Liquified Natural Gas Pump With Hydrostatic Journal Bearings
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In view of the inflammability of LNG (liquefied natural gas), submerged type pumps are normally used to protect against leaks. The overhaul interval of an LNG pump, therefore, is subject to the life of its ball bearings, because LNG is an extremely low temperature, low viscosity and low boiling point liquid. The use of LNG to fulfill the extremely demanding functions of lubrication and cooling of ball bearings results in substantial reduction in the bearings’ maximum load tolerance. The limits inherent in the ball bearings, consequently, will make it difficult to extend the life of the bearings beyond that currently achieved, if the bearing system itself depends on the use of conventional ball bearings. Since the service life of a ball bearing is not necessarily constant, there is a possibility of sudden break-down. Thus, new LNG pumps were developed using a form of bearing for which service lifetime is not primary importance, though it nevertheless has a long service life and high reliability, and is small and light. Details are given later. Two types of the static bearings were developed: a hydrostatic slide bearing and a hydrostatic guide bearing, incorporating a hydrostatic journal bearing surrounding the ball bearing. The hydrostatic slide bearing supports the shaft by virtue of the film pressure of the LNG discharged from the pump itself. Therefore, the shaft with ball bearings in the usual way, the buffer effect of LNG in the hydrostatic guide section substantially reduces the radial load of the ball bearings. A hydrostatic journal bearing does not bear any of the thrust load. A structure has, therefore, been developed in which a disk-piston-type thrust balancing system can be used to enable self-balancing of the shaft thrust load, thereby reducing the thrust load to zero. The downstream side of the disk piston is designed to allow a variable clearance whereby, when it is at its maximum, the total flow area upward thrust force exceeds the downward thrust force. In this way, the two thrust forces always balance each other automatically. A high lift impeller was designed, the number of operational stages was reduced, and, radial diffusers were used to enable the use of shorter shafts. For the stage housing, a piston ring type sealing is used, a single layer cylinder casing method was developed that enables a multistep seal provided by the piston rings at each stage. This allowed the pump diameter to be reduced. The use of hydrostatic slide bearings (hydrostatic guide bearings), a thrust balancing mechanism, and the reduction of shaft length has enabled substantial extension to be made to the overhaul interval of pump. Pumps with hydrostatic slide bearings:100,000 hours or more. Pumps with hydrostatic guide bearings:50,000 hours or more. The use of hydrostatic slide bearings results in some reduction in the shaft bearing performance, due to wear of the face of the bearing. Measurement of the amount of wear, however, enables the remaining life of the bearing to be predicted, and this in turn facilitates cost reductions in terms of the failure diagnosis system employed. Furthermore, since there is little likelihood of sudden failure, maintenance schedules can be organized in a systematic way. The use of high lift impeller and single wall cylindrical radial diffuser has enabled the production of a compact, lighter pump unit, which is easy to disassemble and reassemble. This technology is applicable to pumps for other liquefied gas, for example, LPG, LEG (liquefied ethylene gas), etc.
Katayama, Tatsuo; Okada, Akihisa (1992). Liquified Natural Gas Pump With Hydrostatic Journal Bearings. Turbomachinery Laboratories, Department of Mechanical Engineering, Texas A&M University. Available electronically from