Design replaces existing pressure-actuated lift-off seals in turbopumps and eliminates low pressure drains, thereby increasing overall efficiency.
This invention is a magnetically actuated seal in which either a single electromagnet, or multiple electromagnets, are used to control the seal’s position. This system can either be an open/close type of system or an actively controlled system.
A lift-off seal (LOS) is a type of shaft seal used in a turbopump that does not allow propellants to enter the turbine during pre-start operations, such as when a cryogenic turbopump is being chilled-in or when the pump is being primed prior to start. Typically, lift-off seals are pressure activated and a low constant pressure in the seal’s secondary seal cavity is needed to provide the delta- P necessary for the seal to open. This is typically accomplished with an overboard drain cavity. The LOS must remain closed during pre-start operations. This prevents cryogenic liquid from chilling-in the turbine, which would result in excessive thermal shock, and subsequent turbine blade cracking. During the start-transient, the LOS must open to prevent propellant gasification and sometimes to provide coolant to the turbine disk. If it opens too soon, however, the turbine pressure can be higher than the pump pressure, and result in hot gas ingestion into the pump or bearings. If it opens too late, the seal surface speed becomes excessive, and results in excessive wear and premature failure of the seal.
The magnetically actuated LOS is more reliable and requires no low-pressure secondary seal cavity or overboard drain (thereby improving efficiency). An electromagnet is used to open and close the seal at an exact prescribed instant during the transient. Additionally, with the magnetically actuated seal, the particular instant can be different between the start transient and shutdown transient. This allows for more desirable and predictable transient performance of the turbopump as well as more certain wear performance of the seal.