Moccasin Penstock
Overview
The Moccasin Penstock conveys Hetch Hetchy water from Moccasin Tunnel to the Moccasin Powerhouse. This project will enhance the reliability of the penstock system, which is critical both for power generation and drinking water supply. An original plan to rehabilitate the nearly 100 year-old penstocks has been revised in order to pursue a plan for their replacement.
A penstock is a pipeline that delivers water to drive an electricity-generating turbine. The existing system is above ground, but a penstock can be located either above or below ground.
Details
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Project Background
The Moccasin Penstock conveys Hetch Hetchy water nearly one mile from Moccasin Tunnel to the Moccasin Powerhouse. The lower 1,084-foot section of welded steel pipe replaced the original penstocks when the new Moccasin Powerhouse was completed in the 1960s. The upper 4,000 feet of penstock dates back to 1924 and has been in service for nearly a century.
Condition assessments based on external inspection and imaging identified a number of deficiencies along the original pipe. This project is intended to enhance the reliability of the penstock system and will include: repair or replacement of some sections of corroded pipe; repair or replacement of four badly cracked concrete anchors and damaged penstock saddles; installation of new manways and a rollout pipe section to provide better access for inspection and maintenance; and recoating the outside pipe, where needed, to reduce future corrosion.
Based on a preliminary study, the project scope was expanded to include a combination of a drop shaft, a tunnel and above grade pipes. The study also considered the replacement of the butterfly valve pneumatic actuator with an electronic actuator, which will include new controls with SCADA connectivity; and a new backup generator.
1925 Photo of original penstock construction
The project is part of a larger Hetchy Capital Improvement Program (HCIP), a multi-year capital program to upgrade or improve water conveyance, water storage, and power generation facilities in the Sierra Nevada portion of the Hetch Hetchy Regional Water System.
- Project Materials