Maintaining Penstock Protection During a Digital Controls Upgrade: A Review of Approaches and Risks

Presented by Matt Roberts at HydroVision International 2019 in Portland, OR

With few exceptions, hydro is considered a safe industry, with few pieces of equipment whose failure can cause catastrophic damage to life or property. Some of those exceptions are in the cases of penstock safety for high head units. Due to the nature of pressure relief valves (PRVs), their operation is invariably linked to wicket gate operation and penstock safety, which in turn means that governor control modification will affect PRV operation. They must be jointly considered when upgrading mechanical equipment to digital control, as digital controls come with a new set of failure modes that have the potential to weaken the safety features in place with older mechanical control equipment. Extreme care must be taken when designing PRV control modernizations which include understanding the tolerance for PRV misoperation, possible failsafe states, and various failure modes, all of which must be addressed through in-depth FMEA (Failure Modes and Effects Analysis).

Utilizing various case studies spanning utilities found mostly in the United States where head levels may be higher, including the U.S. Bureau of Reclamation (BOR), Pacific Gas and Electric (PG&E), Portland General Electric (PGE), and Placer County Water Authority (PCWA), this paper will present an array of various PRV operating mechanisms that exist and approaches which have been taken to modernize them with the intent of sharing this information with other utilities that may be looking to modernize the controls on their PRV. Moreover, this paper will discuss the precision and importance of PRV operation as well as how to maintain the original level of safety and reliability that was implemented with mechanical equipment as controls equipment is modernized and digitized.