For many years, the traditional method to let down pressure in a liquid stream used a Joule-Thomson (J-T) valve. An improvement in the overall efficiency of this let-down process can be achieved by replacing the J-T valve with an expander. In doing so, and depending on the machine, the overall plant efficiency may increase up to 8%, reducing operating costs and increasing LNG production.
By using the technology and extensive experience from the proven Cryodynamics® pump design, an extremely reliable and efficient expander was developed. The cryogenic expander is a radial inflow reaction with an induction generator mounted on an integral shaft. The entire unit, including the turbine and generator, are totally submerged in the cryogenic fluid. This design completely eliminates the need for dynamic seals, shaft couplings, generator support structure, and any concern for possible misalignment between the turbine and generator. By eliminating possible leakage from rotating seals, the design is extremely safe, especially when hazardous fluids are being handled.
To ensure reliability and increase mean time between overhauls, our expanders use the same Thrust Equalizing Mechanism (TEM®) as used in our pumps for decades. This system completely eliminates axial thrust loads, dramatically increasing bearing life and reducing machine wear.
Cryogenic expanders can be supplied with variable speed technology together with fixed inlet vane geometry. By introducing variable speed technology to the application, the efficiency of the expander can be optimized over a wide range of flows and desired pressure reductions. This method enables automatic optimization of work transfer and performance to match changes in process conditions. At the heart of the system is the variable speed, constant frequency (VSCF) controller, which can vary the speed of the generator and expander to precisely match process conditions. At the same time, power is produced at constant frequency with unity power factor. The low harmonic sinusoidal output can then be fed directly back into the plant electrical system. In processes where few operating points are expected, cryogenic expanders may be supplied without the VSCF for constant speed operation. In this configuration, the expander can be connected directly to the site’s electrical grid, simplifying the install and reducing installation costs.
All expanders are installed inside a pressure vessel designed and built to applicable pressure vessel codes. The system is suitable for installation in hazardous area locations as required and defined by applicable codes and standards. In addition, each expander is fully tested at the Cryogenic Performance Test Facility simulating actual site conditions. Testing can be performed at full speed and full power with a variety of liquids such as LNG, LPG, or LN2, depending on the temperature and specific gravity of the service fluid. For more information on testing of cryogenic expanders, click here for one of our recently published technical papers which elaborates more fully, the technical aspects of testing these revolutionary machines.
The latest innovation in the Cryodynamics line of expanders is the upward flow, two-phase expander. These expanders allow the expansion of liquefied gases into two-phase mixtures within its final stage. By harnessing the energy of two-phase expansion, the expander’s generator output is increased, boil-off losses are reduced, and the overall liquefaction process is improved even further.