Hydroelectric Generator-Motor Brake Dust Collection: Boundary Condition Selection for Flow Modeling of a Dilute-Phase Pneumatic Conveying System

Abstract

Due to a significantly large mass moment of inertia many large vertical pump-turbines, used at pumped storage plants, for hydroelectric power generation are equipped with mechanical braking systems. After shutdown, these systems allow the unit to be brought to rest in a relatively short period of time. Even units which take advantage of electrical braking use mechanical brakes. The brakes are applied when the unit’s angular velocity is sufficiently small; this reduces wear on the thrust bearing and other components. Additionally, mechanical brakes prevent slow rotation of the unit due to wicket gate leakage and are also used during routine maintenance. The brake dust generated from the braking process can contaminate various sections of the generator-motor, adversely impacting unit performance. To prevent this and improve unit reliability and availability, many larger units are equipped with brake dust collection systems. Brake dust collection systems utilize air to convey brake dust to a filtering apparatus, where the dust is collected and transported safely away from sensitive components. Such a system can be described as a dilute-phase pneumatic conveying system. The most significant design parameters are: pressure drop, solids flow rate, and gas flow rate. These parameters are optimized to ensure the correct saltation and pickup velocities are achieved to convey the material over the required distance. Some system designers employ equal flow as an inlet boundary condition (BC) for designing pneumatic conveying systems of this type. This study examines the validity of using equal flow versus equal pressure as an inlet BC; with the goal of demonstrating that equal pressure is more appropriate and results in a more robust system design. This was accomplished by developing a steady state flow model of the brake dust collection system in PIPENET™. Model results show that the equal pressure inlet BC is not only a more conservative BC, but its application also revealed potential short comings in the system’s ability to attain adequate pickup velocities in various sections of the system. The selection of the appropriate BC is essential to the successful design of a robust pneumatic conveying system.