interference fit, but is constructed by combining impellers connected axially with a geared connection (Hirth coupling) plus a tie rod. This design overcomes the speed limits for interference connections and increases the rotational speed of the impeller by approximately 40%. For a train equipped with an HPRC, the gear can be either a standard parallel axis-type design or an epicyclic gear to reach the highest gear ratio.
The impeller for an HPRC can be open- or closed-type, depending on mechanical properties and efficiency.
The HPRC allows approximately a:
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