Accelerated Lifetime Testing of Main Shaft Seals for Tidal Turbine Rotors

This report briefly discusses the observations and results from accelerated lifetime testing performed by the National Renewable Energy Laboratory (NREL) on the main shaft seal for the Verdant Power fifth-generation (Gen5) underwater tidal energy converter turbine, which successfully performed at the Roosevelt Island Tidal Energy project in 2020?2021. To evaluate a 5-year service interval (SI) for this component, the main shaft seal was operated nearly continuously for 137 days at a rotational velocity of 160 rotations per minute while the test stand recorded water pressure, barrier fluid pressure, temperature, and number of cycles, representing ~ 40% of the SI. An additional separate test was conducted to measure the aging behavior of the rubber drive rings. For the SI evaluation the water pressure reservoir was held constant as 199,9 kPa (29 psi). Barrier fluid pressure remained relatively constant throughout the duration of the test but fell to as low as 69.6 kPa (10.1 psi). No barrier fluid leakage was observed throughout the test. A sudden failure occurred within the seal after the power to the test machine was interrupted for a scheduled building maintenance procedure. Upon restarting, the main shaft seal lost all ability to prevent water ingress. The exact cause is not known but is believed to be either a seal assembly issue or a change in the alignment of the seal components during or following the power outage. Following seal disassembly, one of the graphite sealing rings showed significant wear. Verdant Power, Dovetail Solutions LLC, and Garlock Sealing Technologies reviewed the seal wear for consensus evaluation of results. NREL returned the seal faces to Garlock, and a review of them indicated a misalignment of the test stand, both overall (shaft moving as a whole) and from front to back (more movement on the water side than the air side). Garlock further indicated that minor misalignment is usually absorbable by the seals; so, the noted wear leads to the conclusion of a test stand disruption. Therefore, the operation of the Gen5 seals to the point of power outage is indicative of long-term performance. Based on these results, it is recommended that follow-on testing be conducted through NREL?s Testing Expertise and Access for Marine Energy Research (TEAMER) program to rectify protocol and assembly issues to further evaluate the SI of this component.